NBS Measurement Services:

NEW NIST

Apr i 1 PUBLICATION

20, 1989

James T. Weaver

Thomas P. Loftus

Robert Loevinger

U.S. Department of Commerce

National Bureau of Standards
Center for Radiation Research

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

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

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

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

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

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

Atomic and Plasma Radiation Division

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

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

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

or wavelength standards; develops new measurement techniques and

methods for spectral analysis and plasma properties; and collects,

cotnpnes, and critically evaluates spectroscopic data. The Division

consists of the following Groups:

Radiation Physics Division

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

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

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

tions; conducts theoretical and experimental research with electron,

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

measurement techiques. The Division consists of the following Groups:

Radiometric Ptiysics Division

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

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

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

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

spectroradiometric and spectrophotometric standards and calibration

procedures. The Division consists of the following Groups:

Develops,

Radiation

operates,

Source

and

and

improves

Instrumentation

major NBS radiation

Division

facilities • Accelerator

^

Research

including the electron Linac and race track microtron; develops, • Linac Operations

designs, and builds electronic and mechanical instrunentation for • Electronic Instrimentation

radiation programs and facilities; provides national leadership • Mechanical Instrunentation

in the standardization of nuclear Instrijnentatlon; and develops

new competence in radiation sources and instrumentation. The

Division consists of the following Groups:

Ionizing Radiation Division

Office of Radiation Measurement

Radiation Theory

Radiation Chemistry and Chemical

Dosimetry

Neutron Measurements and Research Neutron Dosimetry

Radioactivity

X-Ray Physics

Dosimetry

Provides primary national standards, measurement services, and basic

data for applications of ionizing radiation; develops new methods of

chemical and physical dosimetry; conducts theoretical and experimental

research on the fundamental physical and chemical interactions of

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

System for Ionizing Radiation; and develops and operates radiation

sources needed to provide primary radiation standards, fields, and

well-characterized beams of radiation for research on radiation

interactions and for development of measurement methods. The Division

consists of the following Office and Groups:

Nuclear Physics Group

Engages in forefront research in nuclear and elementary particle physics;

performs highly accurate measurements and theoretical analyses which

probe the structure of nuclear matter; and improves the quantitative

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

CALIBRATION OF GAMMA-RAYEMITTING BRACHYTHERAPY SOURCES

James T. Weaver

Thomas P. Loftus

Robert Loevinger

Center for Radiation Research

National Measurement Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

NOTE: As of 23 August 1988, the National Bureau of standards (NBS) became the National Institute of Standards and Technology (NIST) when President

Reagan signed into law the Omnibus Trade and

Competitiveness Act.

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued December 1988
Library of Congress Catalog Card Number: 88-600609

National Bureau of Standards Special Publication 250-19

Natl. Bur. Stand. (U.S.), Spec. Publ. 250-19, 60 pages (Dec. 1988)

CODEN: XNBSAV

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

it serves.

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1988

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

The calibration and related measurement services of the National Bureau

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

measuring instruments in achieving the highest possible levels of

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

calibration, special test, and measurement assurance services. These

services allow customers to directly link their measurement systems to

measurement systems and standards maintained by NBS. These services are offered to the public and private organizations alike. They are described in NBS Special Publication (SP) 250, NBS Calibration Services

Users Guide.

The Users Guide is being supplemented by a number of special

publications (designated as the "SP 250 Series") that provide a detailed

description of the important features of specific NBS calibration

services. These documents provide a description of the: (1)

specifications for the service; (2) design philosophy and theory; (3)

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

of measurement uncertainty including random and systematic errors and an error budget; and (6) internal quality control procedures used by NBS.

These documents will present more detail than can be given in an NBS

calibration report, or than is generally allowed in articles in

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

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

more readily available to the user.

This document (SP 250-19), NBS Measurement Services: Calibration of

Gamma-Ray- Emitting Brachytherapy Sources, by J . T. Weaver, T. P. Loftus

,

and R. Loevinger, is the nineteenth to be published in this new series of

special publications. It covers the calibration of small radioactive

sources used for interstitial radiation therapy (brachytherapy) in terms

of the physical quantity exposure, measured in roentgens, or the quantity

air kerma, measured in grays (see test numbers 47010C, 47011C, and 47040S

in the SP 250 Users Guide). Inquiries concerning the technical content of

this document or the specifications for these services should be directed

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

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

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

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

found inside the back cover.

NBS would welcome suggestions on how publications such as these might be

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

new calibration services, special tests, and measurement assurance

programs

.

Joe D, Simmons

Acting Chief

Measurement Services

Chris E. Kuyatt

Director

Center for Radiation Research
Abstract . The calibration of small radioactive sources used for interstitial

radiation therapy (brachytherapy ) is performed in terms of the physical

quantities exposure or air kerma. The calibrations are listed in NBS Special

Publication 250 as 47010C, 470110, and 47040S (formerly 8.4E, 8.4F, and 8.4A).

The calibration provides the air-kerma rate (the exposure rate, before 1987

Oct 1) in vacuum at 1 meter perpendicular to the long axis of the source.

^°Co and ^Cs sources are calibrated by comparison with NBS working standard

sources of the same type, while ^'^^Ir and I25j sources are calibrated by

measurement in a reentrant ionization chamber that was calibrated using NBS

working standard sources of the same type. The working standard sources were calibrated using the NBS graphite cavity ionization chambers except for ^^Sj^

for which the NBS measurement standard was a free-air chamber. The working

standard sources of the two long-lived sources have been measured a number of

times over the years; the reliability of the reentrant chamber for the two

short-lived sources is assured by use of sealed radium sources as a constancy

check. The overall uncertainty (considered to have the approximate

significance of a 95% confidence limit) is given as 2% for all the sources

except for ^^Sj seeds, for which it is given as 5%, 6%, and 7%, depending on the type of seed. The stated uncertainty for ^25j seeds does not include

possible errors due to low-energy x rays not recognized at the time the

standards were established.

Key words : air kerma; air-kerma strength; brachytherapy; calibration;

cesium-137; cobalt-fiO; exposure; iodine-125; iridium-192;

radioactive source; reentrant chamber; standard; uncertainty.

iv
Table of Contents

Page

Abstract i v List of Tables vi

List of Figures viii

Foreword ix

1. Introduction 1 1.1. Description of the service 1

2. Design philosophy and theory 5

3. The long-half-life radionuclides -- ^^^Cs and ^^Co ^ 3.1. The replacement method 8

3.2. Cesium-137 12

3.2.1. Background information 12

3.2.2. Present standards 13

3.3. Cobalt-60 15

3.3.1. Background information 15

3.3.2. Present standards 16

3.4. Uncertainty assessment for ^^''Cs and ^°Co sources 18

4. The short-half-life radionuclides — ^'^^Ir and i^sj 21

4.1. The reentrant chamber 21

4.2. Iridium-192 22

4.3. Iodine-125 25

4.3.1. Working procedures for ^^Sj calibration 28

4.4. Information to be recorded in reentrant-chamber calibrations . 30

4.5. Uncertainty assessment for ^'^'^Ir and ^^^I 32

5. Safety considerations 37

5.1. Radiation safety 37

5.2. High-voltage safety 38

References 39

Appendix A: Adjustment of NBS exposure rates, 1986 Jan 01

Appendix B: Sample calibration report for cesium-137 source Appendix C: Sample calibration report for iridium-192 seed

Appendix D: Sample calibration report for iodine-125 seed

V
List of Tables

Page

1-1. Characteristics of the radionuclides accepted for brachytherapy

calibration 3

1-2. Rate constants for the brachytherapy radionuclides calibrated at

NBS 4

3-1. Engraving on NBS standard ^^"^Cs and ^^Co sources 9

3-2. Design details for ionization chamber used for replacement-method

source calibrations 9

3-3. Typical recombination data for the 2.8-liter spherical aluminum

ionization chamber 11

3-4. Correlation coefficient R, and values of the slope and intercept

for the relation l/I = a + b/V for the normalized data in

table 3-3 11

3-5. Relative exposure rates for ^^'^Cs working standards, based on 1967

measurements 13

3-6. Corrections for number 1 graphite cavity ionization chamber in 1967

and 1974 13

3-7. Measurements of the "500" ^^"^Cs working standard, in mR mVh ... 14

3-8. Comparison of 1967 updated "500" source exposure data with check

measurements performed in 1978 and 1983 14

3-9. Best values for ^^^Cs working standards based on mean of three

measurements of the "500" source in the period 1967-1983 15

3-10. Calibration data for two cobalt-60 sources stored in safe in

Room B145, in mR m^/h 15

3-11. Exposure data for two ICN ^°Co sources 16

3-12. Mean exposure rate for the two ICN ^"^Co sources, adjusted to the

1986 standard 17

3-13. Ratios of exposure data for ICN 100 and ICN 500 derived from

open-air measurements with standard chamber 50-1, and from

replacement measurements in Room B145 17

3-14. Uncertainty analysis for ^^^Cs and ^^Co source calibration by

substitution 19

vi
List of Tables (Continued)

Page

4-1. Constancy check of aluminum reentrant ionization chamber for the

years 1977 through 1987 22

4-2. Dimensions and materials for two types of ^"^^Ir seeds 23

4-3. Reentrant chamber calibration factors at STP for ^'^^r seeds ... 23

4-4. Data from calibration of a stai nl ess-steel -cl ad ^^^Ir source ... 24

4-5. ^2^1 seed types and their associated reentrant ionization-chamber

calibration factors 26

4-6, Effect of noise pick-up in high-voltage connection to aluminum

reentrant chamber 29

4-7. Uncertainty analysis for ^"^^r source calibration with the

reentrant chamber 33

4- 8. Uncertainty analysis for ^^Sj source calibration with the

reentrant chamber 35

5- 1. Rate constants in units convenient for protection calculations . . 38

vii
List of Figures

Page

Figure 1. Calibration range for brachytherapy sources 6

Figure 2. Cross-section view of the polystyrene base that supports the

2.8-liter spherical aluminum ionization chamber 10

Figure 3. Aluminum reentrant ionization chamber used for measurement of

I92ip and 1251 seeds 21

viii
FOREWORD

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

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

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

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

until NBS moved from the Washington to the Gaithersburg site in 1965. At that

time he took on the added responsibility of supervising the x-ray calibration

work. He continued in this capacity until his retirement in June 1984. At

that time, it was necessary to document all the dosimetry calibration

services, as part of a Bureau-wide program. It was evident that Tom was the

ideal person to take on this job, and he was awarded a contract for that

purpose. The result was six separate documents entitled "Review of

loni zati on-Chamber Current-Measurement Techniques in the NBS Dosimetry Group,"

"lonization-Current Measurement Console Equipment," "Pre-Cal ibration Testing

of Exposure Instrumentation," "Gamma-Ray Calibration Ranges," "Exposure

Calibration of Radiation Instruments at the NBS," and "Brachytherapy Source

Calibrations at the NBS."

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

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

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

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

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

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

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

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

the example he set while at NBS.

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

December 1987

ix

1. Introduction

This report describes the status of the calibration service for

brachytherapy gamma-ray sources, as of December 1987. Section 1 defines the

physical quantities exposure and air kerma, in terms of which the calibrations

are performed; the quantities derived from exposure and air kerma, in terms of

which the calibrations are reported; and the exposure and ai r-kerma rate

constants, in terms of which the radiations are characterized for dosimetry

purposes. Section 2 describes the two methods used for brachytherapy calibration, either direct comparison with NBS working standards using the replacement method, or indirect comparison using the reentrant (well-type) ionization

chamber. Section 3.1 describes the replacement method of calibration used for

the two long-lived radionuclides, 137qs 60qo. Sections 3.2 and 3.3

describe the standards and calibration details for 137q5 apjj eOQo sources,

respectively. Section 3.4 covers the assessment of uncertainty for the two

long-lived radionuclides. Section 4.1 describes the reentrant chamber used

for the calibration of the two short-lived radionuclides, ^^^Ir and ^^^l.

Sections 4.2 and 4.3 describe the standards and calibration details for ^"^^r

and I25i sources, respectively. Section 4.4 describes the detailed information that must be recorded with reentrant-chamber calibrations, and section

4.5 covers the assessment of uncertainty, of the two short-lived radionuclides. Section 5 covers radiation and high-voltage safety considerations.

References are given at the end of the report, in the order in which they are

cited. Attachments give the statements issued by NBS at the time that

exposure rates were adjusted, and three sample calibration reports.

1.1, Description of the service

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

Dosimetry Group, ^ receives a variety of small radioactive sources for calibration. These are primarily brachytherapy sources, which are sources used for

interstitial or intracavity radiation therapy. ( Brachy is from the Greek,

meaning "near.") Some of the sources are used for calibration or other non- medical purposes, but they are calibrated like brachytherapy sources. These

calibration services are described in NBS Special Publication 250 [1]^ under

Test Nos. 47010C, 47011C, and 47040S (formerly 8.4E, 8.4F, and 8.4A).

Calibrations are performed in terms of the physical quantities exposure or ai r kerma .

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

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

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

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

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

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

^On August 23, 1988 the National Bureau of Standards became the National

Institute of Standards and Technology. On April 1, 1988 the Dosimetry Group

became the Radiation Interactions and Dosimetry Group.

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

document.

1
Then

X = dQ/dm . (1)

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

of exposure, the roentgen (R), is equal to 2.58 x 10"'+ C/kg (exactly). The

ionization arising from the absorption of bremsstrahl ung emitted by the

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

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

is the ionization equivalent of air kerma.

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

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

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

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

quotient of dE^p 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 the air in that volume element. Then

Kg^P = dE^p/dm . (2)

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

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

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

Kg^.p = X(W/e)/(l-g) , (3)

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

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

lost to bremsstrahl ung.

The currently accepted value of W/e is 33.97 J/C [2]. The currently

accepted values of g for ^°Co and ^^'^Cs gamma rays are 0.32% and 0.16%,

respectively [3]. Then, using SI units, X is converted to K^^-^ by multiplying

by 34.08 and 34.02 for ^^Co and ^^"^03, respectively. The value of g is

negligible for i^zjp i25i^ l^h^ conversion factor is just 33.97.

Any calibration performed in terms of exposure can be converted to a calibration in terms of air kerma by use of the multiplicative constants given

above. Because the SI units of exposure and (especially) exposure rate are very inconvenient, they are never used in practice, and therefore are not used

in this document. The quantity air kerma is being adopted in place of exposure in virtually all countries; at the time of writing (December 1987) NBS

has made the change for brachytherapy calibrations, but not for dosimetry

calibrations using photon beams. The change for brachytherapy calibrations

was made on 1987 October 1, after the draft of this manuscript for internal

NBS use was completed. As a consequence, the material that follows is for the

most part written in terms of exposure, using the special unit of exposure,

the roentgen (R), but it applies equally well to air kerma, since only a

2
multiplicative constant is needed to convert the former quantity to the

latter. While the text is simplified by using the word "exposure" to imply

both physical quantities, air kerma is named explicitly when numerical magnitudes are involved, or when it seems useful to remind the reader that air

kerma is also meant.

