NBS Measurement Services:

Research Information Center

iNalioiml Bureau of Standards

Gaithersburg, Maryland 20899

Calibration of

X-Ray and Gamma-Ray'''

' ' ''''

IVIeasuring Instruments

NBS

Special

Publication

250-16

Paul J. Lamperti

Thomas P. Loftus

Robert Loevinger

U.S. Department of Commerce

National Bureau of Standards
Center for Radiation Research

The Center for Radiation Research is a major component of the National Measurement Laboratory in the National

Bureau of Standards. The Center provides the Nation with standards and measurement services for ionizing

radiation and for ultraviolet, visible, and infrared radiation; coordinates and furnishes essential support

to the National Measurement Support System-for ionizing radiation; conducts research in radiation related

fields to develop improved radiation measurement methodology; and generates, coirpiles, and critically

evaluates data to meet major national needs. The Center consists of five Divisions and one Group.

Atomic and Plasma Radiation Division

Carries out basic theoretical and experimental research into the • Atomic Spectroscopy

spectroscopic and radiative properties of atoms and highly ionized • Atomic Radiation Data

species; develops well-defined atomic radiation sources as radiometric • Plasma Radiation

or wavelength standards; develops new measurement techniques and

methods for spectral analysis and plasma properties; and collects,

compiles, _and critically evaluates spectroscopic data. The Division

consists of the following Groups:

Radiation Physics Division

Provides the central national basis for the measurement of far ultra- • Far UV Physics

violet, soft x-ray, and electron radiation; develops and disseminates • Electron Physics

radiation standards, measurement services, and data for for these radia- • Photon Physics

tions; conducts theoretical and experimental research with electron,

laser, ultraviolet, and soft x-ray radiation for measurement applications; determines fundamental mechanisms of electron and photon interactions with matter; and develops advanced electron- and photon-based

measurement techiques. The Division consists of the following Groups:

Radiometric Pliysics Division

Provides national measurement standards and support services for ultra- • Spectral Radiometry

violet, visible, and infrared radiation; provides standards dissemination • Spectrophotometry

and measurement quality assurance services; conducts research in optical • Radiometric Measurement Services

radiation, pyrometry, photometry, and quantum radiometry; and develops

spectroradiometric and spectrophotometric standards and calibration

procedures. The Division consists of the following Groups:

Radiation Source and instrumentation Division

Develops, operates, and improves major NBS radiation facilities

including the electron Linac and race track microtron; develops,

designs, and builds electronic and mechanical instrunentation for

radiation programs and facilities; provides national leadership

in the standardization of nuclear instrumentation; and develops

new competence in radiation sources and instrunentation. The

Division consists of the following Groups:

• Accelerator Research

• Linac Operations

• Electronic Instrumentation

• Mechanical Instrunentation

Ionizing Radiation Division

Office of Radiation Measurement

Radiation Theory

Radiation Chemistry and Chemical

Dosimetry

Neutron Measurements and Research

Neutron Dosimetry

Radioactivity

X-Ray Physics

Dosimetry

Provides primary national standards, measurement services, and basic

data for applications of ionizing radiation; develops new methods of

chemical and physical dosimetry; conducts theoretical and experimental

research on the fundamental physical and chemical interactions of

ionizing radiation with matter; provides essential standards and measurement support services to the National Measurement Support

System for Ionizing Radiation; and develops and operates radiation

sources needed to provide primary radiation standards, fields, and

well-characterized beams of radiation for research on radiation

interactions and for development of measurement methods. The Division

consists of the following Office and Groups:

Nuclear Physics Group

Engages in forefront research in nuclear and elementary particle physics;

performs highly accurate measurements and theoretical analyses which

probe the structure of nuclear matter; and improves the quantitative

understanding of physical processes that underlie measurement science.
NBS MEASUREMENT SERVICES:

CALIBRATION OF X-RAY AND GAMMARAY MEASURING INSTRUMENTS

Paul J. Lamperti

Thomas P. Loftus

Robert Loevinger

Center for Radiation Research

National Measurement Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued March 1988
Library of Congress Catalog Card Number: 88-600515

National Bureau of Standards Special Publication 250-16

Natl. Bur. Stand. (U.S.), Spec. Publ. 250-16, 137 pages (Mar. 1988)

CODEN: XNBSAV

Commercial products— materials and instruments— are identified in this document for the sole pur- pose of adequately describing experimental or test procedures. In no event does such identification imply recommendation or endorsement by the National Bureau of Standards of a particular product; nor does it imply that a named material or instrument is necessarily the best available for the purpose

it serves.

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1988

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
PREFACE

The calibration and related measurement services of the National Bureau

of Standards are intended to assist the makers and users of precision

measuring instruments in achieving the highest possible levels of

accuracy, quality, and productivity. NBS offers over 300 different

calibration, special test, and measurement assurance services. These

services allow customers to directly link their measurement systems to

measurement systems and standards maintained by NB,S. These services

are offered to the public and private organizations alike. They are described in NBS Special Publication (SP) 250, NBS Calibration Services

Users Guide .

The Users Guide is being supplemented by a number of special

publications (designated as the "SP 250 Series") that provide a detailed description of the important features of specific NBS

calibration services. These documents provide a description of the:

(1) specifications for the service; (2) design philosophy and theory;

(3) NBS measurement system; (4) NBS operational procedures; (5)

assessment of measurement uncertainty including random and systematic

errors and an error budget; and (6) internal quality control procedures

used by NBS. These documents will present more detail than can be

given in an NBS calibration report, or than is generally allowed in

articles in scientific journals. In the past NBS has published such

information in a variety of ways. This series will help make this type

of information more readily available to the user.

This document ( SP 250-16), NBS Measurement Services: Calibration

of X-Ray and Gamma-Ray Measuring Instruments, by P. J. Lamperti, T. P.

Loftus, and R. Loevinger, is the sixteenth to be published in this new

series of special publications. It describes the calibration and

irradiation of x-ray and gamma-ray instruments in terms of the physical

quantity exposure, measured in roentgens, or the quantity air kerma,

measured in grays (see test numbers 460 IOC through 46050S in the SP 250

Users Guide) . Inquiries concerning the technical content of this

document or the specifications for these services should be directed to

the authors or one of the technical contacts cited in SP 250

The Center for Radiation Research ( CRR) is in the process of publishing

21 documents in this SP 250 series, covering all of the calibration

services offered by CRR. A complete listing of these documents can be

found inside the back cover.

NBS would welcome suggestions on how publications such as these might

be made more useful. Suggestions are also welcome concerning the need

for new calibration services, special tests, and measurement assurance

programs

Joe D. Simmons Chris E. Kuyatt

Acting Director Director

Measurement Services Center for Radiation Research

iii
Abst ract . The calibration and irradiation of x-ray and gamma-ray instruments

are performed in terms of the physical quantity exposure. The calibrations

are listed in NBS Special Publication 250 as calibrations 46010C through

46050S (formerly 8.3A through 8.3M). A calibration or correction factor is

provided for radiation detectors, charge sensitivity of a high-gain

electrometer is tested, and passive dosimeters are given known exposures.

Calibration is performed by comparing the instrument against an NBS primary

standard of exposure, which is a free-air chamber for x rays and a cavity

ionization chamber for cesium-137 and cobalt-60 gamma rays. A variety of

quality assurance checks are performed to assure the constancy of the

standards and the accuracy of the calibrations and irradiations. The overall

uncertainty (considered to have the approximate significance of a 95%

confidence limit) is given as 0.7% for exposure rate in the NBS beams, 1% for

calibration of a cable-connected chamber and irradiation of passive

dosimeters, and 1.5% for calibration of a condenser chamber.

Key words: calibration; cavity chamber; cesium-137 gamma rays; cobalt-60

gamma rays; exposure; free-air chamber; ionization chambers; primary standard;

standard; uncertainty estimate; x rays.
TABLE OF CONTENTS

Page

Abstract iv

List of Tables vii

List of Figures x List of Attachments xii

Foreword xiii

1. Introduction 1 1.1 Description of service 1 1.2 Beam qualities for calibration of x- and gamma-ray

measuring instruments 3

2. Design philosophy and theory 7

3. Description of system 7 3.1 General overall view 7 3.2 X-ray exposure calibration standards 8

3.2.1 Wyckoff-Attix (50- to 300-kV) FAC corrections 12

3.2.2 Ritz (20- to 100-kV) FAC corrections 26

3.2.3 Lamperti (10- to 20-kV) FAC corrections 31

3.3 Intercomparison of standard free-air ionization chambers .... 35

3.4 Gamma-ray exposure standards and calibration ranges 35

3.4.1 Cavity-chamber standards, description 36

3.4.2 Gamma-ray sources, location, initial activity, and

description 38

3.4.3 Calibration of gamma-ray beams 38

3.4.4 Useful beam size 42

3.5 loni zation-chamber current-measurement techniques 43

3.5.1 Background and history 43

3.5.2 Electrometers 44

3.6 Ionization current measurement-console equipment 48

3.6.1 X-ray calibration data acquisition system 49

3.6.2 Gamma-ray calibration data acquisition system 57

3.7 Pre-cal ibration tests of exposure instruments 60

3.7.1 Charger/reader scale linearity test 60

3.7.2 Leakage tests of condenser probes 62

3.7.3 Testing for atmospheric communication, condenser probes 62

3.7.4 Testing for atmospheric communication, cable-connected

probes 63

3.7.5 Testing of readers for cable-connected probes 63

3.7.6 Testing of instruments to be calibrated in electrical

units 63

3.7.7 Summary and comments 65

3.8 Capacitor, pressure indicator, temperature indicator calibration 65

3.8.1 Capacitor calibration 65

3.8.2 Temperature indicator calibrations 68

3.8.3 Pressure indicator calibrations 68

V
TABLE OF CONTENTS (Continued)

Page

4. Operating procedures 71

4.1. Administrative procedures 71

4.2. Calibration of i ntegrati ng-type instruments 73

4.2.1. Condenser chambers 73

4.2.1.1. General considerations 73

4.2.1.2. X-ray calibrations 74

4.2.1.3. Gamma-ray calibrations 75

4.2.2. Cable-connected instruments 76

4.2.2.1. General considerations 76

4.2.2.2. X-ray calibrations 76

4.2.2.3. Gamma-ray calibrations 77

4.3. Calibration of current -type instruments 77

4.3.1. X-ray calibrations 77

4.3.2. Gamma -ray calibrations 77

4.4. In-house calibration checks 78

4.5. Test of high-quality feedback electrometers 79

4.6. Irradiation of thermoluminescent dosimeters ... 79

4.6.1. General considerations and procedures 79

4.6.2. Exposure techniques 79

5. Assessment of uncertainty 80

6. Safety Considerations 83

6.1. Radiation safety 83

6.1.1 X-ray calibration ranges 83

6.1.2 Gamma-ray calibration ranges 84

6.2. High-voltage safety 84

6.3. Laser safety 84

References 85

List of attachments 87

vi
LIST OF TABLES

Page

1-1 Calibration conditions for x-ray and gamma-ray measuring

instruments 6

3-1. Important dimensions and parameters for the three NBS free-air

ionization-chamber standards 11

3-2. Secondary photon contribution, Sp, in percent 17

3-3. Data from Handbook 64, and fractional areas, used to calculate

the percent loss in ionization for moderately (M) and heavily (H)

filtered x rays due to inadequate plate spacing in the

Wyckoff-Attix FAC 19

3-4. Data from Handbook 64, and fractional areas, used to calculate

the percent contribution to the ionization in the Wyckoff-Attix

FAC due to scattered photons 20

3-5. Summary of calculations from tables 3-3 and 3-4 for percent loss

and gain of ionization due to lack of plate separation and

scattered photons, respectively 20

3-6. Comparison of x-ray beam fi It rat ions used for data reported in

Handbook 64 with filtrations presently used for conventional

calibration conditions 21

3-7. Data presently used (May, 1986) to compute corrections for the

Wyckoff-Attix standard free-air ionization chamber for conventional calibration conditions 22

3-8, Comparison of the kg values presently used (May, 1986) with kg

values computed from the least squares equation 24

3-9. Products of all exposure-rate-independent corrections for each

x-ray beam quality in current use (May 1986) with the WyckoffAttix FAC, and the products of the corrections based on the data

from HB64 and this review 25

3-10. Mass air-attenuation coefficients and values of kg for the Ritz

FAC 28

3-11. Computation of electron-loss corrections for the Ritz FAC .... 29

3-12. Summary of corrections for the Ritz FAC 30

3-13. Recombination corrections for the Lamperti FAC 33

3-14. Summary of corrections for the Lamperti FAC 35

3-15. Dimensions of spherical graphite ionization chambers 37

3-16. Gamma-ray source locations and nominal activities of sources

at time of purchase 38

vi i
LIST OF TABLES (Continued)

Page

3-17. Constants for computing exposure rate corrected for decay for

the horizontal beam sources 39

3-18. Data and references for calibration of B034 vertical -beam ^ocq source 40

3-19, Data and references for calibration of vertical -beam gamma-ray

sources in Rm B036 41

3-20. Constants for computing useful beam radii for the horizontal and

vertical beam calibration ranges 42

3-21. Beam size defined by 50-percent radiographic density contour for

vert ical -beam gamma-ray sources, tor specific collimator settings 43

3-22. HP Scanner channel numbers and input signals for 300-kV x-ray

machine 50

3-23. Electrometers and capacitors mounted in x-ray standards

measurement console 50

3-24. Comparison of solutions for true kV using first and second-degree

solutions for the constant A 53

3-25. Calibration beam code - filter wheel positions for 300-kV x-ray

machine 54

3-26. HP scanner numbers and input signals for 100-kV calibration range 55

3-27. Calibration beam code-filter wheel positions for the 100-kV x-ray

range 56

3-28. Capacitor values (1986 Jun) in mobile measurement console .... 58

3-29. Constants in "G" program for computing air temperature in

gamma-ray calibration rooms 59

3-30. Victoreen Model 570 electrometer charging data 61

3-31. Electrometer leakage current and currents induced by adding

cables and connectors 64

3-32. Electricity division calibrations 66

3-33. Liquid-in-glass thermometer calibration 68

3-34. Summary of thermistor calibration equations 70

3-35. Pressure indicator comparison summary 70

vi i i
LIST OF TABLES (Continued)

Page

4-1. Standard distance and collimator settings, room B036 76

4-2. Analysis of three NBS check chambers for the period Sep 1969 to

Oct 1987 79

5.1. Uncertainty analysis for exposure rate 81

5.2. Uncertainty analysis for calibration and irradiation 82

ix
LIST OF FIGURES

Page

Fig. 1-1. Graph of generating constant potential against half-value

layer for the beam qualities used in x-ray calibration ... 4

Fig. 1-2. Graph of aluminum homogeneity coefficient against aluminum

half-value layer for the L, M, and H filtrations 4

Fig. 1-3, Graph of copper homogeneity coefficient against copper halfvalue layer for the M, and H filtrations 5

Fig. 3-1. Schematic of 300-kV x-ray set 8

Fig. 3-2. Sectional view of Wyckoff-Attix (50- to 300-kV) free-air

chamber 9

Fig. 3-3. Sectional view of the Ritz (20- to 100-kV) free-air chamber . 9

Fig. 3-4. Schematic cross-sectional views of the Lamperti (10- to

20-kV) free-air chamber 10

Fig. 3-5. The loss of electron-produced ionization (in percentage of

total electron ionization) beyond different radii from a zero-diameter beam of constant potential x-rays 13

Fig. 3-6. The loss of electron-produced ionization (in percentage of

total electron ionization) beyond different radii from a zero-diameter beam of constant potential x-rays 13

Fig. 3-7, The loss of electron-produced ionization (in percentage of

total electron ionization) beyond different radii from zero- diameter x-ray beams of the constant potentials indicated , , 14

Fig, 3-8, The percent contribution of electron-produced ionization

per centimeter for the indicated constant-potential x-ray

beams and filtrations given in figure 3-5 14

Fig. 3-9. The percent contribution of electron-produced ionization

per centimeter for the indicated constant-potential x-ray

beams and filtrations given in figure 3-6 15

Fig. 3-10. The loss of electron-produced ionization per 5 cm for the

indicated constant-potential x-ray beams and an inherent

filtration of approximately 3 mm Cu 15

Fig. 3-11. Cross section of Wyckoff-Attix FAC 16

Fig. 3-12. Percent secondary photon contribution within radii from 5 to 15 cm 18

Fig. 3-13. Fractional recombination correction (k^-l) versus exposure

rate for the Wyckoff-Attix FAC 24

x
LIST OF FIGURES (Continued)

Page

Fig. 3-14. 300 kV spectra. Initial triangular spectrum assumed for

300 kV across x-ray tube 27

Fig. 3-15. Fractional recombination correction (k^-l) versus exposure

rate for the Ritz FAC 32

Fig. 3-16. Fractional recombination correction (k^-l) versus exposure

rate for the Lamperti FAC 34

Fig. 3-17. Cross-sectional view of typical cavity chamber 36

Fig. 3-18. Circuit of a negative-feedback amplifier with resistive

feedback element 46

Fig. 3-19. Circuit of a negative feedback amplifier with a capacitive

feedback element 46

Fig. 3-20. Circuit used for Dosimetry Group calibrations of capacitors 67

Fig. 3-21. Schematic of thermistor calibration setup 69

Fig. 4-1. Detailed construction of the 25-r Victoreen condenser

ionization chamber 74

xi
LIST OF ATTACHMENTS

Attachment Section Page

1 1.2 ISO Publication 4037-1979 (E). (Excerpt) 87

2 1.? lEC Publication 731. 1982. fExcerot) 89

3 1.2 NBS beam codes used prior to 1986 for calibration

of x-ray and gamma-ray measuring instruments 92

4 3.4.2 Sketch of horizontal 20-Ci and 200-Ci ^^Co sources . . 93

5 3.4.2 Sketch of horizontal 30-Ci and 300-Ci ^^'^Cs sources . 94

6 3.4.2 Sketch of vertical 10-kCi ^^Co Theratron "F"

(Super G) head 95

7 3.6.1 Block diagram of data acquisition system (DAS) . . , . 96

8 3.6.1 Universal Voltronics power supply calibration report . 97

9 3.7.3 Schematic drawing of Keithley 6169 chamber98

10 3.7.6 99

11 3.8.2 Liquid-in-glass thermometer calibration report . . . . 100

12 4.1 102

13 41 • 1J. Tvoiral inventory of instruments for x-rav or gamma-ray calibrations, 1986 July 24. (Excerpt) . . . 103

14 4.1 Condenser-type instrument record (Side 1 and 2 of

ra rd ) 105

X -J 4 1 Pii rrpirl" -t" vnp i n<;t riimpnl' rprnrri ^^tHp 1 anri ? nf

106

16 4.1 107

17 4.1 108

18 4.1 Cable-connected chamber Report of Calibration . . . . 109

19 4.1 115

20 4.1 Typical page in data book for TLD irradiation . . . . 119

21 4.1 120

22 4.5 122

xii
FOREWORD

NBS Special Publications 250-16 and 250-19 can be considered monuments

to the skill and dedication of Thomas P. Loftus. Tom came to the Radiological

Equipment Section of NBS in 1951, working under Dr. Scott Smith, Section

Chief, and Dr. H. 0. Wyckoff, Chief, Radiation Physics Laboratory. Tom was involved with and supervised the gamma-ray and brachytherapy calibrations

until NBS moved from the Washington, D.C. to the Gaithersburg , Maryland site

in 1965. At that time he took on the added responsibility of supervising the

x-ray calibration work. He continued in this capacity until his retirement in

June 1984. At that time, it was decided to update the documentation of all

the dosimetry calibration services, as part of a Bureau-wide program. It was evident that Tom was the ideal person to take on this job, and he was awarded

a contract for that purpose. The result was six separate documents entitled

"Review of loni zati on-Chamber Current -Measurement Techniques in the NBS

Dosimetry Group", "lonization-Current Measurement Console Equipment",

"Pre-Cal ibration Testing of Exposure Instrumentation", "Gamma-Ray Calibration

Ranges", "Exposure Calibration of Radiation Instruments at the NBS", and

"Brachytherapy Source Calibrations at the NBS".

The first five documents form the basis for SP 250-16, and the last forms

the basis for SP 250-19. In order to meet the requirements for the NBS SP 250

series of publications, the Loftus material has been reorganized, rewritten in

places, supplemented with additional material, and edited by P. J. Lamperti

(SP 250-16), J. T. Weaver ( SP 250-19), and R. Loevinger (both documents).

The final documents reflect the knowledge, skill, experience, and careful

attention to detail of Thomas P. Loftus. P. J. Lamperti and J. T. Weaver, who

worked under Tom for many years, wish to express here their thanks to him for

the example he set while at NBS.

P.J.L., R.L., J.T.W.

March 1988

X i i i

1. Introduction

This report describes the status of the calibration service for x-ray and

gamma-ray measuring instruments, as of September 1986. Section 1 defines the

physical quantity exposure, in terms of which the calibrations are performed,

and also the properties of the x-ray and gamma-ray beams used in the service.

Section 2 is a brief definition of calibration by substitution. Section 3

describes in detail the calibration system and its measurement standards,

first (section 3.2) the free-air chambers and the x-ray beams, and then

(section 3.4) the cavity-chamber standards and the gamma-ray beams. Sections

3.5 and 3.6 describe the instruments and techniques for measuring current and

charge, and for storing and analyzing the calibration data. Section 3.7

describes the tests applied to the instruments being calibrated, while section

3.8 covers the tests and calibrations performed on NBS laboratory equipment.

Section 4 describes the operating procedures, including the administrative

procedures and methods of handling and calibrating the instruments, and

irradiating the passive dosimeters. The assessment of uncertainty is covered

in section 5, followed by safety considerations in section 6, including

radiation, high-voltage, and laser safety. References are listed at the end

of the report, in the order in which they were cited. Twenty-one attachments

are appended to the report; a list of the attachments gives the number of the

section in which the attachment is cited.

1.1 Description of service

The National Bureau of Standards (NBS), Ionizing Radiation Division,

Dosimetry Group receives a variety of instruments for calibration, test, or irradiation in x- or gamma-ray beams. These services are Test Nos. 4601 OC

through 46050S (formerly 8.3A through 8.3M) in NBS Special Publication 250

[l],i "NBS Calibration Services Users Guide 1986-88," available from the NBS

Office of Physical Measurement Services. Calibration factors or correction

factors are provided for radiation detectors. The charge sensitivity of a high-gain electrometer can be tested at any one set of switch positions in

conjunction with a calibration factor. Passive dosimeters can be given known

exposures of x or gamma radiation. Calibrations and irradiations are performed in terms of the physical quantities exposure and air kerma .

The quantity exposure characterizes an x-ray or gamma-ray beam in terms

of the electric charge liberated in air by ionization of air molecules.

Exposure can be defined qualitatively as the total charge per unit mass liberated in free air by a photon beam. More precisely, the exposure (X) is the

quotient of dQ by dm, where dQ is the sum of the electrical charges on all the

ions of one sign produced in air when all the electrons liberated by photons

in a volume element of air whose mass is dm are completely stopped in air.

Then

X = dQ/dm .

^Numbers in brackets indicate the literature references at the end of this

document

1
The SI unit of exposure is the coulomb per kilogram (C/kg); the special unit

of exposure, the roentgen (R), is equal to 2,58 x lO-'^ C/kg (exactly). The

ionization arising from the absorption of bremsstrahl ung emitted by the

secondary electrons is not to be included in dQ. Except for this small

difference, significant only at high energies, the exposure as defined above

is the ionization equivalent of air kerma.

The quantity kerma characterizes a beam of photons or neutrons in terms

of the energy transferred to any material. In the calibration service under

discussion, consideration is limited to photon beams and air. Air kerma can

be defined qualitatively as the total energy per unit mass transferred from an x-ray or gamma-ray beam to air. More precisely, air kerma (K^^-p)

quotient of dE^p by dm, where dE^-p is the sum of the initial kinetic energies

of all electrons liberated by photons in a volume element of air and dm is the mass of the air in that volume element. Then

•^air = dE^p/dm .

The SI unit of air kerma is the gray (Gy), which equals one joule per

kilogram; the special unit of air kerma is the rad, which equals 0.01 Gy.

The relationship between exposure and air kerma can be expressed as a simple equation:

Kg^-p = X . {W/e)/(l-g)

where W/e is the mean energy per unit charge expended in air by electrons, and

g is the mean fraction of the energy of the secondary electrons that is lost

to bremsstrahl ung.

The currently accepted value of W/e is 33.97 J/C. The currently accepted

values of g for ^°Co and ^^y^s beams are 0.32% and 0.16%, respectively. Then,

using SI units, X is converted to K^^p by multiplying by 34.08 and 34.02 for

^^Co and ^^"^Cs beams, respectively. The value of g is negligible for the

x-ray beams of interest, and the conversion factor is just 33.97.

Any calibration or irradiation performed in terms of exposure can be

converted to a calibration or irradiation in terms of air kerma by use of the

multiplicative constants given above. Because of the inconvenient form of SI

units for exposure and (especially) exposure-rate, the quantity air kerma is

being adopted in place of exposure in virtually all countries. NBS has not

made this change at the time of writing (June 1986), but expects to do so. The material that follows is written in terms of exposure, but it applies

equally well to calibrations and irradiations in terms of air kerma, making

use when necessary of a multiplicative constant to convert the former to the

1 atter.

Only ionization chambers known to be stable and reproducible are accepted

for calibration in this program. Institutions submitting ionization chambers

for calibration are strongly urged to carry out beforehand a careful program

of stability checks involving redundant measurements in highly reproducible

radiation fields, and to repeat those checks after NBS calibration, and again

at suitable intervals. Instruments submitted for calibration, and material

submitted for irradiation, must be shipped in reusable containers.

2
An x-ray tube produces bremsstrahl ung spectra, highly inhomogeneous beams

with photon energies from very low values to a high-energy cutoff given by the

maximum potential applied to the x-ray tube. These beams are customarily

filtered (with aluminum, copper, tin, or lead) in order to reduce the unwanted

low-energy x rays. It is conventional to characterize the "quality" of the

filtered x-ray beam in terms of the thickness of aluminum or copper required

to reduce the exposure rate to 50%, and then again to 25%, of its original

value. These thicknesses are called the first half -value layer (HVL) and the

second HVL. The measurements must be made using good-geometry attenuation in

order to obtain numbers of general significance. The homogeneity coefficient

is the ratio of the first to the second HVL, here multiplied by 100 to provide

more convenient numbers. A value near 100 indicates that the filtration has

produced an approximately homogeneous beam that is approaching monoenergetic

conditions.

1.2 Beam qualities for calibration of x- and gamma-ray measuring instruments

The NBS x-ray beam qualities offered for exposure calibration of x- and

gamma-ray measuring instruments were revised in Jan 1983. Several previously

offered beam qualities were discontinued, a number of new qualities added, and

a total of 32 qualities offered. The beam qualities are divided into groups

according to filtration, i.e., light, moderate, and heavy filtration. The

qualities for each group were chosen so that the x-ray tube constant potentials plotted against HVL fall on or near a smooth curve. Likewise the

aluminum and copper homogeneity coefficients plotted against HVL fall on or near a smooth curve. (The homogeneity coefficient is taken as HC = 100 x 1st

HVL/2nd HVL.) Figure 1-1 is a graph of the generating constant potential

against the aluminum and copper HVLs. Figure 1-2 and 1-3 are graphs of the

homogeneity coefficient against the HVL, for aluminum and copper, respectively.

The beam codes consist of the letter L, M, or H, followed by the generating constant potential in kilovolts. For example, MlOO indicates moderate

filtration and 100-kV constant potential. Table 1-1 gives a complete listing

of beam codes offered.

It has been our experience that calibration factors for ionization

chambers fall on smooth curves when plotted against HVL, provided that all

calibration points have been chosen from a single group, L, M, or H. If

calibration points are chosen from more than one group, discontinuities may

occur, hence no attempt should be made to interpolate between such calibration

factors.

The selection of beam qualities for instrument calibration depends on the

situation of interest. The H qualities are usually used for calibration for

radiation protection, since these beams have the narrowest spectrum at each

generating potential, and probably most nearly approximate radiation that has

penetrated a protective barrier. The M qualities are usually used for

calibration for radiation therapy. The L qualities are for calibration of

instruments used for measurement of unfiltered or lightly filtered beams that

give high exposure rates, as is often the case in radiation biology and

Grenz-ray therapy.

3
HALF VALUE LAYER (mm)

Fig. 1-1, Graph of generating constant potential against half-value layer for

the beam qualities used in x-ray calibration. The two upper curves refer

to copper half-value layers for the moderate (M) and heavy (H) filtrations. The three lower curves refer to aluminum half-value layers for

the light (L), M, and H filtrations. The three S symbols indicate two

special filtrations that do not fit in with the L, M, and H filtrations.

100

0.1 LO

Al HALF VALUE LAYER (mm)

10

Fig. 1-2. Graph of aluminum homogeneity coefficient against aluminum halfvalue layer for the L, M, and H filtrations. The two S symbols indicate

two special filtrations that do not fit in with the L, M, H filtrations.

4
Cu HALF VALUE LAYER (mm)

Fig. 1-3. Graph of copper homogeneity coefficient against copper half-value

layer for the M, and H filtrations. The S symbol indicates a special

filtration that does not fit in with the M and H filtrations.

5
Table 1-1. Calibration conditions for x-ray and gamma-ray measuring instruments.

This is the last page of a computer-generated calibration report.

BEAM ADDED FILTER HALF-VALUE HOMCXSENEITY EFFECT. DIS- EXPOSURE RATE

CODE LAYER COEFFICIENT ENERGY TANCE MIN. MAX.

A T All rD AL cu A T AL CU

I, MM J CMM^ CKEVj (CM) (MR/S) (R/S)

LIO 0 0.029 79 25 0.001 1.7

T T R JjiO U 0 . 050 74 25 0.001 4.2

T on U U . 071 76 A50 0 . 001 3. 3

T "^n O OAR r\ oo 60 S A50 A AA ^ 0 . 001 A Ji 0.4

T AH n Rr» U . OU 0 . 49 57 50 0 . 001 0.4

TLiOU KH U . Doy 0 . 75 58 50 0 . 001 0.4

T on 1 . <iO'± 1 . OO 58 50 0 . 001 0.4

T T nn JjIUU i . y r O 2 . 8 59 50 0 . 001 0.4

pi o in U . doU 0.152 79 50 0. 001 0.5

M30 0.50 0.36 64 50 0.001 0.3

M40 0.786 0.73 66 50 0.001 0.4

nou 1 . 02 0 . 032 66 62 50 0.001 0.4

noU T R T 1 . 51 1 . 68 0 . 052 68 64 0.8 0.2

niUU R r\ 5 .

0

0 . 20 72 55 *1 A

1 .

0

A 19 0.3

in Ro HlDU R r\ 5 . 0

n OR 0 . 25 10 . 2 0 . 67 87 62 1 .

0

0.4

4 . 1

T "1 O 1 . 12 14 . 9 1 . 69 95 69 ^ A

1 .

0

A 99 0.3

I/O Rr^ R r\ o . 2 18 . 5 3 .

2

98 86 ^ A

1 .

0

A A 0.2

HOUIJ 4 . 0 6 . 5 22 5 .

3

100 97 A 0 0.5 0.08

nlU 0 . 105 0 . 148 89 25 0.001 0.003

TT *1 R 0 . 500 0 . 152 87 25 0. 001 0. 003

0.20 1 . 021 0 . 36 88 50 0.001 0. 003

H30 4. 13 1.23 0.038 93 94 50 0.001 0.003

H40 4.05 0.26 2.9 0.093 94 95 50 0.001 0.003

noO 4 . 0 0 . 10 4 .

2

0. 142 92 AA90 38 0.3 0. 065

H60 4.0 0.61 6.0 0.24 94 89 46 0.02 0.005

HlOO 4.0 5.2 13.5 1.14 100 94 80 0.005 0.002

HI 50 4.0 4 .

0

1 . 51 17 .

0

2 .

5

100 AP 95 120 0.03 0.010

H200 4.0 0 . 60 4 . 16 0 . 77 19. 8 4 .

1

100 99 166 0.02 0. 006

H250 4.0 0.60 1.04 2.72 22 5.2 100 98 211 0.03 0.005

H300 4.1 3.0 5.0 23 6.2 99 98 252 0.04 0.003

S75 1.504 1.86 63 50 0.001 0.4

S60 4.0 2.8 0.089 75 70 0.3 0.06

137CS 10.8 662 1.5 0.1

60CO 14.9 1250 1.5 2.5

FOR THE X-RAY BEAM CODES, THE LETTER INDICATES LIGHT, MODERATE, HEAVY, AND

SPECIAL FILTRATION, AND THE NUMBER IS THE CONSTANT POTENTIAL IN KILOVOLTS.

