Calibration of X-Ray and nist_^^

Gamma-Ray Measuring Publication

Instruments 250-58

Paul J. LampertI

Michelle O'Brien
fhe National Institute of Standards and Technology was established in 1988 by Congress to "assist industry in the development of technology . . . needed to improve product quality, to modernize manufacturing processes, to ensure product reliability . . . and to facilitate rapid commercialization ... of products based on new scientific discoveries." NIST, originally founded as the National Bureau of Standards in 1901, works to strengthen U.S. industry's

competitiveness; advance science and engineering; and improve public health, safety, and the environment. One

of the agency's basic functions is to develop, maintain, and retain custody of the national standards of measurement, and provide the means and methods for comparing standards used in science, engineering,

manufacturing, commerce, industry, and education with the standards adopted or recognized by the Federal Government.

As an agency of the U.S. Commerce Department's Technology Administration, NIST conducts basic and

applied research in the physical sciences and engineering, and develops measurement techniques, test methods, standards, and related services. The Institute does generic and precompetitive work on new and

advanced technologies. NIST's research facilities are located at Gaithersburg, MD 20899, and at Boulder, CO 80303.

Major technical operating units and their principal activities are listed below. For more information contact the

Publications and Program Inquiries Desk, 301-975-3058.

Office of the Director

• National Quality Program

• International and Academic Affairs Technology Services

• standards Services

• Technology Partnerships

• Measurement Services

• Information Services

Advanced Technology Program

• Economic Assessment

• Information Technology and Applications

• Chemistry and Life Sciences

• Materials and Manufactunng Technology

• Electronics and Photonics Technology

Manufacturing Extension Partnership

Program

• Regional Programs

• National Programs

• Program Development

Electronics and Electrical Engineering

Laboratory

• Microelectronics

• Law Enforcement Standards

• Electricity

• Semiconductor Electronics

• Radio-Frequency Technology^

• Electromagnetic Technology^

• Optoelectronics^

Materials Science and Engineering

Laboratory

• Intelligent Processing of Materials

• Ceramics

• Materials Reliability^

• Polymers

• Metallurgy

• NIST Center for Neutron Research

Chemical Science and Technology

Laboratory

• Biotechnology

• Physical and Chemical Properties^

• Analytical Chemistry

• Process Measurements

• Surface and Microanalysis Science

Physics Laboratory

• Electron and Optical Physics

• Atomic Physics

• Optical Technology

• Ionizing Radiation

• Time and Frequency^

• Quantum Physics'

Manufacturing Engineering

Laboratory

• Precision Engineering

• Manufacturing Metrology

• Intelligent Systems

• Fabrication Technology

• Manufacturing Systems Integration

Building and Fire Research

Laboratory

• Applied Economics

• Structures

• Building Materials

• Building Environment

• Fire Safety Engineering

• Fire Science

Information Technology Laboratory

• Mathematical and Computational Sciences^

• Advanced Network Technologies

• Computer Security

• Information Access

• High Performance Systems and Services

• Distributed Computing and Information Services

• Software Diagnostics and Conformance Testing

• Statistical Engineering

'At Boulder, CO 80303. ^Some elements at Boulder, CO.
NIST Special Publication 250-58

NIST MEASUREMENT SERVICES:

Calibration of X-Ray and Gamma-Ray

IVIeasuring Instruments

Paul J. Lamperti

Michelle O'Brien

Ionizing Radiation Division

Physics Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-8460

Supersedes NIST Special Publication 250-16 (March 1988)

April 2001

U.S. Department of Commerce

Donald L. Evans, Secretary

Technology Administration

Dr. Karen H. Brown, Acting Under Secretary of Commerce for Technology

National Institute of Standards and Technology

Dr. Karen H. Brown, Acting Director
Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply

recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended

to imply that the entities, materials, or equipment are necessarily the best available for the purpose.

Abstract

The calibration and irradiation of instruments tliat measure x-rays and gammarays are performed in terms of tlie piiysical quantity air-kerma. The calibrations are listed in the NIST Special Publication 250 Calibration Service Users Guide as

calibrations 4601 OC through 46050S. The process for establishing calibration or

correction factors for radiation detectors, charge sensitivity tests for high-gain

electrometers, and irradiations of passive dosimeters is explained in this

document. Calibrations are performed by comparing the instrument to a NIST

primary standard, which include four free-air chambers for x-rays and cavity

ionization chambers for cesium-137 (^^'^Cs) and colbalt-60 (^°Co) gamma rays.

Details of the design and use of these standards are presented in this document,

as well as, a description of the quality assurance checks, which are performed to

assure the constancy of the standards and the accuracy of the calibrations and

irradiations. The overall uncertainty with a significance of a 95 % confidence limit

is given as 0.9 % for the air-kerma rate in the NIST beams, 1 % for the

calibration of a cable-connected chamber and irradiation of passive dosimeters,

and 1 .5 % for the calibration of a condenser chamber.

National institute of Standards and Technology Special Publication 250-58

Natl. Inst. Stand. Technol. Spec. Publ. 250-58, 98 pages (April 2001)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2001

For sale by the Superintendent of Documents, U.S. Government Printing Office

Internet: bookstore.gpo.gov — Phone: (202) 5 1 2- 1 800 —Fax- (202) 512-2250

Mail: Stop SSOP, Washington, DC 20402-0001
Foreword

This edition of the National Institute of Standards and Technology (NIST) Special Publication

250 Series is a revision to the NBS Special Publication 250-16 Calibration of X-Ray and

Gamma-Ray Measuring Instruments document. The original work has been reorganized,

rewritten and supplemented to reflect the changes with the calibration services since 1988. Paul

Lamperti and Michelle O'Brien wish to acknowledge the contributions of R. Loevinger, J. Weaver and the late T. Loftus.

iii
Abstract

Table of Contents

Page

ii

Foreword iii

List of tables iv

List of figures vii

1.0 Scope 1 2.0 References documents 1 2.1 International organization for standardization

2.2 National Institute for Standards and Technology

3.0 Terminology

3.1 Descriptions of terms specific to this document

3.2 Keywords 2

4.0 Calibration service f , : 3

4.1 Description of services 3

4.2 Beam qualities for calibration of x-ray

measuring instruments 4

5.0 Design philosophy and theory 13

6.0 X-ray calibration services 13

6.1 X-ray calibration ranges 13

6.2 X-ray exposure calibration standards 13

6.3 Calculation of air kerma 19

6.4 Wyckoff-Attix (50 kV to 300 kV) free-air

ionization chamber corrections 20

6.5 Ritz (20 kV to 100 kV) free-air ionization

chamber corrections 24

6.6 Lamperti (10 kV to 20 kV) free-air ionization

chamber corrections 27

6.7 Attix (20 kV to 50 kV) free-air ionization

chamber corrections 29

6.8 Comparison of standard free-air ionization

chambers 31

7.0 Gamma-ray air kerma standards and calibration ranges 35

7.1 Cavity-chamber standards 35

7.2 Gamma-ray sources 36

7.3 Calibration of gamma-ray beams 37

7.4 Useful beam size 39

8.0 lonization-chamber current-measurement techniques 41

8.1 Background and history 41

8.2 Electrometers 42

iv
8.3 Ionization current measurement-console equipment 42

8.3.1 X-ray calibration data acquisition system 42

8.3.2 Gamma-ray calibration data acquisition system 45

8.4 Pre-calibration tests of instruments 46

8.4.1 Charger/reader linearity test 46

8.4.2 Testing of readers for cable-connected probes 47

8.4.3 Testing of instruments to be calibrated in

electrical units 47

9.0 Support equipment calibrations 47

9. 1 Capacitor calibrations 47

9.2 Temperature indicator calibrations 47

9.3 Pressure indicator calibrations \^ 48

10.0 Operating procedures 48

10.1 Administrative procedures " 48

10.2 Calibration of integrating-type instruments 49

10.2.1 Condenser chambers 49

10.2.2 Cable-connected instruments 51

10.3 Calibration of current-type instruments 52

10.3.1 X-ray calibrations 52

10.3.2 Gamma-ray calibrations 52

10.4 In-house calibration checks 52

10.5 Test of high-quality feedback electrometers 53

10.6 Irradiation of passive and electronic dosimeters 54

10.6.1 General considerations and procedures 54

10.6.2 Exposure techniques 54

11.0 Assessment of uncertainty 54

12.0 Safety considerations 58

12.1 Radiation safety 58

12.2 High-voltage safety 59

References 60

Attachment 1 : X-ray beam qualities offered at the NBS prior to 1986 63

Attachment 2: Additional references - databooks 65

Attachment 3 : Attix chamber schematics 67

Attachment 4: History of the measurement of current 70

Attachment 5: Typical ionization chamber calibration report 75

Attachment 6: Typical TLD calibration report 81

Attachment 7: Typical electrometer calibration report 84

v
List of Tables

Table 4- 1 . Calibration conditions for x-ray and gamma-ray measuring

instruments

Table 4-2. Mammography x-ray beam-quality parameters

Table 4-3. ISO X-Ray Beam Quality Parameters

Table 6- 1 . Description of the NIST x-ray systems

Table 6-2. Important dimensions and parameters for the NIST standard

free-air ionization chambers

6

10

12

14

14

Table 6-3. Summary of calculations from ref [3] for percent loss and gain

of ionization due to lack of plate separation and scattered photons

in the Wyckoff-Attix free-air ionization chamber

Table 6-4. Comparison of x-ray beam filtrations used for data reported in

ref. [3] with filtrations presently used for conventional

calibration conditions

20

21

Table 6-5. Data used to compute corrections for the Wyckoff-Attix standard

free-air chamber for conventional calibration conditions 22

Table 6-6. Comparison of the ka values currently used to those values

computed from the least squares equation

Table 6-7. Products of all air-kerma-rate-independent corrections for the

Wyckoff-Attix free-air ionization chamber

Table 6-8. Mass air-attenuation coefficients and air attenuation corrections

for the Ritz free-air ionization chamber

23

24

25

Table 6-9. Computation of electron-loss corrections for the Ritz free-air

ionization chamber 26

Table 6-10. Summary of corrections for the Ritz free-air ionization

chamber 27

Table 6-11.

Table 6-12.

Recombination corrections for the Lamperti free-air ionization chamber 28

Summary of correction factors for Lamperti free-air

ionization chamber 29

VI
Table 6-13. Correction factors for the Attix free-air ionization

chamber 30

Table 6-14. Results of the comparison of the Lamperti standard with the

BIPM standard 32

Table 6-15. Results of the comparison of the Ritz standard with the BIPM

standard 32

Table 6-16. Comparison of the Lamperti chamber to the 50 kV NFL

standard 32

Table 6-17. Comparison oi the Ritz chamber to the NFL 50 kV

standard 33

Table 6-18. Comparison of the Ritz chamber to the NFL 300 kV

Standard 33

Table 6-19. Comparison results for the mammography standard 33

Table 6-20. Recent "in-house" comparisons standards 34

Table 7-1. Dimensions of spherical graphite ionization chambers 35

Table 7-2. Gamma-ray source locations and nominal activities of

sources as oi January 1 , 1 999 36

Table 7-3. Constants for computing exposure rate corrected for decay

C jI 1 j11

for the horizontal beam sources 37

Table 7-4. Data and positioning for calibration of vertical-beam

gamma-ray sources in room B036 38

Table 7-5. Constants for computing useful beam radii for the horizontal

and vertical beam calibration ranges 40

Table 7-6. Beam size defined by 50% radiographic density

contour for vertical-beam gamma-ray sources, for specific

collimator settings 41

Table 8-1. Parameters of the straight line fits for the temperature probe

calibrations used in the x-ray ranges 44

Table 8-2. Low-energy Fantak unit voltage calibration 44

vii
Table 8-3. High-energy Pantak unit voltage calibration 45

Table 8-4. Temperature probe parameters for the gamma source

calibration ranges 46

Table 10-1. Analysis of three NIST check chambers for the period

of 1979 to 1997 53

Table 11-1. Uncertainty analysis for tungsten x and gamma-ray

air-kerma rates 55

Table 11-2. Uncertainty analysis for gamma-ray calibrations 56

Table 11-3. Uncertainty analysis for air-kerma rates with the Attix

chamber 56

Table 1 1-4. Uncertainty analysis for mammography calibrations 57

Table 11-5. Explanation of various components of the uncertainty

analyses 57

Table 11.6. Summary of expanded uncertainties 58

viii
List of Figures

Fig. 4-1 . Graph of generating constant potential against half-value layer for

the beam qualities used in the x-ray calibration range 7

Fig. 4-2. Graph of homogeneity coefficients against aluminum half-value

layer for the L, M, H and S filtrations 8

Fig. 4-3. Graph of the homogeneity coefficient against the half-value layer

measured in copper for the M, H and S beam qualities 9

Fig. 4-4. Graph of the HVL's of the mammography beam qualities versus

constant potential 1

1

Fig. 6-1. Schematic of 300 kV x-ray range 15

Fig. 6-2. Schematic of mammography calibration range 15

Fig. 6-3. Schematic of Wyckoff-Atix free-air ionization chamber 16

Fig. 6-4. Sectional view of the Ritz free-air chamber 16

Fig. 6-5. Schematic cross-sectional views of the Lamperti free-air

chamber 17

Fig. 6-6. Schematic of Attix free-air chamber 18

Fig. 7-1. Port detail for horizontal ^°Co, 5 mm diameter source 36

ix

1. Scope

1.1 This document describes the x-ray and gamma-ray calibration services provided by the

Ionizing Radiation Division, Radiation Interactions and Dosimetry Group at the National

Institute of Standards and Technology (NIST). The calibration and irradiation of x-ray and

gamma-ray instruments are performed in terms of the physical quantity air kerma. The

calibrations are listed in NIST Special Publication 250 [1] as calibrations 460 IOC through

46050S. Calibration or correction factors are provided for radiation detectors, charge sensitivity

is tested for high-gain electrometers, and passive dosimeters are irradiated to known quantities of

radiation. Calibrations are performed by comparing the instrument to a NIST primary standard,

which include free-air chambers for x-rays and cavity ionization chambers for cesium- 137

('^^Cs) and cobalt-60 (^Vo) 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 0.9 % for the air-kerma rate in the NIST beams, 1 % for calibration of a cable-connected chamber and irradiation of passive dosimeters, and 1.5 % for calibration of a condenser chamber.

2.0 Referenced documents

2.1 International Organization for Standardization

ISO/IS 4037-1:1996 X and gamma reference radiations for calibrating dosimeters and dose rate meters and for determining their responses as a function of photon energy - Part 1 . : Radiation

characteristics and production methods

2.2 National Institute of Standards and Technology

NBS Special Publication 250-16 Calibration of X-Ray and Gamma Measuring Instruments

SP 250 NIST Calibration Services Users Guide 1998

NBS Handbook 64 Design of Free-Air Ionization Chambers

NBS Handbook 78 Report of the International Commission on Radiological Units and

Measurements

NIST Special Publication 811 Guide for the Use of the International System of Units (SI)

NIST Technical Note 1297 Guidelines for Evaluating and Expressing the Uncertainty of NIST

Measurements

3. Terminology

3.1 Descriptions of terms specific to this document

3.1.1 air kerma - the quotient of dE,^ by dm, where dE,,. 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 air in that

volume element. The SI unit of air kerma is the gray (Gy).

1
3. 1 .2 beam quality - used to refer to a specific x-ray beam with a characteristic half-value layer

and produced by a constant potential kilovoltage.

3.1.3 calibration - the process whereby the response of a dosimeter or measuring instrument is characterized through comparison with an appropriate national standard.

3. 1 .4 calibration factor - the quotient of the air kerma in the absence of the chamber and the

charge generated by that radiation in the ionization chamber, expressed in units of Gy/C.

3.1.5 correction factor - the quotient of the air kerma or exposure in the absence of the chamber

and the electrometer reading with the ionization chamber.

3.1.6 effective energy - the energy of monoenergetic x-ray beam which has the same half-value

layer as the spectrum in question.

3. 1 .7 exposure - 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 are completely stopped

in air. The SI unit of exposure is the coulomb per kilogram (C/kg); the special unit of exposure,

the roentgen (R), is equal to exactly 2.58E-4 C/kg.

3.1.8 half-value layer - (HVL) the thickness of the specified material added as a beam

attenuator that reduces the air kerma rate by one half of the unattenuated beam air kerma rate value.

3.1.9 homogeneity coefficient - (HC) the ratio of the first to the second half-value layer.

3.1.10 monitor instrument - an instrument used to monitor the stability of the air kerma rate during an irradiation.

3.1.11 quarter-value layer - (QVL) the thickness of the specified material added as a beam

attenuator that reduces the air kerma rate to one quarter of the unattenuated beam air kerma rate value.

3.1.12 second half-value layer - the difference between the quarter-value layer and the halfvalue layer.

3.1.13 x-ray unit - system comprising of a high voltage generator, an x-ray tube and an x-ray

controller.

3.2 Key words

air kerma; calibration; cavity chamber; cesium- 137 gamma rays; cobalt-60 gamma rays;

exposure; free-air chamber; half-value layer; ionization chambers; mammography chamber

calibrations; primary standard; standard; uncertainty estimate; x rays.

2
4. Calibration service

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

measuring instruments, as of January 1999. The physical quantities air-kerma and exposure,

calibration by substitution, and the properties of the x-ray and gamma-ray beams used in the

NIST services are described in this document. This service guide explains the details of the

calibration systems and their measurement standards, the instruments and techniques for

measuring current and charge, and the analyses of calibration data. Tests applied to the

instruments being calibrated, as well as the tests and calibrations performed on NIST laboratory

equipment, and the associated assessment of uncertainty are covered. The operating procedures,

including the administrative procedures and methods of handling and calibrating the instruments,

and the irradiation of passive dosimeters, are included in this guide. The document concludes

with a brief discussion of the necessary safety considerations.