In particular, until 1987 October 1, calibrations of brachytherapy

sources were provided in terms of exposure rate at one meter perpendicular

to the long axis of the source, corrected for air attenuation and buildup,

and room scatter. After that date, the recommendation of the American

Association of Physicists in Medicine was adopted [4], and calibrations of

brachytherapy sources were provided in terms of "air-kerma strength", in the

SI units micro-grays meters squared per hour (pGy m^/h). Sample calibration

reports are given in the appendices.

The characteristics of the radionuclides accepted for calibration are given in table 1-1. For ^^^r and ^^^l, only sources that are the same as the

standard sources in size, wall material, and source composition can be

reliably calibrated. For ^^Co and 137q5^ gome variation in size and wall

material is acceptable. The overall uncertainty (considered to have the

approximate significance of a 95% confidence limit) of a brachytherapy

calibration varies from 2% to 7% depending on radionuclide and type of source.

Table 1-1. Characteristics of the radionuclides accepted for brachytherapy

calibration. Columns 2 and 3 are from reference [5].

Radionucl ide

Pri nci pal

photon

energi es

(keV)

Halflife

Range of

acceptable

exposure

rates

(uR m2/s)

Range of

acceptable

ai r-kerma

strengths

(yGy m2/h)

6 0Co

192IP

1251

1173,1332

662

296 - 468

27 - 35

5.2714 ± 5 y

30.0 ± 2 y

73.83 ± 18 d

59.6 ± 2 d

0.3 to 50

0.3 to 50

0.004 to 1 0.004 to 1

10 to 1500

10 to 1500

0.1 to 30

0.1 to 30

Institutions submitting a brachytherapy source for calibration must be

licensed by the Nuclear Regulatory Commission for possession of that source.

The source must be free of contamination. Radioactive sources are received at

NBS through the Health Physics Group, who perform leakage and contamination

checks before the sources are released for calibration. Institutions planning

to submit a brachytherapy source for calibration should first contact the

appropriate person at NBS as given in the latest edition of NBS Special

Publication SP 250, with regard to waiting time, turn-around time, purchase

order, shipping address, and other matters listed there.

3
Brachytherapy sources are gamma-ray-emitting radionuclides encapsulated

in a suitable metal, ordinarily chosen to be innocuous to tissue. They come

in a variety of forms, often referred to as "seeds," "tubes," or "needles,"

but sometimes coming in the form of wire. The seeds are very small, about

1 mm or less in diameter and a few millimeters in length; the tube sources may

be several millimeters in diameter and 10 to 30 millimeters in length; needles

are one or two millimeters in diameter and up to 50-mm long, pointed on one

end. These sources are characterized by the manufacturer and the user in

terms of activity, or activity per unit length. The exact value of the

activity is not relevant to the NBS calibration, but is useful in establishing

the approximate exposure rate before measurement and in controlling radiation

safety. The rate constants for the relevant radionuclides are given in

table 1-2.

Table 1-2. Rate constants for the brachytherapy radionuclides calibrated at

NBS 16']. These values do not account for the attenuation due to

encapsulation, except that the radium value assumes equilibrium

with daughter products and filtration by 0.5 mm Pt. Divide the

tabular values in column 2 by 3.6 to obtain values in the units

( yR m2/mCi s) for comparison with table 1-1.

Exposure- Air-kerma

Radionuclide rate constant rate constant

(R m2/Ci h) (yGy m2/MBq h)

60Co 1.307 0.311

137CS 0.3275 0.0777

I92ir 0.4002 0.0948

1251 0.1326 0.0314

Radium 0.825 0.195

The values of the rate constants given in table 1-2 are approximate, and

should not be taken as applicable to a particular encapsulated source. The

user of a brachytherapy source will take the product of the stated activity

times an assuned exposure rate constant as the exposure rate at a specified

distance. As a result, the exposure rate at one meter measured by NBS divided

by the exposure rate constant stated by the manufacturer should agree with the

stated activity, within the desired limits of accuracy. If the activity and

the exposure rate constant are both stated by the manufacturer when a source

is submitted to NBS, their product can be compared to the NBS calibration, and

this serves as a measurement-assurance test for the manufacturer and also for

NBS.

Radium is not accepted for calibration at NBS, but is included in

table 1-2 for comparison with other radionuclides, because its exposure rate

constant is so well known. It is commonly used as a laboratory standard of

instrument stability because of its long half-life, approximately 1600 y [6].

4
2. Oesign philosophy and theory

Rrachytherapy sources are provided with calibrations in terns of exposure

rate at one meter perpendicular to the axis of the source, corrected to vacuum. The calibration is based on measurements of other brachytherapy

sources using the NBS primary standards of exposure. The measurements using

the primary standards were performed in air at distances of about one meter or

less, and were corrected to one meter and for air attenuation and buildup, and

for room scatter, as appropriate. Thus the calibration of a brachytherapy

source is based on an indirect comparison with an NBS primary exposure

standard: the user source is compared with an NBS source that has in turn

been compared with an exposure standard. The comparison with an NBS source is

either direct, or indirect using a reentrant chamber, depending on the halflife of the brachytherapy source.

The long-half-life sources ^°Co and 137qs calibrated by direct

comparison with NBS working-standard sources, using an external ionization

chamber, at distances between one-half and one meter. The comparison is

carried out in a concrete calibration range (fig. 1), so an appreciable amount

of scattered radiation reaches the detector. The validity of the calibration

depends on the fact that the NBS standard source and the source being

calibrated have the same radionuclide and the same position, and are similar

in size, shape, and encapsulation, so presunably the scattered radiation is

the same in spectrum and in relative intensity for the two sources. The

working-standard sources were calibrated in open-air geometry using the NRS

cavity-chamber exposure standards.

The short-half-life sources i^^jp Qf^d I25i are calibrated using a reentrant (well-type) chamber (figs. 1 and 3) that was standardized using

sources that had been calibrated against NBS exposure standards; the stability

of the chamber is monitored by radiim sources. For "^^^Ir, the original

standardization of the chamber was performed with a group of ^^^Ir sources

that had been calibrated in open-air geometry using the NBS cavity-chamber

standards of exposure. For ^^^l, the photon energy is too low for use of the

cavity-chamber standards, so the chamber was standardized using a group of

I25i sources that had been calibrated with an NBS free-air chamber. The

reentrant chamber has been standardized for three kinds of i^sj seeds and two

kinds of ^^^Ir seeds, which vary enough in their spectra so that there is a measurable difference in their calibration factors.

5
6
Figure 1. Calibration range for brachytherapy sources.

y

A. Spherical ionization chamber for ^^Co and ^^'^Cs sources.

B. Source holder for ^^Co and ^^^Cs sources.

C. Slide for source holder. Distance from source center to chamber center is

variable from 0.5 m to 1 m.

D. Reentrant chamber for ^'^^Ir and ^^sj source.

E. Wooden supports.

F. 70 mm foam cushion.

G. Steel support rails.

H. Steel support plate, 15 mm by 41 mm.

I. Thermistor cable for temperature measurement. Thermistor can be placed

adjacent to either chamber.

J. Collection-potential cables.

K. Low-noise signal cables.

a. 30 cm, concrete barrier thickness.

b. 95 cm, end of barrier to chamber center.

c. 235 cm between chamber centers.

d. 35 cm, chamber center to end of barrier.

e. 46 cm, barrier height.

f. To storage area.

g. 94 cm to concrete wall (not shown).

h. 92 cm, width of concrete trough.

i. wall 2.7 m to ceiling, mirror 0.5 m high,

j. wall 7 m long, mirror 4.7 m long.

7
3. The long-half-life radionuclides -- ^^^Cs and ^QCo

3.1. The replacement method

Small-source calibration by the replacement method consists of

measurement of the radiation from a standard source S, and then, after

replacing the standard with the unknown source U, measurement of the

radiation from the unknown. The standard and the unknown source must be the

same radionuclide, must be placed in the same source holder, and must be

the same distance from the detector. The exposure rate at unit distance

from the unknown, (^D^)^, is directly proportional to the exposure rate at

unit distance from the standard, (XD^)^, with the constant of proportionality

equal to the ratio of the mean signal for the unknown, and the mean signal

Ig for the standard, appropriately corrected for background b, for atmospheric

temperature T, and pressure P. With the subscripts u and s referencing the

unknown and standard respectively, the general equation is:

. , 273.15 + T,, I - b . (^^')u - 273.15 . t;' P^ (XD2)^ (4)

where T is the temperature in degrees Celsius. Since the exposure data for

the standard is corrected, at the time of the original measurement, for room reflection and air attenuation, the same conditions pertain to the

calibration of the unknown. The measurements are carried out in the concrete

calibration range shown in figure 1. The sources and the detector are spaced

between 50 and 100 cm apart, depending on the source strength. The

measurement procedures and equipment details described here are common to the

calibration of both small ^°Co and small ^^^Cs sources.

The standard sources used in the replacement method of calibration, and

the information engraved on each source, are given in table 3-1.

The radiation detector used for replacement-type calibrations is an ionization chamber. For convenience, design details for the chamber are given

in table 3-2, and illustrated in part in figure 2. The chamber is of guardring design with the guard conducting surface interposed between the chamber

wall and the collection electrode. This design allows the use of high

potentials for the efficient collection of ions and prevents the addition of

high-voltage-induced insulator-leakage currents to the chamber ionization

current. An additional feature of the electrode-insulator configuration is

the recessed high-voltage insulating surface. The insulator, being below the

guard surface, cannot be "seen" by the collection electrode. This arrangement

minimizes high-voltage "soakage" effects, often experienced in ionization

current measurements, which are believed to be due to electrostatic distortion

of the electric field by charge trapped on the high-voltage insulator.

8
Table 3-1. Engraving on NBS standard ^^'^Cs and ^^Co sources.

Engraving on Source identity Cylinder wall End wall

SOCo "500" ICN

2-9-76

995

500 MC

CO-60

60Co "100" CO-60

100 MC

996

2-9-76

ICN

I37cs "500" Csl37 N.C.C.

D49

I37cs "50" Csl37 N.C.C.

G46

I37cs "5" Csl37 N.C.C.

B48

Table 3-2. Design details for ionization chamber used for replacement-method

source calibrations.

Chamber*

Shape sphere

Material . aluminum alloy

Outside diameter 178 mm

Inside Diameter 175 mm

Wall thickness 1.59 mm

Volume 2.79 liter

Collection electrode

Shape cylinder

Material copper

Length (inside) 140 mm Diameter 3.2 mm Volume 0.001 liter

*Chamber dimensions are computed using the measured chamber outside diameter

of 7.0 in and taking the wall thickness as 1/16 in.

9
Ttft 83 798 CONNECTOR WITH frre insolation

Figure ?.. Cross-section view of the polystyrene base that supports the

2.8-liter spherical aluminum ionization chamber. It also provides

high-voltage insulation, and supports the guard ring and the

collection electrode. The heavy lines indicate where the polystyrene surfaces have been coated with colloidal graphite to make

them conductive. The outside diameter of the sphere is 178 mm. From reference [7].

The chamber collection electrode extends into the sphere to within 30 mm

of the chamber wall. There is no record that special precaution was taken to

round off the end of the collection electrode so that at very high collection

potentials there is a possibility of corona from sharp corners; however, the

electric field between the end of the electrode and the surface of the sphere

is only about 250 V/cm for the usual operating conditions. A much higher

electric field exists between the collection electrode and the chamber wall

where the electrode enters the chamber. At that point, the electric field is

equal to about 7.8 kV/cm for a collection potential of 1.1 kV.

Typical data showing the recombination characteristics of the 2.8-liter

chamber are given in table 3-3. Two i^y^g sources different in activity by

about a factor of 10 were positioned, in turn, at a distance of one meter from

the chamber for these measurements. Assuming a linear relationship between

the reciprocal of the ionization current and the reciprocal of the collection

potential, predicted corrections for recombination from regression analysis

differ by only 0.1% for these disparate currents, as shown in table 3-4. It can be concluded that the increase of current with the increase of collection

potential represents initial recombination, which is independent of current,

and no recombination correction is appropriate for relative measurements.

Subsequent studies of the 2.8-liter chamber, e.g., that recorded in databook

870, page 25 (OB 870:25), 3 April 1986, confirm this conclusion for 3.8 pA and

33 pA, and show that the recombination correction is not more than a few

tenths percent up to 185 pA. It is important in performing this type of

experiment to proceed by changing the collection potential without changing

the source position in order to eliminate the additional uncertainty of source

non-uni formi ty

.

^Databook references in this document are for the convenience of NBS personnel

and can be ignored by the general reader.

10
Table 3-3. Typical recombination data for the 2.8-liter spherical aluminum

ionization chamber (HB 806:27). The polarity of the chamber wall

is negative. The table gives the reciprocal of the normalized

current.

Collection Normalized values for two ionization currents,

potential 3.8 pA 33 pA

(V)

225 1.0072 1.0105

350 1.0057 1.0062

550 1.0030 1.0026

700 1.0019 1.0030

900 1.0011 1.0013

1100 1.0000 1.0000

Table 3-4. Correlation coefficient R, and values of the slope and intercept

for the relation l/I = a + b/V for the normalized data in

table 3-3.

3.8 pA 33 pA

R 0.970 0.989

a 0.999 0.998

b 2.011 2.824

Before 1986 June 01, the source holder associated with the 2.8-liter

ionization chamber was a PMfIA (polymethylmethacrylate, Lucite) trough mounted

on an optical bench. The trough was supported on both ends, such that sources

up to about 50 mm in length would fit between the supporting rods. The trough

wall was a few millimeters thick and the vee about 9 mm deep. The attenuation

of I37cs gamma rays in the trough wall was about 2% while for ^°Co radiation

it was 1.4%. These estimates were computed using total attenuation coefficients of 8.28 X 10-3 and 6.08 x 10-3 m2/kg for ^^'^Cs and ^^Co gamma

rays respectively [8]. The density of PMMA is 1.18 g/cm3. In 1986 June the

holder was modified to support a trough from one end only. Using square

plastic tubing of the same material and wall thickness as the earlier trough,

troughs were made from 1/2-inch, 5/8-inch, 3/4-inch, and 1-inch tubing by

cutting away two sides leaving vees with depths of 7, 10, 12, and 15 mm. The vees are about 75 mm long, with about 50 mm of the square tubing left for

attachment to the holder on the optical bench. Because the trough is

supported on one end only, the source is readily viewed in the open vee, in

order to keep track of rotation of the source around its axis during measurement. The attenuating effect of the trough wall, while small, is not

insignificant and sources with active volumes extending above the vee of the

trough should not be calibrated against standards that nest inside the vee.