THE INHERENT FILTRATION IS APPROXIMATELY

1.0 MM BE FOR BEAM CODES LIO-LIOO, M20-M50, H10-H40, AND S75; AND

3.0 MM BE FOR BEAM CODES M60-M300, H50-H300, AND S60.

THE HALF-VALUE LAYERS FOR 137CS AND 60CO ARE CALCULATED.

THE HOMOGENEITY COEFFICIENT IS TAKEN AS 100(1ST HVL/2ND HVL)

.

6
The revised calibration beam qualities brought NBS practice into line

with practice at other national standards laboratories. The H group of

qualities agrees with the "narrow spectrum" qualities recommended by the

International Standards Organization (ISO) in "X and y reference radiations

for calibrating dosimeters and dose ratemeters and for determining their

response as a function of photon energy," ISO Publication 4037-1979 (E)

(attachment 1). The ISO recommendations extend from 300 kV to 40 kV, below

which the H group has been extended to 10 kV in agreement with practice at the

PTB (the standards laboratory of the Federal Republic of Germany). The

M group of qualities is in agreement with the recommendation for radiation

therapy calibration in "Dosimeters with ionization chambers as used in

radiation therapy," lEC Publication 731, 1982 (attachment 2).

The NBS beam codes used since Jan 1983 are shown in table 1-1. The beam

codes S60 and S75, and the 5 beam codes that were discontinued have characteristics that are not consistent with those of the L, M, and H groups. The NBS

beam codes used prior to Jan 1983 are given in attachment 3.

2. Design philosophy and theory

X-ray calibrations are performed by using the substitution method. Using

this method, the exposure or exposure rate is determined at some point in

space by a free-air chamber. The instrument to be calibrated is then placed

at the same point in space as the standard. The response of the instrument is

determined. The calibration factor is the quotient of the x-ray exposure, in

the absence of the chamber, and the charge generated by that radiation in the

ionization chamber:

Calibration Factor = X/Q

The correction factor is the quotient of the x-ray exposure, in the absence of

the chamber, and the electrometer reading with the ionization chamber:

Correction Factor = X/( reading)

Gamma-ray calibrations are performed by using previously calibrated beams and

correcting for decay. Details of the calibration of gamma-ray beams are given

in section 3.4.3.

3. Description of system

3.1. General overall view

Two x-ray calibration ranges are available for instrument calibrations.

One range is used for x-rays generated at potentials of 10 kV to 100 kV, the

other is used for x-rays generated at potentials of 50 kV to 300 kV. X-rays

are produced using constant-potential generators. Gamma-ray calibrations are performed on any one of seven sources, of which three have vertical beams, and

four have horizontal beams. A schematic diagram of the 300-kV x-ray calibration range is given in figure 3-1,

7
3.2. X-ray exposure calibration standards

Standardization of x-ray beams for the quantity exposure is carried out

at NBS by means of three free-air ionization-chamber standards (FAC) [2], [3],

and [4]. Figures 3-2 to 3-4 show cross-sectional views of the three free-air

chambers. The important dimensions of the three free-air chambers are given

in table 3-1. With reference to table 3-1, note that the free-air chambers

become larger as the x-ray energy increases. The chamber size is dictated by

the need, at low x-ray energies, to minimize air attenuation in the path

between the FAC defining diaphragm and the collection plate, and the need, at

high x-ray energies, to compensate for the downstream loss of electrons from

the collection volume by providing sufficient air path length upstream. The

latter also eliminates electrons from the diaphragm. At the higher x-ray

energies, the plate separation is also made larger in an attempt to allow for

complete energy loss for electrons with paths predominantly perpendicular to

the x-ray beam direction.

Lead -i- Steel

y Housing Telescope

_/^X-Ray Tube

Safety Chamber 1

Shutter

Filter

Monitor

/' Chamber

Beam Aperture

Timing Shutter

Test Chamber ^

Carnage

Free Air Chamber

X-Ray Bea m

Centerline

Support Carriage

^ To Computer

Interface

High Voltage

Generators

Fig. 3-1. Schematic of 300-kV x-ray set.

8
OIAPHRAOM

Icm

99.8 Ml _M.3 cm.

, I cm SPACINOS

I ,MV PLATE

4=L

M.9 cm

. Otal. _ em

t OF COLLECTOR PLATE'

Fig. 3-2. Sectional view of Wyckof f-Attix (50- to 300-kV) free-air chamber.

TO ELECTROMETeS OR CROUNO lb I

GUARD

PLATER

GUARD

COLLECTING PLATES ; "mpj ~1 "

X-RAT BEAM

LEAD

STEEL

LINED BOX ^ HPGM VOLTAGE PLATE 7" n

SPRING t-^ CONTACT

Fig. 3-3. Sectional view of the Ritz (20- to 100-kV) free-air chamber.

9
1 . Lead to current measuring system

2. Lead to thermistor readout

3. Thermistor

4. Collector plate (brass)

5. Guard plate (brass)

6. Guard ring (brass)

7. Diaphragm (tungsten alloy)

8. High voltage plate (brass)

9. Supporting insulator (delrin)

10. Grounded case (brass)

1 1 . High voltage lead

12. Lead to midpoint of potential divider

13. Radiation shield (brass)

Fig, 3-4. Schematic cross-sectional views of the Lamperti (10- to 20-kV)

free-air chamber.

10
Table 3-1. Important dimensions and parameters for the three NBS free-air

ionization-chamber standards.

X-ray Ai r Elect ric

t ube r 1 d L e r 1 a L e Lo \ 1 eci. or u 1 a pn ra gm aDsorpc 1 on T 1 e 1 u potent i a 1 sepa rat i on hei ght 1 ength d i a met e r / ( ID) 1 engt h st ren at h (kV) (mm) (mm) (mm) (mm) (mm) (V/cm)

10 to 60 40 50 10.146 4.994/(5S) 39.02 750

20 to 100 90 90 70.030 10.00/(10A) 127.39 555

50 to 300 200 268 100.80 10.00/(108) 308 250

The lO-to-60-kV FAC is designed for use up to 60 kV but is normally used only

up to 15 kV. In column five ID gives the identification number for each

diaphram.

Although strenuous efforts were made, by the designers of the free-air

chambers, to realize experimentally the quantity exposure, various corrections

are still required. The corrections are as follows:

Air attenuation (k^) - The area of the beam, and the exposure rate, are defined at the position of the FAC diaphragm, but the length of the air volume

in which the ionizing electrons of interest are produced is defined by the

length of the collector plate. Correction must be made for attenuation of the

x-rays in the air path between the position of definition and the position of

measurement. The distance for air attenuation measurements is taken to be the

distance between the defining plane of the diaphragm and the center of the

collector. This correction is energy dependent and at low energies

atmospheric temperature and pressure effects become significant.

Photon scatter (kp) - Ionization produced by electrons resulting from

photons scattered out of the diaphragm-defined beam is not included in the

definition of exposure.

Electron loss (kg) - Energetic electrons may leave the collection volume

before expending their entire energy in the form of ionization. The volume is

defined by the area of the collection plate and the distance between the

collection plate and the high-voltage plate (plate separation).

Recombination (kg) - Ions may recombine before they are collected. This

correction is exposure-rate dependent.

Since the quantity exposure is defined as charge per unit mass of air, and the

special unit of exposure, the roentgen, is 2.58 x lO""* C/kg, the calculation

of exposure data from standard free-air chamber measurements of charge is

accomplished using the equation

C = 1 dQZdt . ^ 2.58 X lO-"* pV i

11
where X is the exposure rate (R/s).

dQ/dt is the rate of change of the collected charge (C/s).

V is the product of the diaphragm area and collector plate length,

the volume (m^).

p is the density of air at the ambient conditions of temperature

and pressure.

P = Po P^o /V (^9/m3)

and Pq is the density of air at the reference conditions, T and P are the ambient temperature and pressure, and Tq and Pq are the

reference temperature and pressure. (T and Tq are in kelvins.)

nk^ is the product of all necessary dimensi onl ess correction

factors.

3.2.1. Wyckoff-Attix (50- to 300-kV) FAC corrections

The correction for electron loss is

kg = 1 - J ApE^/100

and the correction for scattered photon contribution is

kp = 1 -

5: ApS^/100 .

In these equations, Ep is the percent electron contribution (a loss) beyond a radius r, Sp is the percent scattered photon contribution (a gain) beyond

radius r, and Ap is the fraction of each annular area that contributes to the

summation. The calculations of the corrections for electron loss, k^, and

photon contribution, kp, are based on data from NBS Handbook 64 (HB64) [2] and

the work of Ritz [5]. Calculations for the percent loss (E^) of electronproduced ionization beyond different radii, from a zero diameter x-ray beam,

utilize figures 8, 9, and 10 in HB64, (reproduced and recaptioned in figs. 3-5

to 3-7 below) while curves for the percent electron contribution to the

ionization per radius increment (AE^/r) at different inner radii are provided

in figures 11, 12, and 13 of HB64 (reproduced and recaptioned in figs. 3-8 to

3-10 below).

12
Fig. 3-5. The loss of electronproduced ionization (in

percentage of total electron

ionization) beyond different

radii from a zero-diameter beam

of constant potential x-rays.

The x-ray potentials were 60 and

75 kV with zero added filter,

100 kV with 1 mm Al added filter,

150 kV with 0.23 mm Cu + 1 mm Al

added filter, 200 kV with

0.5 mm Cu + 1 mm Al added filter,

and 250 kV with 1 mm Cu + 1 mm Al

added filter. The x-ray tube had

an inherent filtration of

approximately 3 mm Al

Fig. 3-6. The loss of electronproduced ionization (in

percentage of total electron

ionization) beyond different

radii from a zero-diameter beam

of constant potential x-rays.

The x-ray potentials were 100 kV

with 0.53 mm Pb added filter,

150 kV with 1.53 mm Sn + 4.0 mm

Cu added filter, 200 kV with

0.7 mm Pb + 4.0 mm Sn + 0.59 mm

Cu added filter, and 250 kV with

2.7 mm Pb + 1.0 mm Sn + 0.59 mm

Cu added filter. The inherent

filtration of the beam was approximately 3 mm Al

13
IjO

ai

.01

_ \ \ \ \\

k \ \

1

11111

\

\ \^ t. \ V

10 20 30 RADIUS, cm

40 SO "To i5 i4" RADIUS, cm

Fig. 3-7. The loss of electronproduced ionization (in

percentage of total electron

ionization) beyond different

radii from zero-diameter x-ray

beams of the constant potentials

indicated. The inherent

filtration of the beam was approximately 3 mm Cu.

Fig. 3-8 The percent contribution of

electron-produced ionization per

centimeter for the indicated

constant-potential x-ray beams

and filtrations given in

figure 3-5.

-3 o^ D ll^'i^-^^ ^'^ ^^^^ ^" interesting pair (as do figures 3.8 and

^.y;. Both figures show the loss of ionization, as a percent of the total ionization beyond different radii for zero-diameter x-ray beams generated at the constant potentials shown on the curves. Figure 3.5 refers to very lightly filtered beams, and figure 3.6 refers to heavily filtered beams essen- tially the same as in table 1-1. The constant potentials 100 kV to 250 kV are the same in the two figures, yet the shapes of the curves are quite different. The experimentally obtained curves represent the resultant of scattering and absorption of photons and electrons, in a series of interactions that appear to be too complicated to allow an easy explanation for the details of the individual curves.

14
« 8 K) It INNER RAOtUS, <m

1.0

<l\< 0.1

.01

_

- \

-

\\V

10 20 90 INNER RADIUS, tm

40 »0

Fig. 3-9. The percent contribution of

electron-produced ionization per

centimeter for the indicated

constant -potenti al x-ray beams

and filtrations given in

figure 3-6.

Fig. 3-10. The loss of electronproduced ionization per 5 cm for

the indicated constant-potential

x-ray beams and an inherent

filtration of approximately

3 mm Cu.

In order to calculate the correction for electron ionization loss, it is

necessary first to find the percent loss in ionization, E^, utilizing figures

8, 9, or 10 in HB64 (figures 3-5 to 3-7) for the x-ray beam quality of

interest, beyond a radius computed from the FAC plate separation and the

diameters of the x-ray beam. For example, the minimum inner radius r^ for the

Wyckoff-Attix FAC is 95 mm for a 10-mm diameter beam (see figure 3-11). It is

always necessary to decrease the plate separation by the diameter of the beam.

It is then necessary to determine the fractions Ap of the annular areas remaining inside the cross section of the FAC collection volume for increasing

inner radii. Next, the products are formed for each fractional area and the

percent contribution to the ionization per centimeter for the inner radius

associated with the fraction. The products are then added algebraically to

the percent loss in ionization beyond the minimum inner radius r^. The net

result is the percent loss in FAC ionization for a particular beam quality due

to inadequate plate spacing.

15
Guardwire Plane

< —lOOmm >

Fig. 3-11. Cross section of Wyckoff-Attix FAC.

16
The calculations of kp requires the use of a revised table (on an errata

sheet) in HB64 for the percent scattered photon contribution (S^) to the

ionization. The revised table is shown in table 3-2.

Table 3-2. Secondary photon contribution, S^, in percent

X-ray tube potential (kV)

Radius

(cm)

60 75 100 150 200 250 300 400 500

(%) {%) (%) {%) {%) {%) {%) (%) {%)

5

10

15

20

25

0.44

0.62

0.85

1.09

1.32

0.41

0.59

0.82

1.06

1.29

0.32

0.49

0.70

0.93

1.14

0.21

0.35

0.54

0.73

0.91

0.20

0.33

0.50

0.68

0.85

0.19

0.31

0.46

0.63

0.78

0.18

0.29

0.43

0.57

0.71

0.16

0.27

0.39

0.52

0.64

0.16

0.27

0.39

0.52

0.64

30

35

40

1.53

1.73

1.92

1.50

1.70

1.89

1.34

1.52

1.70

1.08

1.24

1.40

1.00

1.15

1.29

0.92

1.05

1.18

0.84

0.96

1.07

0.76

0.86

0.96

0.76

0.86

0.96

The fractions of the annular areas inside the collection plate system of

the Wyckoff-Attix FAC were calculated using the following equations:

Annular area A = n(r2^^2 - r^)

Area of a segment = (nr^/2) - [r^ /rj "

""o ^^ sin-i(rQ/rT )

)

where si n-i (r^/r^- ) is in radians.

A^ = 2(Pp^i - Pp), annular area outside cross section of collection

vol ume.

Ar = 1 - A^/A, fraction of annular area inside cross section of

collection volume.

The percent secondary photon contribution Sp changes slowly with distance

from the x-ray beam so it is only necessary to consider large differences in

radii when calculating ApSp. The fractional area inside the cross section of

the volume defined by the collector plate and the guard wire is just the

difference between 2P and the annular area, for the radii 95 mm and 129 mm.

It is important to recognize that the difference in percent photon

contribution, between the data given for 95 mm and 129 mm, is multiplied by

the fractional area to get ApSp between those radii. To aid in interpolation

for Sp, the data of table 3-2 are plotted in figure 3-12.

17
1.0

60 kV

0 5 10 15

BEAM RADIUS, cm

Fig. 3-12. Percent secondary photon contribution within radii from 5 to 15 cm.

Details of the calculations for the loss and gain of ionization in the

Wyckoff-Attix FAC are given in tables 3-3 and 3-4 and a summary of the results

is given in table 3-5. The beam code designations from table 1-1 are used in

table 3-5 to associate the data derived from HB64 with present-day x-ray beam

conditions. The corrections for gain of ionization are calculated from a plot

of corrections derived from data in HB64 vs_ Al half-value layer. The corrections for electron loss are calculated from data in HB64 generated for beam

conditions very nearly the same as present-day conditions. It is assumed that

the differences are not significant. There is one exception to this assumption and that is for H300 for which the HB64 filtrations are not comparable.

For a comparison of the present day and HB64 beam filtrations, refer to

table 3-6.

The percent corrections, ZApEp and EApSp, presently used for the WyckoffAttix FAC and the products of all exposure-rate-independent corrections for

each beam code are given in table 3-7. Comparison of columns 3 and 4 of

table 3-7 with columns 2 and 3 of table 3-5 shows there is good agreement, in

general, between the data calculated in this review from HB64 and the data

presently used. The corrections for electron loss (zApEp) for beam M300 and

H300 are estimated from figure 9 of reference 2, and may be slightly in error

since the filtration of that reference beam is not the same as that of the

present-day M300 and H300 beams. The trends in the corrections in this energy

range have been considered and an uncertainty in zApEp of 0.07% has been

estimated for M300 and 0.15% for H300.2

^Estimated uncertainties are intended to correspond roughly to a 67%

confidence level, as explained in section 5.

18
cQ[ > (ON 1 1 Co Cc

</) <0O

(/)

4-> cu C

0) fO a

c_ QOJa. o)

4-^ Q)

-»

O

to c

tou <U o T3

+J

«/) -a >>

0) (O

t/) C- 3 X M

</) -o

rtj O)

QJ (- {_ <u

(O 4->

M- co

•p— MU

fO

c_ M- •r—> o to c (U

(T3 x:

T3C

to o 2: o O "^cC ^ U. C I—

>> ^X

ro OJ <-

i: +j 4-> E t- <: O <U I 4- T3 4-

<4-

O M- E O

(T3 ^ •-> C (J

rt3

O >»

Q M- 3

00I CO

H300 0m

CMS0

(M

OS0m1—( »0 <*

i-H /-\ u ^ *S w ^< 0 w ^s Of <

ms4 00 <

(14

<u 6o 55V E C o c ^ 1 1 1 1 1 1 . • 00

00 CM iA 1—

CI

CM 1—

0 0 CM 1 _ 1 00 CM 0 CM 1 to ou 0 0 00 CM <-l 0 Q 00 •10 CO in • • • 1 0 00 00 • • 0 in 00 CM CO CM CM • • 10 CM

.-1 °1 0 en

<—

0 1 CN

t-l <r «-l f—

t—

0 0 00 1 1 m (O 00 CM Q• • • 1 CO 00 in f-l 00 • • • 1 \o CO 0 CO0 1

<—

00 in • • • 1 1 00

vO *^ 0 C 1 ON CM vO

CM 0 0 0 p-t • • d d 1 1 ITl r>. 0 On 00 lA en <—

CM 0 0 0 0 • • « 1 0 0 Q 0

NO OD <!

GO vD

m ind

to lA in

w • • « 0 CM CM

»-l

m iTl in in 0 o>

1—( 1 i

«^

>> • -D

0) c

t_ 0

0)

-(->

(- E

fO <4- OJ J3 E

(O >> 0)

fO J3

t_X DC c

fO -0C

£_ <o 0

%-

2:

0)

<o (-0

(T5 M-

+-> (O VO

-0 1 CO >

0) 1 0 j3

(O ro +->

a; (U j= a> I— 00

19
Table 3-4. Data from Handbook 64, and fractional areas, used to calculate the

percent contribution to the ionization in the Wyckof f-Attix FAC

due to scattered photons. No distinction is made in this table

regarding the "M" or "H" type filtration.

Inner

(cm)

Radius

Outer Ap

(cm) {%)

60

(%)

kV

ApV

(%)

100 kV 150 kV

r F r r F r (%) (%) (%) (%)

0 9.5 1.00 0.60 0.60 0.47 0.47 0.34 0.34

9.5 12.9 0.66 0.14 0.09 0.13 0.09 0.11 0.07

2ApS

1 1 0.69 0.56 0.41

200 kV 250 kV 300 kV

0 9.5 1.00 0.32 0.32 0.30 0.30 0.28 0.28

9.5 12.9 0.66 0.10 0.07 0.09 0.06 0.08 0.05

ZA^S

F r 0.39 0.36 0.33

Table 3-5. Summary of calculations from tables 3-3 and 3-4 for percent loss

and gain of ionization due to lack of plate separation and

scattered photons, respectively. The data are for the WyckoffAttix FAC.

Beam Code 2ApE^ (%) EApS^ {%)

M 60 0 0.69

MlOO 0 0.56

M150 -0.16 0.41

M200 -0.36 0.39

M250 -0.48 0.36

M300 -0.81 0.33

H 50 0

H 60 0

k

HlOO -0.08 * H150 -0.77 * H200 -0.81 * H250 -0.23 * H300 ** *

* Same as for "M" beam codes.

* . . . . The 300-kV filtration of 4 mm Cu is not comparable with the composite

filtration used for H300.

20
Table 3-6. Comparison of x-ray beam filtrations used for data reported in

Handbook 64 with filtrations presently used for conventional

calibration conditions.

Handbook 64 NBS SP-250, May 1984

X-ray tube

potential

(kV)

Inherent

filter

(mm)

Pb

(mm)

Added

Sn

(mm)

Filter

Cu

(mm)

Al (mm)

Inherent

filter

(mm)

Added

Pb Sn

(mm) (mm)

Filter

Cu

(mm)

Al (mm)

60 3 Al 3 Be 1 .51

1 f\f\ 100 3 Al 1.0 3 Be 5.0

150 3 Al - 0.23 1.0 3 Be 0.25 5.0

200 3 Al - - 0.50 1.0 3 Be _ _ 1.21 4.1

250 3 Al - - 1.0 1.0 3 Be - 3.20 5.0

300 3 Cu - - - - 3 Be 6.5 - 4.0

50 (No data) 3 Be 0.10 4.0

60 (No data) 3 Be 0.61 4.0

100 3 Al 0.53 3 Be 5.2 4.0

150 3 Al 1.53 4.0 3 Be 1.51 4.0 4.0

200 3 Al 0.7 4.0 0.59 3 Be 0.77 4.16 0.60 4.0

250 3 Al 2.7 1.0 0.59 3 Be 2.72 1.04 0.60 4.0

300 (No data) 3 Be 5.0 3.0 4.1

For the NBS SP-250 beams, the first 6 entries are moderately (M) filtered and

the final 7 entries are heavily (H) filtered.

21
Table 3-7. Data presently used (May, 1986) to compute corrections for the

Wyckoff-Attix standard free-air ionization chamber for conventional calibration conditions. An energy independent correction

factor of 1.0015 for noncoplanarity of the guard-collector plate

system is included in the product nk-j.

lOO(l-kg) 100{l-kp)

Beam Code zAcEp zApSp kpk^ nk,-/k<-

{ly {%)

^

M 60 1.0203 - 0.77 0.9924 1.0140

MlOO 1.0097 - 0.60 0.9940 1.0052

M150 1.0068 -0.15 0.40 0.9975 1.0058

M200 1.0055^ -0.40*^ 0.39^ 1.0001 1.0071

M250 1.0045 -0.50 0.36 1.0014 1.0074

M300 1.0039^ -0.62 0.32^ 1.0030 1.0084

H 50 1.0103 0.63 0.9937 1.0055

H 60 1.0088^ 0.55^ 0.9945 1.0048

HlOO 1.0060^ -0.04 0.41^ 0.9963 1.0038

H150 1.0050 -0.68 0.35 1.0033 1.0098

H200 1.0043 -0.82 0.35 1.0047 1.0106

H250 1.0040 -0.26 0.35 0.9991 1.0046

H300 1.0038^ -0.62^ 0.31^ 1.0031 1.0084

S 60 1.0131 0.70 0.9930 1.0075

^Estimated from a graph of k^^ vs

HVL in mm Cu.

'^Estimated to be essentially the same beam quality as the previous beam code

MFC.

^Calculated using figure 9 of reference 1.

^Predicted from the least squares fit:

ZApSp = 0.9912 + 1.816 x 10-3 logg(HVL in mm Al

)

22
The corrections given in table 3-7 for air attenuation, k^, in the

Wyckoff-Attix FAC are taken from data in Data Book 848 page 110 (DB 848:110)

where reference is made to DB 331 111:92-117 and DB 527:081. The corrections

appear to be nearly a linear function of the logarithm of the half -value-1 ayer

in copper for the beam qualities of interest. The data points cover the range

of Cu HVL's for the "M" and "H" beam qualities except for M60 and H300. The

kg values for those conditions are estimated from a plot of the data on semilog graph paper.

The data points given in DB 848:110 when fitted to a logarithmic curve using the method of least squares provide the following equation:

kg = 1.0067 - 2.0229 x 10-3 log^glHVL in mm Cu)

The correlation coefficient is 0.96. A comparison of the k^ data, and estimated values, with k^ computed using the least-squares-derived equation is

shown in table 3-8.

The only serious discrepancy between the presently used values for k^,

and kg derived from the least squares equation, is for M60 where the presently

used value for kg appears to be about 0.7% too high. A data point for

0.089 mm Cu HVL, which does not fit in very well with the rest of the data,

evidently had an undue influence in estimating kg for the M60 beam quality.

This conclusion is substantiated to some extent by data from figure 15 of HB64

where the air attenuation is shown to be 4.4% per meter at 60 kV for an inherent filtration of 3 mm Al . The inherent filtration for the Seifert 300-kV

x-ray tube is 3 mm Be, and for the M60 beam quality 1.51 mm Al is added,

giving an effective total filtration (assuming 60-kV x rays) equal to

2.5 nm Al . A thin phenolic protective plate covers the beryllium window,

which further adds to the filtration. If the attenuation per meter of air were 5%, kg would be 1.015, which is 0.5% lower than the value used

presently."

The recombination correction, k^, for the Wyckoff-Attix chamber is

computed using a first-degree logarithmic equation, developed using the

OMNITAB POLYFIT program, for data resulting from the work of Wyckoff and

Lamperti. A copy of the graph of the data plotted in 1969 is shown in

figure 3-13. The result can be represented by the equations:

kg = 1 + 6.52 X 10-^ JO. 382^ J in R/min

kg = 1 + 3.11 X 10-3 x°-382^ X in R/s

^At the time of writing (June 1986) an investigation of the M60 discrepancy is

planned.
Table 3-8. Comparison of the kg values presently used (May, 1986) with

values computed from the least squares equation.

vu n V L

( mm ^

^ If III 1 1 prpc pnt

M 60 0.052 1.0203* 1.0128

MlOO 0.20 1.0097 1.0100

M150 0.67 1.0068 1.0076

M200 1.69 1.0055* 1.0057

M250 3.2 1.0045 1.0044

M300 5.3 1.0039* 1.0034

H 50 0.142 1.0103 1.0107

H 60 0.24 1.0088* 1.0096

HlOO 1.14 1.0060 1.0065

H150 2.5 1.0050 1.0049

H200 4.1 1.0043 1.0039

H250 5.2 1.0040 1.0034

H300 6.2 1.0038 1.0031

*E St i mated from graph in DB 848:110

o

1.0 2.0 4.0 6.0 8.0 10 20 30

Exposure Rate, R/Min

Fig. 3-13. Fractional recombination correction (k^-l) versus exposure rate

for the Wyckoff-Attix FAC.

24
Representative values of the recombination correction are as follows:

Exposure rate, X k.

(R/min) (R/s)

1.0 0.0167 1.0006

10.0 0.167 1.0016

100.0 1.67 1.0038

The net result of this review of the corrections for the Wyckof f-Att i

FAC is given in table 3-9 where the products of the presently used corrections

and those computed in this review are compared. The details regarding the

individual corrections are summarized in the following comment section.

Table 3-9. Products of all exposure-rate-independent corrections for each

x-ray beam quality in current use (May 1986) with the Wyckof fAttix FAC, and the products of the corrections based on the data

from HB64 and this review.

nk^/kg

Beam Code Current Review Pi f f erence m

M 60 1.0140 1.0074 0.65

MlOO 1.0052 1.0059 -0.07

Ml 50 1 .0058 1 .0066 -0.08

M200 1.0071 1.0069 +0.02

M250 1.0074 1.0071 +0.03

M300 1.0084 1.0097 -0.13

H 50 1.0055

H 60 1.0048 1.0042 +0.06

HlOO 1.0038 1.0032 +0.06

H150 1.0098 1.0100 -0.02

H200 1 .0106 1 .0096 +0.10

H250 1.0046 1.0036 +0.10

H300 1 .0084

Some comments regarding the Wyckoff-Attix FAC corrections

1. kg. The presently used correction for air attenuation for M60 appears

to be too high, although the comparison is made between two extrapolated

values for k^. This correction should be determined experimentally using

well-known techniques. The presently used corrections at M150 and H60 are almost 0.1% different from the values computed from the least-squares-derived

equat i on.

25
2. kg and k . The methods and sources of data for computing corrections, for ionization loss and gain due to electron loss and scattered-photon

contribution respectively, have been provided. The information has been given

in such detail that it can be checked and, if newer data become available, it can be revised.

In general, the computations in this review provide data for the

electron-loss and scattered-photon gain corrections which are in agreement

with the data now used. Some exceptions, where differences are of the order

of 0,1%, are (comparing tables 3-5 and 3-7) zApS- for M60 and zApSp for H150.

The corrections for M300 assume that the rather large difference in filtration

between the experimental conditions (4 mm Cu) , and the actual filtration for

M300 (3 mm Be + 6.5 mm Sn + 4.0 mm Al ), will not significantly affect the

magnitude of the corrections. The same corrections as are used for M300 are used for H300. While there is some justification for the assumption of

equivalence between the experimental and M300 filtrations, the same is not

true for H300, and calibrations for this condition have been discontinued

until the appropriate data are available. (Refer to fig. 3-14).

3. Non-copl anari ty correction. This correction is based on measurements

made many years ago. The guard-collector plate assembly should be

re-inspected periodically to verify the validity of this correction.

3.2.2. Ritz (20- to lOQ-kV) FAC corrections

The Ritz FAC is used for standardization of x-ray beams for x-ray tube

potentials from 20 to 100 kV [3]. The dimensions of interest for purposes of

developing corrections for this FAC are: L, the length of the ai rpath between

the def i ni ng-pl ane of the diaphragm and the mid-plane of the collection plate;

and the collection-plate-system separation and height, refer to table 3-1.

The largest FAC correction for "low" energy, lightly filtered x rays is

for air attenuation (k^). This correction is determined with the FAC at

particular distances from the x-ray tube, since the intervening air acts as a filter and at low energies can influence the measurements. The procedure for

determining the correction involves removal of the FAC diaphragm and setting a fixed diaphragm in the beam independent of the FAC. For a particular set of

conditions (distance at which the exposure rate is to be determined, x-ray

tube potential, filtration), the ionization current produced in the FAC is

measured with the mid-plane of the collection plate at the position where the

exposure rate is to be determined, (position 1) and again with the mid-plane

moved away from the source by the air absorption distance shown in table 3-1

(position 2). Position 1 is the normal position of the diaphram, and

position 2 is the normal position of the collection plate. The diameter of

the fixed diaphragm must be such that the defined beam in its entirety is

intercepted by the FAC at the two measurement positions. The ratio of the

currents measured at position 1 and position 2 is the air attenuation correction factor for the conditions of measurement. These conditions include the

atmospheric temperature and pressure since the attenuation is dependent on the

density of the air. The attenuation correction is computed for P = 750 mrnHg**

"One atm = 101.325 kPa = 1013.25 mbar = 760 mmHg. Our laboratory barometers

read out in mm of Hg, so all constants in this review are computed using

pressure in units of mm of Hg.

26
100

0 so 100 ISO 200 2S0 300 nieton intrfy (kiV)

Figure 3-14. 300 kV spectra. Initial triangular spectrum assumed for 300 kV across x-ray tube. Spectrum modified by mass attenuation coefficient

from Hubbell [6]. The triangular spectrum is hypothetical and is used to

allow calculation of the other spectra.

and T = 293.15 K (dry air density = 1.189 mg/cm^) to provide a correction

factor representative of normal room conditions. If the experimentally

determined attenuation coefficients y/p are so large that normal variations in room conditions produce significant differences in the correction, then

pressure and temperature must be taken into account. This condition pertains

27
for X ray beams with Al HVL's < 0.22 mm. A list of mass air-attenuation

coefficients, experimentally determined using the Ritz FAC, is given in

table 3-10, Also included in table 3-10 are air-attenuation corrections, k^,

for the reference conditions P = 750 mmHg and T = 293.2 K. (The data for

these calculations are taken from DB 752:075-084, 143-181; DB 848: 170-200;

DB 852:5-8.) Values for k, in table 3-10 differ by less than a few hundredths

of one percent from presently used data given in a similar table recorded in

DB 852:11.

Table 3-10. Mass air-attenuation coefficients and values of k for the Ritz

FAC. The defining plane of the FAC diaphragm is 50 cm from the

X-ray tube target

For the Ritz chamber. L = 127.39 mm.

Beam

Code

1 1 V/ 1 HvL

(mm Al

)

y/p

(cm2/mg) 293.2 K

L20 0.071 8.2293 X 10-3 1.1327*

L30 0.22 3.2392 X 10-3 1.0503*

L40 0.49 1.6816 X 10-3 1.0258

L50 0.75 1.2163 X 10-3 1.0186

L80 1.83 7.3175 X lo-"* 1.0111

LlOO 2.8 4.3435 X 10-t 1.0066

M20 0.152 4.0835 X 10-3 1.0639*

M30 0.36 1.8811 X 10-3 1.0288

M40 0.73 1.1051 X 10-3 1.0169

M50 1.02 7.4192 X 10-4 1.0113

H20 0.36 1.6190 X 10-3 1.0248

H30 1.23 5.8048 X 10-"+ 1.0088

H40 2.9 3.9028 X 10-4 1.0059

These factors can vary significantly with changes in atmospheric temperature

and pressure. For ambient conditions, k^ is calculated using kg = exp(yL).