4.1 Description of service

The NIST, Ionizing Radiation Division, Radiation Interactions and Dosimetry Group

receives a variety of instruments for calibration, test, or irradiation in x- or gamma-ray beams.

These services are assigned test numbers 460 IOC through 46050S in NIST Publication 250 [1]

available from the NIST Technology Services, Office of Measurement Services, Calibration

Program Office. 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. Dosimeters can be given known exposures of x-ray or gamma-ray radiation. Calibrations and irradiations are performed in terms of the physical

quantities air kerma and exposure.

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

energy transferred to any material. For the calibration service described in this document,

consideration is limited to photon beams in air. Air kerma is the total energy per unit mass

transferred from an x-ray beam to air. Air kerma, A^^,^, is the quotient of dE^^ by dm, where dE,^ 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 air in that volume element. Then

K.- —- (4-1) air J y' ' J 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 quantity exposure characterizes an x-ray or gamma-ray beam in terms of the electric

charge liberated through the ionization of air. Exposure is defined as the total charge per unit mass liberated in air by a photon beam and is represented by the equation:

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

3
stopped in air. The SI unit of exposure is the coulomb per kilogram (C/kg); the special unit of

exposure, the roentgen (R), is equal to exactly 2.58E-4 C/kg. The ionization arising from the

absorption of bremsstrahlung emitted by the secondary electrons is not 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 relationship between exposure and air kerma can be

expressed as a simple equation:

X-K...-^^^ air (4-3) 2.58^-4 {WIe)

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

of the initial kinetic energy of secondary electrons dissipated in air through radiative processes.

The currently accepted value of W/e is 33.97 J/C [2] and g is negligible for the x-ray beams of

interest. The currently accepted g values for ^°Co and '^^Cs beams are 0.32 % and 0.16 %,

respectively.

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 perform stability checks involving redundant measurements in highly

reproducible radiation fields before sending their instruments to NIST, and to repeat those

checks after NIST calibration, and again at suitable intervals. Instruments submitted for

calibration, and material submitted for irradiation, must be shipped in reusable containers.

An x-ray tube produces bremsstrahlung spectra, 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 a high purity metal to reduce the

unwanted low-energy x-rays. Three x-ray calibration ranges are used for the calibration

services. Two of the ranges contain x-ray tubes with tungsten anodes. The x-ray beams from

these anodes are filtered with aluminum, copper, tin or lead. Two anode types are offered for the

mammography calibration service, molybdenum (Mo) and rhodium (Rh). The Mo generated x- ray beams are filtered with Mo, Al, or Rh foils, while the Rh beams are filtered with Rh and Al

foils. 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 air kerma rate to 50 % and to 25 % of its original value. These thicknesses are called the half-value layer (HVL) and the quarter-value

layer (QVL). The HVL and QVL measurements must be made using good-geometry attenuation

in order to obtain accurate and reproducible numbers. The homogeneity coefficient (HC) is the

ratio of the first to the second HVL, often expressed as a percent. A HC value near 1 (or 100 %)

indicates that the filtration has produced an approximately homogeneous beam that is approaching monoenergetic conditions.

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

The 32 NIST tungsten x-ray beam qualities are divided into three groups according to

filtrafion, i.e., light (L), moderate (M), and heavy (H) filtradon. The beam codes consist of a

letter L, M, or H, followed by the generating constant potential in kilovolts. For example. Ml00

indicates moderate filtration and 100 kV constant potential. The special (S) series beam codes,

S60 and S75 have characteristics that are not consistent with those of the L, M, and H groups.

4
The qualities for each group were chosen so that relatively smooth curves result for the graph of

tube potential versus HVL. Table 4-1 gives a complete listing of beam codes currently available

at NIST. The NBS beam codes used prior to January 1986 are listed in Attachment 1 . Recent

changes to filtration, due to the x-ray tube replacement, are shown in footnote g. Due to changes

in the physical setup of the low-energy calibration facility, the LI 0, LI 5, HI 0 and HI 5 beam

qualities are now used with a source-to-detector distance of 50 cm, instead of the previous 25 cm

distance. The resulting changes to the half-value layers are given in Table 4-1. Depending on the

energy response and design of the ionization chamber, the calibration factors for a specific

ionization chamber often fall on smooth curves when plotted against HVL In this case, 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 will occur, hence no attempt should be made

to interpolate between such calibration factors. Figure 4-1 is a graph of the HVL against the

generating constant potential. Figure 4-2 is a graph of the HC against the HVL, measured in

aluminum. Figure 4-3 shows the HVL and HC measured in copper against the generating

constant potential.

The mammographic beam qualities offered at NIST were chosen to cover the range of

HVLs of x-ray beams found in clinical settings. The beam codes which name the beam qualities are a combination of the chemical symbol of the anode and the filter respectively, followed by

the constant potential in kilovolts. The letter "x" ends the beam codes which name the exit beam

qualities. The exit beam qualities, which represent the transmission of the x-rays through the

breast, are generated by an additional filtration of 2.0 mm of Al. The mammographic beam

qualities offered are listed in Table 4-2. Figure 4-4 is the graph of the HVL measured in

aluminum against generating constant potential.

The ISO x-ray beam qualities are now available at NIST. A list of all ISO beams offered

at NIST is found in Table 4-3. The NIST H group of qualities agrees with the ISO narrow

spectrum (NS) qualities recommended by the ISO document 4037, referred to in section 2 . The

ISO recommendations extend from 300 kV to 40 kV, below which the NIST H group has been

extended to 1 0 kV in agreement with practice at the national metrology institute of Germany,

Physikalisch-Technische Bundesanstalt (PTB). The NIST M group of qualities is in agreement

with the recommendation for radiation therapy calibration in lEC Publication 73 1

.

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 instrumentation, 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 of radiation therapy instruments. The L qualities are predominately 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. The Mo and Rh

beam qualities are offered to simulate the clinical mammographic beams.

5
Table 4- 1 . Calibration conditions for x-ray and gamma-ray measuring instruments

Beam

code'

Additional Filteration Half-value layer Homogeneity

coefficient

Effective

energy''

Distance

Al Cu Sn Pb Al Cu Al Cu (keV) (cm;

(mm) (mm) (mm) (mm) (mm) (mm)

X-Rav Beam Qualities

LIO^ 0.035 89 50

L15' 0.057 68 50

L20 0.069 73 50

L30^ 0.30 0.22 63 50

L40® 0.53 0.50 59 50

L50^ 0.71 0.76 60 50

L80^ 1.45 1.83 57 50

LlOO 1.98 2.77 57 50

M20^ 0.27 0.15 69 50

M30 0.5 0.36 65 50

M40^ 0.89 0.73 69 50

M50^ 1.07 1.02 66 50

M60 1.56 1.68 66

M80 2.61 2.97 67

MlOO 5.0 5.02 73

M120 6.87 6.79 77

M150 5.0 0.25 10.2 0.67 87 62

M200 4.1 1.12 14.9 1.69 95 69

M250 5.0 3.2 18.5 3.2 98 86

M300 4.0 6 5 22.0 5.3 100 97

HIO'^ 0.105 0.05 91 50

0.5 0.153 86 50

H20^ 1.01 0.36 91 50

H30^ 4.50 1.23 93 50

H40^ 4.53 0.26 2.90 90 50

H50 4.0 0.1 4.2 0.142 92 90 38

H60 4.0 0.61 6.0 0.24 94 89 46

HlOO 4.0 5.2 13.5 1.14 100 94 80

H150 4.0 4.0 1 '51 1 7 0 1 \J\J 9'^ 1 ?0

H200 4.0 0.6 4.16 0.77 19.8 4.1 100 99 166

H250 4.0 0.6 1.04 2.72 22.0 5.2 100 98 211

H300 4.1 3.0 5.0 23.0 6.2 99 98 252

S60 4.35 2.77 72

S75 1.50 1.86 63 50

Gamma-Ray Beam Qualities '"Cs 10.8

14.9

662

1250

a For the x-ray beam codes, the letter indicates light (L), moderate (M), heavy (H) and special filtration (S), and the number

is the constant potential in kilovolts. b The additonal filtration value does not include the inherent filtration. The inherent filtration is approximately 1.0 mm Be

for beam codes LIO-LIOO, M20-M50, H10-H40 and S75; and 7.0 mm Be for beam codes M60-M300, H50-H300 and S60

plus the filtration resulting from the transmission monitors,

c The homogeneity coefficient is taken as 100(lst HVL / 2nd HVL).

d The effective energy is for which it is believed to be a meaningful characterization of the beam quality. e The distance shown is that between the radiation source and the detector center or the reference line. For the beam codes where no distance is listed, the distance ranges between 78 and 200 cm, depending on the desirable beam size, f Changes to the HVL from the 1986 value, result from a change in the distance from 25 cm to 50 cm.

g To match the previous HVL, changes in the additional filtration value were required with the installation of a new x-ray tube in 1997.

6
Fig. 4-1 Graph of generating constant potential against half-value layer for the beam qualities

used in the x-ray calibration range. The curves for half-value layers measured in copper and

aluminum are shown for the light (L), moderate (M), heavy (H) and special (S) filtered beam

qualities. The lines connecting the data points are for ease of viewing.

7
1 I MINI

X s

0.01 0.10 1.00 10.00 100.00

Half-Value Layer (mm Al)

Fig. 4-2. Graph of homogeneity coefficients against aluminum half-value layer for the L, M, H

and S filtrations. The lines connecting the data points are for ease of veiwing.

8
50 I \ ^ I I I I I 1 1 \ \ I Min i \ \

I I I I I I 0.01 0.10 1.00 10.00

Half-Value Layer (mm Cu)

Fig. 4-3. Graph of the homogeneity coefficient against the half-value layer measured in copper

for the M, H and S beam qualities. The lines connecting the data points are for ease of viewing.

9
Table 4-2. Mammography x-ray beam-quality parameters

Beam Tube Additional Half-Value Homogeneity

Code' Voltage Filtration'' Layer'' Coefficient

(kVp) (mm) (mm Al) (Al)

Mo Anode

Mo/Mo23 23 0.032 Mo 0.271 70

Mo/Mo25 25 0.032 Mo 0.296 72

Mo/Mo28 28 0.032 Mo 0.332 74

Mo/Mo30 30 0.032 Mo 0.351 75

Mo/Mo35 35 0.032 Mo 0.392 78

Mo/Rh28 28 0.029 Rh 0.408 80

Mo/Rh32 32 0.029 Rh 0.445 82

Mo/Mo25x 25 0.030 Mo + 2.0 Al 0.566 91

Mo/Mo28x 28 0.030 Mo+ 2.0 Al 0.626 96

Mo/Mo30x 30 0.030 Mo+ 2.0 Al 0.660 95

Mo/Mo35x 35 0.030 Mo+ 2.0 Al 0.748 90

Rh Anode

Rh/Rh25 25 0.029 Rh 0.351 76

Rh/Rh30 30 0.029 Rh 0.438 81

Rh/Rh35 35 0.029 Rh 0.512 86

Rh/Rh40 40 0.029 Rh 0.559 90

Rh/Rh30x 30 0.029 Rh+ 2.0 Al 0.814 96

Rh/Rh35x 35 0.029 Rh+ 2.0 Al 0.898 95

" The beam codes are a combination of the chemical symbol of the anode and the filter respectively, followed by the constant potential in kilovolts. The letter "x" ends the beam

codes which denote "exit" beams. The exit beam qualities, which are intended to represent

the transmission of the x-rays through the breast, are generated by an additional filtration

of 2.0 mm of aluminum.

The inherent filtration is 1 mm Be for all qualities plus the filtration due to the

transmission monitor.

" The half-value layers listed were determined through direct measurements with the

primary standard free-air ionization chamber at a distance of Im.

10
1.0

0.2 I ^ L_ \ \ \ 1 \ 1 \ 1 20 25 30 35 40 45

Constant Potential (kV)

Fig. 4-4. Graph of the HVL's of the mammography beam quahties versus constant potential.

11
Table 4-3. ISO X-Ray Beam Quality Parameters

Beam Added Filtration (mm)" First HVL Second HVL

Code' Al Cu Sn Pb mmAl mmC mmAl mmCu

HKIO 0.04 0 OS HK20 0 1 s U. 1 J 0.13 0 16 HK30 0.39 0 SQ

HK60 'X 1 Q 0.08 0U. 1 1 1 1 HKIOO ^ on 0 1 s 0.31 0 46

HK200 lis 1.72 9 4"? HK250 1 fiO - 2.52 J.J/

HK280 3.45 4 07

HK300 9 S 1 3.46 4 91 WS60 0U.J1 0.18 0U.Z.911 WS80 0W.JS 0.35 0 44

WSllO 9 0 0.96 WS150 11 .U03J 1.88 9 1 3 WS200 9 01 3.09 J3 .J35J WS250 4 01 4.30 4T^. JUSO

WS300 f\ S4 5.23 J.JO S 18

NSIO n HQS 0.051 u. uuu NS15 0\}.'-ty AQ 0.15 U. 1 o NS20 ft QO 0.32

NS25 0.69 NS30 1.16 1 NS40 0 91

' 0.085 0 0Q9

NS60 - 0 0.25 0 96

NS80 9 0 0.59 0 66

NSlOO S 0 1.13 1 IQ

NS120 4*^ .yy QO 11 .ut- 04 1.70 1 8S

NS150 9 JUSO 2.40 NS200 9 04 9 Q8 4.09

NS250 9^.u011 9^.yQ71 5.26 S "^9 NS300 9 QQ 4 qq 6.17 6 30

LKIO 0 30 0.062

LK20 0.43 LK30 0 1 X 1.48 LK35 0 9S 2.16 9 ^f^ LK55 11.1!/ 1 q 0.26 LK70 9 64 0.51 LKIOO (j.dI 1.0 1.27

1.0 4.0 9 04

LK170 1.0 3.0 1.5 3.47 LK210 0.5 2.0 3.5 4.54

LK240 0.5 2.0 5.5 5.26

In the beam codes, the letters indicate low air kerma rate (LK), high air kerma rate (HK), narrow spectrum (NS), and wide spectrum (WS); and the number is the constant

potential in kilovolts. The inherent filtration is a combination of the filtration due to the monitor chamber plus

approximately 1 .0 mm Be for beam codes produced at tube potentials of 30 kV and

below, approximately 7.0 mm Be for HK60 and HKIOO and for all other techniques the inherent filtration is adjusted to 4 mm Al.
5. Design philosophy and theory

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

kerma or air-kerma 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 and the

response of the instrument is determined. The calibration factor is the quotient of the air kerma,

i^air in the absence of the chamber, and the charge, Q, generated by that radiation in the

ionization chamber:

Kir

Calibration Factor = —^ (5-1)

Q

The correction factor is the quotient of the air kerma or exposure, in the absence of the chamber,

and the electrometer reading with the ionization chamber:

Kir Correction Factor = — (5-2a)

electrometer reading

Correction Factor = X

(5-2b)

electrometer 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 7.

6. X-ray calibration services

6.1 X-ray ranges

Three x-ray calibration ranges are available for instrument calibrations. One range is used

for tungsten x-rays generated at constant potentials of 1 0 kV to 1 00 kV, the other is used for

tungsten x-rays generated at potentials of 50 kV to 300 kV. The mammography x-rays are generated at constant potentials of 23 kV to 40 kV. Table 6-1 describes some of the features of

the x-ray systems utilized in each calibration range. A schematic of the 300-kV system is given

in Fig. 6-1. A schematic of the mammography range is given in Fig. 6-2.

6.2 X-ray air kerma calibration standards

Standardization of x-ray beams for the quantity air kerma or exposure is carried out at NIST by

means of four free-air ionization chamber standards. These standards are discussed in Refs.

[3,4,5]. Figures 6-3 to 6-5 show cross-sectional views of the three free-air chambers used in the

tungsten x-ray ranges. Figure 6-6 is a schematic of the standard for mammography. The

important dimensions of the free-air chambers are given in Table 6-2. With reference to Table

6-2, note that the free-air chambers become larger as the x-ray energy increases. At low x-ray

energies, the air attenuation air path between the defining diaphragm and the collector center

needs to be minimized. On the other hand, at high x-ray energies a sufficient air path is needed

13
Table 6-1. Description of the NIST x-ray systems.

Features Unipolar^ Bipolar'' Mammography

Manufacturer Pantak Pantak Gulmay

Output voltage (kV) 5 to 100 5 to 320 5 to 50

Output current (mA) 0.5 to 80 0.5 to 30 0.1 to 40

Output power (kW) up to 3.2 up to 4.2 up to 1.2

kV adjustment (kV) 0.1 0.1 0.1

mA adjustment (mA) 0.01 0.01 0.1

Fixed anode material W W Mo or Rh

Tube window (mm Be) 1 3 1

Focal spot size (mm) 3x3 4x4 ^^ X 4 " and 5 X 5

''Generator and tube were installed in December of 1997.

''Generator was installed in February of 1998 with the existing x-ray tube.

"Two x-ray tubes are used in the mammography calibration range with the same generator.

'^Size of the Mo anode in tube installed in 1994.

^Size of the Rh anode in tube installed in 1994.

Table 6-2. Important dimensions and parameters for the NIST standard free-air ionization

chambers

Chamber X-ray tube

potential

(kV)

Plate

separation

(mm)

Plate

height

(mm)

Collector

length

(mm)

Diaphragm

diameter/ID

(mm)

Air

absorption

length

(mm)

Electric

field strength

(V/cm)

Lamperti 10-60 40 50 10.146 4.994/5S 39.02 750

Attix 10-50 variable

209 max

87=' variable 10.00/ 1 Ou 212.7" variable

Ritz 20- 100 90 90 70.030 10.00/lOA 127.39 55

Wyckof

f- Attix

50 - 300 200 268 100.80 10.00/1 OB 308 250

^ Inner diameter of the cylindrical chamber.