11
The restriction on the length of a source to be calibrated is only that it fit

into the trough. In the worst case, if all the activity of a source capsule

50 mm in length were concentrated at one end (25 mm off center), the

geometrical error, neglecting oblique attenuation in the source and trough

walls, is only 0.03% when the trough is located one meter from the ionization

chamber.

The small-source calibration procedure has evolved from a replacement

technique involving the gold-leaf electroscope, where the unknown and the

standard were measured, ideally in a USSU or SUIJS sequence, procedures that

usually make sequential considerations unimportant. While the old method

depended on a certain sequence of measurement to take care of effects

primarily due to temperature and pressure changes, these factors are now taken

into account by correcting for ambient conditions. However, the effect of

trends in measurements due to unknown factors can be minimized by the proper

selection of measurement sequence and the USSU or SUUS sequences are still to

be recommended.

Perhaps more important than the sequence in which the readings are taken

is determining the uniformity of the unknown source around its axis. If there

is a mark that identifies one orientation, the source should be rotated 90°

between measurements, so that I^j represents the mean of at least four orientations. If there is no such identifying mark, one can be supplied using a marking pencil, or the source can be removed and replaced several times, to

obtain a random sampling of the orientations.

If the source being calibrated is not essentially the same size and shape

as the NBS standard, the question arises whether the differences give rise to

a significant error in the calibration. Even if all other conditions are the

same for the two sources, large variations in size and shape can change the

amount and pattern of radiation scattered from the walls of the concrete

calibration range. Either an attempt should be made to quantify the increased

uncertainty, or a statement of the problem should be included in the calibration report.

3.2. Cesi um-137

3.2.1. Background information

Small sources with activities from about 3 mCi to 500 mCi (110 MBq

to 18.5 GBq) are calibrated for the quantity exposure by comparison with

working standard I37f3 sources. The working standard sources were measured in

1967 [7] using a graphite cavity ionization chamber now identified as chamber 1 [9]. Although several sources were measured in 1967, only three,

the 5-, 50-, and 500-mCi (nominal activities) sources, have continued to be

used for calibrations. In order to have an unambiguous standard, these

working standards were made self-consistent by taking the "500" source as the

reference source against which the "5" and "50" sources were calibrated using

the replacement technique. For detailed information about the graphite

ionization chamber measurements of the three working standards, reference can

be made to [7], but since the "500" source is used as the reference standard,

only that source is used in the following review to show the effect of

updating the standards. The relative exposure rates of the three working

standard sources are given in table 3-5.

12
Table 3-5. Relative exposure rates for ^^'^Cs working standards, based on 1967

measurements.

Source Relative exposure rates

"son" ion

"50" 10.80

"5" 0.932

3.2.2. Present standards

The NBS exposure standards for ^^Co and i^tq^ gamma rays were revised on 1972 May 01, at which time the NBS standards became the mean response of a set

of six spherical graphite ionization chambers [9]. These standards are referred to as "1974 standards," from the date of publication. The chambers

varied in volume from 1 cm^ to 50 cm^ and had either spherical caps or sets of

chambers with different wall thickness, for evaluating attenuation and scatter

of the radiation in the walls. The correction factors for the number 1 chamber, identified in [7] as VIII, are given in table 3-6. The ratio of the

product of the correction factors in the 1974 column to that in the 1967

column is 0.9935, which is the factor used to bring the 1967 brachytherapy

source calibrations into agreement with the 1974 exposure standard. Updated

self-consistent exposure data for the three working standards are given in DB

806:005 (1975 Dec 31), using a half-life of 29.68 y [10]. (The half-life is

di scussed bel ow)

.

Table 3-6. Corrections for number 1 graphite cavity ionization chamber in

1967 and 1974.

Correction factors

1967 [7] 1974 [9]

Collision mass stopping power ratio (C to air) 1.010 1.0143

Wall absorption 1.025 1.0199

Stem scatter 0.999 0.9964

Recombi nati on 1.002 1.0015

Mass energy-absorption coefficient rati 0 1.000 0.9997

Center of electron production 0.998 0.9990

Chamber volume*

0.9962

1.0004

*The volume for chamber number 1 given in reference [7] (identified as VIII)

is 1.130 cm3. The original volume measurements were reevaluated at a later

date and a lower value for volume (1.1295 cm^) was adopted.

The NBS exposure standards were revised again on 1986 Jan 01, as described in appendix A. For i^y^s gamma rays the change consisted of the

introduction of a 0.3% correction for humidity, and a 0.5% change in the

stopping power ratio, giving a net conversion factor of 0.992, to convert 1974

exposure rates to 1986 values.

13
In addition to the original measurement in 1967, the "500" source has

been checked twice in open-air geometry, in 1978 and in 1983. Table 3-7 gives

the original measurements and the exposure rates corrected to the 1974 and

1986 exposure standards.

Table 3-7. Measurements of the "500" i^tqs working standard, in mR m2/h.

Measured exposure rate

Measurement Reference Measured corrected to standard of

date exposure rate 1974 1986

1967 Aug 24 [7] 136.96 136.07 134.98

1978 Apr 29 DB 805:004-7 105.17 105.17 104.33

1983 Dec 03 DB 851:036 92.55 92.55 91.81

Table 3-8 shows the 1986 exposure rates from table 3-7 corrected to 1986

Jan 01 using half-lives of 29.68 and 30.0 y, and normalized to 1978 values.

(The year was taken to have 365.25 days.) Clearly the value 29.68 y brings

the three measurements of the "500" source together slightly better than the

now-accepted value of 30.0 y. (A least-squares fit to the data of table 3-7

gives 29.2 y.) If however it is postulated that in the "500" source there was

a short-half-life contaminant which was essentially gone by 1978, the 30.0-y

half-life fits the data very well. No other plausible explanation has been

found for the 1-percent drift in the exposure rate of the "500" source. The

standard deviation of the mean measurement was in each case less than 0.1%, so no reason has been found for assigning significantly different weights to the

three values.

Table 3-8. Comparison of 1967 updated "500" source exposure data with check

measurements performed in 1978 and 1983. The exposure rates have

been corrected to 1986 Jan 01.

Measurement Days to Normalized exposure rates

date 1986 Jan 01 J^^^ = 29.68 y J^^^ = 30.0 y

1967 Aug 24 6705 1.008 1.011

1978 Apr 29 2804 1.000 1.000

1983 Dec 03 760 1.003 1.001

The best current value of the exposure rate for the "500" source is then

taken to be the unweighted mean of the three determinations. The best values

of the exposure rates for the three working standards are given in table 3-9,

using tables 3-5, 3-7, and a half-life of 30.0 y. Since the standard deviation of each determination is small compared to the 1% drift, the standard

deviation of each value in table 3-9 is computed from table 3-8, and is 0.35%

with two degrees of freedom. Calibrations performed after 1986 Jan 01 are based on the mean of the three measurements of the "500" source given in

table 3-9. Calibrations performed before 1986 were based on the 1967 measurements.

14
Table 3-9. Best values for ^^'^Cs working standards based on mean of three

measurements of the "500" source in the period 1967-1983. The

data are consistent with the 1986 exposure standard, and are corrected to 1986 Jan 01 using a half-life of 30.0 years.

Source Exposure rate

(mR m2/h)

"500" 87.7

"50" 9.47

"5" 0.817

The appropriate half-life for the NBS ^^'^Cs brachytherapy working

standards can only be decided by further open-air measurements. These should

take place at intervals not greater than five or six years, in order that the

uncertainty in the half-life not contribute more than about 0.1% to the

combined uncertainty of calibration, as discussed in section 3.4. A copy of a typical calibration report of a ^^'^ts source is given in appendix B.

3.3. Cobalt-60

3.3.1. Background information

The first calibrations of small ^°Co sources (in the early 1950's) were referenced to 226Ra (encapsulated in 0.5 mm Pt-Ir) with the radium emission

constant taken as 0.84 mR m^/mg h. The calibrations were carried out using a gold-leaf electroscope for which the calibration factor was 0.7325 mR m2/"mg

Ra Eq" h. When the gamma-ray emission constant was changed [11] to 0.826 mR

m2/mg h, the constant for the electroscope was changed to 0.7203 mR m2/"mg Ra

Eq" h. Two sources measured using the electroscope are located in drawers A-1

and A-2. The data given on the safe labels (Room B145) for these sources,

corrected for 4994 days decay, and the result of check measurements (OR

744:025) using the 1974 NBS standards are given in table 3-10.

Table 3-10. Calibration data for two cobalt-60 sources stored in safe in

Room B145, in mR m2/h.

Source Safe label data Safe label data Check data

drawer 1959 Apr 30 corrected to corrected to

1972 nec 31 1972 Dec 31

A-1 101.0 16.72 16.64

A-2 100.4 16.62 16.55

The agreement between the old calibration data for A-1 and A-2 and

measurements of these sources using the 1974 standards is within 0.5%. This

good agreement can be considered in part fortuitous since the original

calibrations were referenced to radiim and radium emission constants, whereas

present ^°Co calibrations are based on standards, independent of radium, using

physical data and corrections developed specifically for ^^Co in 1972 [9].

15
3.3.2. Present standards

Compared to ^^"^Cs, there are now few requests for ^°Co small-source

calibrations. One reason perhaps is the relatively short half-life for ^°Co ^'^^ y)' The short half-life for ^^Co also poses the problem of

maintaining standards with exposure rates in a range useful for small-source

calibrations. For example, A-1 nd A-2 will have exposure rates of about 3 mR

m2/h in 1987.

New ^°Co sources were purchased in 1966 from Atomic Energy of Canada,

Ltd. (AECL), and m 1976 from ICN Biomedicals, Inc. (ICN), and are available

for use as standards for small-source calibrations using the replacement

method. Only two of the seven sources purchased have been calibrated

directly, using one of the standard graphite cavity ionization chambers in an open-air configuration. When and if it is deemed necessary, the remaining

sources can be calibrated against these tvvo sources using the replacement

technique.

The standardized ^°Co sources were manufactured by ICN. They had nominal

activities of 100 mCi (3.7 GRq) and 500 mCi (18.5 GBq) when purchased in 1976.

The calibration data for these two standards in terms of exposure rate at one meter, corrected for air attenuation and room reflection, are given in the

first four columns of Table 3-11.

Table 3-11. Exposure data for two ICN ^^Co sources. The graphite cavity

ionization chamber identified as 50-1 [9] was used in an open-air

configuration for standardization of the sources.

Source Measurement Exposure Time diff. to Exposure rate

ICN date Reference rate 1986 Jan 01 1986 Jan 01

(mR m2/h) (days) (mR m2/h)

100 1978 Sep 29 DB 806: 35 69.20 2651 26.35

100 1983 Aug 20 DB 851: 24 36.46 865 26.41

100 1983 Dec 03 DB 851: 38 35.10 760 26.40

500 1983 Aug 20 DB 851: 24 194.7 865 141.0

500 1983 Dec 03 DB 851: 38 188.2 760 141.6

In the 1986 adjustment to the NBS exposure standards (appendix A), the

^^Co exposure rate was reduced by 1.1%, of which 0.7% was due to a change in

the stopping-power ratio, 0.1% to a change in the energy- absorption coefficient ratio, and 0.3% due to introduction of a humidity correction. The

exposure rates of the 100-mCi and 500-mCi sources, corrected to 1986 Jan 01,

using a half-life of 5.271 y, are given in the last column of table 3-11, and

the mean exposure rates are given in table 3-12.

The standard deviation of the mean shown for the 100 source is from the

data in table 3-11. The ICN 100 source is in safe drawer A-4, and the ICN 500

source is in safe drawer A-3.

16
Table 3-12. Mean exposure rate for the two ICN ^°Co sources, adjusted to the

1986 standard.

Source

ICN

Mean exposure rate

19% Jan ni

(mR m2/h)

100

500

26.39 ± 0.1%

141.3

As shown in table 3-11, the ICN 100 source was measured on three

occasions, with a time difference of about five years between the first and

second measurements. In each case, the 50-1 graphite ionization chamber [9]

and the open-air configuration were used for the measurements. The difference

of about 0.2% between the first and second measurements is an indication of

the good long-term reproducibility achievable with such measurements, where

the standard chamber is the same but the set-up and essentially all of the

other measurement equipment are different. While the two measurements of the

100-mCi source made four months apart were essentially identical, the two

measurements of the 500-mCi source on the same dates differed by 0.4%, giving

an indication of a limitation on the achievable accuracy.

A test of the consistency of the standard chamber measurements for these

two sources using the replacement method showed a difference between the

ratios of the open-air measurements for ICN 100 and ICN 500, and the replacement measurements, of about 0.1% (table 3-13). The difference between the

ratios is in a direction to indicate that recombination corrections may be

required for the replacement measurements, but measurements dedicated to

resolve this question have not been conclusive. Future use of the ICN 500

soCo source will produce an ionization current in the 2.8-liter chamber of the

order of magnitude of 35 pA or less. It is shown in table 3-13 that below

this current there appears to be initial recombination only, and no recombination correction is required for relative current measurements, up to about 40 or 50 pA.

Table 3-13. Ratios of exposure data for ICN 100 and ICN 500 derived from

open-air measurements with standard chamber 50-1, and from

replacement measurements in Room B145.

Measurement

condi ti on Exposure data ratiosICN 500/ICN 100 Reference

Standard chamber, open-air

Replacement, B145

5.354

5.349

Table 3-12

OB 851:30

17
3.4. Uncertainty assessment for ^^^Cs and ""^Co sources

The method of uncertainty assessment used here follows the recommendation

of the Comite International des Poids et Mesures (CIPM). The uncertainty

estimates are of two kinds. Conventional statistical estimates of random

uncertainties are given as standard deviations of the mean, and for conven- ience are designated as "Type A", which can be considered to be objective

estimates. All other uncertainty estimates, designated "Type B", are subjective estimates, based on the extensive experience of the calibration staff.

The Type B uncertainties are estimated so as to correspond approximately to a 67% confidence level, and they are assumed to have roughly the character of

standard deviations. The estimates of each type are combined in quadrature,

and then the two results are combined in quadrature to give a combi ned

uncertai nty , which is in turn multiplied by two to give an overal

1

uncertai nty . The overall uncertainty is considered to have the approximate

significance of a 95% confidence limit.