Note: The experimentally determined attenuation coefficients in column 3

should not be confused with conventional, good-geometry attenuation

coefficients.

The effect on k^, of changes in temperature and pressure from reference

conditions can be calculated as follows:

kg = exp(yL) = exp((u/p) pL)

28
where L is the air absorption length from table 3-1. Because the air density

p is proportional to P/T, and p/p is independent of density, it can be shown

that

dkg/kg = (mD (dP/P - dT/T) .

For Beam Code L20, L = 12.7 cm, and yL = 0.12. Thus a 1% change in P or T

gives a 0.1% change in k , and a smaller change for the remaining beam codes

in table 3-10.

The electron-loss corrections (kg) for the Ritz FAC are taken from data

in the 1959 publication of Ritz [5]5 on the design of free-air ionization

chambers. The dimensions of the Ritz FAC are such that the ionization loss

due to loss of electrons occurs only at the upper end of the x-ray energy

range for which this FAC is used. Therefore, only figures 10 and 14 of

reference [5] are required for calculation of the corrections for Beam Codes

S75 and LIOO. These data, used in conjunction with the fractional areas inside the collection-plate system for different radii, provide the required

corrections for ionization loss. For this calculation, the plate separation

is reduced from 9 cm to 8 cm to account for the 1-cm-diameter FAC beamdefining diaphragm and to simulate the required zero-beam diameter. The

results of the calculations shown in table 3-11 do not differ by more than

0.1% from the corrections for electron loss presently used for these

conditions (DB 852:11). Since no data are published for the L80 x-ray beam

condition, the correction for electron-loss is estimated by interpolation

following the curvature of the data for Ep at 60, 75, and 100 kV with 3 mm Al

added filtration. The estimated correction is 0.12%, which is not significantly different from the presently used correction. A detailed analysis of

the kg corrections required for the Ritz chamber has been performed. The

calculations, which consider x-ray beam penumbra effects on the magnitude of

the corrections, are recorded in a loose-leaf binder entitled "Supplement to

DB776, FAC Comparisons."

Table 3-11. Computation of electron-loss corrections for the Ritz FAC

Radius S75 LIOO

Inner Outer

(cm) (cm) {%) %^ A)

0 4.0 1.00 -0.10 -0.10 -0.60 -0.60

4.0 4.5 0.58 0.035 0.02 0.20 0.12

4.5 5.0 0.28 0.024 0.01 0.13 0.04

^e

= EApER -0.07 -0.44

5A11 the Ritz data are for a target-to-diaphragm distance of one meter. The

assumption is made that, for the present-day operating distance of 0.5 m, the

shorter air path will not significantly affect the Ritz data for electron

loss and scattered photon contribution corrections.

29
Corrections for the scattered-photon contribution to the ionization in

the FAC are derived from Ritz [5], and Allisy and Roux [7]. The percent

scattered-photon contributions within different radii, and appropriate multipliers for several x-ray beam conditions, are given in Ritz [5], figure 15 and

table 1, respectively. The data of Ritz, Allisy, and Roux were combined and,

by means of least -squares fit for kp vs^. Al HVL in mm, the following

logarithmic equation was developed:

kp = 0.9956 + 2 X 10-3 log^pCAl HVL in mm) .

The values of kp for the Ritz FAC listed in table 3-12 are computed from

this equation. For convenience, the products of all rate-independent

corrections for the Ritz FAC are provided in the last column of table 3-12.

1 dU 1 c 3-12. Summary of corrections for the Ritz FAC. 1 ne va i ues gi ven lor k are computed using P = 750 mmHg and T = 293 2°K

Beam Added Al Al

code filter HVL ^a ^e nk^

(mm) (mm)

L20 none 0.071 0.9933 1.0000 0.9933*

L30 0.265 0.22 0.9942 1.0000 0.9942*

L40 0.50 0.49 1.0257 0.9949 1.0000 1.0205

L50 0.639 0.75 1.0186 0.9953 1.0000 1.0138

L80 1.284 1.83 1.0110 0.9960 1.0010 1.0080

LlOO 1.978 2.8 1.0065 0.9964 1.0051 1.0080

M20 0.230 0.152 0.9940 1.0000 0.9940*

M30 0.50 0.36 1.0289 0.9947 1.0000 1.0234

M40 0.786 0.76 1.0170 0.9952 1.0000 1.0121

M50 1.021 1.02 1.0114 0.9955 1.0000 1.0069

S75 1.504 1.86 1.0076 0.9960 1.0007 1.0043

if These products include only k- and k^ and must be multiplied by exp(yL).

The recombination corrections for the Ritz FAC are calculated from an equation of the same form as that developed for the Wyckoff-Attix FAC. The

corrections are based on measurements of ionization currents at several

collection potentials, with exposure rate as a parameter. In accord with the

method of Scott and Greening [8], the inverse of the ionization currents are

plotted against the inverse of the squares of the collection potentials.

30
Extrapolation of the plotted data to 1/E2 = n predicts the inverse of the

saturation ionization current. If the ionization currents are normalized to

the current measured at the normal operating collection potential, the inverse

of the intercept at 1/E2 = 0 is the recombination correction for that

particular exposure rate.^

The Scott and Greening extrapolation procedure for exposure rates ranging

from 70 mR/s to 480 mR/s yields the data shown plotted in figure 3-15. The

OMNITAB POLYFIT program used to fit the data to a first degree logarithmic

equation provides the following equations:

kg = 1 + 9.182 x 10-6 X 1-174^ X in R/mi n

kg = 1 + 1.123 X 10-3 X 1-17^, X in R/s .

With reference to figure 3-15, the plotted points are from data derived

from measurements with a 1-cm diameter FAC diaphragm (the diaphragm in normal use for calibrations) and from measurements made using a 0.5-cm diameter FAC

diaphragm. There appears to be no significant difference in the data sets.

Two points using the 0.5-cm diaphragm at = 20 R/min (solid dots) are considered to be outliers and the original data have not been reviewed.

The data plotted in figure 3-15 are based on measurements recorded

throughout DB 592, 624, and 638. The data plots used for extrapolations are filed in a file cabinet in Rm. R033 under the general title "FAC Recombination." Further analyses of the data are recorded in a three-ring binder

entitled, "Supplement No. 1 to DB 592, 624 and 638. 10-60kV and 20-100 kV FAC

Comparison." The three-ring binder is kept in the bookcase in Rm. R033.

3.2.3. Lamperti (10- to 20-kV) FAC corrections

The Lamperti FAC is designed for x-ray exposure standardization in the

region 10 kV to 60 kV. In practice, the Ritz FAC, with exposure measurement

capabilities overlapping that of the Lamperti FAC, is used for calibrations

down to and including 20-kV x rays. The Lamperti FAC is used only for

measurements at 10 kV and 15 kV.

Lamperti and Wyckoff [4] have discussed in detail the corrections for the

Lamperti FAC. Although many different corrections are identified, the

important corrections are for air attenuation, k^, and scattered photon

contribution to the ionization, k . The corrections for electron loss, k^, in

the 10-kV to 60-kV region are giv^n as "much less than 0.1%" [4]. For x rays

generated at 10 kV and 15 kV, kg should equal unity since the FAC plate

^The extrapolation procedure described was also used to determine recombination corrections for the Wyckof f-Attix FAC but the data were plotted with 1/E

as the abscissa. According to Scott and Greening, this procedure exaggerates

the effects of recombination and is proper only for very low exposure rates.

However, the predicted recombination correction for the Wyckof f-Attix FAC is

only about 0.2% for the maximum exposure rate tested (about 0.33 R/s

(20 R/min)") and the usual operating electric field strength of 250 V/cm.

31
O.OlOf

QOOlO

0.00010

0.00001

i

1 1

— 1 1 "I—1

1 I—TTTTT 1 1 1 1

1

I

1 "1—I 1 11 III'1

1 M1 1 1 "T 1

1 —81 A 1 II 11 1 1 1 ij

6 /

—

o/ —-

-- —

o „ : — 8 - o oV

o / o —

o /

o/

Oro — //0 • -

/ 1 1 1 1 1 1 1 1 1

•

1 1 1 1 ill! I 1 1 Mill

1.0 10 100 1000

Exposure Rate(R/min)

Fig. 3-15. Fractional recombination correction (k^-l) versus exposure rate

for the Ritz FAC.

separation is 4 cm (refer to table 3-1) and the cont i nuous-sl owi ng-downapproximation (CSDA) range [9] for 15-keV electrons is on!}' 0.5 cm (air

density equal to 1.1888 mg/cm^). Actually, kg should be unity for the

Lamperti FAC for x rays up to 50 kV since the CSDA range for 50-keV electrons

is 4.1 cm for normal room temperature and pressure. (This is a very conservative statement because the assumptions are that the entire energy of the

highest energy photon is transferred to an electron, and that the CSDA range

is equal to the practical range. According to Katz and Penfold [10], the

practical electron range would be only about 80% of the CSDA range for 50-keV

electrons.

)

The intercomparison of free-air chambers described by Lamperti and

Wyckoff [4] was carried out for x rays generated by x-ray tube potentials from

20 kV up to 60 kV, with corrections for the effect of scattered photons, k^,

obtained from Ritz [3] and Allisy and Roux [7]. The data from these two

sources differ slightly (=: 0.1%) in the region below 30 kV and the two sets of

data have been combined using the least squares method to arrive at the

following equation:

kp = 0.9975 + 1.034 x 10-3 log

10

(Al HVL in mm)

32
Somerwi! [Ill investigated a systematic di fference between several

national FAC standards, intercompared at the Rureau Internationale des Poids

et Mesures, and found that the FAC scattered-photon contribution correction

for chambers with 4n-mm diaphragm-to-collection-plate distances, d, should be

less than corrections determined from measurements with chambers having

d = 100 mm. (The Ritz FAC has d = 127 mm.) The values of kp were computed

for X rays with Ream Codes LIO, LIB, HIO, and HI 5 using the Sbove equation.

These corrections were reduced by adding 0.15% at LIO and HIO and 0.10% at L15

and HIB because the Ritz and Allisy-Roux measurements were for d = 100 mm. The adjustments to the corrections are the percentages determined by

Somerwi 1

The air attentuation corrections, k-, for the Lamperti FAC are determined

using the Ritz FAC in the two-position, i ndependent -diaphragm technique,

already described. In this case, the distance between the two measurement

positions is made equal to 39 mm since that is the air attenuation path length

in the Lamperti FAC. The Ritz FAC is used because the Lamperti FAC sans- diaphragm aperture is not large enough to encompass the beam defined by the

fixed diaphragm. The measurement data are recorded in DB 752:143-162.

The recombination corrections for the Lamperti chamber were determined

using the procedure suggested by Scott and Greening. The data are recorded

throughout DB 592, 624, and 638, in the three-ring binder entitled "Supplement

No. 1 to DB 592, 624 and 638" and in file folders under the general title "FAC

Recombination", all filed in Rm. B-033. The equation developed from these

studies for determining the recombination correction is:

kg = 1 + 7.08 X 10"^ Jo. 735^ J r/^^-

p

kg = 1 + 1.435 X 10"^ Jo. 735^ J 1^/5

Recombination corrections for exposure rates commonly encountered in

instrument calibration work are provided in table 3-13 and figure 3-16,

Table 3-13. Recombination corrections for the Lamperti FAC

• Exposure rate, X (R/min) (R/s)

10 0.167 1.000

100 1.67 1.0002

1000 16.7 1.0011

The corrections presently used for the Lamperti FAC are given in

table 3-14 where most of the air attenuation corrections are shown to be

dependent on temperature and pressure and the corrections for photon scatter

have been computed from the equation for kp adjusted for the Somerwi 1 correction, kg is equal to unity as are all other exposure-rate independent

corrections identified by Lamperti [4],

33
10 100 1000

Exposure Rate (R/min)

Fig. 3-16. Fractional recombination correction (k--l) versus exposure rate

for the Lamperti FAC.

34
Table 3-14. Summary of corrections for the Lamperti FAC. The air attenuation

data are for a target-to-diaphragm distance of ?.S0 mm. For the

Lamperti FAC, L = 39.0? mm.

Be am codeAO oeo filter

(mm Al)

HVL

(mm Al)

li/p

( cm2/mg)

LlO 0 0. 029 0. 0194 0. 9972 "A" LIB 0 0.050 0.0125 0.9971

HIO 0.105 0.048 0.0140 0.9975 *

HI 5 0.500 0.152 0.9976 1.0245

kg = exp(yL)

3.3. Intercomparison of standard free-air ionization chambers

The present NBS standard free-air ionization chambers have been intercompared with the standards of other nations and internally, to test their

congruity where their measurement capabilities overlap. It is beyond the

scope of this review to discuss in detail the various international intercomparisons except to state that one should expect agreement within 1.2% [21.

Internal i ntercompari sons of the free-air chambers have been carried out

in 1961-62, 1966-67, 1975 and 1976 for the Lamperti and Ritz chambers, and in

1958-59, 1962, 1975-76 for the Wyckof f-Att ix and Ritz chambers. The results

of these intercomparisons were tabulated in 1977 in a report for the Dosimetry

files. The mean difference between the Lamperti and Ritz FAC's in the region

20 kV to 50 kV was found to be 0.41%. The mean difference between the

Wyckoff-Attix and Ritz FAC's was found to be 0.51% in the region 60 kV to

100 kV. No adjustment is made for these differences, since they are well

within the maximum difference of 1.2% estimated by the ICRIJ, and the

intercomparisons give no indication which member of a pair is to be considered

the more reliable. The relatively recent small adjustments to the scattering

corrections for the Lamperti-Ri tz FAC's will not significantly affect the

agreement

3.4. Gamma-ray exposure standards and calibration ranges

There are seven gamma-ray sources available for the calibration of

instruments for the quantity exposure and for delivering known exposures to

passive dosimeters. The sources are collimated and the beams have been

calibrated using appropriate standard graphite cavity ionization chambers.

These chambers have precisely known volumes, so that when exposed to a gamma

ray beam they define the ionization per unit volume of air and, with suitable

corrections for wall absorption and other perturbations, the collected charge

can be interpreted directly in terms of the quantity exposure.

35
3.4.1. Cavity-chamber standards, description

The cavity chambers, used for studies leading to the revised (1972 May 1)

60Co and 137^5 exposure-rate standards, were fabricated from reactor-grade,

high-purity graphite, following the design of Wyckoff [12], shown in

figure 3-17. The spherical shape was chosen in order to allow the standards

to be based on a homogeneous group of chambers of different volumes, to avoid

the distance effect and the complexity of set-up in measurements with

cylindrical chambers, and to present a uniform, symmetrical, chamber aspect to

the source. The chambers have been carefully compared in the gamma-ray beams,

and the NBS standard of exposure for these radiations is the mean response of

all the spherical graphite cavity chambers.

Figure 3-17. Cross-sectional view of typical cavity chamber.

The dimensions of the spherical chambers are given in table 3-15. The

three small-volume chambers, identified as 0.5, 1, and 2, were designed to be

used as a group to determine a wall correction and provide one measurement of

gamma-ray exposure. These chambers have the same nominal outside diameter but

different wall thicknesses and thereby different cavity volumes. They were fabricated using ball end-mills of diameters 3/8, 1/2, and 5/8 in. The wallthickness values given for these three chambers were derived from measurements

of outside diameters and the diameters of the end-mills used in their fabrication. The net volumes given in the table are the differences between the

cavity volumes and the volumes of the collection electrodes. The electrode

diameter for the 50-cm3 chambers is nominally 0.3 cm but for all other

chambers it is nominally 0.1 cm.

36
Table 3-15. Dimensions of spherical graphite ionization chambers

Nnmi na

lie U Hi i1" c "i Hp VJI U [J M 1 u t. vol ume Vol ume vol ume diameter density Radial wall thickness

(cm3) (cm3) (cm3) f cm \ f Q • cm~ 3

)

(cm) (g. crn-2)

0.5 0.440 0.431 2.078 1 .72 0.563 0.968

1 1 .140 1 .131 ? 065 1 73 0.398 0.688

2 2.029 2.019 2.080 1.74 A O /I C

10 10.088 10.069 3.428 1 .72 0.3755 0.647

30 30.262 30.24 4.607 1.74 0.3751 0.653

50-1 51.943 51.634 5.34 1.73 0.3652 0.632

50-2 50.425 50.089 5.58 1.73 0.5085 0.880

50-3 50.460 50.155 5.80 1.73 0.6129 1.060

The 50-cm3 chambers have the same nominal cavity size but different wall

thicknesses and can be used as a group to determine the wall correction. The

wall correction can also be determined by the addition of closely fitting

shells to two of the chambers. With the two methods of wal 1 -absorption

measurement available, each of the 50-cm3 chambers can be considered to

provide an independent measurement of exposure.

Of all the spherical chambers, the 50-cm3 chambers are of the highest

quality, great care having gone into their fabrication to insure close tolerances in all dimensions. Measurements of wall thickness at many locations on the periphery of both halves of each 50-cm3 chamber show that the range of the

wall thickness variation is less than 0.025 g.cm-2 and the largest difference

between the average values for the two halves is 0.016 g.cm"2.

The fabrication of the lO-cm^ and 30-cm3 chambers was performed with less

restriction on the variation of chamber wall thickness with the result that

the average wall thicknesses for the two halves differ by 0.016 cm (0.25 g.cm-2) for the 10-cm3 chamber and 0.052 cm (0.089 g.cm-2) for the

30-cm3 chamber. Although these differences seem to infer uncertainties in the

chamber wall correction of up to 0.3 percent, in fact the chamber response is

related to the average wall thickness and the average is used in plotting the

chamber response versus wall thickness to determine the wall correction. The

wall thicknesses given in table 3-15 for these two chambers are the overall

averages for measurements in a radial direction at a number of positions (see

fig. 3-17), and the corrections are determined for the chambers by the

addition of closely fitting spherical shells to the chamber walls.

The densities for the 0.5-cm3 and 2-cm3 chambers were measured using the

principle of Archimedes, while density for the l-cm^ chamber was inferred from

measurements of another chamber fabricated from the same material. The

densities for the 50-cm3 chambers were determined from a cylindrical block

machined from the same material. The densities for the lO-cm^ and 30-cm3

37
chambers were determined using differential weighings in and out of distilled

water. The differential weighing method and mechanical measurement of

dimensions and weighing of the same graphite block give densities which differ

by only 0.01 g«cm-3. Further details are contained in a report by Loftus and

Weaver [13],

3,4.2. Gamma-ray sources, location, initial activity, and description

The location, year acquired, nominal activity at the time of

purchase, and beam orientation of the gamma-ray sources used for instrument

calibration are given in table 3-16. Drawings showing the horizontal -beam

sources and collimator dimensions are attachments 4 and 5. A sketch of the

Theratron "F" head which contains the ^^Co source located in room B036 is

attachment 6.

Table 3-16, Gamma-ray source locations and nominal activities of sources at

time of purchase

Radionucl ideYear*

acqui red Act i vity Location

Beam

orientation

60Co 1978 10 kCi B036 Vertical

60Co 1965 1 kCi BO 34 Vertical

60Co 1964 200 Ci B021B Hori zontal

60Co 1964 20 Ci B015B Hori zontal

137CS 1957 1,7 kCi B036 Vertical

137CS 1964 300 Ci B021A Horizontal

137CS 1964 30 Ci B015A Hori zontal

These dates are supplied only to indicate the approximate ages of the sources. The activities do not take self-absorption into account.

3,4,3, Calibration of gamma-ray beams

The horizontal -beam calibration ranges were calibrated at various

distances from the sources. The data were fit to a suitable function which

allows accurate computation of exposure rates at selected distances, or of

distances for selected exposure rates. The equation used for this purpose was derived by using the inverse square law with correction for air attenuation

and buildup. An equation that was found to adequately predict the exposure

rate X to within a few hundredths of one percent was of the form

XD2 = Kj(l + KgD + K3D2 + K^^D3)

where D = S + and S is the distance from the source to the reference point

of the chamber, as read on a scale mounted on an aluminum channel. An

38
Al-channel scale is associated with each source. The S are offset

distances, for each source, introduced to improve the fit of the data to the

polynomial in D. In practice, the equation is solved for exposure rate by

rearranging the equation to:

X = (A/D2 + B/D + C + ED)e' XT

where x is the decay constant for the radionuclide, and T is the elapsed time

since the source was calibrated.

Tables of exposure rate vs distance are printed out, using the Digital

Equipment Corp. 11/23 Computer (DEC), for each month with arguments of three

days. In the information at the top of the tables, the letter P is used for

Sq. Although the tables are useful for many purposes, the constants for the

equations are stored in the "G CP" program for the HP9836 computer and

exposure rates are computed, as required, when calibrations are performed

using the mobile measurement console. The nomenclature for the constants is

different in the DEC program for producing tables, from that used by the

HP9836 to compute exposure rates. For ease of translation between programs,

values of the constants for 1984 Dec 21 are given in table 3-17 for each

source, using both sets of letters.

Table 3-17. Constants* for

the horizontal

computing exposure

beam sources. The

rate corrected for

reference date is

decay for

1984 Dec 31

DEC:

HP9836:

Source

A

Kl

(m2 mR/s)

B 102 K2

(m mR/s)

C

10*+ K3

(mR/s)

P 102 K4

(m)

X 105 K5

(d-i) DB:page

B015A-137CS 2.0323 -0.943 -1.8619 -0.818 6.326 750:118

B021A-137CS 19.617 -13.84 123.24 -0.304 6.326 744:203

B015B-60CO 0.5924 -0.1657 -1.345 -0.144 36.010 717:127

B021B-60CO 5.2676 0.4764 67.33 -0.528 36.010 717:125

if The constant E equals zero for these four sources. The constant P is the

offset distance so distance equals the scale reading plus P in the DEC

program. Do not confuse the subscripted K's in the Loftus-Wea ver paper with

the Kl, K2 etc. used in the HP9836 program.

The data for the ^^Co sources in table 3-17 are derived from the data in

Table 17 of Loftus and Weaver [13]. Checks on the calibration of these

sources through the years since 1973 show them to be unchanged. On the other

hand, the checks on the i37cs sources indicate a need for recal ibrat i on since

they were found to have changed from previous calibrations by several tenths

of one percent. The reason for these changes is unknown. The updated

constants for the i37cs source in table 3-17 are based on the newest

calibration data. They are derived from a computer program for non-linear

least-squares function fitting (SAAM) of measurement data.

39
The ^^Co source in Rm. B034 is not used at present for instrument

calibrations. This source, initially 1 kCi, had been used for calibration of

smal 1 -vol ume cable-connected chambers and high-range R-meter probes but as of

1985 Jan, has an activity of only about 150 Ci making exposure times for those

calibrations too long. However, the source can be utilized for exposures of

passive instruments (TLD, Fricke, etc.). The data for two calibrations of

this source are given in table 3-18.

Table 3-18. Data and references for calibration of B034 vertical -beam ^^Co source. Al-Ch. scale reading: 50 cm. Collimator setting:

10 X 10 on the 100 cm TD scale. Distance to source: 97.9 cm. Date 1979 Dec 31

DBrpage Standard chamber and

1 cm3

exposure rate (mR/s)

10 cm3

Mean, all meas.

750:93 56.75 56.65

817:42-50 56.81 56.66

817:93 56.72 56.72

Calibrations of the vertical -beam sources are limited to one or two sets

of conditions. For the ^^Co source located in B034, the reference distance on the Al -channel mounted scale is 50 cm. The offset of the scale-zero from the source center is 47.9 cm. To the best of our knowledge the source consists of one layer of 1 mm x 1 mm cylindrical pellets.

The highest activity ^^Co source available for calibration work is

located in B036. At the end, of 1984, the effective activity of the source was about 3 kCi (computed using XD^ measurements). The source was calibrated

early in 1979 using the 1 cm^ and 10 cm^ graphite standard ionization

chambers. The mean of those measurements corrected for decay is used at the

present time (1986 May). As shown in table 3-19, a later measurement using

the 10-cm3 standard chamber checked the original data.

The vertical -beam 137qs source in Room B036 is calibrated for several

collimator setting-distance conditions. The calibrations rely mainly on measurements with the l-cm^ standard chamber although some measurements with

the 10-cm3 and l-50-cm3 chambers have also been tried. Some of the lO-cm^ and

l-50-cm3 data in table 3-19 are parenthesized to indicate that they are not

included in the source calibration. These data are included in table 3-19 to

call attention to the need for further study of the measurement problems

encountered, and data accumulated, over a period of several years for this source. For example, the agreement between the 1 cm^ chamber and the l-50-cm3

chamber measurements for the 15 x 15 collimator setting show that the

standards are consistent for the large field size. But a reason for the

discrepancies in the data for the 8x8 collimator setting has not been

determined.

In the course of using the standard graphite ionization chambers for NBS

and other-agency source calibrations, the electrodes in the chambers have

either loosened and shorted to the chamber wall or have been over-voltaged.

40
When this occurs, the electrodes are weakened and carbon is deposited on the

high-voltage insulator. The l-cm^, lO-cm^ and 30-cm3 chambers are particularly prone to this problem since the electrodes are of small diameter

(< 1 mm) and, with the present stem design, are difficult to anchor securely

in position. It has been necessary on several occasions to straighten or replace these electrodes and to clean or replace the high-voltage insulators

and chamber stems. Subsequent checks of source calibrations, in particular

using the l-cm^ chamber, indicate that the relationship of that chamber to the

others, established under ideal conditions in 1972, may no longer hold.

Measurements of the high-exposure-rate sources after the last repair of the

l-cm3 chamber were several tenths of one percent higher than the reference

data. This is shown to some extent in table 3-19 where the latest data for

the 8x8 collimator setting is higher than the previous measurements. Some

l-cm3 chamber exposure data not included in table 3-19 indicate a difference

between present and earlier measurements of as much as 0.4%. On the other

hand, measurements at low exposure rates at 1 meter from the ^^"^Cs source in

Rm B021A show agreement between the l-cm^ standard and the l-50-cm3 or the

10-cm3 standard to be within 0.02% and 0.01% respectively. Referring to

DB 817:155, we have

Ratio of exposure rates, B021A, 100 cm:

^ 'Vlin , 0.9998

X (1-50 cm3)

and /

^^^^^^

= 0.9999

X (10 cm3)

The net cavity volume for the l-cm^ chamber following the last repair

(replacement of electrode and refurbishment of the HV electrode) is

1.1297 cm3. Details can be found in DB 817:94 if needed.

Table 3-19. Data and references for calibration of vertical -beam gamma-ray

sources in Rm B036. All data corrected to 1984 Dec 31.

Dist

Standard chamber Exp. rate

Al-Ch. and exposure rate used for

DB:page Source scale Coll • set 1 cm^ 10 cm 3 1-50 cm3 cal ib.

(cm) (mR/s) (mR/s) (mR/s) (mR/s)

799:34 60co 105 6 X 6 468.8 467.5 468.2

817:52 II II II 467.3

750:201 137CS 55 8 X 8 105.18

799:44,60 II II II 105.07

817:121,

131,141 II 11 II 105.21 (104.43) (103.68) 105.21

799:70 II II 15 X 15 108.42 108.21 108.31

817:138 II II II 108.19 107.78

817:121 II 38 10.5 X 10.5 167.16

41
The main difficulty in reproducing exposure calibration data centers

around the l-cm^ chamber. A small -volume standard chamber is required for

high exposure-rate measurements, and in beams that may be uniform only over

small areas (DB 799:10-11 and DB 817:17), but for small-volume chambers

unwanted extra-cameral volumes in the vicinity of the HV insulator and stem

can have a large effect on the chamber response to radiation. Additional

difficulties of another sort arise in comparing data over a period of years

when different measurement equipment may be used, (See for example DB

817:51-52.) The uncertainty statement given by Loftus and Weaver [13] is only

representative in the short term and for optimum conditions. If the same

overall uncertainty (0.7%) is to apply to a long-term exposure standardization, the primary standards must be carefully preserved and not used as working standards, and the associated equipment (electrometer, capacitor,

voltmeter) must be kept in accurate calibration.

3.4,4. Useful beam size

The "useful beam" radius for the gamma-ray sources is defined as the

distance from the center of a radiograph of the beam to the point where the

density of the film is 90 percent of the center density, minus the difference

in distance between the 90 percent and 50 percent density measurements. With

Rqq the radius of the 90-percent density contour, and R^q the radius of the

55-percent density contour, the useful beam radius (UB) is computed as

fol lows:

= K6 X SD + K7 (called Beam size in "G CF" program)

RgQ = K8 X SD + K9 (not specifically named in "G CF" program)

UB = R3, - (R50 - R90)

UB = 2RgQ - Rgg (called useful beam in "G CF" program

In the above equations, K6, K7, etc. are the constants listed in the

"G CF" program and SD is the aluminum channel scale reading. The constants

for each source are listed in table 3-20

Table 3-20. Constants for computing useful beam radii for the horizontal and

vertical beam calibration ranges.

Dist* Source K6 K7 K8 K9 Useful beam

(cm) (cm) (cm) radius (cm)

SD 137CS - B015A 0.188 0.613 0.143 0,044 Variable

SD 6OC0 - B015B 0.157 0.237 0.112 -0.106 II

SD 137CS - B021A 0.207 0.235 0,149 -1.068 II

SD 6OC0 - B021B 0.191 0.682 0,124 -0.236 II

38 137CS - B036 0 6.8 0 4.3 1.8

55 137CS - B036 0 6.4 0 4.05 1.7

105 60CO - B036 0 6.3 0 4.4 2.5

* SD indicates a variable source distance.

42
Referring to attachments 3 and 4, showing the hori zontal -beam sources,

note the cavity surrounding each of the sources. Radiographs of these beams

indicate a "hot" spot in the center so a conservative computation for the

useful beam is justified.

The vertical -beam sources are calibrated at specific distances thus the

constants K6 and K8 in table 3-20 are set to zero. The useful beam radii for

these sources is very small, less than 2 cm for the ^^"^Cs source and 2.5 cm

for the 60Co source. Referring to attachment 5, which shows the Theratron "F"

head which contains the B036 soco source, note that the sketch shows the

distances measured to determine the Al -channel scale offset distance.

Instruments designed for measurement of high-exposure-rate beams are often

sensitive to beam size. This effect occurs because the small-volume chambers,

used primarily to minimize ion recombination, are subject to the influence of

ext ra-cameral and insulator "soakage" effects, which can constitute a significant fraction of the chamber reading.

Some instruments showing the stem effect are the NPL 0.2-cm3 and the

Farmer-NEL 0.6-cm3 chambers, and Victoreen chambers of 100-R range and higher.

The difference in calibration factors for a Victoreen 100-R chamber, depending

on whether or not its stem is protected from radiation, may be as much as 4%.

In order to insure that sensitive chamber stems are protected, calibrations of

high-exposure-rate chambers are performed only for certain distance and

collimator conditions. These conditions are purposely few in number to reduce

the primary standardization work required and to minimize the possibility of error in setup. The beams are nominally square and the dimensions have been

determined using radiographic film. The 50% density contour is taken as defining the beam size for this purpose. The data are given in table 3-21.

Table 3-21. Beam size defined by 50-percent radiographic density contour for

vertical -beam gamma-ray sources, for specific collimator settings.

Al-Ch. Col 1 imator Beam

scale dist. settings size

Source (cm) (cm)

137CS - B036 38 10.5 X 10.5* 13.6 X 13.6

137CS - B036 55 8x8* 12.8 X 12.8

60Cs - B036 105 6x6 12.6 X 12.6

Actual setting determined by gauge blocks.

3.5. lonization-chamber current -measurement techniques

3.5.1. Background and history

Ionization currents as encountered in exposure standardization are currents produced by irradiation of a gas in an ionization chamber. The

ionization chamber may be a free-air chamber, such as one of the national

43
standard chambers, or a cavity chamber, where the gas is surrounded by some

wall material. Ionization chambers, regardless of type, consist of electrodes

that are insulated from one another and that are polarized in order to collect

charge produced in the gas. The ions produced in the air by the beam are swept from the chamber volume by the electric field between the electrodes.

In the normal course of events in dosimetry measurements, ionization

currents very rarely reach 50 nA. (An exposure rate of 1 R/s will produce

about 0.3 nA/cm3 in an ideal ionization chamber.) Included in the measurement

of these currents are currents not produced by the radiation of interest but

by background radiation and insulator leakage. The magnitude and sign of

these extraneous currents must be determined and the measured current

corrected for their effects in order to determine the true ionization current.