'' This is variable; the value shown is what is used for routine use.

14
Telescope

Lead and

steel housing

I

Safety shutter Beam aperture

Test chamber

Monitor chamber

X-ray beam

~ cehTerrme

Beam defining

filter

X-ray tube Timing shutter

Movable

support carriage

Fig. 6-1. Schematic of 300 kV x-ray range.

Collimating

shutter

apparatus

Filter wheel

Translating

x-ray tube

mount

Mo anode

Rh anode

Free-air

ionization

chamber

i

Test chamber

carriage

Free-air

ionization

chamber

X-ray beam

centerline

Generator

Beam

aperture

wheel

Test chamber

] ^

moimts

Translating

support cart

Alignment

laser

Alignment

telescope

Fig. 6-2. Schematic of mammography calibration range.

15
HV plate Guard wires 1 cm spacings

Diaphragm

X-ray

beam

Guard plate Steel box lead lined

Collector plate

Fig. 6-3. Schematic of Wyckoff-Atix free-air ionization chamber.

to electrometer or ground

I I I

Guard

plate

Diaphragm

X-ray

beam

Collecting plate Guard

strips

Steel box

lead lined High voltage plate -Spring contact n

Fig. 6-4. Sectional view of the Ritz free-air chamber.
Sectional Side Elevation

1---

11 -

Fig. 6-5. Schematic cross-sectional views of the Lamperti free-air chamber.

1. Lead to measuring system, 2. lead to thermistor readout, 3. thermistor, 4. brass collector plate,

5. brass guard plate, 6. brass guard ring, 7. tungsten diaphragm, 8. brass high voltage plate, 9. supporting insulator, 10. brass ground case, 11. high voltage lead, 12. lead to midpoint of

potential divider, 13. brass radiation shield, A. shows orientation for the second diagram, section

A-A.

17
Beam

defining

aperture

Signal

Collecting electrode

Exit

Moveable Plate

Guard

Sensitive collecting volume

Entrance

HV bias

ushrod

Stepping motor slide 2

Fig. 6-6. Schematic of Attix free-air chamber.

to compensate for the downstream loss of electrons and eliminate electrons from the diaphragm.

In addition, at the higher energies, the plate separation is made larger to allow for complete or nearly complete energy loss for electrons with paths predominately perpendicular to the x-ray

beam direction.

Although strenuous efforts were made by the designers of the free-air chambers to realize

experimentally the quantity exposure, various corrections are still required. A brief description

of the four corrections follow. The area of the beam and the exposure rate are defined at the

position of the free-air ionization chamber 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, k^, of the x-rays in the air path between the diaphram

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. The photon scatter correction, k^, must be made for the ionization that

results fi-om photons scattered out of the diaphragm-defined beam. This correction is geometry

dependent and needs to be evaluated for each defining aperture. Some energetic electrons leave

the collection volume before expending all their energy in the form of ionization. Electron loss

corrections, k^, are not required for the Lamperti or the Attix chamber due to the short range of

the low energy photons measured with these chambers. The energy dependent electron loss

correction must be applied to the Ritz and the Wyckoff-Attix free-air ionization chambers. Since

some of the ions recombine before being collected, the air-kerma rate dependent recombination

correction, k^, is required.

18
6.3 Calculation of air kerma

For a free-air ionization chamber with volume V, the air-kerma rate is determined by the relation

K-^ — T^^i (6-1)

"air ciair

where

K ^ the air-kerma rate (Gy/s),

I

y

= the mass ionization current measured by the standard (Cs"' kg"'),

PairV = the volume of the standard (m^),

p^,y = the density of air at the ambient conditions of temperature and pressure (kg m ), PT

Pair = Po— (6-2)

p„ = the density of air at reference conditions, 1.293 x 10'^ (kg m"^)

T = the ambient temperature (K)

P = the ambient pressure (Pa)

T„ = the reference temperature (K)

P„ = the reference pressure (Pa),

is the mean energy expended by an electron of charge e to produce an ion pair e

in dry air; the value used at NIST is 33.97 JC"', = is the fraction of the initial electron energy lost by radiative processes lost in air; g

is negligible for the x-ray beams of interest and 0.32 % and 0.16 % for the ^°Co

and '^''Cs respectively,

nk; = the product of all the necessary non-dimensional corrections.

The largest correction at low energies is that due to the air attenuation of the x-ray fluence along

the air path length, L between the reference plane and the center of the collecting volume. The

correction factor is calculated using the following equation:

= exp(^I) = exp(-^pl) (6-3)

P

where— = the mass air-attenuation coefficient, experimentally determined (m^kg"')

P'

p = the density of air at the ambient conditions of temperature and pressure (kg m"^)

and

L = the air absorption length or air path length (m).

19
6.4 Wyckoff-Attix (50 kV to 300 kV) free-air ionization chamber corrections

The correction for electron loss is K

= \ - ILA^jm (6-4)

and the correction for scattered photon contribution is

k^ - \ - (6-5)

In equation (6-4), E,. is the percent electron contribution, a loss, beyond a radius r of an x-ray

beam, 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, and photon contribution, k^, are based on data from Ref. [3] and

the work of Ritz [6]. One should refer to Ref. [3] if calculations are to be made. A summary of

the results of the calculations discussed in Ref [3] are found in Table 6-3 which lists the percent

loss and gain of ionization due to lack of plate separation and scattered photons in the WyckoffAttix chamber. The beam code designations from Table 4- 1 are used in Table 6-3 to associate the

data derived from Ref [3] with present-day x-ray beam conditions. The corrections for gain of

ionization are calculated from a plot of corrections derived from data in Ref [3] versus Al HVL.

The corrections for electron loss are calculated from data in Ref [3] generated for beam

conditions very nearly the same as present-day conditions, (see Table 6-4). It is assumed that the

differences are not significant. The exceptions to this assumption include the H300, M80 and

Ml20. The correction for electron loss is estimated by interpolation.

Table 6-3. Summary of calculations from ref [3] for percent loss and gain of ionization due to lack of plate separation and scattered photons in the Wyckoff-Attix free-air ionization chamber

Beam Code S Ap (%) I Ap S, (%)

M60,H60 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

HlOO -0.08 0.56

H150 -0.77 0.41

H200 -0.81 0.39

H250 -0.23 0.36

H300^ 0.33

^The 300-kV filtration of 4 mm Cu is not comparable with the H300 composite filtration.

20
Table 6-4. Comparison of x-ray beam filtrations used for data reported in ref [3] with titrations

presently used for conventional calibration conditions

Reference [3] X-rayNBS SP-250

tube Inherent

potential filter Pb

(kV) (mm) (mm)

Added Filter Sn Cu

(mm) (mm)

Al

(mm)

Inherent

filter (mm)Pb

(mm)

Added Filter Sn Cu

(mm) (mm)

Al

(mm)

Moderately Filtered Beams (M)

60 3 Al 3 Be 1.51

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

Heavily Filtered Beams (H)

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

The percent corrections, YApE^ and YApS^ used for the Wyckoff-Attix chamber and the

products of all exposure-rate-independent corrections for each beam code are given in Table 6-5.

Comparison of columns 3 and 4 of Table 6-5 with columns 2 and 3 of Table 6-3 shows there is good agreement, in general, between the data calculated in the review from Ref [3] and the data

presently used. Corrections for electron loss, YApE^, for beams M300 and H300 are estimated

from figure 9 of Ref [3], and may be slightly in error since the filtration of these reference

beams 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 a type B uncertainty for lApE^ of 0.07

% has been estimated for M300 and 0.15 % for H300.

The corrections given in Table 6-5 for air attenuation, k^, in the Wyckoff-Attix free-air

ionization appear to be nearly a linear function of the logarithm of the HVL in copper for the

beam qualities of interest, see Ref [1] of Attachment 2. The data points cover the range of Cu

HVL's for the M and H beam qualities except for M60 and H300. The values for those

21
Table 6-5. Data used to compute corrections for the Wyckoff-Attix standard free-air chamber

for conventional calibration conditions

Beam Code

100(1-

S Ap

lOO(l-kp)

S Ap S. ^k^/k,

M60 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

H50 1.0103 0.63 0.9937 1.0055

H60 1.0088'' 0.55' 0.9945 1.0048

HlOO 1.0060" -0.04 o.4r 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'* o.3r 1.0031 1.0084

S60 1.0131 0.70 0.9930 1.0075

^ An energy independent correction factor of 1.0015 for non-planarity of the guard-collector

plate system is included in the product of nkj.

*' Estimated from a graph of k^ versus HVL in mm of 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: I S, = 0.9912 + 1.816 E-3 log,(HVL in mm Al).

conditions are estimated from a semilog curve fit. The data points, found in Ref [1] of Attachment 2, when fitted to a logarithmic curve using the

method of least squares provide the following equation:

= 1.0067 - 2.0229x10"^ log,o(//KZ mm Cu) (6-6)

The correlation coefficient is 0.96. A comparison of the data, estimated and computed using

the least-squares-derived equation is shown in Table 6-6.

The recombination corrections for all the NIST free-air ionization chambers are based on

measurements of ionization currents at several collection potentials, with air kerma-rate as a parameter. In accord with the method of Scott and Greening [9], the inverse of the ionization

currents are plotted against the inverse of the squares of the collection potentials. Extrapolation

of the plotted data to 1/E^ = 0 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/E^ = 0 is the recombination correction for that particular

22
Table 6-6. Comparison of the values currently used to those values computed from the least

squares equation

Beam Code HVL (mm Cu) Present Least Squares

M60 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

H50 0.142 1.0103 1.0107

H60 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

^Estimated from graph found in Ref 1. of Attachment 2.

air-kerma rate. This same analysis technique can be performed w^ith an abscissa of inverse

potential, instead of inverse squared potential, as was the case for the Wyckoff-Attix

recombination correction determination. According to Scott and Greening, this procedure

exaggerates the effects of recombination and is proper only for very low air-kerma rates. However, the predicted recombination correction for the Wyckoff-Attix chamber is only about

0.2 % for the maximum air-kerma rate tested, 2.9 mGy/s. The result can be represented by the

following equation, where the air-kerma rate is in units of Gy/s:

= 1.00038 + l.SlxlQ-^ (6-7)

The product of the currently used correction factors for the Wyckoff-Attix chamber along with

the recently reviewed products of the correction factors are listed in Table 6-7. A

non-coplanarity correction, based on guard-collector plate assembly measurements, is applied to

all free-air chamber measurements. In general, the computations from the most recently

reviewed data for the electron-loss and scattered-photon gain corrections are in agreement with

the existing data. Two exceptions, which have differences on the order of 0.1 %, are the lApSr

corrections for the M60 and HI 50 beam qualities. 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.

23
Table 6-7. Products of all air-kerma-rate-independent corrections for the Wyckoff-Attix free-air

ionization chamber

Beam Code

nk/k.

Current

nk/k^

Review Difference iVo)

M60 1.0140 1.0074 0.65

MlOO 1.0052 1.0059 -0.07

M150 1.0058 1.0066 -0.08

iVlZWu 1 0071 1 OO^^Q 0u.uz09

M250 1.0074 1.0071 0.03

M300 1.0084 1.0097 -0.13

xIjU 1 oo^ ^

H60 1.0048 1.0042 0.06

HlOO 1.0038 1.0032 0.06

H150 1.0098 1.0100 -0.12

HlOO 1.0106 1.0096 0.10

H250 1.0046 1.0036 0.10

H300 1 0084

6.5 Ritz (20 kV to 100 kV) free-air ionization chamber corrections

The Ritz free-air ionization chamber [4] is used for standardization of x-ray beams for

x-ray tube potentials from 20 kV to 100 kV. The dimensions of interest for purposes of

developing corrections for this free-air ionization chamber are the length of the air path between

the defming-plane of the diaphragm and the mid-plane of the collection plate and the

collection-plate-system separation and height, refer to Table 6-2 for the dimensions.

The largest free-air ionization chamber correction for "low" energy, lightly filtered x-rays

is for air attenuation, k^. This correction is determined with the free-air ionization chamber 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 free-air ionization chamber diaphragm and setting a fixed diaphragm in the beam

independent of the free-air ionization chamber. For a particular set of conditions, including the

distance from source, the x-ray tube potential, and the filtration, the ionization in the free-air

ionization chamber 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 a distance equal to the air path length, position 2. Position 1 places the mid-plane in

the normal position of the diaphragm. Position 2 places the mid-plane of the Ritz chamber in the

normal position of the collection plate. The diameter of the fixed diaphragm must be such that

24
the defined beam in its entirety is intercepted by the free-air ionization chamber 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 a pressure of 99.992 kPa and a temperature of 293.15

K, (dry air density of 1.189 mg-cm"^) to provide a correction factor representative of normal

room conditions. If the experimentally determined attenuation coefficients, |i/p, are so large that

normal variations in room conditions produce significant differences in the correction, then the

difference in air density from 1.189 mg-cm"^ must be taken into account. The conditions pertain

for all x-ray beams with HVLs less than 0.22 mm Al. The effect on of changes in temperature

and pressure from reference conditions must be taken into account using equation 6-3. A list of

the air-attenuation coefficients and values for for the Ritz are listed in Table 6-8. See Ref [7]

for a list of calculated mass attenuation coefficients. These have been verified recently using the

Attix chamber for a direct determination of the correction.

Table 6-8. Mass air-attenuation coefficients and air attenuation corrections for the Ritz free-air

ionization chamber

Beam Code HVL

(mm Al) (cmVmg)

Air Attenuation

Correction (kj^

L20 0.071 8.2293E-3 1.1327"

L30 0.22 3.2392E-3 1.0503"

L40 0.49 1.6816E-3 1.0258

L50 0.75 1.2163E-3 1.0186

L80 1.83 7.3175E-4 1.0111

LlOO 2.8 4.3435E-4 1.0066

M20 0.152 4.0835E-3 1.0639"

M30 0.36 1.8811E-3 1.0288

M40 0.73 1.1051E-3 1.0169

M50 1.02 7.4192E-4 1.0113

H20 0.36 1.6190E-3 1.0248

H30 1.23 5.8048E-4 1.0088

H40 ?9 3 9078F,-4 1 00S9

^ The air attenuation corrections were determined with the defining plane of the Ritz chamber at 50 cm, for an air-path length of 127.39 mm.

" These corrections vary significantly with changes in atmospheric temperature and pressure.

For ambient conditions, k^ is calculated using k^ = exp (|iiL).

The electron-loss corrections, k^, for the Ritz free-air ionization chamber are taken from

data in the 1959 publication on the design of free-air ionization chambers [6]. The dimensions of

the Ritz free-air ionization chamber are such that the ionization loss due to loss of electrons

25
occurs only at the upper end of the x-ray energy range for which this free-air ionization chamber

is used. Therefore, only figures 10 and 14 of Ref 6 are required for calculation of the

corrections for beam codes S75 and LI 00. 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 free-air ionization chamber beam defining diaphragm and to simulate the

required zero-beam diameter. The results of the calculations shown in Table 6-9 do not differ by

more than 0.1 % from the corrections for electron loss presently used for these conditions. 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 E, at 60, 75, and 100 kV with 3

mm Al added filtration. The estimated correction is 0.12 %, which is not significantly different

Table 6-9. Computation of electron-loss corrections for the Ritz free-air ionization chamber

Radius S75 LI 00

Inner Outer Ap ApE, ApE,

(cm) (cm) (%) (%) (%) (%)

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

ke -0.07 -0.44

from the presently used correction. A detailed analysis of the corrections is currently

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

the corrections can be found in Ref 3 of Attachment 2. Corrections for the scattered-photon contribution to the ionization in the free-air

ionization chamber are derived from Ritz [6] and Allisy and Roux [9]. The percent scatteredphoton contributions within different radii, and appropriate multipliers for several x-ray beam

conditions, are given in Ritz [6], figure 15 and table 1, respectively. The data of Ritz, Allisy and

Roux were combined and by means of least-squares fit for vs. HVL measured in Al, the

following logarithmic equation was developed:

kp = 0.9956 + IxWhog^^iHYL mm Al) (6-8)

The values of k^ for the Ritz free-air ionization chamber, listed in Table 6-10, are computed from

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

chamber are provided in the last column of Table 6-10.

• The recombination corrections for the Ritz free-air ionization chamber are calculated

from an equation of the same form as was developed for the Wyckoff-Attix free-air

ionization chamber and described in a previous section. The Scott and Greening extrapolation

procedure for air-kerma rates ranging from 0.6 mGy/s to 4.2 mOy/s yields the following

logarithmic equations:

= 1 + %.lU6x\0-^K,{Gyls) (6-9)

26
1 doie D- 1 u. Summary of corrections for the Ritz free-air ionization chamber

Beam Added Al HVL

Code filter (mm Al)

1 a ka K n kj

(mm)

L20 U 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

T C f\ 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 0.73 1.0170 0.9952 1.0000 1.0121

M50 1.021 1.02 1.0114 0.9955 1.0000 1.0069

S75 1.504 0.36 1.0076 0.9960 1.0007 1.0043

The values given for are computed using P=750 mm Hg and T=293.2°K.

"These products include only and k^ and must be multiplied by exp(fiL).

The recombination data was obtained using a 1 cm diameter free-air ionization chamber

diaphragm, which is used for routine calibrations, and from measurements made using a 0.5 cm

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

to develop these equations can be found in Ref 4 of Attachment 2.

6.6 Lamperti (10 kV to 20 kV) free-air ionization chamber corrections

The Lamperti free-air ionization chamber is designed for x-ray air-kerma standardization

in the region of 10 kV to 60 kV. In practice, the Ritz free-air ionization chamber, with air-kerma

measurement capabilities overlapping that of the Lamperti chamber, is used for calibrations

down to and including 20 kV x rays. Typically the Lamperti free-air ionization chamber is used

only for measurements at 10 kV and 15 kV.