Table 3-14 gives the details of the uncertainty estimate. The table

has two parts, corresponding to the two kinds of measurements made during a brachytherapy calibration. Part (a) lists the component uncertainties when

using the NBS primary standard graphite ionization chambers to determine the

exposure rate of an NBS working standard source, referred to in the table as a secondary working standard source. Part (b) in table 3-14 lists the component

uncertainties when measuring any source in the concrete calibration range

described in section 3.1, using the 2.8-liter spherical ionization chamber

described there.

For ^37qs^ weaker NBS working standard sources have been compared

with the strongest (table 3-9) and they are referred to in the table as tertiary working standard sources.

A user source is compared with either a secondary or a tertiary working

standard. Comparison with a secondary working standard involves two measurements in the concrete calibration range, so the resultant uncertainty can be

obtained by a combination in quadrature of the uncertainties listed in (a)

with two times those listed in (b). Comparison with a tertiary working

standard involves four measurements in the concrete calibration range, so the

resultant uncertainty can be obtained by a combination in quadrature of the

uncertainties listed in (a) with four times those listed in (b).

For a radioactive source, the uncertainty in the activity arising from

uncertainty in the half-life can be evaluated by differentiating the equation

representing exponential radioactive decay. The result can be presented in

the form

dA/A = In 2 (t/T) (dT/T) (5)

where dA/A is the fractional uncertainty in the activity due to the fractional

uncertainty in the half-life dT/T, during the elapsed time t/T multiples of

the reference half-life T.

18
Table 3-14. Uncertainty analysis for ^^'^Cs and ^^Co source calibration by

substitution.

A B

{%) (%)

(a) Measurement of secondary working standard

u sing primary graphite standard chambers.

Volume n.l 0.05

Charge 0.04 0.1

Timing 0.03 0.03

Air density 0.03 0.1

Recombination loss 0.07

Humidity* ' 0.1

Leakage and radiation background 0.02

Stopping-power ratio* 0.25

Energy-absorption coefficient ratio* 0.05

Wall absorption 0.1

Stem Scatter 0.02

Mean center of electron production 0.13

Effective measurement point* 0.05

Radial nonuni f ormity - 0.01

Distance (50 cm) 0.1

Correction to vacuum Air attenuation 0.05

Room scatter 0.3

Source nonuni formity 0.2

Half-life 0.14

Long-term reproducibility 0.3

Quadratic sum 0.12 0.62

(b) Measurement of source using 2.8-liter

spherical ionization chamber.

Charge 0.05 0.1

Timing 0.03

Air density 0.05 0.05

Recombination loss 0.1

Leakage and radiation background 0.2

Distance 0.1

Difference in scatter 0.2

Source nonuni formity 0.2

Source size 0.1

Quadratic sum 0.07 0.40

Comparison of user source with

secondary standard tertiary standard

Combined uncertainty 0.85 1.03

2 X combined uncertainty 1.70 2.06

Overall uncertainty (rounded) 2 2

19
Notes to Table 3-14

The uncertainty estimates are at the one-sigma level, and are in percent. The

columns headed A and B refer to the estimates described in the text as Type A

and Type B.

Charge includes uncertainties for both voltage and capacitance.

Air density includes uncertainties for both temperature and pressure.

Recombination loss refers to the uncertainty in correcting for loss of

electrons by ion recombination.

Humi dity refers to the uncertainty in correcting to dry air. The humidity

can be assumed to be the same during the comparison of two sources.

Hal f-1 i fe refers to the primary standard source. From equation (5) and table

1-1, the uncertainty in the activity due to the uncertainty in the half-life

is 0.015%/y for ^^'^Cs and 0.0012%/y for ^OCo. The value shown in the table is

the mean of the three values calculated for table 3-8, for the time intervals

shown to 1986 Jan 01. This value is used here for ^^Co also because it makes

a negligible contribution to the final combined uncertainty, and using

separate uncertainties for the two radionuclides would complicate the table.

Long-term reproducibility refers to the complete measurement system. From

tables 3-8 and 3-11, the uncertainty is estimated to be the value shown.

Difference in scatter refers to the difference in the contribution of

scattered radiation to the measured current, when comparing two sources.

*r)ata from Table 5.1 of reference [12].

All the entries in table 3-14 are uncertainties of multiplicative

factors. While leakage and radiation background are subtractive corrections,

their uncertainty estimates apply to the total current and are considered to

be uncertainties of multiplicative factors.

For the entries in table 3-14, the Type A component is much smaller than

the Type B component, so much so that even if the Type A component were

neglected, the combined and overall uncertainties would not be significantly

changed. This arises from the complexity of the interaction of ionizing

radiation with matter: it is never possible to account fully for the numerous

second-order effects, but it is usual during brachytherapy calibration at NBS

to repeat measurements until the Type A component of the uncertainty is much

smaller than the Type B component.

Based on the assessment of uncertainties described here, the value 2% is

adopted as an estimate of the overall uncertainty in calibration of brachytherapy sources of i^tq^ soqq by i\yQ substitution method. The overall

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

confidence limit.

20
4. The short-ha1f-1 ife radionuclides -- ^^^r and ^^Sj

4.1. The reentrant chamber

A reentrant-type (well-type) ionization chamber, illustrated in figure 3,

is employed in the calibration of small sources of these radionuclides. The

chamber is an aluminum sphere with an internal volume of 3.44 liters and a 7.9-mm wall thickness. The collection electrode, which is a hollow cylinder

surrounding the reentrant tube, is guarded and the high- voltage insulator is

recessed to minimize electrostatic effects.

Figure 3. Aluminum reentrant ionization chamber used for measurement of ^^^r

and I25j seeds. The outside diameter of the sphere is 203 mm. For

^^^Ir, the tube coming down from the top is brass and carries a glass vial with the source at the bottom (section 4.2). For ^^sj^

the tube coming down from the top is aluminum and the seed rests on the bottom (section 4.3). The source position is close to the

center of the sphere. From reference [13].

The ionization chamber is operated with a collection potential of 1100 V

with positive ground. The aluminum sphere is at high potential and one must

be aware, as is the case with all ionization chamber measurements, that a shock hazard is present. Introduction of a current-limiting resistor in

series with the high voltage supply to reduce the shock hazard has the attendant problem of adding noise to the signal, and operation with the chamber

wall at ground requires a triax-connector-type electrometer. In the triax

type of operation, the electrometer isolation from ground must be capable of

withstanding the high collection potential and the operator must be aware that

certain of the electrometer terminals are "hot". This latter condition brings

the hazard to the operator's milieu and can damage equipment not designed to

withstand high potentials.

CUCTWOOC

21
In contrast to the calibration of small sources of ^°Co and ^'^^Cs, where

the reference for calibration resides in source standards, the reference for

exposure standardization of ^^^Ir and ^^Sj sources resides in the calibration

factors for the reentrant chamber. It is important therefore to insure that

the chamber characteristics have not changed over the course of time.

Although the chamber design is such that no problem with constancy is foreseen, the chamber should be tested before use. To this end, a small 226^3 source (identified as N-O.B, stored in safe drawer R-10) placed in a plastic

tube has been used as a constancy check source since 1977. Starting in 1987,

a somewhat larger source (identified as N-5, stored in safe drawer B-7)

has also been used. A summary of the constancy check measurements using these

two sources is given in table 4-1. There is no evidence in that table of a drift in the response of the chamber to the smaller radium source, and the

reproducibility of the measurements has been very satisfactory. It was found

that the readings were slightly different depending on which end of the source

was up. Starting in October 1987 the two sources have been marked with a black marker on one end, and a record has been kept of the source orientation.

The ratio of the currents with the black end up, to that with the black end

down, was found to be 1.0015 and 1.0022 for N-0.5 and N-5, respectively. No

distinction was made between the two ends in the data of table 4-1.

Table 4-1. Constancy check of aluminum reentrant ionization chamber for the

years 1977 through 1987. The currents were normalized to STP {0°C

and 101.325 kPa) and to 1987 Dec 31 using a radium half-life of

1600 y. a is the standard deviation of a single measurement.

Source: N-0.5 N-5

Number Mean

Year meas

.

current 0

(pA) {%)

1977 5 34.52 0.3

1978 9 34.57 0.2

1979 3 34.52 0.2

1980 2 34.60 0.0

1981 4 34.51 0.1

1982 1 34.54

1983 5 34.56 0.1

1984 8 34.54 0.2

1985 19 34.54 0.1

1986 23 34.57 0.2

1987 66 34.56 0.2

Number Mean meas. current a (pA) {%)

34 298.1 0.1

4.2. Iridium-192

The reentrant tube for ^^"^Ir source calibrations [13] is made of brass.

When the tube is screwed into the top of the chamber, the bottom of the tube

is near the chamber center. A small hole in the side of the reentrant tube ensures that the ionization chamber is vented to the atmosphere.

22
Since the aluminum sphere is at high potential, it is necessary to use

an insulating holder when putting ^'^'^Ir sources into the brass reentrant tube.

The holder is a glass vial 65 mm in length with an outer diameter of 9.2 mm

and a wall thickness of 1.4 mm (0.29 g/cm^). A thin plastic tube is used to

insert the vial into the chamber. Although the attenuation of the relatively

high-energy iridium radiation by the glass is not great, the presence of the

glass is required during source standardization because the calibration factor

for the reentrant chamber includes the effect of the glass vial.

Chamber calibration factors have been determined for two types of iridium

sources using standardized seeds measured using a graphite ionization chamber

[9], [13], The sources are different in composition and internal dimensions

but externally they are alike. The two types of encapsulations, platinum and

stainless steel, can be distinguished by testing with a magnet. Details of

the source dimensions and materials [13] are given in table 4-2.

Table 4-2. Dimensions and materials for two types of i^^jp seeds.

Wall

Material Platinum Stainless steel

Outside diameter 0.5 mm 0.5 mm Thickness 0.1 mm 0.2 mm

Approximate length 3 mm 3 mm

Source

Composition Pt 90%, Ir 10% Pt 70%, Ir 30%

Diameter 0.3 mm 0.1 mm

When an ^'^'^Ir source is submitted for calibration, the type of

encapsulation must be specified since the reentrant chamber calibration

factors for the two types of seeds are different. The chamber calibration

factors at standard temperature and pressure (STP: 0°C and 101.325 kPa) for

the seed-type ^^^Ir sources, in glass vials in the brass reentrant tube, are

given in table 4-3. The NBS exposure standards revision on 1986 Jan 01 for

i92ip consisted in the introduction of a 0.3% correction for humidity, and a 0.4% change in the stopping power ratio, giving a net conversion factor of

0.993.

Table 4-3. Reentrant chamber calibration factors at STP for '^^^Ir seeds.

Source Before 1986 Jan 01 1986 Jan 01

encapsulation calibration factor calibration factor

(kR m2/A s) (kR m2/A s)

Platinum 3.301 3.278

Stainless steel 3.209 3.187

23
The ionization currents produced by ^'^^Ir sources in the reentrant

chamber must be corrected for recombination. Tests show that at a collection

potential of 1100 volts, and ionization currents up to about 80 pA, the

recombination correction is constant, 1.004. For ionization currents of about

900 pA, the recombination correction is estimated to be l.OOfi. In the region

80 to 900 pA, linear interpolation is used to estimate the recombination

correction for intermediate currents.

The calibration factors determined in Table 4-3 used a 74.0-day half-life

[13], The elapsed times, between the calibration of the various arrays of

seed and the calibration of the aluminum reentrant chamber with individual

seeds, ranged from about 2.6 to 3.1 half lives for the platinum-encapsulated

seed, and was about 0.12 half-lives for the stainless-steel -encapsul ated

seeds. The recalculation of all the data using the currently accepted half

life of 73.83 days [table 1-1] is not practical. The magnitude of the change

can be estimated from equation (5), taking

We find that the shorter half-life would reduce the expected current by about

0.45% for the platinum-encapsulated seeds, and by a negligible 0.02% for the

stai nl ess-steel -encapsul ated seeds. The uncertainty of the half-life is given

in reference [5] as 0.18 days, at the 1-sigma level, which leads to an uncertainty of about 0.5% for the platinum-encapsulated seeds, and this value

is used in table 4-7. Uncertainty in the decay correction of a source being

calibrated due to a calibration time of a few weeks, is negligible.

While in principle there should be an increase of 0.4 to 0.5% in the

exposure rate of platinum-encapsulated sources, consideration of the small

size of this change relative to the overall uncertainty of the calibration,

and of the inconvenience to the user community resulting from the change,

appears to justify not applying the correction at this time. This correction

is due to the time that elapsed between calibration of the seed arrays with

the primary standard chambers and calibration of the reentrant chamber with

the seeds. It is clear in retrospect that this component of the uncertainty

could have been greatly reduced by performing the work so that the intervals

between these different kinds of measurements were kept much smaller than one half-life.

Typical data resulting from an ^'^^Ir source calibration are summarized in

table 4-4.

Table 4-4. Data from calibration of a stainless-steel -clad ^'^'^Ir source. The

dT = 74.00 - 73.83 = 0.17 days.

data are from DB 806:48-49.

Date Time of

day

Number

of meas.

Mean

current

at STP

(pA)

Std. dev. Mean current

of at STP

the mean 1979 May 14.5

(%) (pA)

1979 May 14

1979 May 15

0950

1425

6

7 31.167

30.956

0.02

0.02

31.14

31.28

24
The mean of the decay-corrected currents is 31.21 pA (using the half-life

73.83 days). The reentrant ionization chamber calibration factor for the

stainless-steel-clad ^'^^Ir sources is 3.209 kR m^/A s, and since the

ionization current is less than 80 pA, the recombination correction is 1.004.

The exposure rate at one meter for this source corrected for scattering and

air attenuation is therefore

)^D2 = 100.6 nR m2/s on 1979 May 14.5 (6)

The manufacturer of this ^^'^Ir source stated to the customer that the activity

per seed (i n a batch of 168 seeds) was 1.217 mCi on 1979 March 26, and that

they used a conversion factor of 4.6 R cm^/mCi h (128 nR m^/mCi s) to compute

exposure data. If one corrects the manufacturer's data to 1979 May 14.5 (49

days, assuming March 26.5) the exposure rate at unit distance is 98.3 nR m^/s

which agrees with the calibration value to about 2%. A copy of a typical

^^'^Ir calibration report is given in appendix C.