The importance of the correction for background and leakage is, of course,

relative to the magnitude of the ionization current but good measurement

technique requires, prior to attempting radiation measurements, that the background and leakage currents be determined. As a rule of thumb, and without

taking special precautions, the leakage current for a good quality ionization

chamber should be less than 5 fA. The measurement of leakage currents will

also include currents due to background radiation so the environment and

special circumstances must be considered in evaluating data. For example, if

tests are to be made of a large-volume ionization chamber in a background

environment found suitable for small -volume chambers, the extra sensitivity of

the large chamber requires separate evaluation of the background environment.

In the past, various types of instruments have been used for radiation

measurements, one of the first being gold-leaf electroscopes, then string

electrometers of various types, direct current amplifiers, AC amplifiers (the

vi brati ng-reed electrometer (VRE) with negative feedback), and presently, the

direct-coupled high-gain negative-feedback electrometers. (There is a good

description of early instrumentation in Hoag and Korff [14].)

3.5.2. El ectrometers

Until high-gain negative-feedback amplifiers were introduced, electrometers used for ionization current measurements were used as null detectors. Charge, or current, was measured by manually nulling the signal on the

most sensitive range of the electrometer. In the null method of measurement,

determination of stray capacitance in the system and voltage calibration of

the electrometer are unnecessary but it is necessary in free-air chambers to

maintain the null such that the collector plate potential is very near the

guard-plate potential. A difference in potential between the guard and

collector plates in a free-air chamber distorts the electric field and deforms

the defined air volume. Measurements of exposure will then be in error, see [15-17] for a more detailed description of this subject.

The VRE was an improvement over direct-current, vacuum-tube electrometers

not only because it utilized negative feedback but, being an AC amplifier, the

problem of zero drift was essentially eliminated. Negative feedback automatically maintains the inner electrode of an ionization chamber near the guard

potential and minimizes field distortion. For many years, VRE's were used at

NBS as null detectors [2,18]. The gain of the VRE's was found on occasion to

be much less than the design gain of 1000; gains as low as 250 were found,

which could lead to reading errors of up to several percent in the collected

44
charge. Loevinger [19] derived corrections for electrometer current

measurements and showed that the gain of several VRE's varied with scale

reading and polarity. Sources of information for the following discussion are given in references 19-24.

The high-gain, negative-feedback amplifier is the type of operational

amplifier used in electrometers for measurement of small currents from ionization chambers. These amplifiers have a high impedance input and, an essential

characteristic for free-air ionization chambers, the input terminal maintains

the collector plate virtually at ground potential. A negative-feedback

electrometer with resistive feedback element is shown in figure 3-18. The

signal current from an ionization chamber impresses a signal on the input

terminal of the operational amplifier, which generates an output voltage V^^^.

Part or all of Vq^^j. is fed back (from the load resistor R|_) through the

feedback resistor to the input terminal, so as to oppose the input signal.

The output voltage is larger than the resultant input voltage V^-^ in the

ratio

where G is the amplifier gain. In modern electrometers, such as the

Keithley 616, G > lO** , so for present purposes V^^ can be neglected compared

to Vq^^^, and

^sig ~ ^out '

For operational amplifiers with large input resistance, for which the input

voltage V^^ « 0, there can be no current through the amplifier, and the

current through the feedback resistor is equal to the ioni zation-chamber

current Ig-jg. Therefore

"sig ^sig f

and finally

^sig " ^out/'^f "

This is the relationship used for measuring current when using a resistive

feedback element.

For low-current measurements high-megohm resistors are required. For

example, a feedback resistor of 20 Mn is required to develop a feedback

voltage of 1 V for the exceptionally large ionization current of 50 nA, Day

and Attix [18] found that the high-megohm resistors of several manufacturers

changed their values by as much as 1% over a period of six months.

Consequently General Radio Corp. air-dielectric capacitors have been used as feedback elements for dosimetry calibration work for many years. A

negative-feedback amplifier with capacitative feedback element C^ is shown in

figure 3-19. In this case,

AVsig = Isig ^^^^f

and we get

Isig = Cf LV^^^/Lt .

45
< Vgig > WWW—

Fig. 3-18. Circuit of a negative-feedback amplifier with resistive feedback

element.

Fig. 3-19. Circuit of a negative feedback amplifier with a capacitive

feedback element.

46
This equation is the basis for all important ionization-current measurements

at NBS. The ionization current I^^g is determined from measurements of V^^.^

using accurate digital voltmeters wnich can be commanded to sample-and-hoTd

readings of VQ^J|. at preset time intervals.

The inverting operational amplifier has other advantages worthy of

mention since they affect the application of these devices to ionization

current measurements. These features are the inherent linearity of the change

in VQ^|. with time, giving AVp^^/At = const., and the ability of the device to

transfer essentially all of the charge produced in the ionization chamber to

the measurement system. The slope AVp^j^/At is constant because the input

terminal being at virtual ground is essentially isolated from the input

circuit, i.e., the magnitude of the voltage on does not affect the ionization current being supplied from the current source. If this were not so, the

charge build-up on would be exponential. The almost complete charge

transfer from the current source is accomplished because aV^^ the potential across the capacitance of the current source, is forced to be'near zero by the

negative feedback and, for an open loop gain of 10,000, aV^^ = V^^j^ x 10"'+.

Then only lO-** Vq^^^ is across the current source capacitance. Other ways to

consider this effect are that the capacitance of the current source is

degenerated by the open-loop gain or, that the effective capacitance of the

electrometer is increased by the open-loop gain.

Personnel safety in the measurement of radiation requires that measuring

instrumentation be located in a radiation-safe environment. In addition, it

is good practice not to allow radiation to strike instruments which may be

susceptible to generation of extraneous currents. These conditions are made

possible through the use of "low-noise" coaxial cables for connection between

ionization chambers and electrometers. Low-noise cables are manufactured by

the Amphenol , Essex, and Microdot companies. The "low-noise" characteristic

of these cables is accomplished by coating the surface of the coaxial

insulator with a conducting material. In this way the metallic braid used as

an electrostatic shield, and the coating, are always at the same potential

regardless of cable disturbance. Additionally, the capacitance between the

coaxial surfaces remains the same even though the braid may not always be in

immediate contact with the surface. Prior to the advent of the low-noise

cable, solid-dielectric lead-shielded connections were made to ionization

chambers, and later, long tubes with spring-loaded central wires were used.

It was necessary to evacuate the air from the tubes to eliminate their

contribution to the measured ionization current.

The use of long, low-noise cables and the discontinuance of manual null

methods were made possible by the use of the high-gain negative-feedback electrometers. Cables between ionization chambers and electrometers in several

of the NBS Dosimetry laboratories are as long as 15 m (50 ft). For a typical

low-noise cable capacitance of 30 pF/ft the total capacitance is 1.5 nF. This

cable capacitance is degenerated to 0.15 pF if the electrometer gain is

10,000. Since, in most measurements, the electrometer feedback capacitors are

1000 pF or greater, the added capacitance in this instance would cause an error of 0.02%. If the feedback capacitance is less than 1000 pF, say 100 pF,

the percent loss of charge trapped on the stray capacitance should be

determined. (The open-loop gain may be as high as 50,000.)

47
Commercial electrometers, such as the Keithley Model 616 used by the

Dosimetry Group, are very versatile instruments. Those features of interest,

for measurement of ionization currents, are the current -and charge capabilities for currents above about 100 fA. The instrument is operated in the

"fast" mode (negative feedback) for the currents of interest here. It has

selectable feedback resistors ranging from 0.1 Mi^ to 10 Tn, and selectable

feedback capacitors ranging from 100 pF to 100 nF. However ionization current

measurement procedures developed over many years in the Dosimetry Group

preclude the use of high-megohm resistors as feedback elements and the use of

air-dielectric capacitors is preferred where possible. Feedback elements of

choice can be selected for this electrometer by setting the range switch to

the "Volts" position. There is no built-in feedback element with the switch

in this position and a capacitor of choice can be connected between the

"guard" terminal at the back of the instrument and the instrument input.

(Refer to Electrometer Amp. and Range Switching Circuit No. 25764E in the

Keithley Mod. 616 Instruction Manual).

The ionization currents usually encountered in Dosimetry Group calibration work are in the range 0.1 pA to 100 pA. It is necessary, therefore, to

have available a range of feedback capacitors in order to efficiently perform

measurements of currents. This has been done for Dosimetry instrumentation by

mounting air-dielectric capacitors of nominal capacitance, 100 pF and 1000 pF,

and good quality 10 nF and 100 nF polystyrene capacitors in chassis, which are then mounted with electrometers in measurement consoles containing other

control and measurement equipment.

The capacitors installed in measurement consoles have either been calibrated by the NBS Electrical Measurements and Standards Division or intercompared with such standards. The General Radio capacitors, of nominal values

100 pF and 1000 pF, were modified originally for mounting on VRE preamplifier

heads and the polystrene capacitors were mounted in metal cylinders for the same purpose. Since the VRE's are no longer used, connections are made to

these capacitors by means of BNC-type connectors. A brass adapter, simulating

the VRE preamp spring-loaded connector, is available in B033 for making

connections to those capacitors not having BNC connections at each end.

Periodically, several of the available capacitors are submitted to the

Electricity Division for calibration. These calibrated capacitors are then

used to "calibrate" the remaining capacitors. A detailed explanation of this

procedure appears in section 3.8.1.

3o6. Ionization current measurement-console equipment

X- and gamma-ray-produced ionization currents are, for the most part,

measured automatically, although capability exists to use manual methods.

There are two automatic data acquisition systems (DAS) in use at the time of

writing. One system is used exclusively for measurements made with the x-ray

sets, and the second system, a portable system, is used for measurements made

with the gamma-ray sources.

48
3.6.1. X-ray calibration data acquisition system

Data are acquired for x-ray standardization by measurement of all conditions relevant to establishing an x-ray exposure at a particular distance from

the x-ray tube target for a shutter-open time Interval. The data acquired are

of three categories: (a) data required for computation of ionization

currents, (b) parameter data, and (c) measurement system test and information

data. In category (a) are: initial and final electrometer feedback voltages,

the atmospheric pressure and pertinent air temperatures, and the shutter-open

time Interval. In category (b) are: the x-ray tube current and potential,

the x-ray tube target-to-reference point distance, and the filter and the

diaphragm wheel positions. In category (c) are: measurements of the collection potentials on the standard free-air chamber, the monitor chamber, and the

chamber being calibrated (if appropriate). Also in category (c) are the

results of tests of accuracy of the system digital voltmeter.

The DAS consists of an input scanner (Hewlett-Packard Model 2901), an integrating digital voltmeter (HP Model 2410C), a coupler (HP Model 2547A) , a preset controller (HP Model 5331A), four Keithley Model 616 electrometers, two

capacitor selector boxes with a total of sixteen calibrated capacitors, two

temperature-measuring devices, and one pressure indictor. Several NIM modules

built by NBS are used to control the system. A block diagram of the DAS is

given in attachment 7.

The HP input scanner, on command, sequentially connects analog signals

from up to 25 sources to the DVM for measurement. The DVM performs each

measurement and provides the information to the coupler which also receives

from the scanner the channel number identifying the source of the signal.

Digital data from the preset timer and the distance switches for the

300-kV range are supplied to the coupler. The rest of the 300-kV range set-up

code (diaphragm, filter, and chamber identity) is supplied directly to a parallel input board (DRV-llJ), located in the Digital Equipment Corp. 11/23

computer expansion box. The function of the coupler is to convert all the

information supplied to it to a form that can be received by the serial inputboard. A different arrangement is used for the 100-kV range where the timer

data and set-up code are all supplied to the coupler. The data provided by

the coupler to the DLV-llJ are in ASCII octal form and are sent serially at a rate of 10 characters per second. The input signals to the scanner for

operation of the 300-kV calibration range, as of 1984 December 31, are given

in table 3-22.

One Important variable not identified automatically is the capacitor used

as the feedback element for each electrometer. These are selected from a capacitor value menu for the 300-kV x-ray range. The capacitor values are entered manually for work done on the 100-kV x-ray range. There are four sets

of these capacitors, one set for each K616 electrometer. The values for these

capacitors, and the instruments with which they are associated are given in

table 3-23.

49
Table 3-22, HP Scanner channel numbers and input signals for 300-kV x-ray

mac hi ne

Channel No. Si gnal

1 Kilovoltage (Seifert control panel)

2 Air temperature, room or inside FAC

3 Feedback voltage, FAC

4 Feedback voltage, monitor chamber

5 Collection potential on monitor chamber

6 Collection potential on FAC

7 Collection potential on test chamber, #1 position

8 X-ray tube current (Seifert control panel)

9 Atmospheric pressure

10 Feedback voltage, test chamber in #1 position

11 Feedback voltage, test chamber in #2 position

12 Positive polarity, standard cell potential

13 Zero, DVM shorted

14 Negative polarity, standard cell potential

15 Collection potential on test chamber, #2 position

16-24 Not used

25 Temperature inside monitor chamber

Table 3-23. Electrometers and capacitors mounted in x-ray standards

measurement console.

(Standard Free-Air Chamber) (Monitor Chamber)

K616 NBS No. 189980 K616 NBS No. 188889

Cap. Type Cap. (pF) Cap. Type Cap. (pF)

A 83.67 B 1 038.4

B 1 050.8 C 10 144

C 9 870 2D 197 871

D 99 060

(Test Chamber #1)

K616 NBS No. 179266

Cap. Type Cap. (pF)

(Test Chamber #2)

K616 NBS No. 179274

Cap. Type Cap. (pF)

A 81.68 A 100.20

B 988.35 B 1 091.8

C 9 808 C 10 571

D 99 761 D 100 187

The A and B type are air-dielectric capacitors. The C and D type are polystyrene-dielectric capacitors. Ref: DB852: 116-117

50
The temperature sensors used to measure room, FAC, and monitor air

temperatures are two-wire thermistors manufactured by Yellow Springs Inc.

(YSI). The sensors for the FAC, and for room air temperature measurement,

are switched to the YSI Mod. 43TN readout depending on the in-beam instrument.

The YSI readout is a bridge circuit which, when unbalanced, provides a voltage

proportional to the temperature. The YSI voltages are monitored on channels 2 (FAC or room) and 25 (monitor). The equations used prior to 1986 Jun 24 for

computing temperatures for the 300-kV range were:

FAC T (°C) = 46.59 (V. + V^) + 30.33 (Ref: DB 848:201)

Mon. Ch T (°C) = 43.23 (V. + V^) + 29.46 (Ref: DB 848:201)

Room T (^C) = 46.75 (V^ + V^) + 30.13 (Ref: DB 848:201)

where i and f refer to the initial and final voltage measurements, and

in volts, for the preset time interval. With the exception of the monitor

chambers thermistor, all temperature indicators have been recalibrated. A

description of the calibration procedure appears in section 3.8.2. The new equations are:

FAC T (°C) = 48.314(V. + V^) + 30.562 (Ref: DB 852:060-061)

Room T (°C) = 49.350(V^ + Vf) + 30.050 (Ref: DB 852:065-066)

The changes in absolute temperature are -.02% and -.05% for the FAC T and the

Room T, respectively. The above constants were incorporated into the computer

programs on or about 1986 Jun 24.

The atmospheric pressure is measured by means of a capacitance-sensitive

device manufactured by the Rosemount Company. The sensor is powered by an NBS

power supply serial no. 2561. The output voltage is calibrated for pressure

measurements in units of millimeters of Hg. The equation used prior to 1986

May 01 for computing the pressure was:

P (mmHg) = 560.85 + 25.54(V, + Vf) (Ref: DB 783:23-25)

The pressure sensor was recalibrated and a description of the calibration

procedure appears in section 3.8.2. The equation used since 1986 May 01 is:

P (mmHg) = 557.52 + 25.891(Vi + \lf) (Ref: DB 852:095)

This represents a change of -.09%. In the preceding equations, the constants

are the result of combining and rounding off the constants stored in the

computer.

The Seifert high-voltage supply was calibrated (1984 Aug 22-23) using a calibrated high-voltage divider (DB 852:45-55). The high-voltage divider was calibrated in March 1983 (Test No. 722/G44568; Seifert Voltage Divider SN

720479). The calibration showed some dependence of the HV on the x-ray tube

current. In the equations below, the x-ray tube current is equal to the tube

current thumbwheel switch setting (maTWS). The relationship between the true

potential (TP) and the control panel thumbwheel kV settings (TWS) was established and found to be (1984 Aug 27):

TWS = -4.75 X 10-1 _ 4.8 x 10-3(mATWS) + (1.01394 + 3.74 x 10"'+ (mATWS))TP

51
The nomenclature in the equation is the same as is used in the DEC

computer program. The number of places used for the constants in the above

equation is sufficient to calculate values for TWS accurate to 0.1 kV. With

the x-ray machine under computer control, the required x-ray tube potential

and current are typed into the computer and the thumbwheel settings are (remotely and effectively) set in accord with the solution of the equation for

TWS. The x-ray tube potential is monitored on channel 1. The sensor is an impedance transformer and the rectified signal is available from a special

module behind the Seifert control panel.

The relationship between the kV analog signal on channel 1 and the true

kV was measured for x-ray tube currents (monitored on channel 8) up to the

maximum current for each x-ray tube potential of interest (Ref. DB 852:55).

The x-ray tube potential is computed using the second equation below

after calculating the x-ray tube current from the voltage signals on channel 8 as follows:

(•^A)calc = ("^ATWS),,ic = C{M^ + Vf)/2 + D

where C = 10.0594 and D = -0.050737

then

Tube Potential (kV) = B(V^ + V^) + A

where A = -0.32637 + (0.010391) (mATWS)^^^^.

and B = 47.7267 - (0.0109295) (mATWS)^^^^

In the solution for the constants A and B for the x-ray tube potential

equation, a linear relationship was assumed, for A and B, with tube current.

While this is correct for B, examination of the data for A (DB 852:55) reveals

that it is the second differences in A which are nearly constant and therefore

a second degree equation would better describe the variation of A with tube

current. Considering the datum at 25 mA(TWS) to be an outlier (the value is

based on only two points), an equation which closely describes A = f(mATWS) is

A = -0.333 - 0.0013944 (mATWS) + 0.0011247 (mATWS)2.

A comparison of calculations for Tube Potential, using the first and

second-degree equations for A, is given in table 3-24.

The differences shown in table 3-24 are of more academic than practical

interest since these signals are used only for monitoring the x-ray tube

potential and current. The potential is set either manually or by computer

and the result is compared with the information from the filter wheel position

to test for consistency and for identification of the appropriate FAC

corrections. The signals on channels 1 and 8 of the scanner monitor the x-ray

tube condition during the course of a calibration.

The filter wheel position associated with the 300-kV machine calibration

beam codes are given in table 3-25. For convenience where the calibration

techniques are the same, the previous beam code is listed. (These are provided for ease of reference in response to inquiries regarding past

instrument calibrations). The beam codes are also listed in table 1-1.

52
Table 3-24. Comparison of solutions for true kV using first and second-degree

solutions for the constant A,

kV signal (V) mATWS

X-ray tube potential (kV)

A, 2nd degree A, 1st degree AkV

b yD .u yo . i +0.1

2.5 5 lift Q 1 1 Q Q i io .y "

0 .0 b 1 A9 7 1/19 7

3 .b b 166.5 166.6 +0.1

/I n

4 .U b 190.4 190.4

4.5 5 214.4 214.3 -0.1

5.0 5 238.0 238.1 +0.1

5.5 5

6.0 5 285 7 ?ft5 ft +0.1

0 ,b b

nU U 1 1 1 CI ciluc 1 Ui inlU mATIJ*^ IMM 1 no

^ .U 1 a lb J J . nu

^ .b ib 1 1 ft R lift 1 iO . 7/ -0.1

3 .0 lb 142.6 142.5 -0.1

3 .b lb 166.4 166.3 -0.1

4.0 15 ion 9 1 on 1 ly u . i -0.1

4.5 15 91 "5 Q ci.o .y

5 .0 15 col tl 9^37 p. -0.1

5.5 15 9<^ 1 K 9A 1 /I -0.1

6 .0 15 9Q(; 'J CO\i .0 OQ C O

«:ob -0.1

2.0 20 95.1 94.9 -0.2

2.5 20 118.9 118.7 -0.2

3.0 20 142.6 142.4 -0.2

3.5 20 166.4 166.2 -0.2

4.0 20 190.1 189.9 -0.2

4.5 20 213.9 213.7 -0.2

53
Table 3-25. Calibration beam code - filter wheel positions for 300-kV x-ray

machine. The letters M or H preceding the number in the Beam

Code column indicate "moderate" or "heavy" filtration. The

number following the letter

x-ray tube.

is the constant potential on the

rllLci wricci rUblUIUil

Beam

D £i c Q n "f" r 1 b c ri L Code

u 1 a

1 M60 MFB

2 S60 MFC

3 M50 - 4 MlOO MFG

5 HlOO -

6 M150 MFI

7 M200 - 8 H300 -

9 M250 MFO

10 H50 HFC

11 H60

12 H150 HFG

13 M300

14 H200 HFI

15 H250 HFK

Calibrations of instruments for lightly filtered x-rays are performed

using either the Lamperti or Ritz FAC which are mounted in the low-energy

calibration range (100 kV). The measurement console equipment for the 300-kV

x-ray machine is also used for the 100-kV range. The switchover to the lOO-kV

range is accomplished by changing input selector cables and switches on a module located in the HP DAS console. The A and C positions are for the

300-kV range and the 100-kV ranges, respectively. The E position can be used

if it became necessary to use the HP DAS for gamma-ray calibrations performed

in rooms B034 and B036. This mode is no longer used since there is a portable

DAS for the gamma-ray calibrations. A description of this system follows in

section 3.6.2. All switches and cables must be set to the same letters to

have proper connections. The HP scanner channel and input signals for the

100-kV range are given in table 3-26.

The power supply for the low-energy-range x-ray tube is a 150-kV supply

built by Universal Voltronics (Ser. No. 2-6-1486). The output of the supply

was calibrated in 1972 by the NBS Electricity Division. A copy of the

calibration report is attachment 8, since there is no reference test number.

On the basis of this calibration, the DVM in the 100-kV control console is set

to provide the required potential on the x-ray tube. The maximum potential

allowed across the x-ray tube is 100 kV although the power supply can be

operated up to 150 kV.

i

54
Table 3-26. HP scanner numbers and input signals for 100-kV calibration

range.

:a n m Q 1

1 ! d r 1 n 1 INO . Si gna

1 X-ray tube HV, analog sig.

2 Room or FAC temp.

3 Feedback voltage, FAC or test ch.

4 Feedback voltage, mon. ch.

5 HV, mon. ch.

6 HV, FAC

7 HV, test ch. (if required)

8 X-ray tube mA, analog sig.

9 Pressure signal

10-11 not used

12 Std. eel 1 , + pola rity

13 HP DVM shorted

14 Std. eel 1 , - polarity

15-16 not used

17 X-ray tube current meter range selected

18-24 not used

25 Mon. temp.

The equation used (Ref: DB 764:004) for computing the high voltage on the

100-kV x-ray tube is:

Tube Potential (kV) = 3.206095 x 10-3 + 1.0233975 V - 1.39388 x lO""* V2

where V is the mean of the analog HV signal measurements taken, on channel 1,

at the beginning and end of the preset exposure interval. The calculated kV

is corrected by 1.5% because the HP DVM in the data acquisition system loads

the signal at the time of measurement.

The x-ray tube current meter has three manually selectable ranges, 2mA,

6mA, and 20mA. The x-ray tube current is determined from signals measured on channels 8 and 17. The signal on channel 17 is a function of the selected

meter range and is labeled TCDIV in the computer program. The equation for

x-ray tube current is:

Tube Current (mA) = (V^- + V^)/2(TCDIV) (V measured on channel 8)

where (TCDIV) = 0.3, However, if the signal on channel 17 is less than

4 volts, (TCDIV) = 1, and if the signal is less than 2 volts, (TCDIV) = 3,

The filter wheel positions associated with the 100-kV x-ray tube

calibration beam codes are given in table 3-27,

55
Table 3-27. Calibration beam code-filter wheel positions for the 100-kV x-ray

range. The letters L, M, or H preceding the numbers refer to the

"Light", "Moderate" or "Heavy" filtration, respectively. The

number following the letter is the constant potential on the

x-ray tube.

Beam Code

Filter wheel position Present Old

1 LIO, L15, L20 L-B, L-C,

2 HIO -

3 M20

4 L40, M30, H15 L-G,

5 H40

6 M50, H20 L-I,

7 S75 L-K

8 LlOO L-M

9 H30

10 M40

11 L30

12 L50

13 L80

14-15

Note that since the same filtration is used for several x-ray tube

potentials, special means of identifying the technique being used are

requi red

Where the same filter wheel position is used for several calibration

energies, IF statements in the computer program operate on the computed x-ray

tube potential. IF statements are required for filter wheel positions 1, 4,

and 6. A value of 20 is hard wired to all filter wheel data making the multitechnique positions 21, 24 and 26. If the filter wheel position is identified

as 21 and the computed kV is greater than 12.5 kV but less than 17.5 kV, then

the code is made equal to 34. Code 34 identifies the beam code as L15 and the

appropriate -data associated with the Lamperti FAC are used to compute exposure

data. If, however, position 21 is identified and the calculated kV is greater

than or equal to 17.5 kV, the code is made equal to 35 and the beam code is

identified as L20. The same procedure is followed for position 4 (24) which

is associated with beam code H15, except where the calculation for kV

indicates otherwise, whereupon it is identified as either M30 (code 36) or L40

(code 37) depending on the magnitude of the kV. Position 6 (26) is M50 unless

the calculated kV is less than 35, in which case the beam code is identified

as H20.

There are four YSI temperature probes used in conjunction with measurements on the 100-kV calibration range. The thermistors for the room, Ritz

FAC, or Lamperti FAC are manually switched one at a time to a single YSI

56
readout and are read on scanner channel 2. The temperature of the monitor

chamber is determined from measurements on channel 25. The equations for

computing these temperatures are:

Monitor temp. = 30.17 + 45.34(V. + V^)

Room temp. = 29.86 + 48.47(V. + V^) (Ref: DB 852:063)

Ritz FAC temp. = 30.05 + 47.92(V. + V^) (Ref: DB 852:064)

Lamperti FAC temp. = 30.17 + 49.48(V^ + V^) (Ref: DB 852:067)

The above constants were incorporated into the computer programs on 1986

Jun 24.

The atmospheric pressure is determined from measurements of the Rosemount

transducer for which the pressure equation was given in the discussion of the

300-kV range.

For both the 100-kV and 300-kV ranges, the shutter-open time is determined by a counter which starts, and stops, counting a 1 kHz (optionally

100 Hz) signal as the edge of the shutter crosses the mid-point of the beam

port. The signal to the timer is a pulse produced by a photodiode when its

light beam is interrupted by a flag on the shutter mechanism. Upon initiating

the exposure, counting commences only on receipt of the photodiode pulse. At

the end of the preset time interval, the shutter is caused to close but the

timer continues to count until the edge of the shutter crosses the portal

mid-point. The timing shutters move quickly so the difference between the

preset time and the actual exposure time is usually less than 0.1 s. The

300-kV machine also has a safety shutter which opens before, and closes after,

the timing shutter. The lead thickness in the timing shutter was minimized

for mechanical purposes but is of sufficient thickness to prevent significant

effects on instrument readings in the interval between operation of the two

shutters.

3.6.2. Gamma-ray calibration data acquisition system

The measurement console and computer controlled equipment described could

also be used for instrument calibrations for gamma rays in Rms. B034 and B035.

For this purpose, it would be necessary to connect the cables on the inputcable-sector board from common to E and to turn all switches to E. Also, it

would be necessary to set up the chamber to be calibrated and make proper

connections from the instrument and control panels in B035. This type of

operation is inefficient, however, since for integration-type-instruments, it

would be necessary to go from room-to-room between readings. In addition, and more importantly, x-ray and gamma-ray calibrations cannot proceed simultaneously. Calibrations as well as other types of measurements using the gammaray sources in rooms B034, B036, B021, and B015 are therefore carried out

using a mobile measurement console consisting of all instrumentation required

for measurement and standardization of ionization currents, and an HP9836-HP3497A computer-controller system which can be made to automatically

57
acquire calibration data for cable-connected instruments, and into which

instrument readings can be typed for other types of instruments. The 3497A is

the data acquisition unit which incorporates, among other capabilities, a scanner, a digital voltmeter (5 1/2 digits) and a timer. The HP 9836 is a computer which instructs the HP 3497A to perform certain measurements. The

computer instructions are in the form of strings of commands addressed to the

HP 3497A. A typical command string in a program named "G CF" is:

OUTPUT 709: "SE1TI2AE1AF0AL2VR3AC0VT2S01"

The address of the HP3497A is 709. (7 is the interface select code and 09 is

the device select code.) The commands are the two letter codes and a number.

The meaning of the above string is as follows:

SEl Sets bit 0 of the service request mask (SRQ) to 1. When bit

0 in Status Register is set to 1, indicating Data Ready, both

bits are true and an SRQ is sent to the controller.

TI2 Two seconds after the 3497 receives the string, a pulse is

output from the timer port.

AEl Enables the external channel increment port.

AFO Sets the first channel to be read at channel 0.

AL2 Sets the last channel to be read, channel 2.

VR3 Sets the voltmeter range to 10 volts.

AGO Closes analog channel 0.

VT2 Triggers voltmeter on an external trigger pulse.

SOI Instruction to system to wait until measurement is complete.

The HP3497A sends a service request (an INTERRUPT 7) on the interface bus

because the bits 0 in the SRQ mask and the STATUS register are true when the

voltmeter has completed the readings on channels 0 through 2. The computer

sends a SERIAL POLL message and the HP3497A returns the STATUS BYTE. The

computer seeing the measurement data have been acquired will then respond in

accord with the actions required by the program.

This mobile console contains a Keithley Model 616 electrometer, a Setra

Model 350A digital barometer, and a Digitec Model 5810 digital thermometer.

Each instrument provides analog signals to the HP3497A. The feedback elements

for the electrometer selector switch in the "Volts" position, are capacitors

mounted in a capacitor-selector chassis. The values for these capacitors are given in table 3-28.

Table 3-28, Capacitor values (1986 Jun) in mobile measurement console.

Capacitor Capacitance

(pF)

A78.05

B-18 1 002.4

C-3 10 213

58
The equations for computing temperature are dependent on the thermistor

used and, for measurements in control room B019, the signals are taken from

YSI readouts mounted in the source control consoles. These equations, the

equation for computing atmospheric pressure from the Setra device, and the

data for converting the analog signals from the thermistor probes to air

temperatures in each calibration range and from the pressure transducer to

atmospheric pressure are stored in the HP9836 "G CF" program. The general

equation for air temperature (Dl) taken from the "G CF" program listing is:

Dl = (Kll) (V(J,2) + V(J-l,2))/2 + K12 (°C)

where \/(J-l,2) is the analog signal measured at the start of an exposure and

V(J,2) is the same measurement at the conclusion of the exposure. The values

of the constants Kll and K12 are given in table 3-29. The disparity in the

values of the constants is due to the different models of bridge-type YSI

readouts used for B015A and B, and B021A and B, and the Digitec digital

thermometer used in the mobile measurement console for B034 and B036.

The equation for computing atmospheric pressure (D2) is listed in the

"G CF" program as:

D2 = 60 (V(J,3) + V(J-l,3))/2 + 802 (mbar)

and D2h = 760 D2/1013.25 (mmHg)

Table 3-29. Constants in "G" program for computing air temperature in

gamma-ray calibration rooms.

Room Kll K12 DB:page

B015A -91.87 29.60 852:070

B015B -92.72 29.85 852:071

B021A -197.96 42.95 852:068-069

B021B -197.45 42.82 852:072

B034 13.82 -0.32 852:073

B036 13.77 -0.31 852:076

Finally, the "G CF" program listing for the pressure and temperature

correction is:

Pt = (760/D2h) (Dl + 273.15)7295.15

Although all standards measurements are acquired automatically, for other

purposes it is sometimes convenient to use manual measurement methods. The

instrumentation is the same as for automatic measurements except for controlling charge-integration time and measurement of . A DVM controller-timer

designed by A. Marella of NBS (Rad. Source and Inst. Div., Drawing ND-3440)

replaces the HP9836-3497 system for manual measurements. The essential

elements of the controller-timer are: an accurate frequency source (Greenway

59
Mod. Y819 (1000 pps.))» a digital counter (ERC Mod. 2306A) , and a comparator

(ERC Mod. 2506-1). The function of the timer is to output a pulse of the

required amplitude and width to initiate a sample-and-hold command to a digital voltmeter with that command capability. The Digitec DVM (either Mod.

266 or 287) requires a positive pulse 10 to 40 ys wide and with an amplitude

between 2.2 and 5,5 V for a sample and hold command. The DVM responds within

60 ms after the occurrence of the leading edge of the pulse.