The corrections for the Lamperti chamber are discussed in detail by Lamperti and

Wyckoff in Ref [5]. Although many different corrections are identified, the important

corrections are for air attenuation, k^ and scattered photon contribution, . The corrections for

electron loss, k^, in the 10 kV to 60 kV region are given as "much less than 0.1 %"[5]. For x rays

generated at 10 kV and 15 kV, k^ should equal unity since the Lamperti chamber plate separation

is 4 cm and the continuous-slowing-down-approximation (CSDA) range for the 15 keV electrons

is only 0.5 cm in air of density 1.1888 mg/cm3) [10]. Actually, k^ should be unity for the

Lamperti chamber for x-rays up to 50 kV since the CSDA range for 50 keV electrons is 4.1 cm

27
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 [1 1], the

practical electron range would be only 80 % of the CSDA range for 50 keV electrons.

The intercomparison of free-air chambers described by Lamperti and Wyckoff [5] 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 [4] and Allisy and Roux

[8]. The data from these two sources differ by about 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 + 1.034xlO"Mog,o {HVL mmAl) (6-10).

Somerwil [12] investigated a systematic difference between several national free-air

ionization chamber standards, intercompared at the Bureau Internationale des Poids et Mesures

(BIPM), and found that the scattered-photon correction for chambers with 40 mm diaphragm-tocollection-plate distances should be less than corrections determined from measurements with

chambers of 100 mm. The values of k^ were computed for x rays with beam codes LIO, L15,

HIO, and HI 5 using the above equation. These corrections were reduced by adding 0.15 % at LIO and HIO and 0.10 % at L 15 and H15 because the Ritz and Allisy-Roux measurements were

for a distance of 100 mm. The adjustments to the corrections are the percentages determined by

Somerwil. The air attenuation corrections, k^, for the Lamperti chamber are determined using the

Ritz chamber in the two-position, independent-diaphragm technique, previously described.

These corrections have recently been directly verified by the Attix chamber. The Ritz and the

Attix chambers were used because the Lamperti chamber aperture design is not large enough to encompass the beam defined by the fixed diaphragm. See Ref. 5 of Attachment 2 for data. The

recombination corrections, k^, for the Lamperti chamber were determined using the procedure

suggested by Scott and Greening. See Ref. 6 of Attachment 2 for Lamperti chamber data. The

equation developed from these studies for determining the recombination correction is, where

the air-kerma rate is in units of Gy/s:

k^ = 0.9996 + 4.573x10"^ A:^-^ (6-11)

Recombination corrections for exposure rates commonly encountered in instrument calibration

work are provided in Table 6-11.

Table 6-11. Recombination corrections for the Lamperti free-air ionization chamber

Air-kerma rate ks

(R/s)

1.5 1.0000

15.0 1.0002

150 1.0014

28
The corrections presently used for the Lamperti chamber are given in Table 6-12 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 adjusted for

the Somerwil correction. The correction, k^, is equal to unity as are all other exposure-rate

independent corrections identified by Lamperti [5].

Beam

code

Added filter (mm Al)HVL

(mm Al) (cmVmg)

K K

LIO 0.0 0.029 0.0194 0.9972 k = exp(|iL)

L15 0.0 0.050 0.0125 0.9971 k = exp(^L)

HIO 0.105 0.048 0.0140 0.9975 k = exp(^L)

H15 0.500 0.152 0.9976 1.0245

6.7 Attix (20 kV to 50 kV) free-air ionization chamber corrections

The free-air ionization chamber dedicated for use in the mammography calibration range

is the Attix chamber, originally designed by Herb Attix [13] in 1961. The chamber was

redesigned [14] and constructed for NIST by the University of Wisconsin Radiation Calibration

Laboratory in 1994. The Attix chamber, a variable-length, cylindrical free-air chamber differs

in design from the other NIST free-air ionization conventional parallel-plate chambers. The

differences contribute to its appropriateness as a standard for the measurement of exposure in the

mammography energy region. The chamber design allows a measurement procedure that

eliminates the need of a correction for field inhomogeneities near the ends of the chamber. This

measurement procedure is based on a subtraction method [13], which involves finding the

difference in collected charge for different electrode lengths. The variable-volume design also

permits the direct measurement of the air attenuation correction, with relative ease. The Attix

chamber has been used to verify air attenuation corrections determined previously for some of

the tungsten x-ray beams and eventually will be used to determine all air attenuation corrections

for the ISO beams, if possible. Laitano and Toni [15] describe the use of this Attix-style free-air

chamber as a national x-ray standard for 100 kVp to 250 kVp x-ray beams.

The Attix chamber is designed for energies up to 50 kVp. The chamber is composed of

an aluminum cylinder with a fixed front plate and a variable position back plate and an off center

electrode. The cylinder and back plate are positioned with precision stepping motor controlled

slides. For a detailed description of the chamber see Ref [14]. As with the conventional

parallel-plate free-air chambers, corrections to the air kerma measurements are minimized

through the design of the chamber, but three corrections are still necessary: air attenuation,

photon scatter, and recombination. The air attenuation and photon scatter corrections k^ are

listed in Table 6-13. Although the plate separation is variable for the Attix chamber, a fixed

airpath length of 2 1 .27 cm is maintained for all the measurements. Experimental determination

of the photon scatter correction for the Attix chamber would be difficult and time consuming, so previous work by V.H. Ritz [4] was used to determine the appropriate correction for the Attix

29
chamber for each available aperture. Table 6-13 lists the photon scatter corrections for a 1 cm

aperture. The recombination correction was determined using the procedure suggested by Scott

and Greening [9]. The recombination correction is calculated from the following equation where

air-kerma rate is in units of mGy/s:

= 1.0002 + Q.m9k,{mGyls) (6-12)

Table 6-13. Correction factors for the Attix free-air ionization chamber

Beam

Code

Half-Value

Layer

(mm Al)

Air Density

P (g/cm3)

u/d

(cmVg)

Air

Attenuation

Correction

Photon

Scatter Correction

Mo/Mo23 U.271 1.179 h-3 O AAO 1 1 AC /I 1.U54 A C\C\AC\ 0.9949

A /T /AT ^ C Mo/Mo25 A AZT 0.296 1 1 OA T~? O 1.180 E-3 1 AO 1 A 1.9819 1 A C 1 1.U51 A AACA 0.9950

A /T /AT" O Mo/Mo28 0.332 1 1 O 1 T? O 1.181 E-3 1.8284 1 r\ An 1.047 A AACA 0.9950

Mo/Mo30 A T C 1 0.35

1

1 1 O 1 T7 O 1.181 b-3 1.676 1 A /I 0 1.043 A AAC 1 0.9951

Mo/Mo35 A T A'^ 0.392 1.175 b-3 1.5693 1 A /I 1.04 A AACO 0.9952

Mo/Rh28 A /I AO 0.408 1 1 O /I O 1.184 h-3 1 C 1 o c 1.5185 1 AO A 1.039 A AAC) 0.9952

Mo/Rh32 3445 1.185 E-3 1 A A 1 1.4416 1 A O T 1.037 A AA C O 0.9953

Mo/Jylo25x (J.JOO 1.1 / / E-3 U.83(J4 1 AT 1 1.021 A AAC C 0.995 J Mo/Mo28x 0.626 1.177 E-3 0.908 1.023 0.9956

Mo/Mo30x 0.66 1.179 E-3 0.8287 1.021 0.9956

Mo/Mo35x 0.748 1.177 E-3 0.7911 1.02 0.9957

Rh/Rh25 0.351 1.184 E-3 1.7098 1.044 0.9951

Rh/Rh30 0.438 1.189 E-3 1.5134 1.039 0.9952

Rh/Rh35 0.512 1.187 E-3 1.2857 1.033 0.9954

Rh/Rh40 0.559 1.185 E-3 1.326 1.034 0.9954

Rh/Rh30x 0.814 1.186 E-3 0.6684 1.017 0.9958

Rh/Rh35x 0.898 1.181 E-3 0.6709 1.017 0.9959

The air kerma determination with the Attix chamber involves the collection of charge

with various plate configurations. By changing the volume of the Attix chamber and knowing

the corresponding change in length of the collecting electrode, the air kerma can be determined

with a minimum of two different plate configurations. Although the minimum number of plate

configurations needed to determine the air kerma is two, four measurements are conducted, with

the fourth being a repeat of the first position. The electrode length is changed by 5 cm with each

30
plate configuration.

For routine measurements, the defining point of the Attix chamber, is positioned at one

meter from the focal spot of the x-ray source. However, for the determination of the HVL's and

the air attenuation corrections, the defining point of the aperture is positioned at 78.73 cm and

the chamber center at one meter from the focal spot. A constant plate separation and chamber

distance firom the focal spot is maintained for the HVL measurements. For the air attenuation

correction measurements, a fixed plate separation is maintained, while the center of the chamber

is moved back 21.27 cm, the air path length. The air attenuation correction for the Attix

chamber, is the ratio of the charge collected when the chamber center is at 100 cm to the charge

at 121.27 cm from the focal spot. Attachment 3 contains schematics which show the chamber

configuration for HVL and air attenuation measurements and the chamber position for a routine

measurement procedure. The average of the three resulting ratios of the change in charge to change in electrode length is calculated and used in the air kerma calculation as a component of

the mass ionization current.

6.8 Comparison of standard fi"ee-air ionization chambers

The NIST standard free-air chambers have been compared with each other and with the

standards of other nations, to test their congruity where their measurement capabilities overlap.

The most recent results from the 1998 BIPM-NIST and the 1998 NPL-NIST comparison are

listed in Tables 6-14 to 6-18. Complete details including the uncertainties of the comparison

can be found in Ref [16] and [17].

The Attix chamber was indirectly compared to the German national standard, using a NIST reference-class ionization chamber. The measurment of air kerma was made with both the

German and the NIST mammography standards and a calibration factor was established for the

NIST reference-class ionization chamber at both institutes. The Attix chamber was also

compared indirectly through the use of the Ritz chamber at the NPL, Teddington, UK. The

results of this indirect comparison are given in Table 6-19.

The latest "in-house" comparison results for the Atdx, Lamperti and Ritz chambers are

listed in Table 6-20. The Wyckoff-Attix and Ritz chamber comparisons have been conducted

periodically since 1958. The Lamperti and Ritz chambers have been compared numerous times

since 1961. From all previous comparisons, the mean difference between the Lamperti and Ritz

chambers in the 20 to 50 kV region was found to be less than 0.4 %. The mean difference

between the Wyckoff-Attix and Ritz chambers was found to be 0.5 % in the region between 60

and 100 kV. No adjustment is made for these differences since they are well within the

maximum difference of 1 .2 % estimated by the ICRU, and the comparisons give no indication

which member of the pair is to be considered the more reliable. Before establishing the Attix

chamber as a primary standard it was compared to the Ritz and the Lamperti chambers. The

complete details of the Ritz to Attix comparison are given in Ref [18]. The comparison results are listed in Table 6-20 and show agreement to be better than 0.35 %. The unpublished

comparison results of the Lamperti to the Attix chamber are also listed.

31
Table 6-14. Results of the comparison of the Lamperti standard with the BIPM standard

Generating potential Half-value layer NIST/BIPM

(kV) (mm Al) 1998

10 0.036 0.9950

30. ; 0.176 0.9961

25 0.250 0.9968

50(b) 1.021 0.9948

50(a) 2.257 0.9938

Table 6-15. Results of the comparison of the Ritz standard with the BIPM standard

Generating potential Half-value layer NIST/BIPM

(kV) (mmAl) 1998

30 0.176 0.9943

25 r 0.250 0.9949

50(b) 1.021 0.9938

50(a) 2.257 0.9956

80 3.01 0.9902

100 4.00 0.9947

Table 6-16. Comparison of the Lamperti chamber to the 50 kV NPL standard

Generating potential Half-value layer NIST/BIPM

(kV) (mm Al) 1998

10 0.036 0.9951

11.5 0.05 1.0006

14 0.07 0.9996

16 0.1 0.9995

20 0.15 0.9992

32
Table 6-17. Comparison of the Ritz chamber to the NPL 50 kV standard

Generating potential

(kV)

Half-value layer

(mm Al)

NIST/BIPM

1998

20 0.15 0.9977

24 0.25 0.9978

34 0.35 0.9989

41 0.5 0.9973

44 0.7 0.9983

50 1.0 0.9983

Table 6-18. Comparison of the Ritz chamber to the NPL 300 kV standard

Generating potential

(kV)

Half-value layer

(mm Al)

NIST/BIPM

1998

50 1 0.9981

80 2.9 0.9941

Table 6-19. Comparison results for the mammography standard

Beam Quality Half-value layer NIST/PTB^

(mm Al)

NIST/NPL''

Mo/Mo25 0.296 1.005

Mo/Mo28 0.332 0.996

Mo/Mo30 0.351 0.999

Mo/Mo35 0.392 1.002

Mo/Mo25x 0.566 0.999

Mo/Mo28x 0.626 0.994

Mo/Mo30x 0.660 1.005

Mo/Mo35x 0.748 1.004

^ These results were obtained through the use of a transfer standard ionization chamber.

These results were obtained through the use of the Ritz chamber and transfered to the Attix

chamber.

33
Table 6-20. Recent "in-house" comparisons standards

Beam quality Half-value layer Ritz/Attix Lamperti/Attix

Mo/Mo23 0.271 0.998

Mo/Mo25 0.296 0.997 0.998

Mo/Mo28 0.332 0.997 0.999

Mo/Mo30 0.351 0.999 0.997

Mo/Mo35 0.392 0.998 0.998

Mo/Rh28 0.408 0.997

Mo/Rh32 0.445 0.997

Mo/Mo25x 0.566 1.001

Mo/Mo28x 0.626 1.001

Mo/Mo30x 0.66 1.001

Mo/Mo35x 0.748 1.000

Rh/Rh25 0.351 0.997

Rh/Rh30 0.438 0.997

Rh/Rh35 0.512 0.999

Rh/Rh40 0.559 0.998 1.004

Rh/Rh30x 0.814 1.000

RhyRh35x 0.898 1.000

L15 0.057 1.007^

L20 0.071 0.999 1.004^

M20 0.152 1.002 1.004^

L30 0.22 0.998

M30 0.36 1.002 1.003^

M40 0.73 1.000

M50 1.02 0.998 0.999'

*This is a comparison of the Lamperti chamber to the Ritz chamber using the new tungsten lowenergy x-ray tube.

34
7.0 Gamma-ray air kerma standards and calibration ranges

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

quantity of air kerma and for delivering known exposures to passive dosimeters. The sources are collimated and the beams have been calibrated using the appropriate graphite cavity ionization

chamber standard. 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 air kerma.

7.1 Cavity-chamber standards

The cavity chambers, used for studies leading to the revised. May 1, 1972, ^°Co and '^^Cs

air kerma-rate standards, were fabricated from reactor-grade, high-purity graphite, following the

design of Wyckoff [19]. A spherical shape was chosen in order to allow the standards to be

based on a homogeneous group of chambers of different volumes. The spherical shape also

reduces the effect of distance errors and the complexity of set-up for measurements with

cylindrical chambers. Additionally, the spherical shape presents a uniform, symmetrical,

chamber aspect to the source. The chambers have been carefully compared in the gamma-ray

beams, and the NIST standard of air kerma for these radiations is the mean response of seven

spherical graphite cavity chambers. The dimensions of the spherical chambers are given in

Table 7-1. Additional details on the construction and corrections for these cavity chambers are contained in a report by Loftus and Weaver [20].

Table 7-1. Dimensions of spherical graphite ionization chambers

Nominal

volume

(cm^)

Volume

(cm^)

Net

volume

(cm^)

Outside

diameter

(cm)

Graphite

density

(gcm-^)

Radial wall thickness

(cm) (gcm-2)

0.5 0.440 0.431 2.078 1.72 0.563 0.968

1 1.140 1.131 2.065 1.73 0.398 0.688

2 2.029 2.019 2.080 1.74 0.246 0.428

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

35
7.2 Gamma-ray sources The location, nominal activity, and orientation of the gamma-ray sources used for

instrument calibrations are given in Table 7-2. Figure 7-1 is a schematic of a irradiator housing

a ''Co gamma-ray source typical of the horizontal gamma-ray source geometries used at NIST.

Table 7-2. Gamma-ray source locations and nominal activities of sources as of January 1, 1999

Radionuclide Activity

(bq)

Location Orientation

^^Co 2.7 E13 B034 vertical

"Co 1.5 E14 B036 vertical

"Co 1.3 Ell B021B horizontal

"Co 9.6 E09 B015B horizontal

'^^Cs 3.1 E13 B036 vertical

'3^Cs 5.8 E12 B021A horizontal

^^^Cs 6.3 Ell B015A horizontal

Fig. 7-1. Port detail for horizontal ^°Co 5 mm diameter source.
7.3 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 air kerma

rates at selected distances, or of distances for selected air kerma rates. The equation used for this

purpose was derived by using the inverse square law with correction for air attenuation and

buildup. The equation that was found to adequately predict the air kerma rate to within a few

hundredths of one percent at the time of the calibration was in the following form:

XD^ = ^i(l + KJD + + K^D^) (7-1)

where

D - S ^ (7-2)

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. The 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 air

kerma rate by rearranging the equation as follows:

X = (A/D^ + B/D + C + ED)e'^^ (7-3)

where X is the decay constant for the radionuclide, and T is the elapsed time in days since the source was calibrated. Tables of exposure and air-kerma rate for days of decay are available

outside each source room. The constants used in equations 7- 1 and 7-3 appear in Table 7-3

.

The constant E used in equation 7-3 is zero for the horizontal gamma sources. The data for the

^°Co sources in Table 7-3 are derived from the data in table 17 of Loftus and Weaver [20].