4.3. Iodine-125

The reentrant ionization chamber designed for exposure calibrations of

i^^ip seeds is also used for measurements of i^sj seeds. However, the brass

reentrant tube could not be used for the ^^^l measurements because of

attenuation in the brass of the low-energy (<35.5 keV) radiation. With the

brass tube, the chamber sensitivity is only about 4 fA per "apparent"^ mCi of

i25i. To put this sensitivity into perspective, the background current is

about 20 fA.

With a reentrant tube fabricated of aluminum with walls 0.64 mm thick,

the chamber sensitivity is increased, for ^^Sj^ 3 factor of about 2400.

The dimensions of the aluminum tube are such that an i^s^ seed, dropped into

the tube, assumes a horizontal position at the bottom of the tube, and is

centered vertically in the spherical chamber. The inside diameter of the tube

is 5 mm so a seed of length 4.5 mm is constrained to lie near the center of

the tube.

While the brass reentrant tube screws into the top of the chamber, the

aluminum tube is designed to be easily placed into, and removed from, the

chamber. This feature is necessary because of anisotropy of the radiation

from the ^^Sj seeds, requiring removal and replacement of the tube many times

in order to randomize the seed orientation in the tube. The calibration

factors for the chamber for the three types of ^^Sj seeds [14] are the

quotients of the exposure rates at unit distance, normal to the plane of

randomized, uniform, arrays of seeds of one type, and the sum of the

ionization currents produced individually in the chamber by each seed of an array. Details regarding the seed type and the chamber calibration factors

are given in table 4-5. The NRS exposure standards revision on 1986 Jan 01

for i25i introduced a 0.2% correction for humidity giving a conversion factor

of 0.998.

'The 3M Company defines "apparent activity in millicuries" as descriptive

of output activity only and not the total quantity of ^^^l contained within

the titanium capsule of the seed.

25
Table 4-5. ^^^l seed types and their associated reentrant i oni zation-chamber

calibration factors.

3-M Co.

Model

No.

Type Details*

Calibration factors

Before 1986 1986 Jan 01

(kR m2/A s) (kR m2/A s)

6701 Gold- 1251 adsorbed on two resin 3.857

marker balls with one gold ball for

seed location by radiography.

6702 No- 1251 adsorbed on three resin 3.708

marker balls. These seeds can have the highest activity.

6711 Silver- I25i adsorbed on silver wire. 4.433

wire Silver characteristic x rays

are emitted.

3.849

3.701

4.424

*The active materials are sealed in titanium seeds 4.5-mm long and 0.8-mm

. outside diameter.

The calibration factor for the chamber does not include a saturation

correction for measurements of a source. A factor of 1.000 for 0.8 pA and

1.004 for 100 pA is given in reference [14], which would indicate the presence

of volume recombination, which is rate dependent. Fourteen sets of measurements, taken over a period of several years (referenced in DB 851:111), involving chamber currents from 8 pA to 530 pA , indicate that rate-independent

initial recombination generally predominates, with a contribution from ratedependent volume recombination. Least squares analysis of the data (excluding

outliers) gives the correction factor for lack of saturation as

k^g^ = 1.005 + 5 X 10-s I ,

where I is the measured current in picoamperes.

Repeated determinations of the recombination correction do not reproduce

well, showing fluctuations up to 1.5% percent; correction factors greater than

\% were considered outliers. The data are analyzed in the conventional manner

by plotting the reciprocal of the collected current against the reciprocal of

the polarizing potential, and in some cases these plots show a curvature that

is opposite to that predicted by the usual equations that represent initial

and volume recombination. The reason for this behavior is not known, but it

is speculated that it is related to the i nhomogeneities of the electric field,

and the strong gradient in the radiation field near the source.

The decay correction used in determining the calibration factors in

table 4-5 are based on a measured half-life, since the half-life used

initially (60.14 ± 0.11 d) was found to be too long. Five seeds were measured

145 days apart, giving a half-life of 58.9 d ± 0.2%. The long-term stability

of the measurement system was found by 10 radium check-source measurements to

be 0.3%, and the two combine to give a random uncertainty of 0.36% [14]. Then

the fractional uncertainty in the measured half-life is dT/T = 0,0036.

26
Due to experimental difficulties, long periods of time elapsed between

the initial free-air-chamber and reentrant-chamber measurements for some of

the 1251 seeds. However, for the most part, the FAC-reentrant chamber

measurement interval was short. The worst cases are for the first series of

gold- and silver-marker-type seeds where the interval between measurements was about 2.5 and 1.3 half-lives, respectively.

The interval between measurements for the first series of no-marker seeds was about 0.22 half-lives. The interval between measurements with the

free-air chamber and the reentrant chamber for each seed type in the second

series was essentially zero. In simmary, using equation (5), the uncertainty

in the calibration factors of table 4-5, due to uncertai nty in the measured

half-life, is

Gol d-marker

Si 1 ver-wi re No-marker

First Series

0.6%

0.3%

0.05%

Second Series

0.01%

0.02%

0.01%

Average

0.3%

0.2%

0.03%

The above averages are the values used in table 4-8 as the uncertainty

component due to the half-life.

Although the currently accepted half-life is 59.6 ± 0.2 d (table 1-1), we continue to use the calibration factors for the reentrant chamber that were

derived from data corrected using the measured half-life (58.9 d). Use of the

accepted half-life would have caused greater disagreement in the observed

measurement with time. With regard to the calibration of an i^sj ^qq^ where

the period of calibration is two weeks or less, the difference in calibration

data depending on the choice of half-lives is less than 0.2%, which is

insignificant in comparison to the overall uncertainties given in table 4-8.

Correction to vacuum was made for attenuation in the air path between the

measurement plane of the free-air chamber (FAC) and the central plane of the

collecting electrode, and for air attenuation in the distance from the source

arrays to the FAC measurement plane. The air path inside the FAC was

12.75 cm, and the source arrays were measured at the following distances from

the FAC measurement plane:

Gold-marker: 75% at 25 cm, and 25% at 50 cm Other seeds: all at 25 cm.

The measurements were made using a 3-mm hole in a lead sheet, at a fixed

distance from the array. This produced a narrow beam that did not touch the

entrance or exit apertures of the FAC. The ion current in the FAC was measured, varying the distance from the array to the measurement plane of

the FAC from about 30 to 50 cm. The measurements were analyzed assuming

exponential absorption, giving an apparent attenuation coefficient of

1.5 X 10"3 cm"i for air at room conditions [14]. This number is significantly

larger than that obtained by calculation using the ^^sj spectrim, which gives

0.4 X 10"3 cm~i. Review of the attenuation data by consultants in statistical

engineering, as part of the preparation of this document, gave a value of

(1.75 ± 0.74) X 10-3 cm-i.

27
For the purpose of calculating the uncertainties in the attenuation

corrections, the value (1.5 ± 0.7) x 10~3 cm~i is adopted. Because of the

assumption of exponential attenuation, the fractional uncertainty in the air

attenuation correction factor is just the uncertainty in the attenuation

coefficient times the length of the air path. For the path length of 12.75

cm, this gives an uncertainty of 0.9% for the air attenuation inside the FAC

,

which applies to all the measurements. For distances of the source array from

the measurement plane of the FAC of 25 and 50 cm, the uncertainties are 1.8

and 3.5%, respectively, which in turn lead to uncertainties of 2.2% for the

air attenuation correction (outside the FAC) for the gold-marker seeds, and

1.8% for the other seeds. These numbers are used in table 4-8.

There is a possibility of significant error in the calculation of tissue

dose based on the NBS exposure calibration of ^25j seeds. This is due to the

presence of the 5-keV x-ray lines originating in the titanium cladding of the

seeds [15"] [1^. These lines contributed to the measurements in the free-air

chamber, but are essentially all absorbed at the surface of the seeds when

implanted in tissue. The error in tissue dose when only the ^^Sj spectrum is

assumed has been estimated to be as large as 10%. Further theoretical and/or

experimental study is required to resolve this question, and to determine

whether a change is necessary in the calibration factor of the reentrant

chamber. The effect of the titaniim K lines is not included in the

uncertainty analysis of table 4-8.

4.3.1. Working procedures for ^^sj calibration

Before it is opened, the source container should be placed in the

mandatory white plastic tray (section 5). The bottle cap should be unscrewed

with the bottle in the lead container, and, with the bottle still in the lead

container, the contents dumped into the tray. The aluminum tube, supported

vertically in the tray, is loaded with a seed using appropriate handling

tongs. The tongs should be inspected after use to insure that the seed has

dropped into the tube, and the presence of the seed in the tube can be

verified by rattling it around. This can be done after loading the seed using

as a handle the short bakelite rod that is threaded into the plastic at the

top of the tube. With the ^^sj seed loaded in the aluminum reentrant tube,

the calibration procedure is as follows:

1. In anticipation of the calibration, the required measurement equipment should have been turned on to warm up for two hours, when using a Keithley Model 616 Electrometer. The Keithley Model 642 Electrometer, which

may also be used, is capable of measuring very small currents but requires up

to 24 hours to be within specifications (offset current 1 x lO'^'^ A) if the

instrument has recently been turned on, overloaded, or subjected to transient

pulses. This extremely low current measurement capability is not usually of

interest in brachytherapy source calibrations.

28
2. With the high-voltaye supply turned off , the hiyh-voltage connection

to the reentrant chamber should be checked. It is good practice to turn the screw that acts as the connector to the chamber wall and to rotate the clip

connecting the high-voltage wire to the screw. Experience has shown that if

these connections remain static over long periods of tiine, the aluminum

oxidizes and develops a high resistance in the connections. As a result,

noise is observed in the measurements. The effect of this type of noise on typical measurements is shown in table 4-6.

Table 4-6. Effect of noise pick-up in high-voltage connection to aluminum

reentrant chamber.

Conditi on

Mean

i oni zat ion

current

(pA)

Number

of meas. Standard

devi ati on

(pA)

Standard

devi ati on of the mean

(%)

Noise

No Noise

115.15

113.64

10

4

1.09

0.02

0.3

0.01

3. If the brass tube is not already installed, screw it into the chamber

finger-tight.

4. Check that the electrometer feedback shorting switch is in the

shorting position. Set the high-voltage polarity switch to negative and turn

on the high-voltage switch. Set the high voltage to 1100 volts.

5. Measure the background leakage current for the measurement system.

Unless new sources of radiation are introduced into the environment of

Room B145, and if the important insulators are clean and dry, the background

leakage current should not exceed 2.5 x lO"^'^ A.

6. Using an appropriate handling device, remove the 226Ra source (N-5 or N-0.5) from the safe drawer (B-7 or B-10, respectively) and place it into the

No. 1 thistle tube (the tube is capped at the bottom). Turn the plastic

thistle tube on its side and shake the source down the tube to determine

whether the blackened end of the source will be up or down when placed into

the reentrant chamber. Record the result. (The radium source may sometimes

flip and reverse ends when dropped into the thistle tube from the top.)

7. Handling the thistle tube with forceps, carry the tube with the

radium source behind the concrete barrier wall to the reentrant chamber and

insert it into the brass tube. Remember that the chamber is at high voltage.

8. Measure the ionization current produced by the radium source in the

reentrant chamber. If the measurement system and chamber have retained their

integrity, the results of the measurements should be in accord with the data

in table 4-1. Ordinarily, four or five measurements are all that are required

to evaluate the condition of the measurement system. A 60-s charge integration time for the N-0.5 radium source produces a feedback potential of about

1.8 V, and for the N-5 source a 70-s time gives about 18 V, when the capacitance of the electrometer is 1 nF. The range of the digital voltmeter used

for electrometer feedback-voltage measurements should be set to maximize the

number of read-out digits. For the Digitec Model 267 DC digital voltmeter,

the 2-V range is selected when the aforementioned integration time and

capacitance are used for the N-0.5, and the 20-V range for the N-5 source.

29
9. With the electrometer feedback shorted, turn off the high voltage

switch and return the source to its drawer in the safe. The source is

unloaded from the thistle tube by letting it slide out into the white plastic

tray.

10. Check that the high voltage is off, then remove the brass tube from

the chamber. The aluminum tube containing the ^^Sj seed can now be placed

into the chamber taking care to insert it vertically to avoid striking the

cup-like collection electrode.

11. Turn on the high voltage. When the high voltage reaches a steady

condition, ionization current measurements can proceed. Normally, the

reproducibility of the measurements for an undisturbed ^^^l seed is such that

no more than two or three measurements are required per seed position. If the

measurements are of good quality, check that the electrometer is shorted and

turn off the high voltage. Wait for the high voltage to decay to near zero. Remove the aluminum tube from the chamber using the bakelite handling rod.

Turn the tube upside down and shake the tube to change the seed orientation.

This operation is, of course, done quickly to minimize radiation exposure.

Carefully reinsert the tube into the chamber. Turn on the chamber high

voltage and when it reaches a steady state, repeat the measurement procedure.

This procedure is repeated until the standard deviation of the mean of the

ionization currents for the randomized seed orientations is <0.4%, but should

include at least 10 independent procedures because sometimes the measurements

show an abnormal distribution.

4.4. Information to be recorded in reentrant-chamber calibrations

The calibration of small sources has been performed manually using a programmable calculator, since the number of calibrations has been small, and

the available computer-OAS systems have generally been occupied with

instrument calibrations. There are many advantages to using computer-DAS

equipment, e.g., automatic measurement of feedback voltages, temperature and

pressure transducer voltages, time of day, time interval, control of shorting

switches, calculation of ionization currents, calculation of temperature and

pressure corrections, corrections for source decay, and automatic recording of

data. At the time of writing (December 1987) a new computer-DAS system has

been acquired and it will be used for brachytherapy source calibrations.

The information that must be recorded in the course of reentrant-chamber

calibration is not only measurement data but all information relating to the

source. The manufacturer will usually provide an estimate of the activity and

the date. In the case of ^^Sj seeds, the information is taken from the label

on the lead container. In a calibration in September 1984, this information

was as follows:

Product

Model

Lot No.

Dosage

By

1251 Seed

6711

151

19.5 mCi

RF

Date 8-20-84

30
Important information always to be recorded is as follows:

1. Person performing calibration.

2. Date.

3. Equipment (Indicate if same, or different from, equipment used

previ ously )

.

4. Capacitance (Identify, reference calibration).

5. HV and polarity of ionization chamber collection potential.

The data to be recorded are the same for manual or computer-DAS systems,,

although the equipment and recording methods are quite different. While the

summary presented here is based on experience with the manual system, the data

to be recorded are as follows, for any system of data acquisition:

1. Time of day for each measurement.

2. Background and leakage data in the form: integration time (At),

feedback potential (Vq) at the start of the integration period,

feedback potential (Vf) at the end of the integration period,

temperature, pressure.