The Greenway oscillators are specified as having frequencies of typically

999.9866 pps. and DVM's such as the Digitec Mod. 266, when compared with the

Fluke DC Voltage Calibrator, have been found to be accurate to 4 1/2 digits.

3.7. Pre-cal i brati on tests of exposure instruments

Considerable time and effort are expended in setting up to calibrate an instrument for the quantity exposure. Additional man-hours and machine time

can be lost if instrument problems are discovered after the calibration

procedure has begun. Therefore all instruments submitted for calibration are tested prior to calibration to discover possible defects and to establish

certain instrument characteristics in a less time-intensive laboratory

facility.

In general, there are two types of instruments submitted for calibration:

(a) ionization chambers associated with exposure readers, and (b) ionization

chambers to be calibrated in electrical units. Type (a) instruments can be

divided into two categories - (1) "condenser" types which consist of a charger/ reader and one or more "condenser" probe ionization chambers (probes);

and (2) cable-connected types. In general , both types have probes with

different ranges, extending the instrument's range from protection-level to

therapy-level exposure rates.

3.7.1. Charger/reader scale linearity test

For all practical purposes, the only charger/ reader sent to NBS for

calibration is the Victoreen Model 570 R-meter, described in section 4.2.1.

The first test of a charger/ reader is for scale linearity but this cannot be

carried out upon arrival. The instrument must be allowed to acclimate itself

to the laboratory environment for at least several hours before testing and,

in fact, because of scheduling, may not be tested for several days.

Along with testing for scale linearity, the general operation of the

charger and_ the quality of the scale and image of the quartz fiber are evaluated. The image of the string, ideally, should be aligned with the scale

markings, and the scale markings and string image should be well-defined from

zero to full-scale reading. Evaluation of the image is somewhat subjective,

but lack of good focus, double images, or string images with high-lights,

should be considered as reasons for returning the instrument to the owner for

adjustment or repair. Although minor adjustments can be made at NBS, they are done reluctantly since other, unforseen, problems may arise. In no case are adjustments attempted without contacting the owner for permission.

The scale linearity test is carried out using the Data Precision Model

8200 NBS No. 500302 voltage source (negative polarity, 1000-V range) and a connector which simulates the stem of a probe. The electrometer is connected

60
to the line, turned on, and allowed to come to equilibrium with temperature

changes caused by the light which illuminates the scale. With the Data

Precision voltage source warmed up and with all voltage switches turned to

zero, the negative polarity output is connected to the probe simulator and the

simulator is inserted and locked into the charger recess. The voltage source

is then turned from "0" to While observing the electrometer scale, the

potential from the voltage source is increased until the image of the string

coincides with the zero marking on the scale. The potential is recorded for

"VO" on the index card and, decreasing the potential, data are recorded for

the 20, 40, 50, 60, 80 and 100 percent of full-scale reading.

According to specifications for the electrojneter, the potentials required

to bring the string image to a zero, and to a full-scale reading, are 525

volts and 275 volts respectively. So the initial setting on the voltage source for a zero reading should be about 525 volts. As a guide as to what

might be expected, charging potentials, the difference in potential between

zero and full-scale reading, and some statistics for 13 randomly chosen

electrometers are given in table 3-30. In those cases where the number of

measurements (n) is large, the data have been accumulated over as many as six

years, and the charging potentials (Vq) and potential differences (Vq-V^qq)

are mean values. The mean V for the 13 electrometers is 525.2 volts with a standard deviation of 7,7 volts (1.5%); and the mean potential difference,

Vp-Vjp-j, is 250.4 volts with a standard deviation of 1.7 volts (0.7%). Note

that the standard deviations for individual instruments are about half the

standard deviations for the group.

Table 3-30. Victoreen Model 570 electrometer charging data. is the

potential required to charge the electrometer to a zero scale

reading. V,qq is the potential for a full-scale reading and S.D.

is the standard deviation.

Serial No.

n S.D. V -V S.D.

(V) (%) (V) (%)

143 6 518.6 3.6 0.7 251.5 0.73 0.3

265 10 526.9 3.1 0.6 247.6 1.13 0.5

997 7 538.6 3.9 0.7 253.6 1.26 0.5

1275 2 522.4 252.3

1285 2 518.6 251.3

1506 4 534.6 3.6 0.7 249.7 0.51 0.2

2261 1 531.0 249.6

2278 2 533.1 251.3

2300 2 521.2 248.9

2302 1 531.0 249.1

2344 4 515.6 3.7 0.7 248.2 0.53 0.2

2346 3 516.6 251.8

3015 6 519.9 3.3 0.6 250.3 0.56 0.2

61
3.7.2. Leakage tests of condenser probes

The condition of the probes associated with the electrometer is

ascertained following the scale-linearity measurements on the electrometer.

The thimbles of all probes are checked for one or more of the following:

(1) physical damage; (2) integrity at joints; (3) tightly screwed-on thimble;

(4) dirt in dust cap that could cause leakage. Assuming the probes are in

good condition, they are charged to a zero reading on the electrometer with

the time and date of charging recorded on the index card kept with the instrument (see section 4.1 for details). The instruments are then stored in a radiation-free area. This procedure provides a means for quickly determining,

on the scheduled calibration date, the leakage condition of each probe.

3.7.3. Testing for atmospheric communication, condenser probes

The Victoreen probes are also tested for communication with the

atmosphere since this is essential if air density corrections are to be

applied to their readings during calibration. This test is carried out by

connecting a probe to a negative-feedback electrometer and placing the probe

in a box which can be pressurized while the probe is being irradiated. The

measurement is carried out with the electrometer in the "current" mode of

operation and readings are taken for room-pressure conditions and at slightly

elevated pressures. The percentage change in the electrometer readings should

equal (nearly) the percentage change in the air pressure. For most probes,

the rate of change of the pressure is of no consequence, however, the pressure

should be changed very slowly for thin-window chambers such as the model 651.

(This precaution must be observed for any thin-window chamber, not just those

manufactured by Victoreen.) Generally, a pressure change of 4 to 5% is

sufficient for this test and conveniently, this change is about the same as the change from normal room pressure to a full-scale reading (785 mmHg) on the

aneroid barometer in the test stand.

A special connector is available for connecting R-meter probes to the

coaxial cable leading to the negative-feedback electrometer. The connector

consists of a modified R-meter probe cap which has been fitted with a BNC

connector. The ground for the connector is extended along, but is electrically isolated from the cap. In order to minimize the contribution of ionization from air volumes in the vicinity of the connector end, the connector is

supported, during the test, in a lead brick. The polarizing potential is

connected to the stem of the probe by an independent connection and what is

usually at ground potential is now at high voltage. This is important in

setting the polarity of the polarizing potential if a triax cable is used in

conjunction with the Keithley Model 6169 Ion Chamber Interface. A copy of the

Keithley drawing is attachment 9, showing the operating modes of the model

6169 interface.

Protection of the modified probe connector from radiation is especially

important when the probe chamber volume is small, such as for "100 R" (» 0.5-cm3) and "250 R" (» 0.2-cm3) chambers. The exposure rate in the

pressure test box, which is about 0.5 m from the source in B021A, is about

0.075 R/s (March 1986). The ionization currents produced in these chambers

for an exposure rate of 0.075 R/s are 1 x lO-n A and 5 x IO-12 a, respectively. These small currents and the usual associated noise in the signal

sometimes make it difficult to discern with good confidence a 4% difference in

62
reading. An additional difficulty in testing the high-range Victoreen probe

is a drift in the reference reading with exposure to radiation. It may be

necessary to cycle high-range probes through several pressure changes before

firm conclusions can be drawn in regard to communication to the atmosphere.

Other problems arise in testing low-range probes. The exposure rate must

be made appropriate for proper operation of low-range probes either by moving

away from the source or by attenuating the beam. In large-volume chambers

(e.g., ?5-mR chambers), if the ionization density is too high, the percentage

increase in reading will not be proportional to the increase in air pressure

because ions are lost due to recombination.

3.7.4. Testing for atmospheric communication, cable-connected probes

Some instruments have ionization chambers cable-connected to an electrometer with a readout in exposure units. The electrometers are used as null

devices or are of the negative feedback type. The scale linearity of these

readers is not tested but the associated ionization chambers are tested for

communication with the atmosphere. A difficulty often encountered with these

tests is incompatabi lity of connectors, requiring a departure from the use of

test station instruments. In this case, the readout associated with the

chamber is used to determine communication with the atmosphere. Depending on the size of the ionization chamber and its appurtenances, it may be necessary

to use one of the special openings in the pressure box or to use another

sealable container. A large tray with sealable lid and openings for a pressure hose and cables is available for special uses.

3.7.5. Testing of readers for cable-connected probes

While the scale linearity of this type of reader is not tested, it is

important to check the collecting potential used for the probe. Some readers

have a switch or button that will permit a "self test". For those instruments

where this convenience is not available, one needs to turn on the reader

alone, then using a high input impedence meter, carefully measure the collecting potential at the probe input connector. Comparison of the observed value

should be made with nominal value given in the instruction book for the

i nstrument.

3.7.6. Testing of instruments to be calibrated in electrical units

The pre-cal ibration testing of ionization chambers to be calibrated in

electrical units is more extensive than for those incorporating an exposurereading electrometer. The tests are for electrical leakage; time required to

equilibrate after application of the collection potential and exposure to

radiation; and the change in reading, for constant exposure rate, with

reduction of the collection potential by half. In addition, if the request

for calibration is for a mean calibration factor for positive and negative

collection potentials, the polarity effects are investigated.

Except in those cases where the ionization chamber is quickly shown to be

defective (high leakage current, large recombination correction, obviously bad

electrostatic shielding) the results of pre-cal ibration tests are, to some extent, dependent on the time allotted to testing. The currents being

measured are extremely small, and changes in measurement data reflect strains

63
in cables and connectors which, in turn, are dependent on the length of time

since the connections were made. Superimposed on these effects are insulator

"soakage" effects caused by the application, and change, of the collection

potential.

Assuming all cable, connector, and chamber insulators are in good condition, the leakage current, after a system has been connected together

overnight, should approach the offset current of the electrometer (a Keithley

Model 616 has been used at the test station). Typical leakage currents for

the electrometer and the electrometer plus cables and connectors are shown in

table 3-31. The data in table 3-31 are the result of measurements taken after

the system had recovered from transients produced by making the various

connections. Under some circumstances, it is difficult to decide when to

bring the tests to a conclusion. To alleviate this problem, a "PreCalibration Ionization Chamber Test Data" form, which allows measurement data

and decisions to be recorded in an organized manner, was prepared by T. P.

Loftus. A copy of this proposed form, on which data from an actual test is

shown, is attachment 10. The data reflect the real-time condition of the

ionization chamber, i.e., the condition existing during calibration, not that

of a quiescent state such as would be be case if the system were allowed to

settle down overnight.

Table 3-31. Electrometer leakage current and currents induced by adding

cables and connectors

Conditions Current (fA)

Model 616 electrometer and Model 6169 interface < 2.0

Triax connector on interface capped off

Triax cable (« 15 m) added and capped off <29

Triax-BNC adapter added and capped off <34

NEL adapter added and capped off <34

Although there are some advantages (reduced noise, single cable) to the

use of triax cables for connecting guarded ionization chambers to electrometers, recent tests indicate that the addition of the triax cable insulation, between the guard and the external shield, can contribute to an increase

in "insulator soakage time". Insulator soakage is the phenomenon observed in

ionization chamber measurements in which a change in collection potential causes a reading transient which disappears exponentially with time. This

soakage is of no consequence for chambers designed to be connected by triax

cable to their electrometers, and for which the collection potential is always

of the same magnitude and polarity. However, in tests designed to evaluate

the recombination characteristics of an ionization chamber, additional

HV-i nsul ation increases recovery time from the transients introduced by the

changing collection potentials. Data showing that low noise coaxial cables

are more suitable for pre-calibration tests are recorded in DB 861:12-14.

64
3.7.7. Summary and comments

Pre-cal ibration testing is performed not only to detect instrument

defects but to provide information required for proper operation during

calibration and to provide data for the final calibration report. The

pre-cal ibration test datum that appears on the R-meter calibration report is

the open or sealed condition. The Roentgen per Coulomb calibration report

contains, in addition to the above item, the leakage current, the ratio of

ionization currents for full and half collection potential, and the magnitude

of the ionization current observed with full collection potential.

The report information could be made more nearly correct in regard to

leakage current if the statement for leakage were changed to the expression

"not greater than" as opposed to stating a specific value. This change would

save time by allowing a determination of leakage currents relevant to the

effect such a current would have on the expected ionization currents during

calibration.

The full and half collection potential current ratio could be made more

relevant if the expected calibration exposure rate were a consideration in the

tests. For low-range ionization chambers, present facilities are adequate and

this consideration could be made a requirement in the test procedure. For the

present, the testing of high-range instruments at full and half potential

needs to be done at the time of calibration.

3.8. Capacitor, pressure indicator, temperature indicator calibration

3.8.1. Capacitor calibration

Periodically, since 1956, several of the available capacitors are submitted to the Electricity Division for calibration. Copies and originals

of these reports are kept in the Dosimetry Group office file in a folder

marked "Intrabureau Calibrations". (Ref. Test No. 521-02-86, 10-30-85; Test

No. 521-07-86, 02-21-86). The letter designations A, B, C and D identify the

capacitors as having nominal values of 100, 1000, 10,000 and 100,000 pF,

respectively. Following the 1985/1986 calibrations, all available capacitors

were compared with either A5, B5, C6, or D4. The data are recorded in

DB 852:082-088. Some typical long-term calibration results are given in

table 3-32. The uncertainty, including both random and systematic, stated by

the Electricity Division is ± 0.02% for A5 and B15, and ± 0.03% and ± 0.05%

for C6 and D4, respectively. (These uncertainties are interpreted by the

Electricity Division as equivalent to three standard deviations.) The results

shown in table 3-31 indicate a reproducibility of 0.02% for capacitors A5 and

B5. Capacitor CI shows a shift of + 0.45% from 1967 to 1980 but only 0.04%

from 1980 to 1983. Capacitor C6 shows a smaller change, 0.02%, from 1983 to

1986.

The circuit shown in figure 3-20 is used for Dosimetry calibrations of

capacitors. The variable voltage source is a Data Precision Model 8200 and

the operational amplifier is a Keithley Model 616 Electrometer (K616). The

electrometer is operated with the Range switch in the "Volts" position, and

the Fast-Normal switch in the "Fast" position. The capacitor to be calibrated

is connected between the feedback (Guard) terminal at the back of the

electrometer, and the input terminal at the front of the electrometer. A

65
Table 3-32. Electricity division calibrations

Serial Number Calibration Date

(month/year)

Capacitance

(PF)

810 1 1 fi . 1 u

J. u/ 81 1O 1 • X u

81 1

(J X • X u 3/70 81.174

8/77 81.19

?/83 81 189

10/85 81 197

XL./ ^ 96? 3

6/57 963 0

4/5Q 963 54

6/59 963 2

9/59 963.2

10/60 963.3

7/63 963.5

6/64 963 6

CI 7/67 10533.2

l?/80 10580 6

2/83 10576.2

C6 2/83 9812.09

2/86 9813.74

calibrated digital voltmeter (HP Model 3456A) is connected between the

feedback and ground terminals to measure V^^j^. The procedure is to clean all

connections using a source of dry gas (Aero Duster^'^, if available) and then

connect the system as shown in figure 3-20. The capacitors and cables should

be positioned where they will not be disturbed. The voltage source, K616, and

DVM are then turned on and warmed up for the recommended times. The

recommended K616 warmup time for critical measurements is two hours, and this

is sufficient for the voltage calibrator and the DVM. The voltage source polarity switch is turned to "-" with the voltage range selected dependent on the relative values of the standard and unknown capacitors. The unknown

capacitor is unshorted using the K616 "Zero Check" switch and the initial

value for V^^j^ is recorded. The voltage from the voltage source is then

changed (aV) and the resulting value of V^^^^ is recorded. The procedure is

repeated as often as is necessary to determine that the measurements are reproducible.

66
VRRIRBLE

VOLTRGE

SOURCE

Fig. 3-20. Circuit used for Dosimetry Group calibrations of capacitors.

The charge introduced to the K616 is aQ=(C aV)5+-cI* With a high-gain

electrometer, essentially all of the charge is transferred to C^J so C^j can be

computed as follows:

Cu = (C AV)std/AVouf

Some precautions to be observed in this procedure are: (a) the integrity of

the system must be established, i.e. one must have confidence in the accuracy

of the voltage standard and DVM; (b) excessively large potentials should not

be used on the standard capacitor, and (c) Vqjj^ readings should be taken

shortly after the voltage is changed on C^^^ but after transients have died

away. The last precaution relates to the offset current in the electrometer

which can cause Vq^^ to change with time. The specifications for the K616

give the offset current as less than 5 x lO"!** A. Assuming that 5 x 10-^'+ A

is the offset current and that is approximately 1 nF, Vq^^ will change by 1 mV every 20 seconds. If aVq^^^ is 1 V, after 20 seconds the reading would be

in error by 0.1%.

67
3.8.2. Temperature indicator calibrations

A liquid-in-glass thermometer, marked Fishers 815553, was calibrated by

the Temperature and Pressure Division in June of 1985 (Ref. Test No. 311-5-85,

6-17-85). The results of this calibration are shown in table 3-33.

Table 3-33. Liquid-in-glass thermometer calibration

Thermometer Reading Correction (IPTS-68)*

21.000 -.031

23.000 -.111

25.000 -.031

International temperature scale - 1968

In order to facilitate the temperature indicator calibrations, a calibrated liquid-in-glass thermometer was used to calibrate the Dosimetry

Group's quartz crystal thermometer, HP Model 2801A. Attachment 11 is a copy

of the liquid-in-glass thermometer calibration report. The results of the

quartz crystal thermometer calibration are in DB 852:058. The resulting probe

corrections were less than 0.05°C. The procedure for calibrating the

individual temperature indicators was to place each indicator in a glass test

tube which, in turn, was submerged in a gallon jug filled with circulating

water, see figure 3-21. The temperature of the water was lowered by introducing cold water. The temperature of the water was raised by introducing hot

water or by energizing a heater wire wrapped around the outside of the jug. A

tube inserted into the jug and connected to a fish pump provided the necessary

circulation. Since the jug was well insulated, a stable temperature could be

achieved without undue waiting. The temperature of the water bath was varied

from about 19°C to 26°C. The output data for each temperature probe (either

voltage or temperature indication) were recorded, with multiple readings taken

at each temperature. These data were then fitted using a least squares

technique. The resulting equations are shown in table 3-34. The details

appear in DB 852:060 to 077. It should be noted that the individual

thermistors do not have any individual markings, and are identified only by

their location.

3.8.3. Pressure indicator calibrations

An aneroid barometer, Wallace and Tierman, Model FA 139, Serial Number

XX11242, NBS ID P-8020, was calibrated by the Temperature and Pressure

Division in November 1985 (Ref. W.O. No. 536-7007, 11-04-85).

Calibrations of individual pressure indicators used at the various sources were made by placing the calibrated barometer alongside the instrument

to be tested and connecting both to a variable pressure device. The pressure

was varied from about 710 mmHg to about 780 mmHg. This range of pressures is

somewhat larger than normally expected. Data was taken with increasing and

with decreasing pressure, the comparisons being made directly or via voltage

signals. The data were fitted by the method of least squares, and the results

are shown in table 3-35.

68
Fig. 3-21. Schematic of thermistor calibration setup.

69
Table 3-34. Summary of thermistor calibration equations

Thermistor Location Calibration Equation DB 852:page

B021A t = -197.96 V +42.951 68-•69

B015A t = -91.871 V +29.6 70

B015B t = -92.716 V +29.854 71

B021B t

= -197.45 V +42.815 72

B034 t

= -13.824 V -0.32232 73

B034 t

= -103.73 V +30.464 77

B036 t = -13.767 V -0.31241 76

Wyckoff-Attix FAC t = +96.628 V +30.562 60-61

JUL) KV, rOSltlOn Ifd. t = +yo. /UU \;V +oU3d bb- c c 00

100 kV, Position 1,2 t

= +96.947 V +29.855 63

Ritz FAC t

= +95.838 V +30.047 64

Lamperti FAC t= +98.969 V +30.169 67

(V = signal i n volts; t temp in °C)

^^Portable computer readout

YSI readout in console

Digitec Model 5810

Table 3-35. Pressure indicator comparison summary

Pressure Indicator

and Location Equation Reference

Setra MO 350A,

SN 26718 on mobile

computer

Rosemont MO 1332A10

SN 1688, B033

Wallace and Tiernan

NBS 151998, B019

Wallace and Tiernan

SN MM 14869, B033

Wallace and Tiernan

NBS 157053, B036

P=45.176 V + 600.13

P=51.782 V + 557.52

P=1.008 (Reading) - 6.18

P=1.008 (Reading) - 6.63

P=1.007 (Reading) - 5.57

DB 852:93-94

DB 852:095

70
4. Operating procedures

4.1. Administrative procedures

The recommended procedure for requesting an NBS calibration service is

outlined in NBS SP 250. In practice, however, customers request calibration

service in a variety of ways. Typically a new (first-time) customer will

establish contact with the Dosimetry Group by telephone or letter, requesting

information regarding techniques offered, charges, backlog time, turnaround

time, and shipping/mailing information. At this stage, there is generally an opportunity to discuss with the prospective customer appropriate qualities of

radiation for the type of service being requested, methods of shipment to

reduce the risk of damage, alternate places to have instruments calibrated,

etc. On occasion, contact is with a purchasing agent or buyer. Experience

has shown that it Is highly desirable to make a record of the name and

telephone number of the person or persons requesting the service.

Other variations in calibration requests are:

(a) Purchase order with detailed description of calibration request,

including beam quality codes, instrument model and serial numbers, name and telephone number of person to be contacted;

(b) Purchase order with little more than the company name and the

statement "calibration required";

(c) Instrument arrives without a purchase order;

(d) Instrument arrives with purchase order fitting into category a, b, or e; and

(e) A letter, with or without sufficient information that can serve as a purchase order.

Regardless of the method used to request a calibration service, a full

description of the service being requested should be obtained by whatever

means possible.

Initial requests for thermoluminescent dosimeter (TLD) exposures can be

more general in nature, since experience has shown that the details of the

exposures requested accompany the dosimeters.

The customer's purchase order is then sent to the Office of Measurement

Services, requesting a test folder. (At the time of writing, 1986 June, it is

sent: OMS, Physics Building, Room B362, Attn: Vickie Weeden.) Each test

folder has a unique number and is used as one of the identifiers on the

calibration report. After a test folder is received from the Office of

Measurement Services, a sequential number is assigned Indicating the

customer's place on the calibration Inventory list.

Information from the purchase order and the test folder number are entered on a calibration inventory form (attachment 12). The calibration

inventory list is updated periodically using the program "INV" and the HP9836

computer. Attachment 13 shows a typical calibration Inventory list. TLD

Irradiations are not entered on this inventory.

71
When instruments arrive for calibration, they are unpacked and inspected

for damage. Shipping damage is reported to the NBS Shipping Department. When

an instrument arrives in a state of disrepair that is obvious by visual

inspection, the customer is notified, and a decision is made whether to return

the instrument to the customer, or if the repair is minor, have NBS personnel

perform the repair. An index card is filled out, according to the type of

instrument, condenser or current type (attachments 14 and 15), using information from the purchase order, or any other source. Each card is then assigned

a Dosimetry Group number (DG No.) obtained from the binder marked "Calibration

Log" kept in the Dosimetry Group Office. The index card is then filed in the

drawer marked "For Test Station" located in room 8033. The precal i brati on tests are described in section 3.7.

Requests for TLD exposure service are typically for periods of one year,

and occasionally for a period of two years. The initial handling of the

purchase order is the same as for instrument calibration. It has been found

convenient to close these test folders approximately every three months. A

copy of the original purchase order is then used to open a new test folder for

the next three-month period continuing until the purchase order expiration

data has been reached.

Most of the time in a calibration procedure is spent in the generation

and review of the calibration report. The data-taking techniques used for

calibrating instruments are described in sections 3.5 and 3.6. After the data

have been taken and analyzed locally, they are transmitted to the NBS Cyber

855 computer via a telephone line or over the NBS NET. At present (June

1986), there are two different programs available for sending data from the

laboratory computer to the NBS Cyber 855. One program is used for the data

taken using the x-ray sets, and another for the gamma-ray data. Detailed

instructions are contained in a binder marked "Computer Instructions for Data

Transfer, Editing, and Report Generation" and located in room B033. A

duplicate binder is kept with the portable HP9836 computer and another with

the Digital Equipment Corporation terminal. Model VT220, located in room C207.

Once the data has been transmitted to a file in the main computer, there

exists the possibility to make corrections to the data file by using the

computer's edit feature. Both of the laboratory computers, the HP9836 and the

DEC 11/23, have editing features that make possible the editing of data prior

to transmission to the NBS main computer. The computer program stored on the

NBS Cyber 855 then generates data reduction and analysis pages (replacing the

data books used in the past), calibration summary pages, and a final calibration report. The calibration summary pages, the data reduction and analysis

pages, and the final report are printed out using the laser printer located in

C29, building 245. Examples of each of these appear as attachments 16-19.

After a calibration report, summary pages, and associated data reduction

and analysis pages have been generated, a series of reviews takes place. The

data reduction and analysis pages are reviewed and checked for various indicators. For example: appropriate beam size for the chamber being calibrated;

spelling of the customer's name and address; chamber and charger/reader name,

model and serial number; ratio of the calculated x-ray tube potential and the

stored value for the kV at that particular beam code; reproducibility of the

data (see column 18 of the attachment); magnitude of the leakage currents;

etc. Errors detected at this stage of review can be corrected by editing the

data file and rerunning the calibration report program. Instructions for this

72
procedure are contained in the binders marked "Computer Instructions for Data

Transfer", etc. Secondly, the calibration summary pages are used to compare

present results with previous results, with the customer's instrument when

applicable, and always with the NBS check chamber calibrated at the same time

as the customer's (as discussed in section 4.2.3). Finally, the report itself

is reviewed, and any typographical errors corrected or necessary additions

made to the calibration- or correction-factor tables to indicate switch

positions or other items of note. The final version of a report contains no erased or retyped information. The reviewer initials the report, then brings

it to the Dosimetry Group office for review by the group leader.

After the group leader has reviewed and initialed the report, it is sent

to the Division Office for signature. Upon return, three copies are made.

One copy plus the original is mailed to the customer and a notation made of

the data sent and to whom in the "Calibration Log", one copy is filed in the

"DG" file, and one copy goes into the test folder. At tnis time, if all the

requested calibration work has been completed, fees are computed, the NBS Form

64 is filled out and three copies made of it. The original goes to Accounts,

A800, Admin. Bldg., one copy goes to the Center Office, one copy goes in the

DG file, and one copy goes into the test folder. The test folder is then

* signed and returned to the Office of Measurement Services.

TLD exposure calibration reports are generated in a different manner.

The data are taken and entered in a data book (attachment 20 is a typical

page). After review, the data are transferred to a TLD calibration report

form (attachment 21) and a DG number is assigned. The secretary then types

the appropriate information into the word processor and produces the final

report. The report is reviewed for correctness, initialed, then sent for

signatures and handled in the same manner as the instrument calibration

reports

Shipping request forms are made out after the Division Chief signs off on

a report and returns it to the Group office. The instrument is packed either

in its original container or in a more suitable one if necessary.

The index cards are then filed in the "Completed Calibrations" file

drawers located in B033, according to type of calibration.

4.2. Calibration of integrating-type instruments

4.2.1. Condenser chambers

4.2.1.1. General considerations

This type of instrument consists of a charger/reader and a variety of

removable condenser probes. As mentioned in section 3.7.1, the only

charger/reader sent to NBS for calibration is the Victoreen Model 570 R-meter

along with appropriate condenser chambers (probes). The Victoreen condenser

R-meter has been so widely used in radiation dosimetry for such a long period

of time, that it merits a special description. A diagrammatic section through

a Victoreen condenser chamber is given in figure 4-1. It consists of an ionization chamber with an air volume of a few cubic centimeters mounted at one end of a shielded stem containing a solid dielectric storage condenser.

73
Connection with the chanter is made through a contact at the other end of the

stem, this being covered by a close-fitting cap when the chamber is in use. For charging the chanter, or reading its potential after irradiation, the cap

is removed and the stem is plugged into a socket in the charger/reader. This

charger/reader contains a quartz-fiber electrometer, which is observed through

a low-power microscope having a scale calibrated directly in roentgens. The

probes are of various volumes, allowing for a variety of total exposures

ranging from 10 mR to 250 R full scale. The wall thickness varies according

to probe model. The calibration techniques appropriate for each model are given in a folder marked "Calibration Technique", located in B033. For those

models where the wall thickness is insufficient for the energy of the

radiation requested, an equilibrium shell must be added.

Fig. 4-1. Detailed construction of the 25-r Victoreen condenser ionization

chanter.

4.2.1.2. X-ray cal ibrations

X-ray calibrations are performed using the substitution method: the

standard is used to measure the exposure rate at a given point in space, then

the instrument to be calibrated is placed at that point and exposed for a known period of time. The probe's response is normalized to 760 mmHg pressure

and 22°C. The ratio of known exposure to the normalized response is a dimensionless number, the correction factor.

Prior to calibration, the chamber is first aligned in the x-ray beam.

Generally this can be done while the x-ray set is warming up. A variety of

holders are available, depending on the probe being calibrated. (In the

drawer marked "Chamber Holder", south wall of B033 for 300 kV set; north wall

cabinet in 100-kV set area.) The chamber is placed in a holder and the laser

beam used to determine vertical and horizontal alignment. The cross-hair

reticle of a telemicroscope is set to the defining plane of the appropriate

free-air chamber prior to final alignment of the probe. For the Wyckof f-Att ix

(50- to 300-kV FAC), the defining plane is the white line on the aperture

holder. For the Ritz chamber (20- to 100-kV FAC), the defining plane is

exactly 15.00 mm (toward the FAC center) from the white line on the aperture

holder. For the Lamperti chamber (10- to 20-kV FAC), the defining plane is

exactly 20.00 mm (toward the FAC center) from the scribed line on the plastic

insert that fits into the removable shield and touches the front face of the

aperture. Adjustment of the chamber to the defining plane is accomplished by

sighting through the telemicroscope (previously aligned on the defining plane)

while using the motorized slide to adjust the chamber center or defining plane

to the FAC plane. Control is accomplished using a control box located in the

74
vicinity of each telemi croscope. Condenser-type probes are right cylinders,

or have rounded-top cylindrical thimbles or thin end windows. For the first

two types, the chamber reference plane is the mid-plane of the thimble. For

the latter, the grooved line on the thimble is the reference plane. The thinwindow chamber is calibrated with its axis parallel to the beam axis, while

the other two types are calibrated with the chamber axes perpendicular to the

beam axi s.

A further consideration in x-ray calibration is choice of an appropriate

beam size. For this purpose a parameter called the "useful beam size", is

compared to the largest dimension of the active volume. The general practice

is to use a "useful beam size" that is only a few centimeters larger than the

active volume size so as to minimize irradiation of inappropriate volumes in

the probe stem. For x-rays, as opposed to high energy gamma-rays, this is of

secondary importance since the chamber stem attenuates the radiation

considerably. The "useful beam size" for the lOO-kV x-ray calibration range

is defined as twice the distance from the center of a radiograph of the beam

to the point where the density of the film is 90 percent of the center density, minus the difference in distance between the 90 percent and the

50 percent density measurements. The "useful beam size" for the 3nO-kV x-ray

calibration range is calculated from plots of normalized density readings of

radiographs made at 78 cm and 220 cm from the x-ray tube target using

apertures ranging from 0.94 cm to ?.54 cm diameter. A limit of 0.5 percent

change in density from the density at the beam center appears to have been

used as the limit for the "useful beam size." The data used for these

calculation are contained in a black loose-leaf binder marked "Seifert + New

100-kV BOC's" and is kept in room B033.

Leakage measurements are taken prior to calibration. If the leakage is a significant fraction of the expected exposure reading, either the probe is

cleaned with canned dry gas, rechecked and found to be corrected, or it is not

calibrated. In addition, leakage measurements are made at the time of calibration. Again if, the leakage is found to be a significant fraction of the

expected reading, the insulators are cleaned using canned dry gas. (The gas

is cold due to expansion, so some time must be allowed for the chamber to

equilibrate with room temperature.) If the cleaning procedure is not

successful, the calibration is terminated.

4.2.1.3. Gamma-ray calibrations

The instruments to be calibrated using gamma-ray beams are of the same

type as described in section 4. and are calibrated in essentially the

same way as in the x-ray beams. The primary difference is that in the gammaray beams, an NBS standard is not used at the time of calibration. Instead, a previously determined value of the exposure rate is corrected for decay to the

time of calibration. This procedure is described in section 3.4.