Checks on the calibration of these sources through the years show them to be unchanged.

Table 7-3. Constants^ for computing exposure rate corrected for decay for the horizontal beam

sources

Source A B C

(Location) K, K2 K3 K4 K5

m^-mR/s 10^ 10' mR/s lO^m 10'

d-' m-mR/s

Cs'^^ (BO 15A) 2.0323 -0.943 -1.8619 -0.818 6.326

Cs'" (B021A) 19.617 -13.84 123.24 -0.304 6.326

Co'° (B015B) 0.5924 -0.1657 -1.345 -0.144 36.010

Co'° (B021B) 5.2676 0.4764 67.33 -0.528 36.010

^The constant E equals zero for these four sources. The constant is the offset distance so distance equals the scale reading plus . ^The reference date is December 31, 1984. See Ref [7] of Attachment 2.

Calibrations of the vertical-beam sources are limited to one or two sets of conditions. For the

37
''Co source located in B034, the reference distance on the Al-channel mounted scale is 100 cm. The offset of the scale-zero from the source center is 47.9 cm. The highest activity ^°Co source

available for calibration work is located in B036. At the end of 1989, the effective activity of the source was about 444 T Bq The source was calibrated early in 1 990 using the standard graphite

ionization chambers. The mean of those measurements corrected for decay is used at the present

time. As shown in Table 7-4, a later measurement using the 10 cm-' standard chamber checked

the original data.

Table 7-4. Data^ and positioning for calibration of vertical-beam gamma-ray sources in room

B036

Source Distance'' Collimator

setting

Standard chamber and

rate (mGy/s)

air-kerma Calibration

air-kerma rate (mGy/s)

(cm) 1 cm^ 10 cm^ 1-50 cm^

"Co 105 6x6 4.122 4.110 4.117

"Co 105 6x6 4.109

'^^Cs 55 8x8 0.9233

'''Cs 55 8x8 0.9223

'''Cs 55 8x8 0.9236 (0.9167) (0.9101) 0.9236

'''Cs 55 15 X 15 0.9517 0.9499 0.9508

55 15x 15 0.9497 0.9461

'^^Cs 38 10.5 X 10.5 1.467

^ All data is corrected to December 31, 1989, see Ref [8] of Attachment 2. No data is included

for the source in B034 because the source was replaced in the last quarter of 1999 and new

parameters have been established.

Aluminum channel scale.

The '^^Cs vertical-beam source in Room B036 is calibrated for several collimator setting

and distance conditions. The calibrations rely mainly on measurements with the 1 cm^ standard

chamber although some measurements with the 10 cm^ and a 50 cm^ chamber have also been

tried. Some of the 10 cm^ and a 50 cm^ data in Table 7-4 are parenthesized to indicate that they

are not included in the source calibration. These data are included in Table 7-4 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 a 50 cm^ chamber measurement for the 15x15 collimator setting show that the

standards are consistent for the large field size. In the course of using the graphite standard ionization chambers for NIST and

other-agency source calibrations, the electrodes in the chambers have either loosened and

shorted to the chamber wall or excessive voltage has been applied. When this occurs, the

38
electrodes are weakened and carbon is deposited on the high-voltage insulator. The 1 cm\ 10

cm^ and 30 cm^ chambers are particularly prone to this problem since the electrodes have

diameters less than 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 1 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-air kerma-rate sources after the last repair of the Icm^ chamber were several tenths of one percent higher than the reference data. This is shown to some extent in Table 7-4 where the

latest data taken on the '^''Cs source for the 8x8 collimator setting is higher than the previous

measurements. Some 1 cm^ chamber exposure data not included in Table 7-4 indicate a difference between present and earlier measurements of as much as 0.4 %. On the other hand,

measurements at low air kerma rates at 1 meter from the '^^Cs source in Rm B021 A show

agreement between the 1 cm^ standard and a 50 cm^ or the 10 cm^ standard to be within 0.02 %

and 0.01 % respectively. See Ref. [9] of Attachment 2. The net cavity volume for the 1 cm^

chamber was measured to be 1.1297 cm^ following the last repair, which involved the

replacement of electrode and refurbishment of the high voltage electrode. See Ref. [10] of

Attachment 2. The main difficulty in reproducing air kerma calibration data involves the Icm^ chamber.

A small-volume standard chamber is required for high air kerma rate measurements, and in

beams that may be uniform only over small areas. See Ref. [1 1] of Attachment 2 for details.

However, small-volume chambers have unwanted extra-cameral volumes in the vicinity of the

high voltage insulator and stems that can have a large effect on the chamber response to radiation, therefore data is taken at both polarities. Additional difficulties of another sort arise in

comparing data over a period of years when different measurement equipment may be used, see

for example Ref. [12] of Attachment 2. The uncertainty statement given by Loftus and Weaver

[20] is only representative in the short term and for optimum conditions. In order to maintain the

overall uncertainty of 0.7 %, the primary standards are carefully preserved and not used as working standards.

7.4. 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 % of the center

density, minus the difference in distance between the 90 % and 50 % density measurements.

With R^O' the radius of the 90 % density contour, and R50, the radius of the 50 % density contour,

the usefiil beam radius (UB) is computed as follows:

i?,, = K6 X SD + K7 (7-4)

i?on = K8 X SD + K9 (7-5)

UB = R.. - (R50 (7-6)

UB = 2i?on - R50 (7-7)

39
In the above equations, the K's are constants and the SD is the aluminum channel scale reading

in cm. The constants for each source are listed in Table 7-5. 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 distances listed in Table 7-4 thus the constants K6

and K8 in Table 7-5 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 ^°Co source.

Table 7-5. Constants for computing useful beam radii for the horizontal and vertical beam

calibration ranges

Scale

Distance

(cm)

Source Location K6 K7 K8

(cm)

K9

(cm)

Useful

beam size

(cm)

variable '^^Cs B015A 0.188 0.613 0.143 0.044 variable

variable ''Co B015B 0.157 0.237 0.112 -0.106 variable

variable '^^Cs B021A 0.207 0.235 0.149 -1.068 variable

variable "Co B021B 0.191 0.682 0.124 -0.236 variable

38 '''Cs B036 0 6.8 0 4.3 1.8

55 '^^Cs B036 0 6.4 0 4.05 1.7

105 ^°Co B036 0 6.3 0 4.4 2.5

100 "Co B034 0 6.8 0 4.7 2.6

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 extra-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 cm^ and the Farmer-NEL 0.6

cm^ 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

7-6.

40
Table 7-6. Beam size defined by 50% radiographic density contour for vertical-beam gammaray sources, for specific collimator settings

Source Room Scale Collimator Beam

Location Distance settings size

(cm) (cm') (cm)

^"Cs B036 38 10.5 X 10.5'' 13.6 X 13.6

•^^Cs B036 55 8x8^ 12.8 X 12.8

''Co B036 105 6x6 12.6 X 12.6

''Co B034 100 6x6" 6.8x6.8

" Approximate settings determined by gauge blocks.

''On 100 cm source distance scale in room B034.

8.0. lonization-chamber current-measurement techniques

8.1. Background and history

Ionization currents in air-kerma standardization measurements are produced by the

irradiation of a gas in an ionization chamber. The ionization chamber may be a free-air chamber,

such as one of the national 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. In an ideal ionization chamber, an air-kerma rate of about 9 mGy/s will produce

about 0.3 nA/cm^. 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 made on 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. For background information on the history of

electrometer use refer to Attachment 4.

41
8.2. Electrometers

The Keithley Model 616 electrometer' was used for many years by the NIST Radiation

Interactions and Dosimetry Group due to the versatility and the ability to use external calibrated

capacitors. Keithley offers several other models, the 617 and the 6512, that provide the

appropriate features for NIST calibration work. The Radiation Interactions and Dosimetry Group

has used NIST calibrated air-dielectric capacitors of nominal capacitance, 100 pF and 1000 pF,

and good quality 10 nF and 100 nF polystyrene capacitors in a chassis, which were then mounted

with electrometers in measurement consoles containing other control and measurement

equipment. In recent years, the Keithley 617, 617-HiQ and the 6512 with their internal

capacitors have been integrated into the measurement control systems, where at one point only

Keithley 616 and 642 resided. The accuracy and stability for data collected with the internal

capacitors of the newer electrometers in the charge mode, eliminates the need for external

capacitors. These electrometers are routinely calibrated using the NIST calibrated capacitors.

This is discussed in a later section.

8.3. 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. Due to recent upgrades, there

are two types of data acquisition systems (DAS) in use in the x-ray and gamma-ray calibration

ranges. One system is used exclusively for measurements made with the x-ray sets and the

second is a portable system used for measurements made with the gamma-ray sources.

8.3.1. X-ray calibration data acquisition system

Data are acquired for x-ray standardization by measurement of all conditions relevant to establishing the x-ray air-kerma rate at a particular distance from the x-ray tube target. The data

acquired consists of data for the computation of ionization currents, parameter data, and

measurement system test and information data. Data used to determine the ionization currents

includes the electrometer charge measurements, the atmospheric pressure and pertinent air

temperatures, and the shutter-open time interval. The parameter data includes the x~ray tube

current and potential, the x-ray tube target-to-reference point distance, and the beam defining

filter thickness and the diaphragm diameters. Test data includes the measurements of the collection potentials on the standard free-air chamber, the monitor chamber, and the chamber being

calibrated, if appropriate. All data is acquired through National Instrument's LabView

interfacing. All charge measurements are corrected for the temperature and pressure at the time

of the charge collection.

The temperature sensors in the 300-kV range used to measure room, free-air chamber,

and monitor air temperatures are two-wire thermistors manufactured by Yellow Springs Inc.

Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of

Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best

available for the purpose.

42
(YSI). The sensors are connected to YSI model 43TN readouts. The YSI readout is a bridge

circuit which, when unbalanced, provides a voltage proportional to the temperature. There are four YSI temperature probes used in conjunction with measurements on the 100 kV calibration

range. The thermistors for the Ritz free-air ionization chamber and the Lamperti free-air

ionization chamber are manually switched to a single YSI readout. The parameters of the

calibration equations for some of these probes are found in Table 8-1. These probes are calibrated at least annually and the required fit parameters are entered into the data acquisition

software. Shown in Table 8-1. are the most recent parameters. In the Mammography Facility

the Hart 5613 platinum resistance thermometers and Hart 1502 digital thermometers are used for

air temperature measurements in the vicinity of the Attix free-air chamber, the monitor chamber

and the test chambers. The calibration correction is applied internally to the 1502 digital meters

therefore no calibration equation is given here.

The atmospheric pressure for the low and high-energy tungsten sets is measured by the

Setra digital barometer and powered by a NIST power supply serial no. 2561. The output

voltage is calibrated for pressure measurements in units of millimeters of Hg. A conversion to the unit of pascal is applied in the calculation of the calibration factor. The equation used for

computing the pressure is: P{mmHg) = -3 ASS +94. 336(voltage) (8-1)

In the mammography range, a Setra 370 barometer is interfaced through RS232 to the data

acquisition software. Any necessary correction is applied directly through software as part of the

air kerma calculation.

The low-energy Pantak generator was calibrated in December of 1997 and the highenergy Pantak in February of 1998, using a calibrated (Test No. 722/G44568) high-voltage

divider (Seifert Voltage Divider SN 720479). The procedure for a voltage calibration requires

the use of the calibrated voltage divider, a calibrated high precision digital multimeter, and a 10

000 ohm resistor. The Pantak high resolution voltage display reading, corresponding to the

calibrated tube voltage was also determined. Since the calibrations showed some dependence of

the voltage on the tube current, the results are given as a function of current for each tube voltage

setting. Found in Tables 8-2 and 8-3 are the parameters of each linear fit ( = mx+h) where x is the display reading of the HP voltmeter used to monitor x-ray tube current and y is the ratio used

to calculate the appropriate voltage HP voltmeter display for the desired nominal voltage.

The tube voltage of the mammography x-ray system has been measured using a Dynalzer

voltage divider, and verified with spectrometer measurements. No adjustment is required to be

made to the MPl controller kV display. Without an available NIST calibrated voltage divider,

the tube voltage has been established to within ± 50 eV of the display setting to the best of our

ability.

All time signals are controlled by a National Instrument data acquisition card which

contains two 24-bit, 20 MHZ counters/timers. The signal to the timers 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 clock continues to count until the

edge of the shutter crosses the portal mid-point. The timing shutter operation in both the 100 kV

and the 300 kV x-ray range has recently been modified to use air cylinder controlled solonoids.

The 300 kV machine also has a safety shutter which opens before, and closes after, the timing

43
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.

The charge measurements are acquired through the use of Keithley 617, 617-HiQ and

6512 electrometers. The internal capacitors of the electrometers are calibrated annually and any

necessary correction is applied as part of the air kerma calculation. Five electrometers, three

6512 and two 617-HiQ, are maintained for use in the low and high-energy range. Four

electrometers are maintained in the mammography range, two standard capacitance range

electrometers (20 nC), a 617 HIQ (20 ^C), and a midrange 617 (200 nC). The HIQ is dedicated

for use with the monitor chamber and the midrange 617 is used for collecting charge on the

Attix chamber. The two 20 nC capacitance range electrometers are used for the customer

chambers.

Table 8-1. Parameters of the straight line fits for the temperature probe calibrations used in the

Temperature probe

location

slope" intercept"

Wyckoff-Attix chamber -101.384 30.252

300 kV Test chamber -101.904 31.246

Lamperti chamber -92.611 28.740

Ritz chamber -100.007 29.540

100 kV Test chamber -105.031 30.766

'Parameters from calibration performed in September of 1999.

Table 8-2. Low-energy Pantak unit voltage calibration

Nominal Voltage Slope Y - Intercept

10 -0.134 12.472

15 -0.092 12.510

20 -0.068 12.09

30 -0.045 12.518

40 -0.034 12.524

50 -0.026 12.513

55 -0.025 12.521

60 -0.023 12.513

70 -0.021 12.513

75' -0.020 12.511

80 -0.016 12.500

100 -0.024 12.502

44
Table 8-3. High-energy Pantak unit voltage calibration

Nominal Voltage olope Y - Intercept

50 0.222 40.456

55 0.193 40.503

60 0.176 40.500

70 0.146 40.491

80 0.129 40.455

100 0.102 40.431

110 0.084 40.449

120 0.084 40.432

125 0.077 40.451

150 0.070 40.432

170 0.062 40.417

200 0.059 40.384

210 0.055 40.379

240 0.059 40.333

250 0.054 40.325

280 0062 40.260

300 0.065 40.211

8.3.2 Gamma-ray calibration data acquisition system

Calibration and measurements using the gamma-ray sources in rooms B034, B036, B021, and

BO 15 are conducted using a mobile measurement console consisting of all instrumentation

required for measurement and standardization of ionization currents. This data acquisition

system is a Visual Basic interfacing system, which can automatically acquire all or some of the

calibration data for cable-connected instruments and passive or other types of cable-connected

instruments, such as those with their own readout. The mobile console contains a Keithley

Model 616 electrometer, a Setra Model 350A digital barometer and a Digitec Model 5810 digital

thermometer. Each cable connected instrument has as analog output signal. The feedback

elements for the electrometer selector switch in the "volts" position are capacitors mounted in a

capacitor-selector chassis. The equations for computing temperature are dependent on the

thermistor used and, for measurements in control room BO 19, the signals are taken from YSI

readouts mounted in the source control consoles. The equation used 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 computer program. The temperature straight line fit paramaters follow in Table 8-4 .

45
Table 8-4. Temperature probe parameters for the gamma source calibration ranges

Temperature probe

location

slope intercept

B021-B -0.201 42.830

B021-A -0.202 42.974

B015A -0.095 29.767

B015B -0.094 29.855

B036 1.006 -0.296

B034 1.001 -0.316

8.4. Pre-calibration tests of instruments

Other parameters of a device's performance can be best determined at the time of

calibration, for example, recombination and leakage. 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.

8.4.1. Charger/reader scale linearity test

For all practical purposes, the only charger/reader sent to NIST for calibration is the

Victoreen Model 570 R-meter. Once the instrument is allowed to acclimate to the laboratory

environment, the charger/reader is tested 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 highlights, should be

considered as reasons for returning the instrument to the owner for adjustment or repair.

Although minor adjustments can be made at NIST, 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 a connector which simulates the stem of a probe and a precision voltage source with a negative polarity on the 1000 V range. The

electrometer is connected to the line, turned on, and allowed to come to equilibrium with

temperature changes caused by the light which illuminates the scale. With the 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 output of the voltage source is then varied while observing the electrometer scale. The

46
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) % of full-scale reading. This

data is then used for the calibration report.

8.4.2. 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 impedance meter, careftilly 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 instrument.

8.4.3. Testing of instruments to be calibrated in electrical units

The pre-calibration testing of ionization chambers to be calibrated in electrical units involves

only a test for communication to the atmosphere. Experience has shown that rarely does an

instrument fail to communicate with the atmosphere due to modem construction techniques.

Additional testing of this type of instrument for leakage and settling down time are performed

during the course of the calibration. An estimate of the recombination effect is made during

calibration, at the highest air-kerma rate, by making measurements at full and half collecting

potential.

9.0 Support equipment calibrations

9.1. Capacitor calibrations

Periodically, since 1956, the reference capacitors are submitted to the NIST Electronics

and Electical Engineering Laboratory, Electricity Division for calibration. The capacitors range

in capacitance from (100, 1000, 10 000 and 100 000) pF. Some typical long-term calibration

results have an uncertainty stated by the Electricity Division of no more than ± 0.05 %. The

uncertainty is interpreted by the Electricity Division as equivalent to three standard deviations.

Typical reproducibility for the last forty years is 0.02 %.