3. Several computations of background drift rate, (V^ - VQ)/At, with

record of mean value used in the computation.

4. Upon introduction of the source into the reentrant chamber, integration time (At) used in exposure measurement.

5. Time of day for each measurement.

6. Vq and Vf (indicate sign).

7. Temperature and pressure. »

8. Net ionization current for each measurement corrected to STP.

9. The mean net STP-corrected ionization current with the standard

deviation of the mean in percent.

10. Record that the high voltage has been turned off .

11. Record that the source has been moved.

12. Record that the high voltage has been turned on^.

13. Repeat steps 5 through 12 until the percent standard deviation of the

mean for the net ionization currents (at reference temperature and

pressure) is <0.4% (if necessary, correct for decay). For ^^Sj

seeds, this repetition should be carried out at least 10 times, to

obtain the randomized-seed-position net ionization current.

31
14. Apply the appropriate recombination correction to the computed

ionization current.

15. Multiply the mean corrected ionization current by the appropriate

reentrant chamber calibration factor to determine the exposure (or

air kerma) rate at one meter ( in vacuo ) from the seed.

16. Correct the manufacturer's activity value to the reference date for

the calibration and compute the exposure rate from the activity and

the rate constant stated by the manufacturer. This should ayree with

the measured exposure or kerma rate within several percent, and serves to check both the N6S calibration and the manufacturer's

measurements. If the manufacturer does not specify a rate constant,

use a value from table 1-2, which will serve as a rough check to

detect a major discrepancy.

It is good practice to repeat calibrations of brachytherapy sources on one or more days. For ^^Co, ^^''Cs, and ^'^^Ir, an overall mean value with

appropriate statistics can usually be reported with good confidence. However,

1251 measurements are not as reproducible as measurements of other sources.

It is necessary, therefore, to report individual mean values of the

measurements at different times in order to describe adequately the character

of the i25j 5eefj during calibration. Reasons suggested for the anomalous

behavior of ^^Sj seeds are: mobility of the resin balls or silver wires,

migration of activity in or out of the carriers, and short half-life

contami nation.

A copy of a typical calibration report for an ^^sj seed is given in

appendix 0. The text of each calibration report is modified as required for

other seed types.

4.5. Uncertainty assessment for ^"^^Ir and ^^sj

Uncertainties of brachytherapy sources calibrated by use of a reentrant

chamber are evaluated in several steps. First the uncertainty of the exposure

rate of the array of sources is estimated, next the uncertainty in the use of

the array of sources to standardize the reentrant chamber is estimated, then

the uncertainty in the measurement of a single source in the reentrant chamber

is estimated, and finally the uncertainties are combined to obtain the final

estimate. The general method of uncertainty assessment, leading to an estimated overall uncertainty, is described in section 3.4.

Tables 4-7 and 4-8 give the details of the uncertainty estimates. As in

table 3-14, all the entries in tables 4-7 and 4-8 are considered to be

estimates of uncertainties of multiplicative factors.

The overall uncertainty for ^^^Ir comes predominantly from the Type B components, as is the case with ^^'^Cs and ^°Co brachytherapy calibrations.

There are two kinds of I92jp sources, but no distinction is made between them

in the uncertainty estimate in table 4-7 except for the half-life (decay

correction) entry. On the other hand, the i^sj seeds are much less reproducible, as noted above. As a result, the three kinds of ^^Sj seeds show

appreciably different overall uncertainties.

32
Table 4-7. Uncertainty analysis for ^^^Ir source calibration with the

reentrant chamber.

A B (%) {%)

Measurement of array of seeds using

graphite standard chambers

Volume 0.01

Charge 0.03 0.05

Timing 0.01

Air density 0.03 0.05

Recombination loss 0.03

Humidity 0.1

Leakage and radiation background 0.02

Stopping power ratio 0.25

Energy absorption coefficient ratio 0.2

Stem scatter 0.02

Wall absorption 0.1

Mean center of electron production 0.02

Effective measurement point 0.05

Radial nonuni formi ty 0.01

Distance 0.04

Correction to vacuum

Ai r attenuation 0.1

Room scatter 0.3

Half-life

Platinum encapsulated (2.82 half-lives) 0.5

Stainless steel encapsulated (0.12 half-lives) 0.02

Array cover attenuation 0.1

Reproducibility in exposure measurement 0.2

Quadratic sum for graphite chamber measurement

Platinum encapsulated seeds 0.21 0.70

Stainless steel encapsulated seeds 0.21 0.49

Calibration of aluminum reentrant

chamber using individual seeds of the array

Charge 0.03 0.05

Timing 0.01

Air density 0.05 0.05

Recombination loss 0.1

Humidity 0.1

Leakage and radiation background 0.02

Reproducibility in reentrant chamber 0.1

Quadratic sum for reentrant chamber calibration 0.12 0.16

33
Table 4-7. Continued

(%) {%)

Measurement of single unknown source 1n

reentrant chamber

Charge 0.05 0.1

Timing 0.01

Ai r density 0.05 0.1

Recombination loss 0.1

Leakage and radiation background 0.02

Reproducibility in reentrant chamber 0.1

PI asti c cover 0.

3

Ouadratic sum for unknown measurement 0.12 0.35

PI ati num Stainless steel

Combined uncertainty 0.84 0.68

2 X combined uncertainty 1.7 1.4

Overall uncertainty 2

Notes to table 4-7 .

The uncertainty estimates are at the one-sigma level, and are in percent. The

columns headed A and B refer to the uncertainty estimates described in the

text as Type A and Type B. Charge includes uncertainties for both voltage and capacitance.

Air density includes uncertainty for both temperature and pressure.

Ha1 f-1 i fe refers to the uncertainty in the decay correction during the

interval between the measurement with the standard chamber and with the

reentrant chamber. The uncertainty in the currently accepted half-life was

used, for the intervals shown in the table. No uncertainty estimate is

included for the difference between the currently accepted half life (73.83 d)

and the value actually used (74.0 d)

.

Plastic cover refers to the uncertainty introduced by the thin plastic ribbon

or tubing in which the seeds are furnished. Although we request that the

tubing beyond the ends of the seed be cut away, some material must be left to

avoid damaging the seed. This extra length causes the seeds to lie at

slightly different levels in the rounded bottom of the glass vial. The error

estimate is for the attenuation in the tubing and the change in the chamber

response due to the different seed positions.

Overall uncertainty is rounded upward to an integral value.

34
Table 4-8. Uncertainty analysis for I25j source calibration with the

reentrant chamber.

A 1

1

AU-•mark Ag-•wi re INO-mark

An

DD

AA

DD A

D

ts (/o) (/o) [lo) (A)

Source array

Exposure rate U . 1 ) / 0.0 7 0.33 0.0/ 0.33

Leakage 0.4 0.2 0.2

Ai r attenuati on FAC 0.9 0.9 0.9

KeprOGUCiDI \\Zy OT

measurements 0.6 0.8 0.3

Di stance 0.2 0.2 0.2

Correction to vacuum 2.2 1.8 1.8

Half-life 0.3 0.2 0.03

Quadratic sum 0.60 2.46 0.80 2.07 0.31 2.06

Calibration of reentrant

chamber

Charge 0.03 0.05 0.03 0.05 0.03 0.05

Timing 0.01 0.01 0.01

Air density 0.05 0.05 0.05 0.05 0.05 0.05

Recombination loss 0.4 0.4 0.4

Humidity 0.1 0.1 0.1

Leakage and radiation

background 0.02 0.02 0.02

Reproducibility in

reentrant chamber 2.2 1.4 0.4

Quadratic sum 2.20 0.42 1.40 0.42 0.40 0.42

Calibration of unknown

sourceCharge 0.03 0.1 0.03 0.1 0.03 0.1

Timing 0.01 0.01 0.01

Air density 0.05 0.1 0.05 0.1 0.05 0.1

Recombination loss 0.4 0.4 0.4

Humidity 0.1 0.1 0.1

Leakage and radiation

background 0.02 0.02 0.02

Reproducibility in

reentrant chamber 0.4 0.4 0.4

Quadratic sum 0.40 0.44 0.40 0.44 0.40 0.44

Total quadratic sum 2.32 2.53 1.66 2.16 0.65 2.15

Combined uncertainty 3.43 2.72 2.25

2 X combined uncertainty 6.86 5.44 4.50

Overall uncertainty 7 6 5

35
Notes to Table 4-8.

The uncertainty estimates are at the one-signa level, and are in percent. The

columns headed A and B refer to the estimates described in the text as Type A

and Type B.

The columns headed Au-mark, Ag-wire, and No-mark refer to the gold-narker,

silver-wire, and no-marker seeds, respectively.

Exposure rate is from table 5.1 of reference [12] except for the added

leakage, air attenuation for free air chamber, and reproducibility of

measurements. The gold-marker seeds were measured at distances of 50 cm and

25 cm while the other two types of seeds were all measured 25 cm. The 50-cm

data have leakage corrections which are larger than 10%, while the 25-cm data

have corrections less than 4% and often less than 2%. However part of the

leakage error is assumed to be embedded in the reproducibility error. The

leakage error is taken as 1% for the 50-cm measurements and 0.2% for the 25-cm

measurements (the 50-cm data contributes 25% to the mean value).

Air attenuation FAC refers to the correction for attenuation in the 12.75-cm

air path within the free-air chamber, as described in section 4.3.

Reproducibility of measurements estimates are from reference [14],

Correction to vacuum refers to the corrections for air attenuation in the air

path between the source array and the free-air chamber, outside the FAC, as described in section 4.3.

Hal f-1 i fe refers to the uncertainty in the calibration factor due to the

uncertainty in the measured half-life, as described in section 4.3.

Uncertainty in the calibration of the user's source due to uncertainty in the

half-life, resulting from about two-weeks calibration time, is considered

negl i gi bl e.

Charge includes uncertainties for both voltage and capacitance.

Air density includes uncertainties for both temperature and pressure.

Overall uncertainty is rounded upward to an integral value.

36
Based on the assessment of uncertainties described here, the following

are adopted as the overall uncertainties for sources calibrated with the

reentrant ionization chamber:

The overall uncertainty is considered to have the approximate significance of

a 95% confidence limit.

5. Safety considerations

The main considerations are radiation safety and high-voltage safety,

5.1, Radiation safety

Handling brachytherapy sources is probably the most hazardous calibration

activity performed by the Dosimetry Group. The small number of sources calibrated per year makes automation of source handling uneconomic. The sources are moved from their shipping containers (generally called "pigs") to

small lead transfer pots, and then to their measurement positions, by means of

long-handled tongs or tweezers. These devices must not be too inconvenient,

as would result from very long handles, since calculation and experience show

that the dose to the operators 's hands is minimized by rapid handling.

The handling of any physically small source such as a seed of ^'^^Ir or

I25i requires special care. The seeds are small, the color of their

encapsulation material is not easily distinguishable from the color of working

surfaces, and the handling tools available can drop or accidentally propel a seed away from the handling area. Moreover, if a seed is dropped behind the

concrete barrier in the vicinity of the source storage safe, the background

radiation from the safe makes it difficult to locate the source by means of a radiation detector. It is mandatory therefore that all seed-type sources be

handled only over a white tray with sides several inches high. This is good

practice for other types of sources as well.

Manipulation of brachytherapy sources should whenever possible be carried

out behind lead L-shields with lead-glass windows. These shields protect the

body and (to some extent) the face of the individual, while the hands are free

to reach behind the shield as necessary to carry out the manipulations. The

work should be performed in suitable trays, in case a source is dropped. If a source is dropped on the floor and its position is not immediately evident,

the room should be secured with suitable signs and the Health Physics Group

called in to locate the source with portable detectors.

Reading engraved identification marks is often difficult. It should be

done with the source behind a lead or concrete shield, using either a telescope or a TV camera.

192IP

^2^1, no-marker

^^^l, silver-wire

gold-marker

2%

5%

6%

7%

37
Radiation badges must be worn at all times while working with brachytherapy sources. In particular, finger badges must be worn when the sources

are being manipulated in any manner. The rate constants listed in table 1-2

can be used to estimate exposures to the hands and body and to judge safe

handling times. Because the most critical exposures are to the hand at

distances of a few centimeters, and the activity of brachytherapy sources can

be given in units of millicuries or megabecquerel s , it is convenient to

convert the rate constants in table 1-2 to the values given in table 5-1.

Table 5-1. Rate constants in units convenient for protection calculations.

Exposure Air-kerma

Radionuclide rate constant rate constant

(R cm2/mCi h) (mGy cm2/MBq h)

60Co 13.1 3.1

3.3 0.8

I92ir 4.0 1.0

1251 1.3 0.3

Radi urn 8.25 2.0

5.2 High-voltage safety

Most of the ionization chambers used in brachytherapy calibration, and in

standardizing NBS brachytherapy sources, have high voltage on their outside

electrodes. The convenience of this arrangement is considered to outweigh the

hazard involved. At tenti veness to safe working procedures is dictated when

working around ionization chambers that have exposed high-voltage electrodes.

In any event, brachytherapy calibrations are performed only by a few highly

experienced members of the Dosimetry Group.

38
Re ferences

[1] NBS, NBS Calibration Services Users Guide 1986-88 . NBS Special

Publication 250, G. A. Uriano, et al., Editors, U.S. Department of

Commerce, National Bureau of Standards, U. S. Government Printing Office,

Washington, D. C. July 1986.

[2] Boutlllon, 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] Boutlllon, M., g-values from Berger and Seltzer tables (1982). Report

CCEMRI (I)/85-18 to Section I of CCEMRI (Consultative Committee for

Standards for the Measurement of Ionizing Radiation), BIPM (International

Bureau of Weights and Measures), Pavilion de Breteull, F-92310 Sevres,

France (1985).

[4] AAPM, Specification of brachytherapy source strength . AAPM Report No.

21, American Association of Physicists in Medicine, American Institute

of Physics, New York, 1987.

[5] NCRP, A handbook of radioactivity measurements procedures . NCRP

Report 58, 2nd edit., National Council on Radiation Protection and

Measurements, Bethesda, 1985.

[6] NCRP, Specification of Gamma-Ray Brachytherapy Sources . NCRP Report

No. 41, National Council on Radiation Protection and Measurements,

Washington, D.C. 1974.

[7] Loftus, T. P., Standardization of cesium-137 gamma-ray sources in terms

of exposure units (roentgens). J. Res. Nat. Bur. Stand. (U.S.) 74A , 1-6

(1970).

[8] Hubbel 1

, J. H. , Photon cross sections, attenuation coefficients, and

energy absorption coefficients from 10 keV to 100 GeV. Nat. Stand. Ref.