The source-to-chamber distance is set by sighting the tel emicroscope on the appropriate scale distance. The standard scale-distance/collimator-beam

settings in room B036 are given in table 4-1.

75
Table 4-1. Standard distance and collimator settings, room B036

Source Scale Di stance Col lima tor

(cm) {cm2)

137CS 38 10 X 10

137CS

6 0Co 105

55 8x8

6x6

All the horizontal sources in rooms R015A, B015R, B0?>1A, and R0?1B allow

scale distances from 35 cm to 300 cm, and have a fixed collimator size. The

probe to be calibrated is adjusted to the beam center-line using the laser

beam associated with each source. The chamber is then centered in the

tel emicroscope scale-reticle. An exception to this technique is when the

probe is larger than 10 cm. The technique for set-up then involves measuring

the probe in the direction of the beam using a metric calipers and determining

the radius. The probe is then placed in the beam, aligned as above, and

adjusted so that the end of the chamber is coincident with the tel emicroscope

cross-hairs. The scale distance, entered as SD in the computer program,

becomes the scale distance of the cross-hairs plus the probe radius.

4.2.2. Cable-connected instruments

4.2.2.1. General considerations

This type of instrument consists of an electrometer-type readout

connected by cable to the probe. Details of the theory of operation, measurement techniques, modes of operation, etc., are best described by the instruction manual for each model. If no manual is provided with the instrument, it

is possible that a copy may be available in a Dosimetry Group file of manuals

located in room B033. All units provide a collecting potential for the probe.

The value of the collecting potential can be obtained from the instruction

manual; good operating procedure dictates that this parameter be tested prior

to calibrating the probe, preferably upon arrival of the instrument at NBS.

If the potential is found to be outside specifications, it may require

replacing the batteries (sometimes special types) or returning the instrument

to the customer for repair.

4.2.2.2. X-ray calibrations

Alignment of these types of probes is the same as the condenser chambers.

A limited variety of probe holders are on hand for this type of instrument.

There are occasions when no specific chamber holder exists for a chamber to be

calibrated; on these occasions, available holders must be adapted (or

modified) accordingly. In all instances, care must be taken to reduce or eliminate scattering material from the beam. Details of the data-taking

procedure appear in section 4.2.

76
The calibration procedure for this type of instrument is only slightly

different from that for condenser-type. Having a direct readout makes it

convenient to take into account any small leakage/background effects. The

procedure used is to unground the reader for the same length of time to be

used in the exposure measurements. One can then correct the final value by

either adding or subtracting the "leakage". The reason this technique is used

for this type of instrument, and not for the condenser-type, is that the

readouts on the cable-connected type allow for a more precise reading (the

electrometers of condenser-types are generally readable only to 1%).

4.2.2.3. Gamma -ray calibrations

This type of calibration is basically the same as for x-rays, and essentially no different from the gamma-ray calibration of condenser chambers.

4.3. Calibration of current-type instruments

Detailed description of current-type instruments and calibration

techniques is given in section 3.5.

4.3.1. X-ray calibrations

Section 3.6 gives a description of the data-acquisition system, ionization current measuring systems, and x-ray-set operation. The setup techniques

for these types of chambers is basically the same as for both of the

i ntegrating-type probes described above. The additional steps that need to be

taken are as follows:

After mounting and aligning the chamber in the fixture and the chamber

collecting potential has been turned on, either manually or under computer

control (see section 3.6), a hand-held voltmeter is used to check the

collecting voltage on the chamber itself. This insures that a connection has

been made. Also with certain Triax-to-RNC adapters a short-circuit occurs if

the triaxial portion of this adapter (at high voltage) is grounded. It is

good practice to insulate this adapter and tape it in place so that motion of

the carriage does not cause the cable to shift. It is also important to keep

all connections out of the radiation beam, thereby reducing errors due to

contributions from ext ra-cameral volumes.

4.3.2. Gamma -ray calibrations

Section 3.4 contains a general description of the gamma-ray sources and

calibration techniques used for these types of instruments. The calibration

techniques used for current-type instruments are basically the same as the

techniques described above for the gamma-ray calibration of integrating type

instruments. As with x-ray calibrations, it is necessary to check the chamber

collecting potential at the chamber itself. Care should be taken in shielding

the cable connections and preamplifiers from the beam and from scattered

radiation. Errors of as much as 5% can be introduced, even in the scattered

beam, if lead shielding is not employed.

77
4.4. In-house calibration checks

The long-term reliability of NBS dosimetry calibrations depends on the

stability of the NBS exposure standards. For gamma rays, the NBS exposure

standard is the mean response of six spherical graphite ionization chambers,

which are periodically intercompared. For x-rays, the NBS exposure standard

is the response of the appropriate free-air chamber; the three free-air

chambers are also periodically intercompared. The in-house calibration checks

are intended to check both the stability of the NBS standards and the

reliability of the calibration procedures.

There are two ways to check the validity of a calibration. The first of

these is to calibrate an NBS probe or chamber that is either the same as or similar to the customer's and on which NBS has a calibration history. There

are two loose-leaf binders (identified as to instrument type) in Room B033

where such histories are kept.

The second check is examination of previous calibrations of the

customer's instrument at the same beam quality. Any significant discrepancy,

> 0.2% for current -type probes, > 1% for i ntegrat i ng-type instruments, is cause for investigation. Such discrepancies seem to occur approximately once

per year out of about 130 routine calibrations, on the average. When there

are several previous calibrations of the customer's instrument at any one beam

quality, one can estimate the reproducibility and decide whether the current

value is acceptable.

For the gamma-ray check chambers, a computer program provides the average

value of the previous 10 calibrations and a comparison of that average value

with the current value. If accepted by the operator this latest calibration

becomes part of a new average of 10 calibrations, subsequently called the

"previous 10". (The criteria for acceptance are generally the same as for

customers' instruments: within 0.2% for current -type chambers and within 1%

for i ntegrati ng-type chambers.) At the end of each new set of 10 calibrations, the gamma-ray calibration program prints out the results, dates, model

numbers, serial numbers, and source identification so that it may be placed in one of the above-mentioned binders.

For the x-ray check chambers, a record is maintained of all calibrations,

and the previous three calibrations are compared with the current calibration

to detect any trend or measurement discrepancy.

Any discrepancy arising with an NBS check chamber of the order mentioned

above should, without exception, give rise to a thorough investigation of the

calibration procedure. Alignment, temperature indications, distance, etc., are to be checked again. If the discrepancy cannot be resolved, a recalibration is necessary.

The calibration check procedures outlined above carry the danger that a very slight drift in the calibration factor of a check chamber would not be

detected, but would accumulate to a significant value over an extended time.

In order to investigate this possibility, the entire calibration histories of

three different chambers were analyzed for trends and outliers using linear

regression. The results showed trends in the calibration factors of no more

78
than 0.09% per year, with an average of 0.04% per year. Twice the standard

deviation of the regression is used as a limit to define outliers. Table 4.?

sinmarizes the results, which are contained in a folder in Room B033. It appears to be worthwhile to expand these analyses to include all the data

available and to update them periodically.

Table 4.2. Analysis of three NRS check chambers for the period

Sep 196Q to Oct 1987.

Ch amb e r Shonka/Wyckof f Exradi n Vi ct oreen

Model No. /Serial No. 3CM3,2 .5MM/344 A2/134 415A/121

Beam Qua! ity Code 60Co 137CS MlOO M50 M50

Number of points 151 66 18 29 53

Overall trend (%/year) 0.04 0.04 -0.03 -0.09 0.01

Stand, dev. of trend (%) 0.00 0.01 0.06 0.01 0.02

Stand, dev. of regr. {%) 0.1 0.1 0.3 0.1 0.6

Number of outliers 4 2 1 2 1

4.5. Test of high-quality feedback electrometers

NRS provides a test service for high-quality feedback electrometers that are used in conjunction with current-type ionization chambers. The procedure

involves electrically testing the electrometer at one feedback-capacitor

position and computing a correction factor, Kq. A typical report form is

attachment 22. As a check on this electrical test, the customer's

current-type chamber is calibrated for one beam quality with both the NRS

system and with the customer's system. Agreement is usually within 0.2%.

4.6. Irradiation of thermoluminescent dosimeters

4.6.1. General considerations and procedures

Thermoluminescent dosimeters received at NBS for irradiation in terms of

exposure have been the type used for personnel protection. Exposures

requested have usually been in the 200 mR to 1000 mR range. Until 1985, all

the exposures requested were for ^^"^Cs gamma rays. Recently there have been

requests to provide exposures at x-ray energies ranging from 50 kV to 150 kV.

4.6.2. Exposure techniques

For both x- and y-ray exposures, the dosimeters are mounted on a scatterfree frame at a distance such that the beam is uniform over the area of

irradiation. Gamma-ray exposures are performed at a scale distance of 300 cm,

where the useful beam size is 25.0 cm diameter. The exposure is obtained from

the charts associated with each source, corrected for decay.

79
All irradiation details are recorded in a data book, see DB 828, pp. 177

for a typical record. After review of the results in the data book, the badge

identification numbers, exposure levels, and date of irradiation are transferred to a TLD report form, from the desk in B033, along with a DG number

obtained from the calibration log in the Dosimetry office. The secretary

types the information into the skeleton form residing in the memory of the

word processor. Upon completion of this step, the report is then checked for

correctness, initialed by the person who performed or who supervised the

irradiation. The report is then sent to the Group Leader for review and

initialing, and to the Division Chief for signature. Upon return from the

Division Chief, three copies are made. One copy plus the original is mailed

to the customer (noting P.O. number on envelope), one copy is for the DG file,

and one copy for the test folder.

5. Assessment of uncertainty

Uncertainty of dosimetry calibration and irradiation is assessed in

several steps. First the uncertainty in the exposure rate of the x-ray and

gamma-ray beams is estimated, next the uncertainty in instrument calibration

or irradiation in each beam is estimated, and finally the exposure-rate

uncertainty is combined with the calibration or irradiation uncertainty to

obtain a final estimate.

The method of uncertainty assessment used here follows the recommendation

of the Comite International des Poids et Mesures (CIPM). The uncertainty

estimates are of two kinds: Conventional statistical estimates of random

uncertainties are given as standard deviations of the mean, and for convenience are designated as "Type A", which can be considered to be objective

estimates. All other uncertainty estimates, which are designated "Type B", are subjective estimates, based on the extensive experience of the calibration

staff. The Type B uncertainties are estimated so as to correspond approximately to a 67% confidence limit, i.e., they are estimated with the intent

that there be about one chance in three that the "correct" value is outside

the chosen limits; they are assumed to have roughly the character of standard

deviations. The Type A and Type B estimates are combined according to the

usual rule for combining standard deviations, i.e., by taking the square root

of the sum of the squares, which is referred to here as the "quadratic sum".

The quadratic sum of the two types of uncertainty is then considered to be the

combined uncertainty , which is in turn multiplied by two to give an overal

uncertai nty . The overall uncertainty is considered to have the approximate

significance of a 95% confidence limit.

Table 5-1 presents the details of the assessment of uncertainty in the

exposure rates determined for x-ray beams by free-air ionization chambers, and

as determined for gamma-ray beams by the set of spherical graphite cavity

ionization chambers. Table 5-2 presents the details of the assessment of

uncertainty in the calibration of current -type ionization chambers and

condenser-type chambers used with their electrometers, and also in the irradiation of passive (thermoluminescent) dosimeters. All the entries in

tables 5-1 and 5-2 are uncertainties of multiplicative factors, except for

radiation background, which is a subtractive correction. Since the uncer- tainty for radiation background makes a negligible contribution to the final

uncertainty, a separate computation is unnecessary.

80
Table 5-1. Uncprtainty analysis for exposure rate.

Gamma - ray beam X-ray hpam

A R A R

Volume 0.06 0.05 0.04 0.01

Charge 0 .03 0.1 0 .03 0 .] Timi ng 0.04 0.1 0.04 0.1

Air density 0.01 0.08 0.01 0.08

Recombination loss, 0.1 0.1

Humi di ty 0.1 0.1

Leakage 0 .01 0.01

Radiation background neg 0.01

Stopping-power ratio 0 .25 NA

Energy-absorption coefficient ratio 0.05 NA

Stem scatter 0.05 NA

Mean origin of electrons 0.1 NA

Effective measurement point 0 .05 NA

Axial nonuni formi ty 0.02 NA

Radial nonuni formity 0 .01 NA

Air attenuation, kg NA 0.07

Scattered photons, k NA 0.07

Electron loss, k^ NA 0.0]

Electric field distortion NA 0.2

Polarity difference NA 0.03 0.1

Penetration of aperture NA 0.04

Penetration of chamber face NA 0.01

Quadratic sum 0.08 0.36 0.07 0.33

Combined uncertainty 0.37 0.34

2 X combined uncertainty 0.74 0.68

Overall uncertainty 0.7 0.7

Notes to table 5-1.

The uncertainty estimates are in percent. The columns headed A and B refer to

the estimates described in the text as Type A and Type B.

Charge includes uncertainties for both voltage and capacitance.

Ai r density includes uncertainties for both temperature and pressure.

Recombination loss refers to the uncertainty in correcting for loss of

electrons by ion recombination.

Humidity refers to the uncertainty in correcting to dry air.

Radiation background refers to uncertainty in the extent to which the relatively large-volume free-air chambers might be sensitive to ambient radiation

background. The much smaller volume cavity chambers are insensitive to radiation background in the NRS calibration laboratory, and the uncertainty is

assumed negligi ble.

81
Table 5-2. Uncertainty analysis for calibration and irradiation.

Current-type Condenser Irradi ation

chambers chambers —————TM—~=-—

A B A B A B

Exposure rate 0.08 0.36 0.08 0.36 0.08 0.36

Scale reading NA 0.2 0.5 NA

Charge 0.04 0.1 NA NA

Timi ng n no. n 1 n 1 0.1

Air density 0.01 0.08 0.01 0.08 NA

Recombination loss NA NA NA

Humidi ty 0.05 0.05 NA

Leakage 0.02 0.1 NA

Radiation background npn neg NA

Di stance 0.01 0.01 0.1

Radial nonuni formi ty NA NA 0.2

Quadratic sum 0.1 0.4 0.22 0.64 0.08 0.44

Combined uncertainty 0..41 0.68 0.45

2 X combined uncertainty 0.82 1.36 0.9

Overall uncertainty 0..8 1.5 1.0

Notes to table 5-2.

The uncertainty estimates are in percent. The columns headed A and B refer to

the estimates described in the text as Type A and Type B.

Exposure rate is from table 5-1.

Charge includes uncertainties for both voltage and capacitance.

Air density includes uncertainties for both temperature and pressure.

R ecombination loss is not corrected for in a calibration so uncertainty is not

applicable in this table. Recombination loss in the usually much higher

exposure rate of the user's beam must be determined by the user.

Humidity refers to the uncertainty arising from the difference between the

hunidity at the time of calibration and at the time of use.

Radiation background and its uncertainty are assumed negligible, since the

relatively small volume chambers being calibrated are insensitive to the

ambient background in the NBS calibration laboratory.

Di stance refers to uncertainty in setting the secondary instrument at the same

distance as the primary standard that established the exposure rate. It does

not refer to uncertainty in the magnitude of the source-detector distance.

82
Estimates of the uncertainty vary slightly for different radiation

sources, beam qualities, measurement distances, methods of measurement, etc.

A full presentation of all these variations would result in many tables,

varying only slightly. In order to simplify the situation, the tables have

been assembled by choosing the largest value for each alternative situation.

Similarly, table 5-1 shows that the uncertainty in the exposure rate

determined by the free-air chambers is slightly smaller than the uncertainty

in the exposure rate as determined by the cavity chambers, so the latter

uncertainty has been used as the first entry in table 5-2.

For all the entries in tables 5-1 and 5-2, the Type A component is much

smaller than the Type B component, so much so that even if the Type A

component were neglected, the combined and overall uncertainties would not be

significantly changed. This arises from the complexity of the interaction of

ionizing radiation with matter: it is never possible to account fully for the

numerous second-order effects, but it is usual during exposure calibration at

NBS to repeat measurements until the Type A component of the uncertainty is

smaller than the Type R component. (The measurement of volume, table 5-1, is an exception, but this is not a routine measurement, and in any event both

components are small.) As a result, it is not worthwhile examining in detail

the random. Type A uncertainties, beyond noting that the number of degrees of

freedom varies between 5 and 15, depending on whether the measurement is part

of a particular calibration, or is carried out to evaluate some aspect of the

measurement system.

Based on the assessment of uncertainties described here, the following

rounded values of the overall uncertainties have been adopted:

exposure rate: 0.7%

calibration of cable-connected chambers: \%

irradiation of passive dosimeters: 1%

calibration of condenser chambers: 1.5%

6. Safety Considerations

The main safety consideration is radiation protection. As described

below, every effort is made to avoid any possibility of radiation exposure,

even though it would be highly unlikely that serious exposures could occur

accidentally. Another safety consideration is exposed high voltage, such as exists on ionization chambers during calibration. There is no danger of high

voltage related to the x-ray generators since the equipment now in use has no exposed high voltage. To a much lesser degree, there is consideration for the

safe use of low-wattage laser beams.

6.1. Radiation safety

6.1.1. X-ray calibration ranges

First and foremost, the two x-ray source ranges are designed to eliminate

any possible exposure to x-radiation. The 100-kV x-ray tube is interlocked

with its power supply in such a way that if the tube is moved from a safe

position, i.e., away from a lead shutter, the high-voltage is turned off (or

cannot be turned on). Some unavoidable leakage of radiation occurs between

83
the shutter assembly and the x-ray tube window at an angle of 90° from the

beam centerline. Prudence dictates that the x-ray set be turned off or a lead

sheet be used to block the radiation while working in this particular area. The 300-kV x-ray tube is enclosed in a housing of 19 mm Pb and 6.4 mm steel.

There is a 25-mm Pb safety shutter and a l?.7-mm timing shutter in front of

the beam portal. At full power, 300 kV, 14 mA, radiation leakage is

negligible. Both x-ray calibration ranges are protected by lead- lined doors

that are interlocked in a fail-safe manner with the shutters. This means that

the shutter or shutters cannot be opened if the door interlock is open; if the

shutter is open and the closed doors are opened, the shutter or shutters

close. Where no door exits, as in one area of the 300-kV x-ray range, a light

beam is used for protection. In addition, a time-delay device inside the

300-keV x-ray range must be actuated upon leaving or the shutter cannot be

opened. As a further indication of radiation danger, two red lights are turned on whenever the shutter or shutters are open. A flashing red light

associated with the 300-kV x-ray set indicates high voltage is on the x-ray

tube.

6.1.2. Gamma-ray calibration ranges

All doors giving access to the gamma-ray calibration ranges have interlocks as per Nuclear Regulatory Commission regulations. The vertical -beam

rooms have a time-delay device inside the room that must be actuated before

leaving the radiation area. In addition to this, a roped-off area exists at

the entrance to B036 to discourage loitering in the vicinity, since the

scattered radiation from the large ^^Co source is sufficient to be above

acceptable levels for long-term exposure in the area outside the door. In

addition to the above safety features, a radiation detector with indicator

lights and an audible signal is in each gamma calibration range. At each

entrance to a gamma-ray calibration range, a set of two red lights indicates a "beam on" condition.

All radiation areas in the building are marked with striped tape and film

badges must be worn by all personnel.

6.2. High-voltage safety

The only danger that exists from high voltage has to do with the chambers

that are being calibrated, and the x-ray calibration range monitor chambers.

To prevent dangerous electric shock, almost all power supplies contain

current-limiting resistors in the high-voltage circuit. Common sense is

dictated when working around ionization chambers that have exposed highvoltage electrodes. Appropriate warning signs are posted.

6.3. Laser safety

The 0.5-mW lasers used for alignment purposes do not pose any serious

danger to eyes. Nevertheless, appropriate warning signs are posted.

84
REFERENCES

1. NBS Calibration Services Users Guide 1986-1988, G. A. Uriano, et al.,

(Eds), NBS Special Publication 250, 1986.

2. Wyckoff, H. 0. and Attix, F. H,, Design of free-air ionization chambers,

National Bureau of Standards Handbook 64, 1957.

3. Ritz, V. H., Standard free-air chamber for the measurement of low energy

X rays (20-100 kilovolts-constant-potential ) , J. Res. Nat. Bur. Stand.

(U.S.), 64C, 49-53 (1960).

4. Lamperti, P. J. and Wyckoff, H. 0., NBS free-air chamber for measurement

of 10 to 60 kV X rays, J. Res. Nat. Bur. Stand. (U.S.), 69C, 39-47

(1965).

5. Ritz, V. H., Design of free-air ionization chambers for the soft x ray

region (20-100 kV), Radiology, 73, 911^922 (1959).

6. Hubbell, J. H., Photon mass attenuation and energy-absorption coefficients from 1 keV to 20 MeV, Int. J. Appl . Radiat. Isot. 33, 1269-1290

(1982).

7. Allisy, A. and Roux, A^, M. , Contribution a la mesure des rayons roentgen

dans le domaines de 5 a 50 kV, Acta Radiologica, 5^, 57-74 (1961).

8. Scott, P. B. and Greening, J. R., The determination of saturation

currents in free-air ionization chambers by extrapolation methods,

Phys. Med. Biol. 8, 51-57 (1963).

9. Berger, M. J. and Seltzer, S. M., Stopping powers and ranges of electrons

and positrons, NBSIR 82-2550-A (2nd Ed.) (1983).

10. Katz, L. and Penfold, A. S., Range-energy relations for electrons and the

determination of beta-ray end-point energies by absorption. Rev. Mod.

Phys. 24, 28-44 (1952).

11. Somerwil, A., Photon scatter correction in low-energy x radiation

measurements, influence of beam path length in standard free-air

chambers. Metrologia 8, 2 (1972).

12. Report of the International Commission on Radiological Units and

Measurements (ICRU), National Bureau of Standards Handbook 78, p. 19,

1959.

13. Loftus, T. P. and Weaver, J. T., Standardization of ^^Co and i37Cs

gamma-ray beams in terms of exposure, J. Res. Nat. Bur. Stand. (U.S.),

78A, 465-476 (1974).

14. Hoag, J. B. and Korff, S. A., Electron and Nuclear Physics , Chap. 17,

pages 404-435, D. Van Nostrand Co., Inc. 250 Fourth Ave., New York, NY,

(1948).

15. Taylor, L. S. and Singer, G. , Note on the guarded-field x-ray ionization

chamber. Radiology _26, 322-325 (1936).

85
16. Taylor, L. S., The precise measurement of x-ray dosage. Radiology ^4,

372 (1930).

17. Boag, J. W., Distortion of the electric field in an ionization chamber

due to the difference in potential between guard ring and collector,

Phys. Med. Biol. 9, 25-32 (1964).

18. Day, F. H. and Attix, F. H., Methods for current measurement with a vibrating reed electrometer. Nat. Bur. Stand. (U.S.), Report 2080

(1952).

19. Loevinger, R., Precision measurement with the total -feedback

electrometer. Phys. Med. Biol . 2;, 267-279 (1966).

20. Keithley, J. F., Yeager, J. R., and Erdman, R. R., Low Level

Measurements , Keithly Instruments, Inc., 28775 Aurora Road, Cleveland, OH

44139 (1984).

21. Frederiksen, T, M., Intuitive IC Op Amps , National Semiconductor Corp.,

Santa Clara, CA. (19847^

22. Instruction Manual, Model 616 Digital Electrometer, Keithley Instruments,

Inc., 28775 Aurora Road, Cleveland, OH 44139 (1977).

23. Instruction Manual, Model 642 Electrometer, Keithley Instruments, Inc.,

28775 Aurora Road, Cleveland, OH 44139 (1980).

24. Rutkowski, G. B., Handbook of Integrated-Ci rcui t Operational Amplifiers , Prentice-Hall, Inc., Englewood Cliffs, NJ (1975).

86
International Standard 4037

INTESNATIONAL organization for STANDARDIZATION»ME)imi'MAPOaMAfl 0PrAHM3AUMfl no CTAMflAfTM3AUMM»ORCANISATlON INTERNATIONALE OE NORMALISATION

X and Y reference radiations for calibrating dosemeters

and dose ratemeters and for determining their response

as a function of photon energy

Rayonnements X ety de rifirence pour I'italonnage des dosimetres et dibitmetres et pour la determination de leur riponse en

fonction de I'inergie des photons

First edition — 1979-05-15

Attachment 1, Section 1.2. ISO Publication 4037-1979 (E). (Excerpt)

U DC 535-34/-36 : 53.089.6 Ref. No. ISO 4037-1979 (E)

Descriptors : dosimeters, exposure flowmeters, calibrating, electromagnetic radiation, reference materials, test equipment, test results. 87
TABLE 2 - Calibration conditiont of fittand X rml^nne* radi«tiont

Smim Mmh enarfv keV"

Rcaolution

%

Comtant

potantial^'

kVcp

Additional filtration^' mm 1ft HVLx 2nd HVLa Homofanaity

coafficiant Laad Tin Coppar mm of copper

33 30 40 0.21 n\j, 1910.75 48 36 60 0.6 0.24 0.29 0,83

65 31 80 2,0 0.59 0.64 0,33 Narrow

83 28 100 5.0 1.16 1,2 0,97 100 27 120 1.0 5.0 1,73 1.74 0.99 spectrum

1 18 36 150 2.5 2.4 2.58 0.93

161 32 200 1,0 3.0 2.0 3.9 4.29 0.91 206 30 250 3.0 2,0 5.2 5.2 1.00 248 34 300 5.0 3.0 6.2

45 48 60 0.3 0.18 0.26 0.69 58 54 80 0.5 0.35 0,52 0.67 Wide

79 57 110 2.0 0.94 1.16 0.81 104 56 150 1.0 1,86 2.14 0.87 spectrum

134 58 200 2.0 3.11 3.53 0.88 169 58 250 4.0 4.3 4.38 0.98 202 58 300 6.5 5.0

NOTE - At a guide it it pointed oot that, for a current of 10 mA and at 1m from the tube, the exposure rate range usually obtained is between

2.6 X 10 * C kg^ h (1 R h ') and 2.6 X 10"3C kg" ^ h^ (10 R h for the narrow spectrum series, and between 2.6 x 10 3 C kg" ' h'

(10 R h ^

I and 2.6 X 10 2 c kg ' h ' (100 R h 'l*' for the wide spectrum series Lower energy photons outside the main portion of the spectrum, whose shape is given in figures 2 to 17, amounting to less than 2 % of the main spectrum, are not ihown

1) The value of the mean energy adopted with a tolerance of -3%. is taken from the results of a comparison of the spectra obtained in France, Germany and the United Kingdom (referertcel.

2) The constant potential is measured under load

3) The total filtration includes, m each case the fixed filtration adjusted to 4 mm of aluminium (see 3.1.3 31

4) The actual value deper>ds on the particular conditions of the installation

I'M t^;-

Attachment 1, continueid.

88
COMMISSION ^LECTROTECHNIQUE INTERNATIONALE

NORME DE LA CEI

INTERNATIONAL E L E C T R 0 T E C H N I C A L COMMISSION

lEC STANDARD

Publication 731

Premiere edition - First edition 1982

Appareils electromedicaux

Dosimetres a chambres d'ionisation utilises en radiotherapie

Medical electrical equipment

Dosimeters with ionization chambers as used in radiotherapy

Available from

American National Standards Institute

International Sales

1430 Broadway

New York, NY 10018

(212) 354-3300

$85 in May 1983

© CEI 1982

Droits de reproduction reserves - Copyright - all rights reserved

Aucune partie de cetle publication ne peui elre reproduile ni ulilisee sous f^o pan of this publication may be reproduced oi utilized in any

quelque torme que ce soil el pai aucun ptocede electronique ou nneca- form ot by any means electronic or mechanical including photocopying

nique y compns la pholocopie el les microfilms sans I accord ecnt de I edileui and microfilm, without permission in writing from the publisher

Bureau Central de la Commission Electrolechnique Internationale

3, ruedeVarennbe

Geneve, Suisse

Attachment 2, Section 1.2. lEC Publication 731, 1982. (Excerpt)

89
731 © lEC 1982

add significantly to the filtration of the beam. The effect on the monitor chamber of backscattered radiation from the reference chamber and from the chamber under test should be

investigated; if this effect is significant, it can be reduced by introducing an additional

diaphragm to shield the monitor chamber (this diaphragm should not limit the useful beam).

A monitor chamber is unnecessary if calibrations are carried out by simultaneous irradi- ation of the reference chamber and the chamber under test; it is also unnecessary in a gammaray beam.

Note. — It is good practice to place all the coinponents just listed as close to the target as possible, consistent with

the need for a narrow beam penumbra.

C4.8 Additional shielding

The effect on the reference chamber or on the chamber under test of scattered radiation:

— from the filter; — from the front of the beam-limiting diaphragm, or — from the X-ray tube housing

should be investigated. If this effect is significant, it should be reduced by suitable shielding.

C4.9 Positioning ofthe equipment

In order to reduce the uncertainty of comparison to ±0.2%, it is necessary to place the REFERENCE POINTS of the reference chamber and of the chamber under test at the same

distance from the target to an accuracy of ±0.5 mm when calibrating at 50 cm from the target.

It is sometimes convenient to have an easily removable and accurately replaceable means of

providing a point on the axis of the useful beam close to the chambers, from which the chambers can be accurately positioned.

It should be possible to adjust and to hold the chambers rigidly; no part of the positioning

device should be in the useful beam.

C5. Radiation qualities

C5. 1 The radiation qualities used for calibration should be similar to those used in radiotherapy.

In order to achieve a range of such radiation qualities, a series of X-ray tube voltages may

be chosen and the additional filtration adjusted until the half-value layer of the beam lies

. : within one of the two areas bounded by the pairs of dashed lines in Figure C I , page 1 37.

Data for Figure CI were supplied by three national laboratories, namely the National

Bureau of Standards (USA), the National Physical Laboratory (United Kingdom) and the Physikalisch-Technische Bundesanstalt (Germany).

Aluminium filtration alone may be used for half-value layers up to about 4 mm Al (about

0.1 5 mm Cu) and copper filtration (with i mm Al filter after the copper filter) for higher half- value layers. Tin filtration may also be used for half-value layers above about 2 mm Cu, with

about 0.25 mm Cu and 1 mm Al filters after the tin. Aluminium of 99.99% purity should be

Attachment 2, continued.

90
731 © lEC 1982

Half- value layerin copper upper graph only

0 03 0 05 01 02 05 10 20 50 (mm)

Half-value layer m aluminium lower graph only

537/X2

Fig. CI. — X-ray tube voltages (constant potential) and half-value layers of radiation qualities

for calibrating exposure meters as used in radiotherapy. For dashed lines, see Subclause C5.1.

Attachment 2, continued.

91
CONVENTIONAL CALIBRATION CONDITIONS FOR X- AND GAMMA-RAY MEASURING INSTRUMENTS

Lightly Filtered X Rays

Aaueu Ha 1 T - va 1 ue Homogenei ty

Beam Lonsuant ui Stance r 1 1 ter Layer Coef f i ci ent uoae rouent 1 a

AlM 1 A

^ C-+ Al u\/i / iSt Al nvL/

[KM) (cm) (mm) (mm) OnA Al UX/I ^no Al HVL

L-B 10 25 0 .029 0.79

L-C 15 25 0 .050 0.74

20 50 0 .071 0.76

L-E*^ 20 50 0.5 .23 0.78

L-G 30 50 0.5 .36 0.64

L-I 50 50 1.0 1.02 0.66

L-K 75 50 1.5 1.86 0.63

L-M 100 50 2.0 2.78 0.59

Moderately Filtered X Rays

Homogeneity

Beam Constant Added Filter^ Half -Value Layer Coefficient

Code Potential Al Cu Al Cu 1st Al HVL/

(kV) (mm) (mm) (mm) (mm) 2nd Al HVL

MFR III lL> , fin 0 0 1.62 n 68

MFC? 60 2.50 0 2.79 0.090 0.79

MFE^ 75 2.51 0 3.391 0.76

MCP 1 fin iUU 3.50 0 5.03 0.20 U . /o

MFI 150 3.49 0.25 10.25 0.66 0.89

MFK. 200 3.49 0.50 13.20 1.24 0.92

MFM^ 250 3.50 1.01 15.80 2.23 0.92

MFO 250 3.47 3.20 18.30 3.25 0.98

Heavily Filtered X Rays

Beam Constant Added Filter^ Half -Value Layer Effective

Code Potential Al Cu Sn Pb Al Cu Energy

(kV) (mm) (mm) (mm) (mm) (mm) (mm) (keV)

HFC 50 2.50 0 0 0.10 4.199 0.14 38

hfe"^ 100 2.50 0 0 0.50 11.20 0.74 70

HFG 150 2.50 4.00 1.51 0 16.96 2.45 117

hfi 200 2.47 0.60 4.16 0.77 19.60 4.09 167

HFK 250 2.50 0.60 1.04 2.72 21.55 5.25 210

GAMMA RAYS

Radionucl ide Energy Calculated Cu Half -Value Layer

(keV) (mm)

137CS 662 10.8

6 0Cq 1250 14.9

^Inherent fi 1 tration approximately 1.0 mm Be.