9.2. Temperature indicator calibrations

A liquid-in-glass thermometer and various probe and meter combinations are periodically

calibrated by the NIST Chemical Science and Technology Laboratory, Process Measurements

Division. In order to facilitate the temperature indicator calibrations, the calibrated

liquid-in-glass thermometer is used to calibrate a quartz crystal thermometer, maintained by the

NIST Radiation Interactions and Dosimetry Group. The quartz thermometer is then used to transfer the calibration to other temperature probes and thermistors in a controlled temperature

bath annually or whenever the need arises.

47
9.3. Pressure indicator calibrations

An aneroid barometer, Wallace and Tierman, Model FA 139, Serial Number XXI 1242,

as well as various other laboratory reference barometers are periodically calibrated by the NIST

Process Measurments Division. Calibrations of individual pressure indicators used at the various sources are made by placing the calibrated barometer alongside the instrument to be tested and

connecting both to a variable pressure device. The instrument readings are compared over a range of pressures that is somewhat larger than normally expected. Data is taken with increasing

and with decreasing pressure. The comparisons are made directly or through voltage signals. A

correction factor is obtained from this data for each pressure indicator. This calibration

procedure is conducted annually or more frequently if the need arises.

10.0. Operating procedures

10.1. Administrative procedures

The recommended procedure for requesting a NIST calibration service is outlined in

NIST Calibration Services Users Guide. In practice, however, customers request calibration

service in a variety of ways. Typically a new, or first-time customer will establish contact with

the Radiation Interactions and Dosimetry Group by telephone, letter, e-mail or fax 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

and methods of shipment to reduce the risk of damage. The customer is informed that a purchase

order must be received at NIST before an official calibration is performed. The purchase order can be sent with the instrument to be calibrated or can be sent separately by fax, mail or e-mail.

In addition to an authorization for payment, the purchase order should include a detailed

description of the calibration request, including beam quality codes, instrument model and serial

numbers, name and telephone number of a technical contact. If an incomplete purchase order is received, every effort is made to get a detailed description of the service requested.

Upon receipt of the purchase order, a checklist, acceptance letter and a customer test folder is generated. A copy of the purchase order, the final copy of the calibration report, the

calibration raw data and summary sheets and any documents of correspondence between the

customer or the Office of Measurement Services (O.S.), Calibration Program Office are maintained in the customer's calibration report folder filed by the unique dosimetry group (DG)

number. After copying the purchase order for the customer folder, the original purchase order,

along with a request for a test folder, should be sent to the O.S.. A test folder will be sent by

O.S. and will contain the original purchase order and appropriate forms. The test folder's unique

number is used as one of the identifiers on the calibration report.

When instruments arrive for calibration, they are unpacked and inspected for

damage. Special attention is given to the condition and type of connector. If an adaptor is sent with the chamber this should be noted on the inventory list along with the description of the

chamber. Shipping damage is reported to the NIST 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

NIST personnel perform the repair.

48
After the instrument has been cahbrated the calibration report is generated. Currently the

reports are generated in WordPerfect or MicrosoftWord. Templates are available to simplify this

procedure and to ensure consistency in the reporting format. Sample reports are found as Attachment 5. The final copy of the calibration report is reviewed and initialed by the preparer and an

additional reviewer and then given to the group leader for review. After the group leader

approves and initials the report, it is sent to the Division Office for signature. Upon return, two

copies are made. The original is mailed to the customer, one copy is filed in the customer folder,

and one copy is added to the test folder. After all requested calibration work is completed the

fees are computed and NIST fonn 64 is generated using the ISSC database. One copy is filed in

the test folder and one in the customer folder or DG file folder, one copy is kept in the

Calibration Log and one copy and the original is sent to the Administrative Officer for the

Radiation Interactions and Dosimetry Group. The test folder is then signed and returned to the

calibration program. Shipping request forms are prepared after the Division Chief signs off on a

calibration 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.

TLD exposure calibration reports are generated in a different manner than the instrument

calibration reports. The data are taken and entered in a data book. After review, the data are transferred to a TLD calibration report form, see Attachment 6 and a DG number is assigned.

The report is reviewed for correctness, initialed, then sent for signatures and handled in the same

manner as the instrument calibration reports.

10.2. Calibration of integrating-type instruments

10.2.1. Condenser chambers

10.2.1.1. General considerations

This type of instrument consists of a charger/reader and a variety of removable condenser

probes. As mentioned previously, the only charger/reader sent to NIST 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 Victoreen condenser chamber 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. Connection with the chamber 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 chamber, 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. For those models where the wall thickness is insufficient for the energy of the radiation requested, an equilibrium shell must be added.

49
10.2.1.2. X-ray calibrations

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 to the same calibrated x-ray source under the same conditions. The

probe's response is normalized to 101.325 kPa (760 mm Hg) pressure and 295.15 K (22 °C).

Prior to calibration, the test 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. The test chamber is placed in a holder and the laser beam is 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 Wyckoff-Attix (50 kV to 300 kV) chamber, the defining plane is the white line on the

aperture holder. For the Ritz (20 kV to 100 kV) chamber, the defining plane is exactly 15.00

mm beyond the white line on the aperture holder in the downstream direction from the x-ray

source. For the Lamperti (10 kV to 20 kV) chamber, the defining plane is exactly 20.00 mm

downstream from the source from the scribed line on the plastic insert that fits into the

removable shield and touches the front face of the aperture. For the Attix (10 kV to 50 kV)

chamber, the defining plane is the scribed line in the brass aperture holder. Adjustment of the

test chamber position to the defining plane is accomplished by sighting through the

telemicroscope which was previously aligned on the defining plane of the free-air chamber.

Motorized slides are then used to adjust the test chamber reference point to the standard defining

plane. Control of the motorized slides is accomplished using a control box located in the vicinity

of each telemicroscope. 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 thin window 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 axis.

A further consideration in x-ray calibration is choice of an appropriate beam size. For

this purpose a parameter called the "beam size" is compared to the largest dimension of the

active volume. The general practice is to use a 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 beam size of the x-ray beams, for

all beam defining apertures for the vertical and horizontal beam position has been determined

using an Exradin Al ion chamber, as well as film. The usefiil beam size is the point where the

ratio of the optical density to the center of the film reveals a change of less than 0.5 %. The rafio

of the intensity as measured with the ion chamber of the center to the outer point of the useful

beam should also change less than 0.5 %.

Leakage measurements are taken prior to calibration after a pre-irradiation. If the

leakage is a significant fraction of the expected exposure reading, either the probe is cleaned or

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. Since the gas is cold due to expansion, some time must be allowed

for the chamber to equilibrate with room temperature. If the cleaning procedure is not

successful, the calibration is terminated.

50
10.2.1.3. Gamma-ray calibrations

The instruments to be calibrated using gamma-ray beams are calibrated in essentially the

same way as in the x-ray beams. The primary difference is that in the gamma-ray beams, a NIST

standard is not used at the time of calibration. Instead, a previously determined value of the air- kerma rate is corrected for decay to the time of calibration. As with the x-ray calibrations, a check chamber is used. The source-to-chamber distance is set by sighting the telemicroscope on

the appropriate scale distance.

All the horizontal sources in rooms BO 15 A, B015B, B021A, and B021B 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 telemicroscope 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 metric calipers and determining the radius. The probe is then placed

in the beam, aligned as above, and adjusted so that the front or back of the chamber is tangent to the telemicroscope cross-hairs.

10.2.2. Cable-connected instruments

10.2.2.1. 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. 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 NIST. If the potential is found to be outside

specifications, it may require replacement of often unique type batteries or returning the

instrument to the customer for repair.

10.2.2.2. X-ray calibrations

Alignment of these types of probes is the same as for 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.

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 unlike the electrometers of condenser-types which are generally

readable only to 1 %.

51
10.2.2.3. 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.

10.3. Calibration of current-type instruments

Background information about current-type instruments is given in Section 8.1.

10.3.1. X-ray calibrations

Section 8.3.1 gives a description of the data-acquisition system and the ionization current

measuring systems. The setup techniques for these types of chambers is basically the same as

for both of the integrating-type probes described above, although some additional steps need to be taken. After mounting and aligning the chamber in the fixture and applying the collection

potential, the collecting voltage is checked at the chamber. This insures that the voltage

connection has been made. Also with certain Triax-to-BNC 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 movement of the instrument mount does not cause the cable to

shift or bind. It is also important to keep all connections out of the radiation beam, thereby

reducing errors due to contributions from extra-cameral volumes.

10.3.2. Gamma-ray calibrations

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.

Care should be taken in shielding the signal lead connections from the beam and from scattered

radiation. Errors of as much as 5 % can be introduced, even in the scattered beam, if shielding is not employed.

10.4. In-house calibration checks

The long-term reliability of NIST dosimetry calibrations depends on the stability of the

NIST air-kerma standards. For gamma rays, the NIST air-kerma standard is the mean response

of six spherical graphite ionization chambers, which are periodically compared. For x-rays, the

NIST air-kerma standard is the response of the appropriate free-air chamber; the four free-air

chambers are also periodically compared. The in-house calibration checks are intended to check

both the stability of the NIST standards and the reliability of the calibration procedures.

Two methods are used to verify a calibration. The first is to calibrate a NIST chamber

which has a calibration history and is similar to the customer's chamber. The second check is an

examination of previous calibrations of the customer's instrument at the same beam quality. If

the discrepancy is significant, greater than 0.2 % for current-type probes, and greater than 1 %

for integrating-type instruments, an investigation is warranted. When there are several previous

calibrations of the customer's instrument at any one beam quality, one can estimate the

52
reproducibility and decide whether the current value is acceptable.

For the gamma-ray and x-ray check chambers, a record is maintained of all calibrations,

and the previous calibrations are compared with the current calibration to detect any trend or measurement discrepancy. Any discrepancy arising with a NIST check chamber of the order of

magnitude mentioned above, gives 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 the complete calibration process is repeated.

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 using linear regression.

The results showed trends in the calibration factors of no more than 0.03 % per year, with an

average of 0.01 % per year. See Table 10-1 for the trends in three NIST check chambers from

1979 to 1997.

Table 10-1. Analysis of three NIST check chambers for the period of 1979 to 1997

Chamber Shonka/

Wyckoff

Exradin Victoreen

Model 3 cm^ A4 A4 A4 415A 415A

Serial Number 344 117 117 117 121 121

Beam Quality Code •^^Cs •^^Cs ''Co MlOO M30 M50

Number of data points 122 37 31 20 45 44

% change / year 0.001 0.003 0.004 0.005 0.02 0.03

% Standard Error

of Regression

0.0003 0.007 0.02 0.013 0.03 0.03

% Standard Deviation^ 0.11 0.09 0.17 0.21 0.71 0.6

" Standard deviation of average of all calibration factors.

10.5. Test of high-quality feedback electrometers

NIST 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. Kg A typical

report form is found in Attachment 7. As a check on this electrical test, the customer's

current-type chamber is calibrated for one beam quality with both the NIST system and with the

customer's system. Agreement is usually within 0.2 %, if not, the calibration is reviewed and

possibly repeated.

53
10.6. Irradiation of passive and electronic dosimeters

10.6.1 General considerations and procedures

Dosimeters received at NIST for irradiation in terms of air kerma or exposure are the type used

for personnel protection. The air kerma from x-rays, ''^Cs and/or ^°Co requested by the customer

can be delivered to passive and electronic dosimeters.

10.6.2 Exposure techniques

For both x- and gamma-ray exposures, the dosimeters are mounted either on a scatter-free frame

or a 30 cm x 30 cm x 15 cm methylmethacrylate phantom at a distance such that the beam is uniform over the area of irradiation. The irradiation distance is the source-to-badge-middle

when using the scatter-free frame. When using the phantom, the irradiation distance is the

source-to-phantom front surface. No correction is made for the distance of the badge-tophantom offset. Gamma-ray exposures are performed at a scale distance of 195 cm or 300 cm. The air kerma is obtained from the charts associated with each source, corrected for decay. All

irradiation details are recorded in a data book. After review of the results in the data book, the

badge identification numbers, air-kerma levels, date of irradiation and a DG number are transferred to a TLD report form, see Attachment 6.

11.0 Assessment of uncertainty

The method of uncertainty assessment follows the NIST policy of expressing uncertainty,

as outlined in the NIST Technical Note 1297. Conventional statistical estimates are given as standard deviations of the mean, and 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 extensive experience. The "Type B" uncertainties are estimated

so as to correspond approximately to one standard deviation. The Type A and Type B estimates

are combined according to the usual rule for combining standard deviations, by taking the square

root of the sum of the squares, 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 the

coverage factor, two, to give an expanded uncertainty. The uncertainty is considered to have the

approximate significance of a 95 % confidence limit. Table 11-1. lists the details of the

assessment of uncertainty in the air kerma rates determined for the tungsten x-ray beams by the

free-air ionization chambers, and as determined for gamma-ray beams by the set of spherical

graphite cavity ionization chambers. Table 11-2. lists 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 dosimeters, conducted with

X- and gamma-rays. Table 1 1-3 lists the details of the assessment of uncertainty in air-kerma

rates determined for the mammographic x-ray beams by the Attix chamber. Table 1 1 -4 lists the

uncertainty components for the calibration of instruments used for mammography. Table 11-5

gives a brief description of some of the uncertainty components. Since the estimates of

uncertainty vary lightly with beam qualities, methods of measurement, and rate, in each case the

largest value is used for the estimate. In an official calibration, measurements could be repeated

54
11-1. Uncertainty analysis for tungsten x and gamma-ray air-kerma rates, shown in %

Gamma-ray beam X-ray beam

Uncertainty components Type A Type B Type A Type B

air density 0.01 0.08 0.01 0.08

charge 0.03 0.1 0.12 0.06

humidity 0.07 0.07

volume 0.06 0.05 0.04 0.01

g

0.02 0.02

W/e 0.15 0.15

axial nonuniformity 0.02 NA

eiieciive measurement pomi U.U J

XT A

energy-absorption coefficient ratio A A^ XT A ISA

mean origin of electrons (\ 1 U. 1 XT A JNA

radial nonuniformity A A1 U.U 1 XT A JNA

blCIIl aCaiicr A AS XTA

Sioppmg-power raiio AU.Z9SJ XT A

iimmg A 1 U. 1 XT A

air attenuation, "NT A A (Yl U.U /

electric field distortion NA 0.2

electron loss, k^ NA 0.1

penetration of aperture NA 0.04

penetration of chamber face NA 0.01

polarity difference NA 0.1

recombination loss, k^ 0.1 0.1

scattered photons, NA 0.2

quadratic sum

combined standard uncertainty

expanded uncertainty

0.08

0.39

0.78

0.38 0.19

0.41

0.82

0.37

55
Table 1 1-2 Uncertainty analysis for gamma-ray calibrations, shown in %

Current-type Condenser Passive dosimeters

Uncertainty components Type A Type B Type A Type B Type A Type B

air kerma rate 0.08 0.38 0.08 0.38 0.08 0.38

air density 0.01 0.08 0.01 0.08 NA

charge 0.04 0.1 NA NA

distance 0.01 0.01 0.01

humidity 0.05 0.05 NA

leakage 0.02 0.1 NA

radiation background negligible negligible NA

radial nonuniformity NA NA 0.02

recombination loss NA NA NA

scale reading NA 0.2 0.5 NA

timing 0.04 0.1 0.04 0.1 0.1

quadratic sum 0.1 0.4 0.22 0.64 0.08 0.44

combined standard

uncertainty 0.41 0.68 0.45

expanded uncertainty 0.82 1.36 0.9

Table 1 1-3 Uncertainty analysis for air-kerma rates with the Attix chamber, shown in %

Uncertainty components Type A Type B

air density 0.01 0.08

aperture area 0.01 0.01

charge 0.20 0.06

humidity 0.07

plate separation 0.01 0.07

air attenuation 0.20 0.01

0.02

photon scatter 0.20

polarity 0.1

recombination 0.06

W/e 0.15

quadratic sum 0.31 0.29

combined standard uncertainty 0.42

expanded uncertainty 0.84

56

j

i
Table 1 1-4 Uncertainty analysis for mammography calibrations, shown in %

Uncertainty components Type A Type B

air kerma rate 0.31 0.29

charge 0.12 0.06

air density 0.10 0.08

humidity 0.05

distance 0.02

quadratic sum 0.33 0.31

combined standard uncertainty 0.46

expanded uncertainty 0.91

Table 1 1-5 Explanation of various components of the uncertainty analyses

uncenamiy Type oimpiiiiea uescnpiion oi L^omponeni

v^uiiipuiiciii

air attenuation A The average oi many air attenuation direct measurements using the Attix cnamoer. B Obtained by calculating the worst realistic error in air path length. This involves

UlC UIICCI lallliy Ui CdlaUllalllllg a ICpiUUULIUlc all paUl Icll^lll dllU lllC CilCCl Oil lllc air-attenuation correction for a small distance error.

air density A Based on typical changes in air density during a measurement set. B Results from the calibration of the barometers and the thermometers.

aperture B The ability to align aperture to center of volume is considered negligible. Since

alignment the beam is uniform at center, an insignificant change in the positioning would

not result in a measurable change to charge.

aperture area A The standard deviation of the measurements of each aperture used.

B Based on the NIST length standard.

charge A The average standard deviation of the primary chamber charge combined with the monitor chamber charge standard deviation.

B Results from the calibration of the electrometers.

g B See Ref 2. humidity B Refers to the uncertainty in correcting to dry air. photon scatter B The photon scatter corrections were determined by Ritz with a stated uncertainty

of+/-0.2%, see Ref 4. plate A For the Attix chamber, the stepper motor accuracy of 400 steps/rev and the lead

separation screw pitch of 1 mm/rev leading to 0.0025 mm out of 50 mm . B Based on the lead screw accuracy of 0.008 in/ftused for the Attix chamber.

recombination A The average of the standard error associated with curve fits of the recombination

data. timing A The standard deviation of various shutter times.