Data Ser. 29, Nat. Bur. Stand. (U.S.), 85 pages (August 1969).

[9] Loftus, T. P., and Weaver, J. T. , Standardization of ^^Co and ^^'^Cs

gamma-ray beams in terms of exposure. J. Res. Nat. Bur. Stand. (U.S.)

78A , 465-476 (1974).

[10] Gorblcs, S. G. , Kunz, W. F. , and Nash, A. E., New values for the halflives of cesium-137 and cobalt-60 nuclides. Nucl eoni cs _2]^, No. 1, 63-65

(1963).

[11] Attlx, F. H. , and Ritz, V. H. , A determination of the gamma-ray emission

of radium. .1. Res. Nat. Bur. Stand. (U.S.) 59, 293-305 (1957).

[12] Lamperti, P. J., Loftus, T. P., and Loevlnger, R., Calibration of x-ray

and gamma-ray measuring instruments at the National Bureau of Standards.

National Bureau of Standards Special Publication 250-16 (1987).

[13] Loftus, T. P., Standardization of 1r1dium-192 gamma-ray sources in terms

of exposure. J. Res. Nat. Bur. Stand. (U.S.) 85 , 19-25 (1980).

39
[14] Loftus, T. P., Exposure standardization of iodine-125 seeds used for

brachytherapy. J. Res. Nat. Rur. Stand. (U.S.) 89, 295-303 (1984).

[15] Kubo, H., Exposure contribution from Ti K x rays produced in the titanium

capsule of the clinical 1-125 seed. Med. Phys. J12_, 215-220 (1985).

[16] Williamson, J. F., Monte Carlo evaluation of specific dose constants in

water for ^^^l seeds. Med. Phys., in press.

40
APPENDIX A

Attachment concerning adjustment of NBS exposure rates.

Exposure rates were adjusted at NBS on 1986 Jan 01 to take into account

recent refinements in some of the physical parameters . Exposure rates in

all reports issued after 1986 Jan 01 are lower than the exposure rates in

reports issued before that date. The changes in percent are as follows:

NBS primary standard: Cavity Chambers Free-air chambers

Radiation: 60,Co 137 Cs 192 Ir 125i and x

Humidity -0 .3 -0. 3 -0. 3 -0. 2 Energy- absorption coefficient ratio -0 .1 0. 0 0. 0 N. A. Stopping-power ratio -0 .7 -0. 5 -0. 4 N. A.

Total change -1 .1 -0. 8 -0. 7 -0. 2

Laboratories with instruments calibrate^d :prior to 1986 Jan 01 can brin,

their instruments into agreement with the new exposure rates by reducing

the earlier calibration factors by the above total changes in percent.

Humidity . Before 1986 no correction was made for the effect of water vapor

in the ambient air, since early NBS work had indicated a negligible effect.

More recent work at several national laboratories has determined this

effect with good accuracy. The values used are taken from ICRU Report 31,

page 31, Figure 5.14. These factors are applied to the exposure rate

determined by the NBS primary standard. No correction is made for the

effect of water vapor on the reentrant chamber, since it is assumed that

both the calibration of the chamber with the standard seeds and the

calibration of unknown source take place in air with a relative humidity

between 10% and 70%.

Energy-absorption coefficient ratio . The most recent values (J.H. Hubbell,

Rad. Res. 70, 58, 1977, and Int. J. Appl. Radiat. Isot. 33, 1269, 1982) were compared with the earlier values used in establishing NBS exposure

standards (J.H. Hubbell, Nat. Stand. Ref. Data Ser. 29, Nat. Bur. Stand.

(U.S.), 1969). Only one appreciable change was found.

Stopping-power ratio . The most recent values (ICRU Report 37, 1984) were compared with the earlier values used in establishing NBS exposure

standards (M. J. Berger and S. M. Seltzer, NASA SP-3012, 1964). The

comparison for ^'-'Co gamma radiation was made by the staff of the Bureau

International des Poids et Mesures, and confirmed at NBS. The comparisons

for the other radionuclides were made at NBS.

The last previous change in NBS exposure standards was made on 1972 May 01.

At that time the exposure rates for cobalt-60 and cesium-137 gamma rays were lowered by 0.7% and 0.6%, respectively, due to adoption as an NBS

exposure standard of six spherical graphite cavity ionization chambers

(Loftus, T.P., and Weaver, J.T., J. Res. Nat. Bur. Stand., 78A- ^65, 1974).

A-1

APPENDIX B

DG 7890/86

TFN 234668

DB 851:156-163

1986 May 9

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

GAITHERSBURG , MARYLAND 20899

Report of Exposure -rate Measurement

of

One Cesium- 137 Source Engraved 3M151267

Submitted by

Colossal Medical Complex

Everytovm, AD 12343

B-1
DG 7890/86

1986 May 9 Page 2 of 3

The cylindrical source engraved 3M151267 is approximately 20 mm in length

and 3 mm in diameter. No determination of radionuclidic purity was performed

.

The strength of this source is given in terms of the exposure rate at unit

distance in vacuum, in a direction perpendicular to the long axis of the source. A 2. 8 -liter aluminum ionization chamber was used to compare the source with cesium- 137 working standards that had been calibrated using

standard graphite ionization chambers. Details of the original

measurements and procedures are given in: T. P. Loftus , "Standardization

of Cesium-137 Gamma Ray Sources in Terms of Exposure Units (Roentgens)", J. Res. Nat. Bur. Stand. 74A, 1-6 (1970).

This source was measured on two different days . The measurements agree to

within 0.1 percent. The exposure rates of the NBS working standards were corrected for decay using a half- life of 30.0 years, taken from NCRP Report

58, 2nd edition (1985). The mean exposure rate at one meter on 1986 Mar

27 was

10.25 mR-m^/h

with an overall uncertainty of 2%.

The uncertainty associated with this calibration is obtained by combining

the component uncertainties in quadrature. The random uncertainty

components are calculated as standard deviations of the mean of replicate

readings; other components are estimated at about the 67% probability level

and are assumed to have the approximate character of standard deviations.

The overall uncertainty is two times the quadratic sum of all the component

uncertainties; it is considered to have the approximate significance of a one-sided 95% confidence limit.

Information on technical aspects of this report may be obtained from J. T. Weaver, Radiation Physics, C214, National Bureau of Standards,

Gaithersburg, MD 20899, (301) 975-5586.

Calibration performed by J . T. Weaver

Report approved by R. Loevinger

For the Director

by

Randall S. Caswell

Chief, Ionizing Radiation Division

Center for Radiation Research

National Measurement Laboratory

B-3 is the same as A-1

B-2
Attachment concerning adjustment of NBS exposure rates.

Exposure rates were adjusted at NBS on 1986 Jan 01 to take into account

recent refinements in some of the physical parameters. Exposure rates in

all reports issued after 1986 Jan 01 are lower than the exposure rates in

reports issued before that date. The changes in percent are as follows:

NBS primary standard: Cavity Chambers Free -air chambers

Radiation: 60co 137cs 192i^ ^25i and x

Humidity -0.3 -0.3 -0.3 -0.2

Energy- absorption coefficient ratio -0.1 0.0 0.0 N.A.

Stopping-power ratio -0.7 -0.5 -0.4 N.A.

Total change -1.1 -0.8 -0.7 -0.2

Laboratories with instruments calibrated prior to 1986 Jan 01 can bring

their instruments into agreement with the new exposure rates by reducing

the earlier calibration factors by the above total changes in percent.

Humidity . Before 1986 no correction was made for the effect of water vapor

in the ambient air, since early NBS work had indicated a negligible effect.

More recent work at several national laboratories has determined this

effect with good accuracy. The values used are taken from ICRU Report 31,

page 31, Figure 5.14. These factors are applied to the exposure rate

determined by the NBS primary standard. No correction is made for the

effect of water vapor on the reentrant chamber, since it is assumed that

both the calibration of the chamber with the standard seeds and the

calibration of unknown source take place in air with a relative humidity

between 10% and 70%.

Energy-absorption coefficient ratio . The most recent values (J.H. Hubbell,

Rad. Res. 70, 58, 1977, and Int. J. Appl . Radiat. Isot. 33, 1269, 1982)

were compared with the earlier values used in establishing NBS exposure

standards (J.H. Hubbell, Nat. Stand. Ref. Data Ser. 29, Nat. Bur. Stand.

(U.S.), 1969). Only one appreciable change was found.

Stopping-power ratio . The most recent values (ICRU Report 37, 1984) were compared with the earlier values used in establishing NBS exposure

standards (M. J. Berger and S. M. Seltzer, NASA SP-3012, 1964). The

comparison for °^Co gamma radiation was made by the staff of the Bureau

International des Poids et Mesures , and confirmed at NBS. The comparisons

for the other radionuclides were made at NBS.

The last previous change in NBS exposure standards was made on 1972 May 01.

At that time the exposure rates for cobalt-60 and cesium-137 gamma rays

were lowered by 0.7% and 0.6%, respectively, due to adoption as an NBS

exposure standard of six spherical graphite cavity ionization chambers

(Loftus, T.P., and Weaver, J.T., J. Res. Nat. Bur. Stand., 78A. ^65, 1974).

B-3

APPENDIX C

DG 8798/87

TFN 240725

DB 875:151-161

1987 Dec 17

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

GAITHERSBURG. MD 20899

Page 1 of 3

REPORT OF AIR-KERMA-RATE MEASUREMENT

Iridium-192 Seed

Manufactured by

For

Best Industries, Inc.

Springfield, VA 22153

Oversize Hospital

Metropolis, AB 12345

Received

Calibrated

1987 Nov 18

1987 Nov 27 Dec 03

The composition of the active material according to the manufacturer is 30%

iridium and 70% platiniom. The active material is 0.1 mm in diameter and is

clad with stainless steel 0.2 mm thick. The source is 3.0 mm long and

0.5 mm in diameter. The source is in a thin nylon tube, the ends cut to

only slightly exceed the ends of the seed. No determination of

radionuclidic purity was performed.

The strength of this seed is given in terms of "air-kerma strength", which

is the product of air-kerma rate and the square of the distance in vacuum,

in a direction perpendicular to the long axis of the seed. A reentrant

ionization chamber that had been calibrated using standardized seeds of the same composition and dimensions but without a nylon tube, was used for the

measurements. The standard seeds had been measured using a graphite

ionization chamber. Details of the calibration of the reentrant chamber

are given in: T. P. Loftus , "Standardization of Iridium-192 Gamma-Ray

Sources in Terms of Exposure", J. Res. Nat. Bur. Stand. 85, 19-25 (1980)

and "Calibration of Gamma-Ray- Emitting Brachytherapy Sources at NBS", to be

published as NBS Special Publication 250-19 (1988).

The mean source strength is determined from measurements for repeated

placements of the seed in the reentrant chamber, where the seed assumes a random, horizontal position at the bottom of a glass vial inserted into the

brass reentrant tube for each placement.

This seed was measured on two different days . The data were corrected for

decay using a half-life of 73.83 days, from NCRP Report 58, 2nd edition

(1985). The results of these measurements are:

Measurement

date

Number of seed

placements

Stand, dev.

of the mean

(%)

Air-kerma strength

1987 Nov 30.5

(GymVs)

1987 Nov 27

1987 Dec 03

0.0

0.0

1.312 10"

1.311 10-

C-1
DG 8798/87

1987 Dec 17

1 Page 2 of 3

The American Association of Physicists in Medicine has recommended that

air-kerma strength be given in the units micrograys meters squared per hour

("Specification of Brachytherapy Source Strength", AAPM Report No. 21, Am.

Inst, of Phys . , NY, June 1987). Adopting those units, and combining all

measurements, the mean air-kerma strength on 1987 Nov 30.5 Eastern Standard

Time (noon Nov 30) was

47.2 MGym^/h

with an overall uncertainty of 2%.

The uncertainty associated with this calibration is obtained by combining

the component uncertainties in quadrature. The random uncertainty

components are calculated as standard deviations of the mean of replicate

readings; other components are estimated and are assumed to have the

approximate character of standard deviations. The overall uncertainty is

two times the square root of the quadratic sum of all the component

uncertainties; it is considered to have the approximate significance of a 95% confidence limit.

For the photons from iridium- 192, the fraction of the secondary electron

energy dissipated as bremsstrahlung is negligible, and air kerma k^^j. is

related to the exposure X by the equation ^

j. = X(W/e) where W/e is the

energy expended in air per unit charge. Then the air-kerma strength in

;uGy-m^/h is divided by W/e = 0.00876 Gy/R to convert to exposure strength

in nR-m^/h.

The manufacturer states an average reading of 13.12 mCi on 1987 Nov 16.

Using the conversion factor supplied by the manufacturer,

4.6 R»cm^/(mCi«h) and correcting for 14.00 days of decay, a value of

5.39 mR-m^/h = 46.4 /xGym^/h is obtained, which is within 2 percent of the

NBS value.

Information on technical aspects of this report may be obtained from

J. T. Weaver, Radiation Physics, C214, National Bureau of Standards,

Gaithersburg, MD 20899, (301) 975-5586.

Calibration performed by J . T. Weaver

Report approved by R. Loevinger

For the Director

by

David M. Gilliam, Deputy Chief

Ionizing Radiation Division

Center for Radiation Research

National Measurement Laboratory

C-3 is same as A-1

C-2
Attachment concerning adjustment of NBS exposure rates.

Exposure rates were adjusted at NBS on 1986 Jan 01 to take into account

recent refinements in some of the physical parameters. Exposure rates in

all reports issued after 1986 Jan 01 are lower than the exposure rates in

reports issued before that date. The changes in percent are as follows:

NBS primary standard: Cavity Chambers Free-air chambers

Radiation: 60co 137cs 192i^ ^25i and x

Humidity -0.3 -0.3 -0.3 -0.2

Energy-absorption coefficient ratio -0.1 0.0 0.0 N.A.

Stopping-power ratio -0.7 -0.5 -0.4 N.A.

Total change -1.1 -0.8 -0.7 -0.2

Laboratories with instruments calibrated prior to 1986 Jan 01 can bring

their instruments into agreement with the new exposure rates by reducing

the earlier calibration factors by the above total changes in percent.

Humidity . Before 1986 no correction was made for the effect of water vapor

in the ambient air, since early NBS work had indicated a negligible effect.

More recent work at several national laboratories has determined this

effect with good accuracy. The values used are taken from ICRU Report 31,

page 31, Figure 5.14. These factors are applied to the exposure rate

determined by the NBS primary standard. No correction is made for the

effect of water vapor on the reentrant chamber, since it is assumed that

both the calibration of the chamber with the standard seeds and the

calibration of unknown source take place in air with a relative humidity

between 10% and 70%.