Discontinued beam codes.

Inherent filtration approximately 1.5 mm Al

Attachment 3, Section 1.2. NBS beam codes used prior to 1986

for calibration of x-ray and gamma-ray measuring instruments.

92
?oTt detail for 20 Ci Co60 source X 6mm diameter.

Source

Port detail for 200 Ci Co60 source X 5mm diameter.

Attachment 4, Section 3.4.2. Sketch of horizontal 20-Ci and 200-Ci

^°Co sources.

93
PORT DETAIL FOR 30 Ci C3137 SOURCE X 15tnm DIAMETER.

PORT DETAIL FOR 300 Ci CsI37 SOURCE X 25mm DIAMETER.

Attachment 5, Section 3.4.2. Sketch of horizontal 30-Ci and 300-Ci

i3^Cs sources.

94
3 . 6mm

Zero on scale

Distance from source center

to scale zero = 412.5mm.

Sketch of lOkCl C060 THERATRON "F" (Super G) head.

Attachment 6. Section 3.4.2. Sketch of vertical 10-kCi eoco Theratron "F" (Super 6) head.

95
*6S

4f

? i I

a. SO)

a.

1

3 E ft

o

o _

<O

t III

si" SS'

§1

EH,

to*

t

(03«J01ll3Ziij>-i

"5 V «•

E

I

« ? w 1°^ jctOCn

|i1N3 i3S3H nvriNVi -cnnD—

s r »

Attachment 7, Section 3.6.1. Block diagram of data acquisition system (DAS).

96
150 KILOVOLT POWER SUPPLY

Universal Voltronics, Serial No. 2-6-1486

Subm i 1 1 e d by

National Bureau of Standards

Divis ion 243

Section 02

Voltage measurements on thic unit were made in September

1972 and the following results obtained:

Console Setting Measured Voltage

(kilovolts') (kilovolts

)

5 5 ,. 18

5 ., 07 5 ,, 25

10 ..07 10 ,.45

20 19 .. 55

20 ., 07 19 ,. 62

30 29 .. 90

40 40 ,. 24

50 50 ,. 6

60 60 .. 9

70 69 ,, 9 80 80 ,, 1 90 90 ,. 4

100 100 ,. 6

100 100 ,. 7 The results given above are estimated to be accurate to

within ±0.1 percent of the quoted value or ±0.05kV, whichever is

greater.

These accuracies refer to the values at the time of

measurement and are not indications of long-term stability.

Attachment 8, Section 3.6.1. Universal Voltronics power supply calibration report.

97
Attachment 9, Section 3.7.3.

98

Schematic drawing of Keithley 6169

chamber'-el ectrometer interface.
PRECALIBRATION IONIZATION CHA14BER TEST DATA

Owner:

Date: 83 Oct lO qB 841: 137-138

Chamber type: m-DAL I Mod 7733 SN

MEASUREMENT SEQUENCE ABCO A Comments 8 Comments

Coll. Pot. (V) + 100 +50 Chamber has

Initial leakage (A) +34 fA -22 fA been Irradiated

Final leakage (A) -3 fA Pre-radiation -16 fA

Time between initial

and final leakage (s)

700 100

Magnitude of ionization current (A) 300 pA 300 pA

Percent change in ion

current from first to

last meas. (X) 0.02 0.01

Time between first and

last ion. current mea- surement (s)

60 0

Mean ion current (A) 316.55pA 316.62 pA

Ratio of ion currents

for full and half

coll. pot. 0.9998

MEASUREMENT SEQUENCE ABCD C 0

Coll. Pot. (V) -50 -100

Initial leakage (A) -98 fA Before irrad.

Final leakage (A) -30 fA it -50V

Time between initial

and final leakage (s) 400

Magnitude of ionization current (A) 300 pA 300 pA

Percent change in ion

current from first to

last meas. (X)

0.44

leadings changing. Running

}ut of time. 0.12

Changing. Ran

out of time.

Time between first and

last ion. current mea- surement (s)

690 210

Mean ion current (A)

Ratio of ion currents

for f ul 1 and half

coll. pot.

(Negati

for ca

/e collection p:

Libration repor

:ential da

.)

:a not required

Attachment 10, Section 3.7.6. Proposed precalibration test form.

99
U.S. DEPORTMENT OF COMMERCE

NRTIONPL BUREOU OF STRNDPRDS

NOTIONPL MEPSUREMENT LPBORPTORY

GftlTHERSBURG, MD. 2iZi899

REPORT OF CPLIBRPTION

LIQUID-IN-GLPSS THERMOMETER

TESTED FOR: NflTIONPL BUREAU OF STPNDPRDS

DIVISION 533. IZI2 MftRKED: FISHER 815553

RftNGE: +19 TO +35 DEGREES C IN (3.02 DEGREE

THERMOMETER CORRECTION

REPDING (IPTS-68)**

21. 000 C -. 031 C

23.000 -.111

25.000 -.031

**PLL TEMPERPTURES IN THIS REPORT PRE BPSED ON THE INTERNPTIONPL

PRPCTICPL TEMPERPTURE SCPLE OF 1968, IPTS-6B. THIS TEMPERPTURE SCPLE

WPS PDOPTED BY THE INTERNPTIONPL COMMITTEE OF WEIGHTS PND MEPSURES PT

ITS MEETING IN OCTOBER, 1968, PND IS DESCRIBED IN "THE INTERNPTIONPL

PRPCTICPL TEMPERPTURE SCPLE OF 1968 PMENDED EDITION OF 1975,''

METROLOGIP 12, NO. 1, 7-17 (1976).

ESTIMATED UNCERTPINTIES IN THE PBOVE CORRECTIONS DO NOT EXCEED 0.03

DEGREE UP TO 35 DEGREES C . FOR P DISCUSSION OF PCCURPCIES PTTPINPBLE WITH SUCH THERMOMETERS SEE

NPTIONPL BUREPU OF STPNDPRDS MONOGRPPH 150, LIQUID-IN-6LPSS

THERMOMETRY.

IF NO SIGN IS GIVEN ON THE CORRECTION, THE TRUE TEMPERPTURE IS HIGHER

THPN THE INDICPTED TEMPERPTURE; IF THE SIGN GIVEN IS NEGPTIVE, THE TRUE

TEMPERPTURE IS LOWER THPN THE INDICPTED TEMPERPTURE. TO USE THE CORRECTIONS PROPERLY, REFERENCE SHOULD BE MPDE TO THE NOTES GIVEN BELOW.

THE TPBULPTED CORRECTIONS PPPLY FOR THE CONDITION OF TOTPL IMMERSION OF

THE BULB PND LIQUID COLUMN. IF THE THERMOMETER IS USED PT PPRTIPL IMMERSION, PPPLY PN EMERGENT STEM CORRECTION PS EXPLAINED IN THE PCCOMPPNYING

STEM CORRECTION SHEET.

NOT TESTED OVER THE ENTIRE RPNGE OF SCPLE NOR PT P SUFFICIENT NUMBER OF

POINTS.

TEST NUMBER 311-5-85

COMPLETED 6-17-85

Attachment 11, Section 3.8.2. Liquid-in-glass thermometer

calibration report,

100
(lit* 174) U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

WASHINGTON. D.C. 20234

CORRECTION FOR EMERGENT STEM

This item correction ihcet it doiljfned to explain

the proccis of computing the correctiong for emergent

item,, but has not been ipeciflcally adapted to the Individual thermometer with which It ii lnued.

If a total-lmmeralon thermometer ii actually used

with A part of the liquid column In the capillary

emergent from the bath, a item correction must bo

calculated and applied as explained below, makini;: use of the formulas given.

The general formula used in computing the correction for emergent stem li

Stem correctlon = fcx»«('i — 0,

where fc—the differential expansion coefficient of mercury (or other liquid In the thermometer such as alcohol, toluene, pentane,

etc.) in the particular kind of glass of which the thermometer is made; (for

numerical values see below)

nrznumber of degrees emergent from the bath; (i=temperature of the bath;

f = mean temperature of the emergent stem. Example l.—Suppose the observed reading was

84.76' C and the thermometer was immersed to the

20* mark on the scale, so that 65* of the column projected into the air, and the mean temperature of the emergent column was found to be 38' C, then

Stem corroclionrrO.OOOlGxCG (Hf)- —38' ) - + 0 40* C. The true IrmpcrHturc is therefore the obiervcd

reading, 84.70" C, f tabular correction as Interpolated

from the report, + emergent stem correction (+0.40' C).

KxAMPi-r. 2.—Suppose the observed reading was

780* F and the thermometer wan immersed to the 200* mark on the scale, so that 580* of the column

projected into the air, and the mean temperature of the emergent column was found to be 170* F, then

Stem correctionrr0.00009x580(780' —170') = -f-32- F

as a flrnt approximation.

Since the result shows that the bath temperature was approximately 780' F-f-32' F, a second approximation should be made, using t, = 812' F instead of

r, = 780' F. This givesStem correction =0. 00009x580(812' —170* ) = + 34' F. The true temperature is therefore the observed

reading, 780' F, + tabular correction as interpolated

from the report, + emergent stem correction (+34' F).

It will be noted that if the average temperature of the stem is below that of the bulb, the sign of the cor- rection will be +, while if the temperature of the stem is above that of the bulb, the sign of the correction will be — .

EXPLANATORY NOTES ON THE EMERGENT STEM CORRECTION

Som« th«rmom«t«ri tr« pointed tnd (r*duat«d bj (h« m«krr (o VAMIK.S or * Foil MKKCHUY-IN-OI-AaS Til KR MOM KTKRB

r»«d correct, or ipproilmttelr correct, temperaturn wtien the bulb «nd the entire liquid Indei In the item are eipoied to the temperiture to be measured, while other thermometer! are to pointed and craduated tbat the/ will read correct, or approil- mately correct, temperaturea when the bulb and only a ihcrt lenfth of the atem of the thermometer are Immeraed In the bath,

the temperature of which li to b* roeaaured. Thermometera of the former data are known as •'total-lmmeralon thermometera," and

thoie of the latter claia at "partlal-lmmeralon thermometera."

Tolal-lmmerilon thermometera are teated under the condition

of total Immeralon and the correclloni reKultlng from «uch « teat

will lerre to reduce the obserred readings of the thermometer to true tem|>eralurei only If the thermometer la utrd aa a total- Immersion thermometer. If such a thermometer Is actually ii»ed as a partlal-lmraeralon thermometer. I.e.. with a part of the mercury column emergent Into the space above the hath, and

with the emerrenl stem therefore either colder (or warmer) than

the bulb, the thermometer will obTlously read lower (or hislicr) than It would under the condition of total Immersion. Hence. If a tolal-lmnieralon Ihermonieler Is so used, a socallcil stem

correclldii must be applied to llic <iIi»<tt<-i| resiling In addldnn

lo itic correcllofi taken from llie scconipAiiyl"K mlilr of corr.c

(Ions This stem correction Is very large If llir numl«er of degrceK emergent snil the illlT<»reiice of trmi>cr«lure lielwccn the linlli and the spsce abo»e II are Urge It may amount In more than

'20* C iSn' K) fur niessurenienl" ninite w I Mi n mercury

tlirrinomelf r at 400* (" (75<f K)

The coemclenl t Is rtlfferenl for illff.renl kinil. i.f glan. nixl e»en fur I lie .sine k I nil of gls«» H illff.T. fur illffcrriil l.ni|Mrs

Inn- InlrrtnU, I . , .llffiTriit xiliic. uf 'I \'»liic« fur k fur liMi mlilr;, nsril I he r inonif I r Ir «l««»e» f'T use In rn I

. ii I ii 11 n «

• I'm ri.r rrit l.in . srr l«(iiil«lrcl n« f.ii:.'*.

Knr (• sills t licrinoinelrrs Mean

temp.

«,-(-' 2

k for "normal"

f last 0* 0. 000108

100 138

130 138 200 130 20O 101

.SOO 104

.SBO 400

430

for "boroalllcate" class 0.OOO104

104 103

107 170

174 178

18;t 188

Kor Kahreobelt thermometers Mean

temp.

I. + I 2

kfor "normal"

glass

k for "boro

silicate" glass 0' 0 000088 0. 000081 200 88 01 300 88 02 400 80 OS 300 00 03 000 02 07 700 000 100 HOO 103

If tlir kliul ut gliiHH iif wlilcli (lie lliernioiueirr Is msUc Is known, (lie value of k (o Im- usciI In coin(iutlng the slrin correction inrt}' lie taken from (lie ntiiM-e (able. If (tie kliul of glass Is ni>( kiiiiwli. iiHc i (l.dillllll fur ('i'IhIiih <ir (l (KKIiltl fur )-'« li reii liel (

( her niiiniel iTM. llt^-li^riute tlieriiininrlerH iire nii\\ genersll) made

nf "iiiirniid" nr "ImroMlllcnle" iilrtsses. If ft ( Iicr iiuinie (er Is gradiiiilei( iinl\ III nli.iiK 4,MI' (' (KMI* K). I( limy lie iiiaile iif eUlier

nf Hie nli.ive KlllHses; If l( U griulilaled (o BlKI* C {\IX2' K) and

Is itcdiiillv iiHiilile Ml (lin( (einiHTiiliire. I( Is inaile of one of Hie

liiiriixUli'ii le t;l<i~>-es or ii xliiilliir u^'i"

'I'lie eipiiiistoliM of ll(]iiliU Hiieli iis nli'olioj lolnem-. eir. I'nrv

i|illle ni|ilillv ivllli llie (eiiUHTii 1 11 re. so (liiK k viirli'N loii »! .le r n lil

> for illlTereiil leniiNTii I II re liilerviilH. II |i|iriii I mil I e nieiii cor reellon for -\iih ( lierinoinel.-rH niii.i lie en I en I ii I eil li.i (iiklin; 1 In

IIk' llliove ei|llilll.oi (I mil for CeUlim I lie r in olne I e r a or 11 IKHIII for I'nlirenlieli ( liernioniedTM The liillle of t, (lie I in I e in (n- ni I ii re of (lie ellirrgenl Irni. Is llie iiiokI illltleiill of ( lii' leriiiH In I lie s lio \ e forninla ( ii eallniale

II iniii lie unlle iii'eii ni I el \ nieiiniireil liy (lie use of siieelnl espll

liir.i (Iiernionielers TIiIk however, very rarel)* done eicepi In

(tie (esdiii; liilMirn ( or> . iiinl (hell iinl) when the s(eiii eorrecdon

nii>( lie ilelerniliieit wllli eon >! lie r n III e preelsliiii (lo 10 iwri-eni or

liellerl III 1:1 iiiriil (lie \nl f f niiiy lie ile ( e r 111 1 ne it lo s silfR

I'leiil II |i|iro\ I liiii d.oi III Jiiilciiielil iir (ireferadll In o > | >e 11 il 1 11 g nil lotliliiri llierii Ii f I lo>e lii'.lili' III,' rniergi'iil >l. Ill Mllh llie

lioll' "f II ivllliiri llieriii..lllelei .onieHliill lirnr.i 1.. die

of I lie ImI I h I loi o I I li. ll.|Olil liieiil,. II,.

Attachment 11, continued

101
CALIBRATION INVENTORY FORM

OWNER'S NAME (31 characters) SEQ NO TFN INST. RECEIVED

CHAMBER NAME (31) MODEL (15)

3

// NO

SN (15) NO OF PTS

X G

ELECTROMETER / READER (31) MODEL (15) SN (15) CONTACT

**************************************************

OWNER'S NAME (31 characters) SEQ NO TFN INST. RECEIVED

Kir-fla^ 4P(S (^)/ no

CHAMBER NAME (31) MODEL (15) SN (15) NO OF PTS

X G

^S-'O.IHA Z^Z _o _i_

ELECTROMETER / READER (31) MODEL (15) SN (15) CONTACT

^r^SS3IS93f

*****************************************************************

Attachment 12, Section A . 1. Calibration inventory form.

102
IHUEHTORY OF IHSlRtllEHIS FOR X RflY OR GflTtlfl RIIV CflLIBRRTIONS

LflSI EDII 1986:07:24:10:23:28 BV PJl

SEQ HO TST FLO IHST REC TST STfl HOTIFY SEHO HOTIFY TO COME IHSTRUMEHT BACKLOG POIHTS FOR J IHST TAKEN X D G n MODEL SH HPTS OOHE GPTSOOHE 1180 OEHE TOM BOB U 1 1 1 nil L.LU

SEO HO TST FLO IHST REf TST STI) HOTIFY SEHD HOTIFY TO COME IHSTRUMEHI BACKLOG u^L^LUU POINTS 1 Villi J RTLETT HUCLEHR 688 238671 YES FOR I IHST TAKEN X 0 G n MODEL SH XPTS DOHE GPTSDOHE 1180 OEHE TOM BOB OTT MAILED

OICIOREEH .025R CHW1BER 188 13151 0 0 10// / / 1 1 UKTOREEH .25R

" 130 13198 0 0 10// / / 1 1 UICTOREEH 2.5R

° 552 13192 0 0 2 0 / / / / I 1 UKTOREEH 25R

" 553 13106 0 0 2 0// / / 1 1 UICTOREEH lOOR

" 621 13370 0 0 2 0 / / / / 1 1 UICTOREEH R-flETER 570 3766 CONTACT RALPH JACOBS 301 871 2693 COHTACT RALPH JACOBS 617 716 6161 UKTOREEH R-«ETER 570 1026 CONTACT M OREUS 206 655 5015

HILL POUER CO

SEQ HO TST FLO IHST REC TST STH HOTIFY SEHD HOTIFY TO COME IHSTRUMEHI BACKLOG POIHTS

693 238786 FOR I IHST TAKEH X 0, 6 8 MODEL SH XPTS DOHE GPTS OOHE 1180 OEHE TOM BOB OTT HAILED

CONTACT flL B0LIH6ER 701 875 1971 X225

SEQ HO TST FLO IHST REC TST STA HOTIFY SEHD HOTIFY TO COHE IHSTRUHEHT BACKLOG POIHTS

TU ELECTRIC

CBPIHTEC CHHHBER

CAPIHTEC CHAMBER

CAPIHIEC CHAMBER

691 239173 MODEL SH PH-500 CII52.1911

PR-06C CII.61657 PH-30 CII30.5838

FOR I INST TAKEN X 0, G 8

XPTS OOHE GPTS OOHE 1180 OEHE TOM BOB OTT HAILED

0 0 10 ////// 0 0 10 I I I I I I

0 0 10 I I I I I I

SEQ HO TST FLO IHST REC TST STA HOTIFY SEHD HOTIFY TO COME IHSTRUMEHI BACKLOG POIHTS

HflUHL SURFACE UEAPOHS CTR

UICTOREEH 1R CHAMBER

UICTOREEH 25R CHAMBER

UICTOREEH .25R CHAMBER

UICTOREEH R-HETER

698 CHTRCT YES MODEL SH

227 12787

70-5 51 130 59

570 3838

FOR I IHST TAKEH

XPTS OOHE GPTSOOHE 1180 OEHE TOH

0 0 10 III

0 0 10 III

0 0 10 III

COHTACT OAHIEL SAH 391 1566

X 0, G 11

OTT HAILED

/ /

/ /

/ /

EGae IDAHO

HQH TEST

SEQ HO TST FLD IHST REC TST STB

699 239390 YES DOHE MODEL SH

NOTIFY SEHO NOTIFY TO COME IHSTRUMEHI BACKLOG POIHTS

FOR I IHST TAKEN X 0. G 11

XPTS DONE GPTSDOHE 1180 OEHE TOM BOB OTT MAILED

0 0 0 0 I I I I I I

COHTACT OR CARLSON FTS 583 2113

UHIUERSITY OF UISCOHSIH

EXRADIH CHAMBER

SEQ HO TST FLO INST REC TST STA HOTIFY SEHD HOTIFY TO COME IHSTRUMEHI BACKLOG POIHTS

706 239527 FOR I IHST TAKEN X 0, 6 11

MODEL SH XPTS OOHE GPTS OOHE 1180 OEHE TOM BOB OTT MAILED

R2 119 0 0 0 0 ////// COHTACT STEUE 60ETSCH 608 262 2170

Attachment 13, Section 4.1. Typical inventory of instruments for

x-ray or gamma-ray calibrations,

1986 July 24. (Excerpt)

103
1986:07:24: 10:23:28PJL

BACKLOG BACKLOG

SEQ NO. X-RAY POINTS GAMMA POINTS

0 0

BALTIMORE GAS & ELECTRIC 710 0 16

BARTLETT NUCLEAR 688 0 0

EG&G IDAHO 699 0 14

KODAK 713 0 21

MILL POUER CO 693 0 8

NAUAL SURFACE WEAPONS CTR 698 0 11 POINT BEACH 712 0 20

PRINCETON UNIVERSITY 709 0 14

TU ELECTRIC 694 0 8

UNIVERSITY OF WISCONSIN 706 0 14

WISCONSIN ELECTRIC 711 0 18

NUMBER OWNERS 12

BACKLOG X-RAY POINTS 1 BACKLOG GAMMA POINTS 22

TOTAL NUMBER POINTS BACKLOGED 23

Attachment 13, continued.

104
PRIORITY U • Oir*NTM(NT or COMMf net n-METER ANO SIMILAR UNIT* MAKE

W «^ JL

MODEL SERIAL NOJ,

OWNER RECE 1 VEO REPORT NO.

,

G J* i / 0 ^

REPORT DATE

ADDRESS ' f TEST FOLDER

V

Cy*^l ytP'Cf

CONTACT! P^,o/ S^j^

TEL. HOt-^q - DATE ift' I/O. ^

OB REF COMMENTS t V-0

V-20 SWITCH SWITCH

V-40 till.

v-';o 10 SETTING 10 SETTING

v-60

v-80

v-100

vO-vlOO

CHAMBERS

RANGE

MODEL

SERIAL NO.

CORRECTION FACTOR ' BEAM DIAMETER (cm)

CS137 EQUILIBRIUM SHELL

ROOM

CORRECTION FACTOR

COflO ^^"^ DIAMETER (Cm)

EdUILIBRIUM SHELL

ROOM

NBS-199 (R*v. 3-84) USrOMU.MRK.rws s

8 §8

Z X X

8

Attachment 14, Section 4.1. Condenser-type instrument record (Side 1 and 2 of card)

.

105
Make Modal Sarlal Number Owner. City. Slala. and ZIP Coda Priority No

Pravloutly calibrated No

T««l Fold*f Numb«r Data Racalvad Report Number Report Data

Chantb«r Informatton Readout Infofmaliefl Wall collacting potanllal and polarity: Make Model Serial Number

Slant orlanl. to baam: [^'parpart. 0 parallal Switch Name Switch Setting

Idantltylng mark toward aourca: / . /

,

Range

Mode

CofDmantt! Sensitivity Input Bsckground

OVM Range

OB REF:

For Information, plaaaa contact NBS-200 U.S. DEPARTMENT OF COMMERCE

(Rov. 3-84) National Buraau of Standards

GUARDED-FIELD IONIZATION CHAMBER RECORD

Name Telephone Number

BQC

CALIBRATION

FACTOR

R/C

BOC

CALIBRATION

FACTOR

R/C

...

PRC-CALIBRATION TEST RESULTS

L-10 H-80 1. Initial loakaga, tamto ampa: 7.2 L-15 H-100

L-20 H-150

L-30 H-200 2. Quard-to-c«ntr«l - electrode leakage:

L-40 H-250

L-50 H-900

L-eo S-40 3. Equlllbretlon time. T - TJred. on) - T(.M9 V-flnel) <

L-100 S-75 M-20

M-30

M-40 4. Current tor current ratioa:

M-50

M-«0 ° HKJp^n Seeled M-100 M-150 6. FullV/HaHV Neg V/Poe V

(neg) (poe) «•"'» <»'••'

].oo (

M-200

M-300

H-10

H-1S

H-20 H-30

H-40

H-50

Attachment 15, Section 4.1. Current-type instrument record (Side 1 and 2 of card)

106
NBS 533. CALIBRATION SUMMARY

CHAMBER NAME: ELECTROMETER:

OWNER:

0.250 R CHAMBER

VICTOREEN R-METER

MODEL: 130

MODEL: 570

STEM PERPENDICULAR TO BEAM. THE 0.25 RNUMBER

PREVIOUS CALIB: YES

CHAMBER OPEN TO ATMOSPHERE WHEN TESTED

SERIAL NUMBER

SERIAL NUMBER

TIME: DO 8770/87

FACED THE SOURCE OF RADIATION.

3464

2990

15:50:23

CR DATE: 1987 NOV 10 RECEIVED: 1987 OCT 14

1 2 3 4 5 6 7 8 9 10 1

BEAM BEAM CALIB EXPOSURE LEAKAGE EXPOSED CORR TUBE CORR AL QUALITY SIZE DIST RATE READING FACTOR POT FACTOR HVL CODE (CM) (CM) (R/S) (R/S) (?5) W FS (KV) RATIO

137CS 62.3 300.0 2.0E-03 .000E+00 .00 50.0 • 1 .032 1 .0149

60CO 37.9 195.0 9.4E-04 .000E+00 .00 50.0 • 1 .017 1 . 0000

BQC REF. CR PAGES

137CS 1 1 2 60CO 5 5 6

3 LAST PREVIOUS FACTORS RESPECTIVE DATES AND r. FULLSCALE

CHAMBER DATA IN CDATA: MODEL RANGE(R) L CH DIM(CM) CT TO STEM(CM) STEM LENGTH(CM) OD(CM) EST LTR(?5)

130 .250 12.50 6.00 13.60 5.09 .00 ALL ZEROS USUALLY MEANS NO DATA FOR THAT POINT.

C.F. PREFIX INDICATES IF CHAMBER WALL WAS INCREASED AND POTENTIAL USED ON WALL IN RESPECT TO C. ELECTRODE:

( NO INCREASE,-!-) (» INCREASED,-)-) (+ NO INCREASE, -t-) (P INCREASED .-h) (- NO INCREASE,-) (N INCREASED,-)

COMMENTS: CR CHECKED BY

Attachment 16, Section 4.1, Calibration summary page (typical)

107
o»

a b. > — — o < 9 CM <« «0 00

®

CM CM <M CM <S rO

<o <o <o ^ <o <o

CM

Ui

a.5 g

MO

CD<

to

>-

®

— K)

Q 9 A

A

AA

^9 9 9 9 V 9 9 9

to K> K) K) «© in in lO in If) tf) <M fM (M CM <M <M 9 9 9 9 9 9

tf)

OO

^9

ooo>cor«.(o<<< ••loiototoo-aa,

CM CMN

CMCMCMCMCMCM9SO

MCMCMCMMCM + T + U U UJ »^®(MfOCMP«.lO«0

eoeoo>o>o>o>M«ocM m m m

mmmmtnineotoao • • • CO CO * to lO ^ K> ^ ^ X -J oeooooootcooo' 90>9999^^ O 0> O O 9 O uiZ

^9 ^9 ^9 ^9

^9 ^9 ^3

^) 4^ ^3 ^3 ^3 ^9 O ® O ® ® O

^" ^' ^' ^ ^'

J (O »-W'09»~'~<J)9 90)9999 *9 90>9999r^

COMCMf^m—

K

mm <o in inm UJ

CM CM CM CM CM CM 5 9 ^ r** 9 ^ o

r>> fx GO 00 1^ • lOfoioioioiomoo 9 CM

^9 *"'"'"'"'"'"

CM II

in in

m in in in

O»f^0O'-»OMKK

00 CO flO o> o> 0>

^> ^> ^> 999999CL<< <0<0«0«0<0<0 Q.CL

»- ??? 9 — '-'OrOfOUiUJUJ 999999r^(0(0 9999999^9 999999lOr^(0

«o r» '"'"'"'^'"'"fOflOK) 9 ^ ^ ^ ^ «- 0>0>0>0>0)(»l I i Ok o> o> 0> O) O) UI X -i

^3 I ^3 ^9 ^9 £ t u z

<0 C4 I I

00 CO

<0 <D

CM CM

CO

in «- •(MO

ro

—

CO »0 r* 9 —

(O 9 CO LJ UI 0> 0> O O 0> 0> < < 0> 9 0> £ £ I > > < < £ £ o o o

108
DG 8766/87

TFN G45520

1987 NOV 02

PAGE 1 OF 6

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

GAITHERSBURG, MD 20899

REPORT OF CALIBRATION

EXRADIN CHAMBER

MODEL A4 SERIAL NUMBER 136

MANUFACTURED BY EXRADIN INC.

LOCKPORT. IL 60441

SUBMITTED BY

RECEIVED AT NBS ON 1987 OCT 05

THE CALIBRATION FACTORS GIVEN IN THIS REPORT ARE QUOTIENTS OF THE

X- OR GAMMA-RAY EXPOSURE AND THE CHARGE GENERATED BY THAT RADIATION

IN THE IONIZATION CHAMBER. THE AVERAGE CHARGE USED TO COMPUTE THE

CALIBRATION FACTOR IS BASED ON MEASUREMENTS WITH THE WALL OF THE

IONIZATION CHAMBER AT THE STATED POLARITY AND POTENTIAL. LEAKAGE

CORRECTIONS WERE APPLIED IF NECESSARY. IF THE CHAMBER WAS OPEN TO THE

ATMOSPHERE THE MEASUREMENTS WERE NORMALIZED TO ONE STANDARD ATMOSPHERE

AND 22 DEGREES CELSIUS. USE OF THE CHAMBER AT OTHER PRESSURES AND

TEMPERATURES REQUIRES NORMALIZATION OF THE ION CURRENTS TO THESE

REFERENCE CONDITIONS. THE NORMALIZING FACTOR F IS COMPUTED FROM THE

FOLLOWING EXPRESSION:

F = (273.15 + T)/(295.15 H)

WHERE T IS THE TEMPERATURE IN DEGREES CELSIUS, AND

H IS THE PRESSURE EXPRESSED AS A FRACTION OF A STANDARD

ATMOSPHERE. (1 STANDARD ATMOSPHERE = 101.325 KILOPASCALS = 1013.25

MILLIBARS = 760 MILLIMETERS OF MERCURY)

THE EXPOSURE RATE AT THE CALIBRATION POSITION WAS MEASURED BY A

FREE-AIR IONIZATION CHAMBER FOR X RADIATION, AND BY GRAPHITE CAVITY

IONIZATION CHAMBERS FOR COBALT-60 AND CESIUM- 137 GAMMA RADIATION.

THE GAMMA-RAY EXPOSURE RATES WERE CORRECTED TO THE DATE OF CALIBRATION,

FROM PREVIOUSLY MEASURED VALUES, BY DECAY CORRECTIONS BASED ON

HALF-LIVES OF 5.27 AND 30.0 YEARS, FOR COBALT-60 AND CESIUM-137

RESPECTIVELY.

Attachment 18, Section 4.1. Cable-connected chamber Report of Calibration.

109
DG 8766/87

1987 NOV 02

PAGE 2 OF 6

THE OVERALL UNCERTAINTIES OF THE CALIBRATION DESCRIBED IN THIS

REPORT IS 1%, OF WHICH 0.7% IS ASSIGNED TO THE UNCERTAINTY IN THE

EXPOSURE RATE OF THE NBS BEAM. THE OVERALL UNCERTAINTY WAS FORMED BY

TAKING TWO TIMES THE QUADRATIC SUM OF THE STANDARD DEVIATIONS OF THE

MEAN FOR COMPONENT UNCERTAINTIES OBTAINED FROM REPLICATE DETERMINATIONS,

AND ASSUMED APPROXIMATIONS OF STANDARD DEVIATIONS FOR ALL OTHER

UNCERTAINTY COMPONENTS; IT IS CONSIDERED TO HAVE THE APPROXIMATE

SIGNIFICANCE OF A 95% CONFIDENCE LIMIT.

THE CALIBRATION FACTOR IS GIVEN TO FOUR DIGITS TO PREVENT

ROUNDING ERRORS UP TO 0.5 PERCENT WHEN THE FIRST DIGIT IS UNITY.

INFORMATION ON TECHNICAL ASPECTS OF THIS REPORT MAY BE OBTAINED

FROM P. J. LAMPERTI, RADIATION PHYSICS C214. NATIONAL BUREAU

OF STANDARDS, GAITHERSBURG , MD 20899, (301) 975-5591.