B Set by the oscillator in the shutter timer or feature of computer board clock.

W/e B See Ref 2.

57
to maintain optimal conditions. Table 1 1-6 gives a summary of expanded uncertainties for each

type of calibration.

Table 1 1 .6 Summary of expanded uncertainties

Tungsten X-rays and Gamma Rays Mammography X-rays

air kerma rate 0.8 % air kerma rate 0.9 %

cable connected chambers 1 %

passive dosimeters 1 % ionization chambers 1 %

condenser chambers 1.5%

The Type A components are generally less significant than the Type B components. This

arises from the complexity of the interaction of ionizing radiation with matter: it is never

possible to account fiilly for the numerous second-order effects, but it is usual during air-kerma

calibrations at NIST to repeat measurements until the Type A component of the uncertainty is smaller than the Type B component. The measurement of volume is an exception, but this is not

a routine measurement and both components are small.

12. 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 exposure to high

voltage, such as exists on ionization chambers and standard 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 in a normal operating mode. All radiation areas in the building are marked with striped tape and dosimeters must be worn by all personnel.

12.1. Radiation safety

12.1.1 X-ray calibration ranges

First and foremost, the three 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 Recent changes to the shielding of this tube eliminated the leakage radiation that once

occurred between the shutter assembly and the tube window at 90°. Flashing red lights signal

any malfunction to the shutter and an audible area radiation detector has been recently

implemented as a back-up precaution. 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 lead safety shutter and a 12.7 mm lead timing shutter in

front of the beam portal. 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. Where no door exits, as in one area of the 300 kV x-ray

58
range, a light beam is used for protection. In addition, a time-delay device inside the 300 kV

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. The mammography range shielding door is interlocked with the x-ray tube shutter.

If the door is opened or is not fully closed, the shutter will return to the shielding position. An

audible alarm will sound if the shutter is not fully closed. Red lights illuminate when the shutter

is opened. Additional lead shielding surrounds the x-ray tube, which must be in the shielding

position for power to be applied to the x-ray tube. No area radiation monitor is used due to the

extremely narrow beam and the low scatter conditions resulting from the low energy x-rays used

in the mammography range.

12, 1 .2. Gamma-ray calibration ranges

All doors giving access to the gamma-ray calibration ranges have interlocks as required

by the 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. Automatic

shielding doors protect occupants in the control area of B036 from the sources. In addition to the

above safety features, a radiation detector with indicator lights and an audible signal is in each

gamma calibration range. A second radiation detector located in the vertical beam area, between

the shielding door of the vertical beams and the outer door, alarms whenever the interlock is broken during an irradiation. At each entrance to a gamma-ray calibration range, a set of two red

lights indicates a "beam on" condition.

12.2. High-voltage safety

The only danger that exists from high voltage comes from the free-air ionization

chambers, the customer chamber, 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 high-voltage electrodes. Appropriate warning signs are posted. The risk of high

voltage from the Attix chamber is minimized by surrounding the chamber with protective cover.

59
REFERENCES

1. Marshal, J.L.,ed. NIST Calibraiton Services Users Gide, NIST (U.S.) Spec. Publ. 250; 1998

January. Available on-line: http://www.ts.nist.gov/calibrations.

2. Boutillon, M. and Perroche-Roux, A.M., Re-evaluation of the W value for electrons in dry

air, Phys. Med. Biol., 32, 213-219 (1987).

3. Wyckoff, H. O. and Attix, F. H., Design of free-air ionization chambers, National Bureau of

Standards Handbook 64, 1957.

4. 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).

5. Lamperti, P. J. and Wyckoff, H. O., NBS free-air chamber for measurement of 10 to 60 kV x

rays, J. Res. Nat. Bur. Stand. (U.S.), 69C, 39-47 (1965).

6. Ritz, V. H., Design of free-air ionization chambers for the soft x ray region (20-100 kV),

Radiology, 73, 911-922 (1959).

7. 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).

8. AUisy, A. and Roux, A, M., Contribution a la measure des rayons roentgen dans le domaines

de5 a 50 kV, Acta Radiologica, 55, 57-74 (1961).

9. Scott, P. B. and Greening, J. R., The determination of saturation currents in free-air ionization

chambers by extrapolation methods, Phys. Med. Biol. 8, 5 1-57 (1963).

10. Berger, M. J. and Seltzer, S. M., Stopping powers and ranges of electrons and positrons,

NBSIR 82-2550-A (2nd Ed.) (1983).

11. 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).

12. 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).

13. Attix, F. H.," Electronic equilibrium in free-air chambers and a proposed new chamber

design," Naval Research Laboratory Report No. 5646, 1961.

14. Coletti, J.G., Pearson, D. W. and DeWerd, L.A., "Mammography exposure standards:

Design and characterization of free-air ionization chamber," Rev. Sci Instrum. 66, 2574-

2577(1995).

15. Laitano, R.F. and Toni, M.P. "The primary exposure standard of ENEA for medium energy

60
x-rays. Characteristics and measurement procedures," Report RT/PROT (83) 27,

Laboratorio di Metrologia della Radiazioni lonizzani ENEA, CRE Casaccia, c.p. 2400

Roma, Italia, 1984.

16. Bums, D.T., Lamperti, P. and O'Brien, M. "Comparison of the NIST and BIPM Air-Kerma

Standards for Measurements in the Low-Energy X-Ray Range," J. Res. Natl. Inst. Stand.

Technol. 104, 135 (1999).

17. Lamperti, P., O'Brien, M., Sander, T. and Williams, T. "Comparison of the NIST and NPL

Air-Kerma Standards for Measurements in the Low-Energy X-Ray Range,"J.Res. Natl.

Inst. Stand. Technol. 105, 701 (2000).

18. Coletti, J. G., Pearson, D. W., DeWerd, L.A., O'Brien, C. M. and Lamperti, P.J. "Comparison of exposure standards in the mammography x-ray region," Med. Phys.

24(8), 1263-1267 (1997).

19. Report of the International Commission on Radiological Units and Measurements (ICRU),

National Bureau of Standards Handbook 78, p. 19, 1959.

20. Loftus, T. P. and Weaver, J. T., Standardization of 60Co and 137Cs gamma-ray beams in

terms of exposure, J. Res. Nat. Bur. Stand. (U.S.), 78A, 465-476 (1974).

21. 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).

22. Taylor, L. S. and Singer, G., Note on the guarded-field x-ray ionization chamber. Radiology

26, 322-325 (1936).

23. Taylor, L. S., The precise measurement of x-ray dosage, Radiology 14, 372 (1930).

24. Boag, J. W., Distortion of the electric field in an ionization chamber due to the difference in

potenfial between guard ring and collector, Phys. Med. Biol. 9, 25-32 (1964).

25. Day, F. H. and Attix, F. H., Methods for current measurement with a vibrating reed

electrometer. Nat. Bur. Stand. (U.S.), Report 2080 (1952).

26. Loevinger, R., Precision measurement with the total-feedback electrometer. Phys. Med. Biol.

2, 267-279 (1966).

27. Keithley, J. F., Yeager, J. R., and Erdman, R. R., Low Level Measurements, Keithley

Instruments, Inc., 28775 Aurora Road, Cleveland, OH 44139 (1984).

28. Frederiksen, T. M., Intuitive IC Op Amps, National Semiconductor Corp., Santa Clara, CA.

(1984).

29. Instruction Manual, Model 616 Digital Electrometer, Keithley Instruments, Inc.. 28775

61
Aurora Road, Cleveland, OH 44139 (1977).

30. Instruction Manual, Model 642 Electrometer, Keithley Instruments, Inc., 28775 Aurora

Road, Cleveland, OH 44139 (1980).

31. Rutkowski, G. B., Handbook of Integrated-Circuit Operational Amplifiers, Prentice-Hall,

Inc., Englewood Cliffs, NJ (1975).

62
Attachment 1 : X-Ray Beam Qualities offered at NBS Prior to 1986
1 dulc r\- 1

.

X-ray beam qualities offered at NBS prior to 1986

Lightly Filtered X-Rays

Beam Constant Distance Added Half-Value Layer Homogeneity

Code r oiennai Filter^' Coeriicient

V

;

(mm Al) (mm Al) (Al)

1 0 75 0 0.03 n TO U. / y

1 J 0 0.05 n 7/1 U. /4

L-L) 50 Q 0.07 U. /o

L-E'' 20 50 0.5 0.23 0.78

L-G 30 50 0.5 0.36 0.64

L-I 50 50 1.0 1.02 0.66

L-K 75 50 1.5 1.86 0.63

T -Vf 1 nn 50 2.0 2.78

Moderately Filtered X-Rays

Added Filter'^ Half-Value Layer Homogeneity

Beam Constant Al Cu Al Cu Coemcient

Code r oieniidi

(k'V^ (mm) (mm) (mm) (mm)

MrrJ \J\J 0 0 1.62 U.oo 2.50 0 2.79 0.09 nu. 70/y

iVlr r!, /5 2.51 3.39 u. /o

MFG 100 3.50 0 5.03 0.20 0.73

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

MFD Zjv 3.47 3.20 18.30 3.25 0.98

Heavily Filtered X-Rays

Added Filter'^ Half-Value Layer

Beam Constant Effective

Code i oieniidi Al Cu Sn Pb Al Cu Energy

(KV ) (mm) (mm) (mm) (mm) (mm) (mm) (keV)

HFC 50 2.50 0 0 0.10 4.20 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

^ Inherent filtration approximately 1 .0 mm Be.

^ Discontinued beam codes.

Inherent filtration approximately 1.5 mm Al.

64
Attachment 2 : Additional references - data books
The references listed in this attachment are NIST registered laboratory data books (DB) or binders which are located in B033 of the Radiation Physics building. The references are

identified by the data book identification number and the page number.

1. DB848 page 110 (DB 848:1 10), DB 331:92-1 17 and DB 527:081

2. DB852:11

3. A loose-leaf binder entitled "Supplement to DB776, Free-Air Ionization Chamber

Comparisons."

4. See DB592, DB624, and DB638. The data plots used for extrapoladons are filed in a file cabinet in Rm. B033 under the general title "Free-Air Ionization Chamber

Recombination." Further analyses of the data are recorded in a three-ring binder entitled,

"Supplement No. 1 to DB 592, DB624 and DB638; 10-60 kV and 20-100 kV Free-Air

Ionization Chamber Comparison."

5. DB752:143-162.

6. See DB592, DB624 and DB638. Also found in binder entitled "Supplement No.l to DB592,

DB624 and DB638" and in file folders under the general title "Free-Air Chamber

Recombination," all filed in Rm. B-033.

7. DB750:1 18, DB744:203, DB717:127 and DB717:125.

8. DB799:34, DB817:52, DB750:201, DB799:44 and 60, DB817:121,131 and 141, DB799:70,

DB817:138and DB817:121.

9. DB817:155.

10. DB817:94.

11. DB799:10-llandDB817:17.

12. DB817:51-52.

66
Attachment 3 : Attix chamber schematics
.21.27 cm

Chamber Center

1 m from Source

-^1.27 cm

Fig. A3-1. Attix chamber configuration for half-value layer and air attenuation measurements.

68
Chamber Center

2 1 .27 cm from Aperture

Aperture

1 m from source

15.5 cm

Fig. A3-2. Attix chamber configuration for the two smallest positions of routine calibrations.

Chamber Center

21.27 cm from Aperture

Aperture

1 m from source

8 cm

25 cm

10.5 cm'

Fig.A3-3 Attix chamber configuration for the two largest positions for an air-kerma

measurement.

69
Attachment 4 : The history of the measurement of current

70
Until high-gain negative-feedback ampHfiers 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 Ref.

[21-23] 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

essendally 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 NIST as null detectors [3,24]. 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 charge. Loevinger [25] 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 Ref. [25-31].

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 resisdve feedback element is shown in Fig. A3-1. The signal current l^^^ from

an ionization chamber impresses a signal on the input terminal of the operational amplifier,

which generates an output voltage Vq. Part or all of Vq is fed back (from the load resistor RJ

through the feedback resistor Rf to the input terminal, so as to oppose the input signal. The

output voltage is larger than the resultant input voltage Vj in the ratio

G = Vo^yr (A3-1)

where G is the amplifier gain. In modem electrometers, such as the Keithley 616, G > 104, so

for present purposes V, can be neglected compared to Vq, and

Kig=^o- (A3-2)

For operational amplifiers with large input resistance, for which the input voltage Vi~ 0, there can be no current through the amplifier, and the current through the feedback resistor is equal to the ionization chamber current Li„. Therefore

Kis=Vp (A3-3)

and finally

^sig-^o^^f (A3-4)

This is the relationship used for measuring current when using a resistive feedback element.

71
Fig. A3-1 . Circuit of a negative-feedback amplifier with resistive feedback.
For low-current measurements high-megohm (MQ.) resistors are required. For example,

a feedback resistor of 20 Mfl is required to develop a feedback voltage of 1 V for the

exceptionally large ionization current of 50 nA. Day and Attix [24] 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 Corporation air-dielectric capacitors have been used as feedback elements for dosimetry calibration work for many years. A negative-feedback

amplifier with capacitative feedback element Cf is shown in Fig. A3-2. In this case,

^K>,-I..s^"Cp (A3-5)

and results in

^sig=CjAVJAt. (A3-6)

This equation is the basis for all important ionization-current measurements at NIST. The

ionization current Ig^g is determined from measurements of Vq using accurate digital voltmeters

which can be commanded to sample-and-hold readings of Vq 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 AVq / At = constant, and the ability of the device

to transfer essentially all of the charge produced in the ionization chamber to the measurement

system. The slope AVq / 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 Cf does not affect

the ionization current being supplied from the current source. If this were not so, the charge

build-up on Cf 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, = Vq 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. 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.

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. 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

73
gain may be as high as 50 000.

Fig. A3-2. Circuit of a negative feedback amplifier with a capacitive feedback element.
Attachment 5 : Typical ionization chamber calibration report
National Institute of Standards and Technology

REPORT OF AIR-KERMA CALIBRATION

OF

ADDRESS 1 ADDRESS 2

ADDRESS 3

Radiation Detection Chamber: Make, Model, SN

Measurements performed by Michelle O'Brien

Report reviewed by Paul Lamperti

Report approved by Stephen M. Seltzer

For the Director

National Institute of Standards and Technology

by

Bert M. Coursey, Chief

Ionizing Radiation Division

Physics Laboratory

Information on technical aspects of this report may be obtained from Michelle O'Brien,

National Institute of Standards and Technology, 100 Bureau Drive Stop 8460, Gaithersburg,

MD 20899, (301) 975-2014. Report format revised 5/00.

Nisr

Nationol Institute of Standard: and Technology

Technology Adminrslrolion, U.S. Deporlmenl of Commerce

DG:XXXXX/00

TF:XXXXXX-00

Report Date: 0/0/00

Page # of

#

76
National Institute of Standards and Technology

REPORT OF AIR-KERMA CALIBRATION

OF

ADDRESS 1 ADDRESS 2

ADDRESS 3

Radiation Detection Chamber: Make, Model, SN

Chamber orientation: the cavity was positioned in the center of the beam with the stem of the chamber

perpendicular to the beam direction

Chamber collection potential: -000 volts with respect to the inner electrode (- charge was collected)

Chamber rotation: the high voltage wire faced the source of radiation

Environmental conditions: the chamber is assumed to be open to the atmosphere

Average leakage: less than 0.02 % of signal

A detailed study of ionization recombination was not performed and no correction was applied to the

calibration factor(s). Ifthe chamber is used to measure an air-kerma rate significantly different from that used

for the calibration, it may be necessary to correct for recombination loss.

Beam

Code

Half -Value Layer Equilibrium

Shell

Added

Calibration Factor

(Gy/C)

295.15 K (22 "C) and

101.325 KPa (1 Atm)

Air- Kerma

Rate

(Gy/s)

Calibration

Distance

(cm)

mm Al mm Cu

NOA^ES X.XXX EX X.XX EX

NO/YES X.XXX EX X.XX EX

NOrfES X.XXX EX X.XX EX

NO/YES X.XXX EX X.XX EX

|\)lSr DG: XXXXX/00

National Institute o( Standards and Technology TF:XXXXXX-00

Technology Adminislrolion, U.S. Deportmenl of Commerce RcpOFt DstC* 0/0/00

Page # of

#

77
Conventional Calibration Conditions

for X- and Gamma-Ray Measuring Instruments

OCell 11 code

Additional Filtration ^ Half-value layer Homogeneity

coefficient

i^llCL'll vc energy

Al

(mm)

Cu

(mm)

Sn

(mm)

Pb

(mm)

Al

(mm)

Cu

(mm)

Al Cu (keV)

X-Ray Beam Qualities

LIO 0.035 89

L15 0.057 68

L20 0.069 73

L30 0.30 0.22 63

L40 0.53 0.50 59

L50 0.71 0.76 60

L80 1.45 1.83 57

LlOO 1.98 2.77 57

M20 0.27 0.15 69

M30 0.5 0.36 65

M40 0.89 0.73 69

M50 1.07 1.02 66

M60 1.56 1.68 66

M80 2.61 2.97 67

MlOO 5.0 5.02 73

M120 6.87 6.79 77

M150 5.0 0.25 10.2 0.67 87 62

M200 4.1 1.12 14.9 1.69 95 69

M250 5.0 3.2 18.5 3.2 98 86

M300 4.0 6.5 22.0 5.3 100 97

HIO 0.105 0.05 91

H15 0.5 0.153 86

H20 1.01 0.36 91

HBO 4.50 1.23 93

H40 4.53 0.26 2.90 90

H50 4.0 0.1 4.2 0.142 92 90 38

H60 4.0 0.61 6.0 0.24 94 89 46

HlOO 4.0 5.2 13.5 1.14 100 94 80

H150 4.0 4.0 1.51 17.0 2.5 100 95 120

H200 4.0 0.6 4.16 0.77 19.8 4.1 100 99 166

H250 4.0 0.6 1.04 2.72 22.0 5.2 100 98 211

H300 4.1 3.0 5.0 23.0 6.2 99 98 252

S60 4.35 2.77 72

S75 1.50 1.86 63

Gamma-Ray Beam Qualities

137cs 10.8 662

14.9 1250

^The additional filtration value does not include the inherent filtration. 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.