Energy-absorption coefficient ratio . The most recent values (J.H. Hubbell,

Rad. Res. 70, 58, 1977, and Int. J. Appl. Radiat. Isot. 33, 1269, 1982)

were compared with the earlier values used in establishing NBS exposure

standards (J.H. Hubbell, Nat. Stand. Ref. Data Ser. 29, Nat. Bur. Stand.

(U.S.), 1969). Only one appreciable change was found.

Stopping-power ratio . The most recent values (ICRU Report 37, 1984) were compared with the earlier values used in establishing NBS exposure

standards (M. J. Berger and S. M. Seltzer, NASA SP-3012, 1964). The

comparison for ^'^Co gamma radiation was made by the staff of the Bureau

International des Poids et Mesures , and confirmed at NBS . The comparisons

for the other radionuclides were made at NBS

.

The last previous change in NBS exposure standards was made on 1972 May 01.

At that time the exposure rates for cobalt-60 and cesium-137 gamma rays

were lowered by 0.7% and 0.6%, respectively, due to adoption as an NBS

exposure standard of six spherical graphite cavity ionization chambers

(Loftus, T.P. , and Weaver, J.T., J. Res. Nat. Bur. Stand., 78A . 465, 1974).

C-3

APPENDIX D

DG 8844/88 U. S. DEPARTMENT OF COMMERCE Page 1 of 2 TFN 241000 NATIONAL BUREAU OF STANDARDS

DB 875:162-200 GAITHERSBURG , MD 20899

DB 882: 4-12

1988 Mar 08

REPORT OF AIR-KERMA-RATE MEASUREMENT

Model 6711 Iodine-125 Seed Lot No. ISW-804

Manufactured by The 3M Company

Medical Products Division

New Brighton, MN 55112

For Nonesuch University Hospital

Old Newtown, AC 12321

Received:

Calibrated:

1988 Jan 26

1988 Jan 29-Feb 29

The model 6711 seed is a welded titanium capsule containing a silver wire

on which the radionuclide lodine-125 is adsorbed. The overall length of

the seed is 4.5 mm and the outside diameter is 0.8 mm. No determination of

radionuclidic purity was performed.

The strength of this seed is given in terms of "air-kerma strength" , which

is the product of air-kerma rate and the square of the distance in vacuum,

in a direction perpendicular to the long axis of the seed. A reentrant

ionization chamber that had been calibrated using standardized Model 6711-

type seeds was used for the measurements. The standard seeds had been

calibrated using a free-air ionization chamber. Details of the calibration

of the reentrant chamber are given in: T. P. Loftus, "Exposure

standardization of iodine-125 seeds used for brachytherapy , " J. Res. Nat.

Bur. Stand. 89, 295-303 (1984) and "Calibration of Gamma-Ray-Emitting

Brachytherapy Sources at NBS", to be published as NBS Special Publication

250-19 (1988).

The mean source strength is determined from measurements for repeated

placements of the seed in the reentrant chamber, where the seed assumes a random, horizontal position at the bottom of the reentrant tube for each

placement

.

This seed was measured on four different days. The data were corrected for

decay using a half- life of 59.6 days, from NCRP Report 58, 2nd edition

(1985). The results of these measurements are:

Measurement Number of seed Stand, dev. Air-kerma strength

date placements of the mean 1988 Feb 16.5

(%) (Gy.mVs)

1988 Jan 29 12 0.2 9.52 x 10"^

1988 Feb 05 12 0.2 9.54 x 10"^

1988 Feb 22 12 0.2 9.59 x lO'^

1988 Feb 29 12 0.2 9.59 x 10'^

D-1
DG 8844/88

1988 Mar 08

Page 2 of 2

The American Association of Physicists in Medicine has recoiiunended that air

kerma strength be given in the units micrograys meters squared per hour

("Specification of Brachytherapy Source Strength", AAPM Report No. 21, Am.

Inst, of Phys . , NY, June 1987). Adopting those units, and combining all

measurements, the mean air-kerma strength on 1988 Feb 16.5 Eastern Standard

Time (noon Feb 16) was

34.4 /iGymVh

with an overall uncertainty of ±6%, which does not take into account

possible errors due to titanium K x rays (H. Kubo , Med. Phys. 12_, 215,

1985; J. F. Williamson, submitted to Medical Physics).

The uncertainty associated with this calibration is obtained by combining

the component uncertainties in quadrature . The random uncertainty

components are calculated as standard deviations of the mean of replicate

readings; other components are estimated and are assumed to have the

approximate character of standard deviations. The overall uncertainty is

two times the square root of the quadratic sum of all the component

uncertainties; it is considered to have the approximate significance of a 95% confidence limit.

For the photons from iodine-125, the fraction of the secondary electron

energy dissipated as bremsstrahlung is negligible, and air kerma K^^^. is

related to the exposure X by the equation K^^j. = X (W/e) where W/e is the

energy expended in air per unit charge. Then the air-kerma strength in

/xGym^/h is divided by W/e = 0.00876 Gy/R to convert to exposure strength

in /iR«m^/h.

The manufacturer states the apparent activity as 34.42 mCi on 1988 Jan 25.

Using the conversion factor supplied by the manufacturer, 40.3

nR«m^/(mCi«s) , and 22 days decay, a value of 1074 nR«m^/s = 33.9 /iGym^/h

is obtained, which is within 2 percent of the NBS value.

Information on technical aspects of this report may be obtained from

J. T. Weaver, Radiation Physics, C214, National Bureau of Standards,

Gaithersburg, MD 20899, (301) 975-5586.

Calibration performed by J . T. Weaver

Report approved by R. Loevinger

For the Director

by

Randall S. Caswell, Chief

Ionizing Radiation Division

Center for Radiation Research

National Measurement Laboratory

D-3 is same as A-1

D-2
Attachment concerning adjustment of NBS exposure rates.

Exposure rates were adjusted at NBS on 1986 Jan 01 to take into account

recent refinements in some of the physical parameters. Exposure rates in

all reports issued after 1985 Jan 01 are lower than the exposure rates in

reports issued before that date. The changes in percent are as follows:

NBS primary standard: Cavity Chambers Free -air chambers

Radiation: 60co 137cs 192ir ^25x and x

Humidity -0.3 -0.3 -0.3 -0.2

Energy- absorption coefficient ratio -0.1 0.0 0.0 N.A.

Stopping-power ratio -0.7 -0.5 -0.4 N.A.

Total change -1.1 -0.8 -0.7 -0.2

Laboratories with instruments calibrated prior to 1986 Jan 01 can bring

their instruments into agreement with the new exposure rates by reducing

the earlier calibration factors by the above total changes in percent.

Humidity . Before 1986 no correction was made for the effect of water vapor

in the ambient air, since early NBS work had indicated a negligible effect.

More recent work at several national laboratories has determined this

effect with good accuracy. The values used are taken from ICRU Report 31,

page 31, Figure 5.14. These factors are applied to the exposure rate

determined by the NBS primary standard. No correction is made for the

effect of water vapor on the reentrant chamber, since it is assumed that

both the calibration of the chamber with the standard seeds and the

calibration of unknown source take place in air with a relative humidity

between 10% and 70%.

Energy-absorption coefficient ratio . The most recent values (J.H. Hubbell,

Rad. Res. 70, 58, 1977, and Int. J. Appl. Radiat. Isot. 33, 1269, 1982)

were compared with the earlier values used in establishing NBS exposure

standards (J.H. Hubbell, Nat. Stand. Ref. Data Ser. 29, Nat. Bur. Stand.

(U.S.). 1969). Only one appreciable change was found.

Stopping-power ratio . The most recent values (ICRU Report 37, 1984) were compared with the earlier values used in establishing NBS exposure

standards (M. J. Berger and S. M. Seltzer, NASA SP-3012, 1964). The

comparison for ^^Co gamma radiation was made by the staff of the Bureau

International des Poids et Mesures , and confirmed at NBS. The comparisons

for the other radionuclides were made at NBS.

The last previous change in NBS exposure standards was made on 1972 May 01.

At that time the exposure rates for cobalt- 60 and cesium- 137 gamma rays

were lowered by 0.7% and 0.6%, respectively, due to adoption as an NBS

exposure standard of six spherical graphite cavity ionization chambers

(Loftus , T.P. , and Weaver, J.T., J. Res. Nat. Bur. Stand., 78A . 465, 1974).

D-3

NBS-n4A (REV. 2-8C) U.S. DEPT. OF COMM. 1, PUBLICATION OR 2. Performing Organ. Report No, 3. Publication Date BIBLIOGRAPHIC DATA

R P Pn R T MO

Decenber 1988 SHEET (See instructions) NBS/SP-250/19

4. TITLE AND SUBTITLE

NBS Measurement Services: Calibration of Gamma-Ray-Emittinr Br achy therany Sources

5. AUTHOR(S)

J. T. Weaver, T. P. Loftus, and Fx.. Loevinger

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GArrHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

same as item 6

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 88-600609

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most si gnificant information. If document includes a si gnificant

bibliography or literature survey, mention it here) The calibration of small radioactive sources used for interstitial radiation therapy

(brachytherapy) is performed in terms of the physical quantities exposure or air

kerma. The calibrations are listed in NBS Special Publication 250 as 47010C, 47011C,

and 47040S (formerly 8.4E, 8.4F, and 8.4A). The calibration provides the air-kerma

rate (the exposure rate, before 1987 Oct 1) in vacuum at 1 meter perpendicular to the

long axis of the source. ^°Co and ^^^Cs sources are calibrated by comparison with NBS

working standard sources of the same type, while ^^^ir and ^^^I sources are calibrated

by measurement in a reentrant ionization chamber that was calibrated using NBS working

standard sources of the same type. The working standard sources were calibrated using

the NBS graphite cavity ionization chambers except for ^^^I, for which the NBS

measurement standard was a free-air chamber. The working standard sources of the two

long-lived sources have been measured a number of times over the years; the

reliability of the reentrant chamber for the two short-lived sources is assured by use

of sealed radium sources as a constancy check. The overall uncertainty (considered to

have the approximate significance of a 95% confidence limit) is given as 2% for all

the sources except for ^^^I seeds, for which it is given as 5%, 6%, and 7%, depending

on the type of seed. The stated uncertainty for ^^^I seeds does not include possible

errors due to low-energy x rays not recognized at the time the standards were

established.

12. KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

air kerma; air-kerma strength; brachytherapy; calibration; cesium-j.37; cobalt-60;

exposure; iodine-125; iridium-192; radioactive source; reentrant chamber; standard;

uncertainty

13. AVAILABILITY

ricl Unlimited

I I For Official Distribution. Do Not Release to NTIS

[xl Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

[]2 Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

60

15. Price

•fy U. S. GOVERNMENT PRINTING OFFICE: 1989 —2'»2-31 1 —92617 USCOMM-DC 6043-P80



THE SP 250 SERIES ON NBS MEASUREMENT SERVICES*

SP 250-1 Spectral Radiance Calibrations SN003-O03-02792-8 $3.50

SP 250-2 Far Ultraviolet Detector Standards PB87227609

SP 250-3 Radiometric Standards in the Vacuum

Ultraviolet SN003-003-02806-1 $6.50

SP 250-4 Fricke Dosimetry in High-Energy Electron Beams

SN003-003-02816-9 $2.75

SP 250-5 Alpha-Partide Calibrations SN003-003-02823-1 $2.00

SP 250-6 Regular Spectral Transmittance SN003-003-02805-3 $3.25

SP 250-7 Radiance Temperature Calibrations SN003-003-02827-4 $2.25

SP 250-8 Spectral Reflectance PB881 09905

SP 250-9 Calibration of Beta-Partide-Emitting

Ophthalmic Applicators

SN003-003-02817-7 $2.00

SP 250-1 0 Radioactivity Calibrations with the "Av" Gamma Ionization Chamtjer, and Other

Radioactivity Calibration Capabilities

SN003-003-02824-0 $2.25

SP 250-1 1 Dosimetry for High-Dose Applications

SNOOa-003-02854-1 $2.75

SP 250-12 Neutron Personnel Dosimetry

SN003-003-02811-8 $2.50

SP 250-13 Activation Foil Irradiation with Californium

Fission Sources SN003-003-02866-5 $2.25

SP 250-14 Activation Foil Irradiation by Reactor Cavity

Fission Sources SN003-003-02861-4 $3.25

SP 250-15 Photometric Calibrations PB881 53747

SP 250-16 Calibration of X-Ray and Gamma-Ray

Measuring Instruments SNOOa-003-02862-2 $7.00

SP 250-17 The NBS Photodetector Spectral Response

Calibration Transfer Program

SN003-003-02857-6 $3.25

SP 250-18 Neutron Source Strength Calibrations SNOOa-00^2863-1 $3.25

SP 250-19 Calibration of Gamma-Ray-Emitting

Brachytherapy Sources

SP 250-20 Spectral Irradiance Calibrations SN003-003-02829-1 $5.50

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 Sen/ice

SN003-0Oa-02891-6

SP 250-24 Standard Cell Calibrations SN003-003-02825-8 $2.75

SP 250-25 Calibration Sen/ice for Inductive Voltage Dividers

SP 250-26 Phase Angle Calibrations SN003-003-02871-1 $5.00

SP 250-27 AC-DC Difference Calibrations SP 250-28 Solid-State DC Voltage Standard Calibrations SN003-003-02842-8 $2.00

SP 250-29 Traceable Frequency Calibrations SN003-003-02844-4 $2.25

SP 250-30 GOES Satellite Time Code Dissemination:

Description and Operation

SN003-003-02845-2 $2.75

SP 250-31 Mass Calibrations SP 250-32 A Calibration Sen/ice for 30 MHz

Attenuation and Phase Shift SN003-003-02875-4

SP 250-33 A Calibration Sen/ice for Voltage Transfomiers and High-Voltage Capacitors

SN003-003-02880-1 $1.25

SP 250-34 High Vacuum Standard and Its Use

* Those entries containing a stock number (003-003 ) and price can be purchased from the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402. GPO will accept checks, money orders, VISA, and Mastercharge. For more infomation, or to place an order, call (202) 783-3238. Be sure to use the stock number in all orders.

Entries containing PB numbers can be purchased from the National Technical Information Sen/ice, Springfield, VA 22161. NTIS will accept American Express in addition to the payment methods listed for GPO. For more information call (703) 487-4650; to place an order

call (800) 336-4700. Be sure to use the PB number on all orders.

Titles without stock numbers are in preparation.
U.S. Department of Commerce

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Official Business

Penalty for Private Use $300