REPORT REVIEWED BY

REPORT APPROVED BY R. LOEVINGER

FOR THE DIRECTOR

BY

DAVID M. GILLIAM, DEPUTY CHIEF

IONIZING RADIATION DIVISION

CENTER FOR RADIATION RESEARCH

NATIONAL MEASUREMENT LABORATORY

Attachment 18, continued.

110
DG 8766/87

1987 NOV 02

PAGE 3 OF 6

NATIONAL BUREAU OF STANDARDS REPORT OF CALIBRATION

DEPARTMENT OF THE AIR FORCE

NEWARK AFS, OH 43055

»^i^^^»^*%1^1^•t^^^1^1^1(1^1^1^1H^1^1^1^

* SEE NOTE BELOW ON *

* EXPOSURE RATES * EXRADIN CHAMBER

MODEL A4 SERIAL NUMBER 136

OPEN TO THE ATMOSPHERE WHEN TESTED

WALL POTENTIAL WAS -500 VOLTS WITH RESPECT TO INNER ELECTRODE

BEAM HALF -VALUE LAYER

CODE AL CU

(MM) (MM)

137CS 10.8

60CO 14.9

CALIBRATION FACTOR

22 DEG C AND 1 ATM

*1.235E+08 R/C

*1.219E+08 R/C

DIST

(M)

1.95

1 . 50

BEAM

SIZE

(MM)

C406

C293

EXP

RATE

(R/S)

4.8E-03

1.6E-03

DURING CALIBRATION THE CAVITY WAS POSITIONED IN THE CENTER OF THE BEAM

WITH THE STEM PERPENDICULAR TO THE BEAM DIRECTION. THE WHITE LINE

FACED THE SOURCE OF RADIATION.

9.E-15 AMPERES WAS THE LEAKAGE CURRENT MEASURED BEFORE CALIBRATION.

1.002 WAS THE RATIO OF THE CURRENT MEASURED FOR FULL COLLECTION

POTENTIAL TO THE CURRENT FOR HALF COLLECTION POTENTIAL FOR A CURRENT

OF 2.5E-10 AMPERES. A DETAILED SATURATION STUDY WAS NOT CARRIED

OUT AND NO CORRECTION FOR LACK OF SATURATION WAS APPLIED TO THE DATA.

* THE CHAMBER WALL THICKNESS WAS INCREASED FOR THIS BEAM QUALITY BY

ADDITION OF THE SHELL SUPPLIED WITH THE CHAMBER.

NOTE: ON 1986 JAN 01 THE EXPOSURE RATES OF NBS EXPOSURE STANDARDS WERE

ADJUSTED. DETAILS ARE GIVEN AT THE END OF THE REPORT.

CHECKED BY

Attachment 18, continued.

111
DG 8766/87

1987 NOV 02

PAGE 4 OF 6

EXPLANATION OF CHAMBER CALIBRATION TABLE

THE BEAM CODE IDENTIFIES IMPORTANT BEAM PARAMETERS. FOR X RADIATION

THE LETTERS L, M, H, AND S STAND FOR LIGHT, MODERATE, HEAVY, AND

SPECIAL FILTRATION, RESPECTIVELY, AND THE NUMBER FOLLOWING THE LETTER

IS THE CONSTANT POTENTIAL ACROSS THE X-RAY TUBE. FOR GAMMA RADIATION

THE BEAM CODE IDENTIFIES THE RADIONUCLIDE.

THE HALF -VALUE LAYERS IN ALUMINUM AND IN COPPER HAVE BEEN DETERMINED

BY A FREE-AIR CHAMBER FOR X RADIATION. THE CU HVLS FOR 60CO AND 137CS

ARE CALCULATED. THE CALIBRATION FACTORS OR CORRECTION FACTORS ARE

LISTED IN ORDER OF INCREASING AL HVL.

THE CALIBRATION FACTOR OR CORRECTION FACTOR IS DEFINED ON THE FIRST

PAGE OF THIS REPORT. IF THE CHAMBER WAS OPEN TO THE ATMOSPHERE, THE

FACTOR HAS BEEN NORMALIZED TO THE TEMPERATURE AND PRESSURE SHOWN AT

THE TOP OF THE COLUMN. WHEN THE ENTRY IS A CALIBRATION FACTOR, IT

IS IN SPECIAL UNITS (ROENTGENS PER COULOMB). WHEN THE ENTRY IS A

CORRECTION FACTOR, IT IS DIMENSIONLESS . IF THE CORRECTION FACTOR

IS FOLLOWED BY A NUMBER, THE NUMBER GIVES THE APPROXIMATE PERCENT

OF FULL-SCALE ELECTROMETER READING AT WHICH THE CALIBRATION WAS

PERFORMED.

THE DISTANCE SHOWN IS THAT BETWEEN THE RADIATION SOURCE AND THE

DETECTOR CENTER OR THE REFERENCE LINE. FOR THIN-WINDOW CHAMBERS WITH

NO REFERENCE LINE, THE WINDOW SURFACE IS THE PLANE OF REFERENCE.

THE BEAM SIZE IS THE PERPENDICULAR DISTANCE FROM THE CENTER LINE

OF THE CALIBRATION BEAM TO THE 50 PERCENT INTENSITY LINE. FOR

CIRCULAR FIELDS THE LETTER C PRECEDES THE DIMENSION. FOR SQUARE

FIELDS THE LETTER S PRECEDES THE DIMENSION, AND THE CHAMBER AXIS IS

PERPENDICULAR TO A SIDE OF THE SQUARE. IF NO LETTER PRECEDES THE

DIMENSION, A SPECIAL FIELD WAS USED AND ITS DIMENSIONS ARE GIVEN

IN A NOTE AT THE BOTTOM OF THE TABLE.

THE EXPOSURE RATE AT WHICH THE CALIBRATION WAS PERFORMED IS GIVEN IN

THE LAST COLUMN. IF THE CHAMBER IS USED TO MEASURE AN EXPOSURE RATE

THAT IS SIGNIFICANTLY DIFFERENT FROM THAT USED FOR THE CALIBRATION IT

MAY BE NECESSARY TO CORRECT FOR RECOMBINATION LOSS.

THE EFFECTIVE ENERGY IS GIVEN ON THE NEXT PAGE OF THIS REPORT FOR

THOSE BEAMS FOR WHICH IT IS BELIEVED TO BE A MEANINGFUL

CHARACTERIZATION OF THE BEAM QUALITY. FOR GAMMA RADIATION THE

EFFECTIVE ENERGY IS THE PHOTON ENERGY.

FOR X RADIATION THE EFFECTIVE ENERGY IS COMPUTED FROM GOOD-GEOMETRY

COPPER ATTENUATION DATA. THE INITIAL SLOPE OF THE ATTENUATION CURVE

IS USED TO DETERMINE AN ATTENUATION COEFFICIENT, AND THE PHOTON ENERGY

ASSOCIATED WITH THIS COEFFICIENT IS THE "EFFECTIVE ENERGY".

THE ENERGY VS ATTENUATION-COEFFICIENT DATA USED FOR THIS PURPOSE ARE

TAKEN FROM J. H. HUBBELL, INT. J. APPL. RADIAT. ISOT. 33, 1269 (1982).

FOR BEAM CODES H50 TO H300, THE EFFECTIVE ENERGY IS WELL REPRESENTED

BY (EFFECTIVE ENERGY) = 0.861V - 6.1, WHERE V IS THE CONSTANT

POTENTIAL IN KILOVOLTS.

Attachment 18, continued.

112
DG 8766/87

1987 NOV 02

PAGE 5 OF 6

CONVENTIONAL CALIBRATION CONDITIONS FOR

X- AND-GAMMA RAY MEASURING INSTRUMENTS

BEAM ADDED FILTER HALF-VALUE HOMOGENEITY EFFECT. DIS- EXPOSURE RATE

CODE LAYER COEFFICIENT ENERGY TANCE MIN . MAX.

AL CU SN PB AL CU AL CU

(MM) (MM) (MM) (MM) (MM) (MM) (KEV) (CM) (MR/S) (R/S)

LIO 0 0 . 029 79 25 0 .001 1 . 7

L15 0 0 . 050 74 25 n nm 4

L20 0 0 . 071 76 50 0 .001 3 . 3

L30 0 . 265 0 .22 60 50 0 . 001 0 .4

L40 0 . 50 0 .49 57 50 0 . 001 0 . 4

L50 0 . 639 0 . 75 58 50 0 .001 0 .4

L80 1 . 284 1 . 83 58 50 0 .001 0 .4

LlOO 1 . 978 2 .8 59 50 0 .001 0 . 4

M20 0 . 230 0 . 152 79 50 0 .001 0 . 5 M30 0 . 50 0 . 36 64 50 0 .001 0,. 3

M40 0 .786 0 .73 66 50 0,. 001 0,, 4

M50 1 . 021 1 .02 0 . 032 66 62 50 0,.001 0,,4

M60 1 . 51 1 .68 0 . 052 68 64 0,. 8 0,. 2

MlOO 5 . 0 5 . 0 0 . 20 72 55 1 . 0 n , 3

M150 5 .0 0 .25 10 .2 0.67 87 62 1

.0 0,. 4

M200 4 . 1 1 . 12 14 . 9 1 .69 95 69 1

,0 0,. 3

M250 5 . 0 3 . 2 18 . 5 3.2 98 86 1

. 0 0

2

M300 4 .0 6.5 22 5 . 3 100 97 0., 5 0., 08

HIO 0 . 105 0 . 148 89 25 0.,001 0.,003

H15 0 .500 0 . 152 • 87 25 0.,001 0. 003

H20 1 .021 0 .36 88 50 0.,001 0. 003

H30 4 . 13 1 .23 0.038 93 94 50 0,,001 0. 003

H40 4 .05 0 .26 2 . 9 0.093 94 95 50 0. 001 0. 003

H50 4 .0 0 . 10 4 .2 0. 142 92 90 38 0. 3 0. 065

H60 4 .0 0 .61 6 .0 0.24 94 89 46 0. 02 0. 005

HlOO 4 .0 5 . 2 13 . 5 1 . 14 100 94 80 0. 005 0. 002

H150 4,.0 4 . 0 1 .51 17,.0 2.5 100 95 120 0. 03 0. 010

H200 4 .0 0 .60 4. 16 0 .77 19,.8 4. 1 100 99 166 0. 02 0. 006

H250 4,.0 0 .60 1.04 2 .72 22 5.2 100 98 211 0. 03 0. 005

H300 4,. 1 3.0 5 .0 23 6.2 99 98 252 0. 04 0. 003

S75 1. 504 1.86 63 50 0. 001 0. 4

S60 4,.0 2,, 8 0.089 75 70 0. 3 0. 06

137CS 10.8 662 1

5 0. 1 60CO 14. 9 1250 1

5 2. 5

FOR THE X-RAY BEAM CODES, THE LETTER INDICATES LIGHT, MODERATE, HEAVY, AND

SPECIAL FILTRATION, AND THE NUMBER IS THE CONSTANT POTENTIAL IN KILOVOLTS.

THE INHERENT FILTRATION IS APPROXIMATELY

1.0 MM BE FOR BEAM CODES LIO-LIOO, M20-M50, H10-H40, AND S75 ; AND

3.0 MM BE FOR BEAM CODES M60-M300, H50-H300, AND S60

THE HALF-VALUE LAYERS FOR 137CS AND 60CO ARE CALCULATED.

THE HOMOGENEITY COEFFICIENT IS TAKEN AS 100(1ST HVL/2ND HVL)

Attachment 18, continued.

113
DG 8766/87

1987 NOV 02

PAGE 6 OF 6

ATTACHMENT CONCERNING ADJUSTMENT OF NBS EXPOSURE RATES.

EXPOSURE RATES WERE ADJUSTED AT NBS ON 1986 JAN 01 TO TAKE INTO

ACCOUNT RECENT REFINEMENTS IN SOME OF THE PHYSICAL PARAMETERS.

EXPOSURE RATES IN ALL REPORTS ISSUED AFTER 1986 JAN 01 ARE LOWER THAN

THE EXPOSURE RATES IN REPORTS ISSUED BEFORE THAT DATE. THE CHANGES

ARE AS FOLLOWS:

NBS PRIMARY STANDARD:

RADIATION

CAVITY CHAMBERS

60CO 137CS 192IR

FREE-AIR CHAMBERS

1251 AND X RAYS

HUMIDITY

ENERGY-ABSORP. COEF . RATIO

STOPPING-POWER RATIO

-0.3

-0.1

-0.7

-0.3

0.0

-0.5

-0.3

0.0

-0.4

-0.2

N.A.

N.A.

TOTAL CHANGE -1 . 1 -0.8 -0.7 -0.2

LABORATORIES WITH INSTRUMENTS CALIBRATED PRIOR TO 1986 JAN 01 CAN

BRING THEIR INSTRUMENTS INTO AGREEMENT WITH THE NEW EXPOSURE RATES BY

REDUCING THE EARLIER CALIBRATION FACTORS BY THE ABOVE TOTAL CHANGES

IN PERCENT.

HUMIDITY. BEFORE 1986 NO CORRECTION WAS MADE FOR THE EFFECT OF WATER

VAPOR IN THE AIR, SINCE EARLY NBS WORK HAD INDICATED A NEGLIGIBLE

EFFECT. MORE RECENT WORK AT SEVERAL NATIONAL LABORATORIES HAS

DETERMINED THIS EFFECT WITH GOOD ACCURACY. THE VALUES USED ARE

TAKEN FROM ICRU REPORT 31, PAGE 31, FIGURE 5.14. THESE FACTORS ARE

APPLIED TO THE EXPOSURE RATE DETERMINED BY THE NBS PRIMARY STANDARD.

NO CORRECTION IS MADE FOR THE EFFECT OF WATER VAPOR ON THE INSTRUMENT

BEING CALIBRATED. SINCE IT IS ASSUMED THAT BOTH THE CALIBRATION AND

THE USE OF THAT INSTRUMENT TAKE PI.ACE IN AIR WITH A RELATIVE HUMIDITY

BETWEEN 10% AND 70%, WHERE THE HUMIDITY CORRECTION IS NEARLY CONSTANT.

ENERGY-ABSORPTION COEFFICIENT RATIO. THE MOST RECENT VALUES

(J.H. HUBBELL, RAD. RES. 70, 58, 1977, AND INT. J. APPL. RADIAT. ISOT.

33, 1269, 1982) WERE COMPARED WITH EARLIER VALUES USED IN ESTABLISHING

NBS EXPOSURE STANDARDS (J.H. HUBBELL, NAT. STAND. REF . DATA SER. 29,

NAT. BUR. STAND. (U.S.), 1969). ONLY ONE APPRECIABLE CHANGE WAS FOUND.

STOPPING-POWER RATIO. THE MOST RECENT VALUES (ICRU REPORT 37, 1984)

WERE COMPARED WITH THE EARLIER VALUES USED IN ESTABLISHING NBS

EXPOSURE STANDARDS (M.J. BERGER AND S.M. SELTZER, NASA SP-3012, 1964).

THE COMPARISONS FOR 60CO GAMMA RADIATION WAS MADE BY THE STAFF OF THE

BUREAU INTERNATIONAL DES POIDS ET MESURES , AND CONFIRMED AT NBS. THE

COMPARISON FOR THE OTHER RADIONUCLIDES WERE MADE AT NBS.

THE LAST PREVIOUS CHANGE IN NBS EXPOSURE STANDARDS WAS MADE ON 1972

MAY 01. AT THAT TIME THE EXPOSURE RATES FOR COBALT-60 AND CESIUM-137

GAMMA RAYS WERE LOWERED BY 0.7% AND 0.6% RESPECTIVELY, DUE TO

ADOPTION AS AN NBS EXPOSURE STANDARD OF SIX SPHERICAL GRAPHITE CAVITY

IONIZATION CHAMBERS (LOFTUS, T.P., AND WEAVER, J.T., J. RES. NAT. BUR.

STAND., 78A, 465, 1974).

Attachment 18, continued.

114
DG 8770/87

TFN NA

1987 NOV 10

PAGE 1 OF 9

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

GAITHERSBURG . MD 20899

REPORT OF CALIBRATION

VICTOREEN R-METER

MODEL 570 SERIAL NUMBER 2990

MANUFACTURED BY VICTOREEN INSTRUMENT CO.

CLEVELAND, OH 44104

SUBMITTED BY BROOKHAVEN NATIONAL LABORATORY

UPTON, NY 11973

RECEIVED AT NBS ON 1987 OCT 14

THE CORRECTION FACTORS GIVEN IN THE TABLE OF CALIBRATION DATA

ARE QUOTIENTS OF THE X- OR GAMMA-RAY EXPOSURE AND THE ELECTROMETER

READING WITH A GIVEN IONIZATION CHAMBER. IF THE CHAMBER WAS OPEN TO

THE ATMOSPHERE, THE READINGS WERE NORMALIZED TO ONE STANDARD

ATMOSPHERE AND 22 DEGREES CELSIUS. USE OF THE CHAMBER AT OTHER

PRESSURES AND TEMPERATURES REQUIRES NORMALIZATION OF THE READINGS TO

THESE REFERENCE CONDITIONS. THE NORMALIZING FACTOR F IS COMPUTED

FROM THE FOLLOWING EXPRESSION:

F = (273.15 + T)/(295.15 H)

WHERE T IS THE TEMPERATURE IN DEGREES CELSIUS, AND

H IS THE PRESSURE EXPRESSED AS A FRACTION OF A STANDARD

ATMOSPHERE. (1 STANDARD ATMOSPHERE = 101.325 KILOPASCALS = 1013.25

MILLIBARS = 760 MILLIMETERS OF MERCURY)

THE EXPOSURE RATE AT THE CALIBRATION POSITION WAS MEASURED BY A

FREE-AIR IONIZATION CHAMBER FOR X RADIATION, AND BY GRAPHITE CAVITY

IONIZATION CHAMBERS FOR COBALT-60 AND CESIUM- 137 GAMMA RADIATION.

THE GAMMA-RAY EXPOSURE RATES WERE CORRECTED TO THE DATE OF CALIBRATION,

FROM PREVIOUSLY MEASURED VALUES, BY DECAY CORRECTIONS BASED ON

HALF-LIVES OF 5.27 AND 30.0 YEARS, FOR COBALT-60 AND CESIUM-137

RESPECTIVELY.

Attachment 19, Section 4.1. Condenser-chamber Report of Calibration

115
DG 8770/87

1987 NOV 10

PAGE 2 OF 9

THE OVERALL UNCERTAINTIES OF THE CALIBRATION DESCRIBED IN THIS

REPORT IS 1.5%, OF WHICH 0.7% IS ASSIGNED TO THE UNCERTAINTY IN THE

EXPOSURE RATE OF THE NBS BEAM. THE OVERALL UNCERTAINTY WAS FORMED BY

TAKING TWO TIMES THE QUADRATIC SUM OF THE STANDARD DEVIATIONS OF THE

MEAN FOR COMPONENT UNCERTAINTIES OBTAINED FROM REPLICATE DETERMINATIONS,

AND ASSUMED APPROXIMATIONS OF STANDARD DEVIATIONS FOR ALL OTHER

UNCERTAINTY COMPONENTS; IT IS CONSIDERED TO HAVE THE APPROXIMATE

SIGNIFICANCE OF A 95% CONFIDENCE LIMIT.

IF SUITABLE CONNECTIONS WERE ACCESSIBLE IN THE ELECTROMETER,

THE DIFFERENCES IN POTENTIAL BETWEEN CERTAIN SCALE READINGS WERE

DETERMINED AND ARE GIVEN IN THE ELECTROMETER CALIBRATION TABLE.

THE CHARGE-READ PROCEDURE FOR VICTOREEN R-METERS, FOR THIS

CALIBRATION, WAS AS FOLLOWS:

1. INSERT THIMBLE CHAMBER AND ADJUST TO ZERO READING.

2. UNLOCK CHAMBER AND REMOVE TO THE POSITION WHERE THE LOCKING

PIN IS AT THE EDGE OF THE CHARGING SOCKET. NOTE ANY DEFLECTION

OF ELECTROMETER READING FROM ZERO FOR THIS REFERENCE POSITION.

3. FOLLOWING EXPOSURE INSERT CHAMBER TO THE REFERENCE POSITION,

IF NECESSARY ADJUST FIBER TO DEFLECTION NOTED, THEN INSERT

CHAMBER FULLY TO DETERMINE READING.

INFORMATION ON TECHNICAL ASPECTS OF THIS REPORT MAY BE OBTAINED

FROM P. J. LAMPERTI, RADIATION PHYSICS C214, NATIONAL BUREAU

OF STANDARDS, GAITHERSBURG , MD 20899, (301) 975-5591.

REPORT REVIEWED BY

REPORT APPROVED BY R. LOEVINGER

FOR THE DIRECTOR

BY

DAVID M. GILLIAM, DEPUTY CHIEF

IONIZING RADIATION DIVISION

CENTER FOR RADIATION RESEARCH

NATIONAL MEASUREMENT LABORATORY

Attachment 19, continued.

116
DG 8770/87

1987 NOV 10

PAGE 3 OF 9

NATIONAL BUREAU OF STANDARDS REPORT OF CALIBRATION

BROOKHAVEN NATIONAL LABORATORY

UPTON, NY 11973

ELECTROMETER MODEL

ELECTROMETER

READING

N

(% FS)

20

40

50

60

80

100

SERIAL NUMBER

POTENTIAL

DIFFERENCE

V(0)-V(N)

(V)

49.0

97.6

122.0

146.6

195.4

245.0

RELATIVE

SENSITIVITY

S(N)/S(50)

1.004

1.000

1.000

1.001

1.001

1.004

533 V = V(0) WAS THE POTENTIAL MEASURED FOR AN ELECTROMETER READING AT

N = 0. THE VALUE OF V(0) GENERALLY SHOWS A LARGER VARIATION THAN

V(0)-V(N). LONG-TERM MEASUREMENTS ON SIMILAR INSTRUMENTS INDICATE THAT

THE PERCENT STANDARD DEVIATION OF V(0)-V(N) SHOULD NOT EXCEED 1.0%

THE ELECTROMETER SENSITIVITY IS S(N) = (V(0)-V(N))/N.

IF THE RELATIVE SENSITIVITY S(N)/S(50) DIFFERS SIGNIFICANTLY FROM

UNITY, THE ELECTROMETER SCALE IS NON-LINEAR. IN THIS CASE, THE

ELECTROMETER READING SHOULD BE MULTIPLIED BY THE APPROPRIATE RELATIVE

SENSITIVITY.

IF THE CORRECTION FACTOR (COLUMN 4 OF THE CHAMBER CALIBRATION TABLE)

WAS DETERMINED FOR A SCALE READING OTHER THAN 50%, THE RELATIVE

SENSITIVITY SHOULD BE RENORMALIZED TO UNITY FOR THAT SCALE READING

BEFORE CORRECTING FOR SCALE NON-LINEARITY.

CHECKED BY

Attachment 19, continued.

117
DG 8770/87

1987 DEC 03

PAGE 4 OF 9

NATIONAL BUREAU OF STANDARDS REPORT OF CALIBRATION

BROOKHAVEN NATIONAL LABORATORY

UPTON, NY 11973

VICTOREEN 0.25R CHAMBER

MODEL 130 SERIAL NUMBER 3464

OPEN TO THE ATMOSPHERE WHEN TESTED

ELECTROMETER MODEL 570 SERIAL NUMBER

BEAM HALF-VALUE LAYER CORRECTION FACTOR DIST BEAM EXP

CODE AL CU 22 DEG C AND 1 ATM SIZE RATE

(MM) (MM) CF % FS (M) (MM) (R/S)

137CS 10.8 *1.03 50 3.00 C623 2.0E-03

137CS 10.8 1.01 50 3.00 C623 2.0E-03

60CO 14.9 *1 .02 50 1 . 95 C379 9.4E-04

* SEE NOTE BELOW ON * * EXPOSURE RATES *

2990

DURING CALIBRATION THE CAVITY WAS POSITIONED IN THE CENTER OF THE BEAM

WITH THE STEM PERPENDICULAR TO THE BEAM DIRECTION. THE RANGE NUMBER

FACED THE SOURCE OF RADIATION.

* THE CHAMBER WALL THICKNESS WAS INCREASED FOR THIS BEAM QUALITY BY

ADDITION OF THE SHELL SUPPLIED WITH THE CHAMBER.

NOTE: ON 1986 JAN 01 THE EXPOSURE RATES OF NBS EXPOSURE STANDARDS WERE

ADJUSTED. DETAILS ARE GIVEN AT THE END OF THE REPORT.

CHECKED BY

Pages 5 and 6 have the same form as page 4.

Pages 7 to 9 are the same as pages 4 to 6 of attachment 1^.

Attachment 19, continued.

118
e

DATABOOK 828

o>r<

ror

0Q05J 00

>$u> 'r\e.K

181

[HOS

^5". 8 8 -^R

t3 oo.o-^^

<X> 33 "2.

OOC>SC?

Exp. T /9V. /9/

^or<-<,s) Pa Soa 5 k; 8<f V

:3 <Ci'»-\

Co m

37. H

1^3^.J

:i7.5 /a 9./ 75/. J

Attachment 20, Section 4.1. Typical page in databook for TLD irradiation.

119
DG 8782/87 Page 1 of 3 DB 874:032

1987 Nov 18

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

GAITHERSBURG , MD 20899

REPORT OF IRRADIATION

Irradiation of 15 Landauer TLD Badges

Submitted by

Irradiated on 1987 November 17.

The TLD badges were supported on a plastic frame and held such that the

radiation was incident normal to their front surfaces. The badges were

positioned at a distance of 3.00 m from a cesium-137 gamma-ray source. The exposure rate was 2.00 niR/s . The overall uncertainty of the

irradiations described in this Report is 1%, of which 0.7% is assigned to

the uncertainty in the exposure rate of the NBS beams. The overall

uncertainty was formed by taking two times the quadratic sum of the

standard deviations of the mean for component uncertainties obtained from

replicate determinations, and assuimed approximations of standard

deviations for all other uncertainty components; it is considered to

have the approximate significance of a 95% confidence limit.

Details of the irradiations are given in the attached table.

Information on technical aspects of this report may be obtained from P. J. Lamperti, Radiation Physics C214, National Bureau of Standards,

Gaithersburg, MD 20899, (301) 975-5591.

Irradiation performed by E. L. Bright

Irradiation supervised by P. J. Lamperti

Report approved by R. Loevinger

For the Director

by

David M. Gilliam, Deputy Chief

Ionizing Radiation Division

Center for Radiation Research

National Measurement Laboratory

Attachment 21, Section 4.1. TLD Report of Irradiation

120
DG 8782/87

1987 Nov 18

Page 2 of 3

NATIONAL BUREAU OF STANDARDS REPORT OF IRRADIATION

Submitted by

Badge

Numbers* Exposure

(mR)

00171 150

00157 150

00226 150

00334 150

00090 150

00057 275

00086 275

00204 275

00117 275

00291 275

00266 475

00065 475

00251 475

00078 475

00261 475

There were 2 control badges: serial numbers 00135 and 00103.

Irradiations performed between 0839 h and 0851 h.

*The badges are labeled with several groups of numbers. The numbers

given in the table are sufficient for purposes of identification.

Checked by

Page 3 is the same as page 6 of Attachment 18.

Attachment 21, continued.

121
DG 8775/87

TFN N/A

DB 819:172-173

1987 Nov 10

Page 1 of 2

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

GAITHERSBURG , MD 20899

REPORT OF TEST

Keithley Digital Dosimeter

Model 35614 Serial Number 20368

Manufactured by Keithley Instrument Company

Cleveland, OH 44137

Submitted by

Received at NBS on 1987 November 10

The referenced electrometer has been tested for use with the ionization

chamber covered by Report of Calibration DG 8776/87. The system was tested for the following combination of switch positions:

Switch Position

Function 10" ^C

Range 20 V

Current Suppress Off

When the electrometer measurements were corrected by Kq , where

' Kq = 1.002,

measurements with the system using Beam Code ^°Co were found to be

consistent with the calibration factor in DG 8776/87. Kg was determined

by injecting a known charge into the electrometer input and observing the

corresponding change in the charge reading. The value of the injected

charge is believed to have an uncertainty of less than 0.1%.

Attachment 22, Section 4.5. Electrometer Report of Test.

122
DG 8775/87

1987 Nov 10

Page 2 of 2

The exposure in air at the reference point of the ionization chamber,

with the chamber replaced by air, is given by

X=KqFQN=1.002FQN

where

N is the chamber calibration factor in terms of exposure per unit

charge, for the stated conditions of calibration;

Q is the change in charge on the electrometer system as indicated by

the digital panel-meter readings, using the tested combination of switch

positions; and

F is a factor that normalizes the measurements to the temperature and

pressure reference conditions for N. F is defined in Report of

Calibration DG 8776/87.

Information on technical aspects of this report may be obtained from P. J. Lamperti, Radiation Physics C214, National Bureau of Standards,

Gaithersburg, MD 20899, (301) 975-5591.

Measurements performed by E. L. Bright

Measurements supervised by P. J. Lamperti

Report approved by R. Loevinger

For the Director

by

David M. Gilliam, Deputy Chief

Ionizing Radiation Division

Center For Radiation Research

National Measurement Laboratory

Attachment 22, continued.

123
mS-114A (REV. 2.8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

PUBLICATION OR

REPORT NO.

NBS/SP-250/16

2. Performing Organ. Report No 3. Publication Date March 1988

4. TITLE AND SUBTITLE

NBS MEASUREMENT SERVICES: CALIBRATION OF X-RAY AND GAMMA-RAY MEASURING INSTRUMENTS

5. AUTHOR(S)

Paul G. Lamperti , Thomas P. Loftus, Robert Loevinger

6. PERFORMING ORGANIZATION (If joint or other than NBS, see /n struct/on

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

Gaithersburg, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

SAME AS ITEM //6 ABOVE,

10. SUPPLEMENTARY KOTES

Library of Congress Catalog Card Number: 88-600515

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information, if document includes a significant

bi bHograpt\y or literature survey, mention it here)

The calibration and irradiation of x-ray and gamma-ray instruments are performed in terms of the physical quantity exposure. The calibrations are

listed in NBS Special Publication 250 as calibrations 46010C through 46050S

(formerly 8.3A through 8.3M). A calibration or correction factor is provided

for radiation detectors, charge sensitivity of a high-gain electrometer is

tested, and passive dosimeters are given known exposures. Calibration is

performed by comparing the instrument against an NBS primary standard of

exposure, which is a free-air chamber for x rays and a cavity ionization

chamber for cesium-137 and cobalt-60 gamma rays, A variety of quality

assurance checks are performed to assure the constancy of the standards and

the accuracy of the calibrations and irradiations. The overall uncertainty

(considered to have the approximate significance of a 95% confidence limit) is

given as 0.7% for exposure rate in the NBS beams, 1% for calibration of a cable-connected chamber and irradiation of passive dosimeters, and 1.5% for

calibration of a condenser chamber.

12. KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

calibration; cavity chamber; cesium-137 gamma rays; cobalt-60 gamma rays;

exposure; free-air chamber; ionization chambers; primary standard; standard;

uncertainty estimate: x rays. _____

13. AVAILABILITY

Unlimited

I I For Official Distribution, Do Not Release to NTIS

[Y] Order From Superintendent of Documents, U.S. Government Printing Off ice, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

137

15. Price

USCOMM-DC 6043-PeO

^.U.S.GOUERNmENT PRINTING orF ICE : 1938-201 -597 : 82552


THE SP 250 SERIES ON NBS MEASUREMENT SERVICES*

SP 250-1 Spectral Radiance Calibrations 003-003-02792 $3.50

SP 250-2 Far Ultraviolet Detector Standards 003-003-02810-0 $425

SP 250-3 Radiometric Standards in the Vacuun

Ultraviolet 003-003-02806-1 $6.50

SP 250-4 Fricke Dosimetry in Hi^Energy Electron Beams

003-003-02816-9 $2.75

SP 250-5 Alpha-Partde Callt)ratlons SN003-003-0283-1 $2.00

SP 250-6 Regular Spectral Transmlttanoe 003-003-02805-3 $3.25

SP 250-7 Radiance Temperature Calltxations 003-003-02827^ $225

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SP 250-9 Calllxation of Beta-Partlde-Emlttlng

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SP 250-15 Photometric Calibrations SN003-003-02835-5 $4.25

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Measuring instruments

SP 250-17 The NBS Photodetector Spectral Response

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SP 250-18 Neutron Source Strength Calibrations

SP 250-19 Calibration of Gamma-Ray-Emitting

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SP 250-20 Spectral irradlance Calibrations 003-003-02829-1 $5.50

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* Those entries containing a stocl< numtter (003-003 ) and price can be purchased from the Superintendent of Documents, U.S. Govemnrwnt Printing Office, Washington, DC 20402. GPO will accept checks, money orders, VISA, and Mastercharge. For more Infomiation, or to place an order, call (202) 783-3238. Be sure to use the stock number in all orders. Titles without stock numbers are in preparatkxi.
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