Nisr

National Institute of Standards and Technology

Technology Adminislralion, U.S. Deportmenl of Commerce

DG: XXXXX/00

TF:XXXXXX-00

Report Date: 0/0/00

Page # of

#

78
Explanation of Terms Used in tiie Calibration Procedures and Tables

Air Kerma: The air-kerma rate at the calibration position is measured by a free-air ionization chamber for X radiation and by graphite cavity ionization chambers for ^"Co and ''^Cs gamma radiation, and is expressed

in units of grays per second (Gy/s). The gamma-ray air-kerma rates are corrected to the date of cahbration

(from previously measured values) by decay corrections based on half-lives of 5.27 years for ^'^Co and 30.0

years for '"Cs. For a free-air ionization chamber with measuring volume V, the air-kerma rate is determined

by the relation:

where

>air I

I / (p^^y) is the ionization current, measured by the standard, divided by the mass of air in the

measuring volume

fF^ir is the mean energy expended by an electron of charge e to produce an ion pair in dry air, the

value used at NIST is WJe = 33.97 J/C

g^i^ is the fraction of the initial kinetic energy of secondary electrons dissipated in air through radiative processes, the values used at NIST are 0.0032 for ^°Co, 0.0016 for '"Cs and

0.0 (negligible) for x rays with energies less than 300 keV, and

n A:, is the product of the correction factors to be applied to the standard.

Air kerma in grays (Gy) is related to exposure {X) in roentgens (R) by the equation:

x=

2.58^-4 W.

To obtain exposure in roentgens, divide air kerma in grays by 8.79E-3 for ^''Co gamma rays, 8.78E-3 for '"Cs

gamma rays, and 8.76E-03 for x rays with energies less than 300 keV.

Beam Code; The beam code identifies important beam parameters and describes the quality ofthe radiation

field. NIST offers four types of reference beam qualities, as well as the ISO reference radiation qualities.

NIST beam codes are referred to as L, M, H, and S beams, which stand for light, moderate, heavy, and special

filtration, respectively. The number following the letter is the constant potential across the x-ray tube. For

gamma radiation, the beam code identifies the radionuclide.

Calibration Distance; The calibration distance 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 at the stated distance is appropriate for the chamber dimensions.

Calibration Factor; The calibration factors given in this report are quotients ofthe air kerma and the charge

generated by the 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. With the assumption that the chamber is open to the atmosphere, the measurements are normalized to a pressure of one standard atmosphere (101.325 kPa) and a temperature of 295.15 K (22 °C).

|\)[SJ- DG: XXXXX/00

National Institute of Standards and Technology TF:XXXXXX-00

Technology Administration, U.S. DeparlmenI of Commetce RcpOFt DfltC* 0/0/00

Page # of

#

79
Use of the chamber at other pressures and temperatures requires normalization of the ion currents to these

reference conditions using the normalizing factor F (see below).

Effective Energy: The effective energy is shown for those beams where it is considered a meaningful

characterization of the beam quality. The effective energy for gamma radiation is the mean photon energy

emitted by the radionuclide, and for x radiation it is computed from good-geometry copper attenuation data. The initial slope of the attenuation curve is used to determine the attenuation coefficient, and the photon

energy associated with this coefficient is given as the "effective energy." The energy vs attenuationcoefficient data used for this purpose were taken from J. H. Hubbell, Int. J. Appl. Radiat. Isot. 33, 1269

(1982). For beam codes H50-H300, the effective energy is well represented by the equation: effective

energy = 0.861 F - 6.1 keV where Fis the constant potential in kilovolts.

Equilibrium Shell ; Material added to the nominal wall thickness of the chamber to ensure electronic

equilibrium.

Half-Value Layer: The half-value layers (HVL) in aluminum and in copper have been determined by

measurements with a free-air chamber for x radiation, and have been calculated for the copper HVLs of ''"Co and '"Cs.

Homogeneity Coefficient: The homogeneity coefficient is the quotient ofthe first HVL and the second HVL,

generally expressed as a percent.

Humidity: No correction is made for the effect of water vapor on the instrument being calibrated. It is assumed that both the calibration and the use of that instrument take place in air with a relative humidity

between 1 0 % and 70 %, where the humidity correction is nearly constant.

Normalizing Factor F: The normalizing factor F is computed from the following expression:

F= (273.15 + T) I (295.15//) where Tis the temperature in degrees Celsius, and //is the pressure expressed

as a fraction of a standard atmosphere. (1 standard atmosphere = 101.325 kilopascals = 1013.25 millibars = 760 millimeters of mercury)

Uncertainty: The expanded, combined uncertainty ofthe calibration described in this report is 1 %, ofwhich

0.8 % is assigned to the uncertainty in the air-kerma rate of the NIST beam. The expanded, combined

uncertainty is formed by taking two times the square root ofthe sum ofthe squares ofthe standard deviations

ofthe 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. Details of the uncertainty analysis are given in: Lamperti, P. J., Loftus, T.P., and Loevinger, R., "Calibration of X-Ray and Gamma-Ray Measuring Instruments at the

National Bureau of Standards," NBS Special Publication 250-16 (1987).

National Institute of Standards and Technology

Technology Adminislrotion, U.S. Department of Commerce Nisr DG: XXXXX/00

TF:XXXXXX-00

Report Date: 0/0/00

Page # of

#

80
Attachment 6: Typical TLD calibration report
National Institute of Standards and Technology

REPORT OF '''Cs IRRADIATION

OF

ADDRESS 1 ADDRESS 2

ADDRESS 3

Measurements performed by Ronnie Minniti

Report reviewed by Paul Lamperti

Report approved by Stephen M. Seltzer

For the Director

National Institute of Standards and Technology

by

Bert M. Coursey, Chief

Ionizing Radiation Division

Physics Laboratory

Information on technical aspects of this report may be obtained from Michelle O'Brien,

National Institute of Standards and Technology, 1 00 Bureau Drive Stop 8460, Gaithersburg,

MD 20899, (301) 975-5586. Report format revised 5/00.

(VJlSf DG: XXXXX/00

National Institute of Standards and Technology TF:XXXXXX-00

Technology Administration, U.S- DeporlmenI of Commerce T> _4. t-» j. r\ lf\ /ryrt Report Date: 0/0/00

Page # of

#

82
National Institute of Standards and Technology

REPORT OF Cs IRRADIATION

OF

ADDRESS 1 ADDRESS 2

ADDRESS 3

Dosimeter Type:

Badge Orientation: front face normal to the incident radiation.

Air Kerma Rate: X.XX EO Gy/s.

TLD Identification Exposed Total Total Irradiation on Exposure Air- Distance

Phantom (mR) Kerma (cm)

(mGy)

Model Number

YES/NO X.XXX EO X.XX EO 000

YES/NO X.XXX EO X.XX EO 000

See the last three pages of attachment 5 for an example of the remainder of this report.

DG: XXXXX/00

National Institute of Standards and Technology TF:XXXXXX-00

Technology Adminislrclion, U.S. Department of Commerce RcpOrt DatC* 0/0/00

Page # of

#
Attachment 7: Typical electrometer calibration report

84
National Institute of Standards and Technology

REPORT OF TEST

OF

ADDRESS 1 ADDRESS 2

ADDRESS 3

Electrometer: Make, Model SN

Measurements performed by Michelle O'Brien

Report reviewed by Paul Lamperti

Report approved by Stephen M. Seltzer

National Institute of Standards and Technology

For the Director

by

Bert M. Coursey, Chief

Ionizing Radiation Division

Physics Laboratory

Information on technical aspects ofthis report may be obtained from Michelle O'Brien, National Institute of Standards and Technology, 100 Bureau Drive Stop 8460, Gaithersburg, MD 20899, (301) 975-2014.

Report format revised 5/00.

National Institute of Standards and Technology

Technology Adminisltolion, U.S. DepartmenI of Commerce Nisr DG: XXXXX/00

TF:XXXXXX-00

Report Date: 0/0/00

Page # of

#

85
THE SP 250 SERIES ON NIST MEASUREMENT SERVICES*

SP 250-1 Spectral Radiance Calibrations PB87 179883 SP 250-2 Far Ultraviolet Detector Standards PB87227609

SP 250-3 Radiometric Standards in the Vacuum Ultraviolet PB87227625

SP 250-4 Fricke Dosimetry in High-Energy Electron Beams

PB881 10374 SP 250-5 Alpha-Particle Calibrations PB881 68620

SP 250-6 Regular Spectral Transmittance PB881 08550

SP 250-7 Radiance Temperature Calibrations PB881 23674

SP 250-8 Spectral Reflectance PB881 09905

SP 250-9 Calibration of Beta-Particle-Emitting Ophthalmic

Applicators

PB881 08535

SP 250-1 0 Radioactivity Calibrations with the "4tt" Gamma

Ionization Chamber and Other Radioactivity

Calibration Capabilities

PB881 23708

SP 250-1 1 Dosimetry for High Dose Applications

PB88201587

SP 250-12 Neutron Personnel Dosimetry

PB87227617

SP 250-13 Activation Foil Irradiation with Californium

Fission Sources PB882 17443 SP 250-14 Activation Foil Irradiation by Reactor Cavity

Fission Sources PB882 17435 SP 250-15 Photometric Calibrations PB881 53747

SP 250-16 Calibration of X-Ray and Gamma-Ray

Measuring Instruments PB882 11826 SP 250-17 The NBS Photodetector Spectral Response

Calibration Transfer Program

PB88201595

SP 250-18 Neutron Source Strength Calibrations PB88211818

SP 250-19 Calibration of Gamma-Ray-Emitting

Brachytherapy Sources PB891 93858

SP 250-20 Spectral Irradiance Calibrations PB88123781

SP 250-21 Calibration of Beta-Particle Radiation

Instrumentation PB88201579

SP 250-22 Platinum Resistance Thermometer Calibrations PB881 38367

SP 250-23 Liquid-in-Glass Thermometer Calibration Service PB89 128888

SP 250-24 Standard Cell Calibrations PB881 23690

SP 250-25 Calibration Service for Inductive Voltage Dividers SP 250-26 NBS Phase Angle Calibration Services PB88225636

SP 250-27 AC-DC Difference Calibrations PB892222616

SP 250-28 Solid-state DC Voltage Standard Calibrations PB881 68703

SP 250-29 Traceable Frequency Calibrations PB881 68364

SP 250-30 GOES Satellite Time Code Dissemination:

Description and Operation

PB881 68760

SP 250-31 Mass Calibrations PB89 153894

SP 250-32 A Calibration Service for 30 MHz Attenuation and Phase Shift PB88238324

SP 250-33 A Calibration Service for Voltage Transformers and High-Voltage Capacitors

PB882252903

SP 250-34 High Vacuum Standard and Its Use

PB891 93841 SP 250-35 The Calibration of Thermocouples

and Thermocouple Materials PB89209340

SP 250-36 A Calibration Service for Current Transformers PB91216770

SP 250-37 Photometric Calibrations PB971 48472

SP 250-38 NIST Leak Calibration Service PB92 149772

SP 250-39 NIST Pressure Calibration Service PB94 164043

SP 250-^0 Absorbed-Dose Calibration of Ionization Chambers

in a ^°Co Gamma-Ray Beam

No PB number

SP 250-41 Spectroradiometric Detector Measurements:

Part I - Ultraviolet Detectors and

Part II - Visible to Near-Infrared Detectors PB98-1 49073

SP 250-42 Sprectroradiometric Detector Measurements:

Part III—Infrared Detectors PB99-1 34371 SP 250-43 Radiance Temperature Calibrations PB98-141047

SP 250-44 Radiation Processing Dosimetry Calibration Services and Measurement Assurance Program

PB98-1 40973

* Entries containing a stock number (SN003-003-) can be purchased from the Superintendent of Documents, U.S. Government

Printing Office, Washington, DC 20402-9325. GPO will accept checks, money orders, VISA, and MasterCard. For more infomation, or to place an order, call (202) 512-1800. Be sure to cite the stock number on all orders.

Entries containing PB numbers can be purchased from the National Technical Information Service, Springfield, VA 22161. NTIS will accept

American Express in addition to the payment methods listed for GPO. For more information or to place an order call (800) 553-6487.

Fax: (703) 321-8547. Be sure to cite the PB number on all orders.

Entries without stock or PB numbers are in preparation.
THE SP 250 SERIES ON NIST MEASUREMENT SERVICES* -Continued

SP 250-45 Radiation Processing Dosimetry Calibration Services: Manual of Calibration Procedures SN003-003-03514-9, PB98-1 40981 SP 250-46 NIST Multifunction Calibration System

SN003-003-03515-7, PB98-131279

SP 250-47 NIST Calibration Service for Capacitance Standards at Low Frequencies SN003-003-03549-1. PB98-1 44687

SP 250-48 Spectral Reflectance SN003-003-03545-9, PB98-149016

SP 250-49 NIST Calibration Services for Gas Flow

Meters: Piston Prover and Bell Prover Gas Flow Facilities SN003-003-03560-2. PB99-1 07260

SP 250-51 Calibration Service of Optoelectronic

Frequency Response at 1319 nm for Combined Photodiode/rf Power Sensor

Transfer Standards SN003-003-03623-4, PB2000-1 03300

SP 250-52 Error Analysis and Calibration Uncertainty of Capacitance Standards at NIST

SN003-003-03633-1 , PB2000-1 03674

SP 250-53 Calibration Service for Spectral Responsivity of Laser and Optical-Fiber Power Meters at Wavelengths Between 0.4 p.n\ and 1 .8 [im

SN003-003-03624-2, PB2000-1 02878

SP 250-54 Optical Fiber Power Meter Calibrations at NIST

SN003-003-03652-8, PB2000-1 06986

SP 250-56 Optical Fiber Power Meter Nonlinearity Calibrations

at NIST

SN003-003-03653-6, PB2000-1 06987

SP 250-57 Measurement Assurance Program for the Spectral

Density of Relative Intensity Noise of Optical Fiber Sources Near 1 550 nm

SN003-003-03658-7, PB2000-1 08256

* Entries containing a stock number (SN003-003-) can be purchased from the Superintendent of Documents, U.S. Government

Printing Office, Washington, DC 20402-9325. GPO will accept checks, money orders, VISA, and MasterCard. For more infomation, or to place an order, call (202) 512-1800. Be sure to cite the stock number on all orders.

Entries containing PB numbers can be purchased from the National Technical Information Sen/ice, Springfield, VA 221 61 . NTIS will accept

American Express in addition to the payment methods listed for GPO. For more information or to place an order call (800) 553-6487.

Fax: (703) 321-8547. Be sure to cite the PB number on all orders.

Entries without stock or PB numbers are in preparation.

AU.S. GOVERNMENT PRINTING OFnCE:2001 -472-068/46009

Technical Publications

Periodical

Journal of Research of the National Institute of Standards and Technology—Reports NIST research and

development in those disciplines of the physical and engineering sciences in which the Institute is active. These include physics, chemistry, engineering, mathematics, and computer sciences. Papers cover a broad

range of subjects, with major emphasis on measurement methodology and the basic technology underlying

standardization. Also included from time to time are survey articles on topics closely related to the Institute's technical and scientific programs. Issued six times a year.

Nonperiodicals

Monographs—Major contributions to the technical literature on various subjects related to the Institute's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed

in cooperation with interested industries, professional organizations, and regulatory bodies.

Special Publications—Include proceedings of conferences sponsored by NIST NIST annual reports, and

other special publications appropriate to this grouping such as wall charts, pocket cards, and bibliographies.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical

properties of materials, compiled from the world's literature and critically evaluated. Developed under a

worldwide program coordinated by NIST under the authority of the National Standard Data Act (Public Law

90-396). NOTE: The Journal of Physical and Chemical Reference Data (JPCRD) is published bimonthly for NIST by the American Institute of Physics (AlP). Subscription orders and renewals are available from AlP,

RO. Box 503284, St. Louis, MO 63150-3284.

Building Science Series—Disseminates technical information developed at the Institute on building

materials, components, systems, and whole structures. The series presents research results, test methods,

and performance criteria related to the structural and environmental functions and the durability and safety

characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a

subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NIST under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations. The standards establish nationally recognized

requirements for products, and provide all concerned interests with a basis for common understanding of the

characteristics of the products. NIST administers this program in support of the efforts of private-sector

standardizing organizations.

Order the following NIST publications—FIPS and NISTIRs—from the National Technical

Information Service, Springfield, VA 22161.

Federal Information Processing Standards Publications (FIPS PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register serves as the official source of information in the Federal Government regarding standards issued by NIST pursuant to the Federal Property and Administrative Services Act of 1949 as amended. Public Law 89-306 (79 Stat. 1 127),

and as implemented by Executive Order 11717 (38 FR 12315, dated May 11, 1973) and Part 6 of Title 15

CFR (Code of Federal Regulations).

NIST Interagency or Internal Reports (NISTIR)—The series includes interim or final reports on work

performed by NIST for outside sponsors (both government and nongovernment). In general, initial distribution

is handled by the sponsor; public distribution is handled by sales through the National Technical Information

Service, Springfield, VA 22161, in hard copy, electronic media, or microfiche form. NISTIR's may also report

results of NIST projects of transitory or limited interest, including those that will be published subsequently in more comprehensive form.