NBS Measurement Services:

Fricke Dosimetry in

High-Energy

Christopher G. Scares

Eric L. Bright

Margarets Ehrlich

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,

compiles, and critically evaluates spectroscopic data. The Division

consists of the following Groups:

Radiation Physics Division

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

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

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

tions; conducts theoretical and experimental research with electron,

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

measurement techiques. The Division consists of the following Groups:

Radiometric Physics 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 quantixn radiometry; and develops

spectroradiometric and spectrophotometric standards and calibration

procedures. The Division consists of the following Groups:

Radiation Source and Instrumentation Division

Develops, operates, and improves major NBS radiation facilities

including the electron Linac and race track microtron; develops,

designs, and builds electronic and mechanical instrinentation for

radiation programs and facilities; provides national leadership

in the standardization of nuclear instrumentation; and develops

new competence in radiation sources and instrtmentation. The

Division consists of the following Groups:

• Accelerator Research

• Linac Operations

• Electronic Instrimentat ion

• Mechanical Instrimentation

Ionizing Radiation Division

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:

Office of Radiation Measurement

Radiation Theory

Radiation Chemistry and Chemical

Dosimetry

Neutron Measurements and Research Neutron Dosimetry

Radioactivity

X-Ray Physics

Dosimetry

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:

FRICKE DOSIMETRY IN HIGH-ENERGY

ELECTRON BEAMS

Christopher G. Scares

Eric L. Bright

Margarete Ehrlich

Center for Radiation Research

National Measurement Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued July 1 987
Library of Congress Catalog Card Number: 87-619834

National Bureau of Standards Special Publication 250-4

Natl. Bur. Stand. (U.S ), Spec. Publ. 250-4, 54 pages (July 1987)

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: 1987

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

The calibration and related measurement services of the National Bureau

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

measuring instruments in achieving the highest possible levels of

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

calibration, special test, and measurement assurance services. These

services allow customers to directly link their measurement systems to

measurement systems and standards maintained by 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 250-4), NBS Measurement Services: Fricke Dosimetry

in High-Energy Electron Beams, by C. G, Soares, E. L. Bright, and M.

Ehrlich, is the fourth to be published in this new series of special

publications. It describes the preparation and calibration of

ferrous-sulf ate (Fricke) chemical dosimeters used for the measurement

of absorbed dose to water in electron beams in the energy range from 5 to 50 MeV (see test numbers 48010M and 48011M 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 night

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

for new calibration ser-vices, special tests, and measurement assurance

programs

.

George A. Uriano Chris E. Kuyatt

Director Director

Measurement Services Center for Radiation Research

1 1

1
ABSTRACT

The N8S Fri cke-Oosimetry Service (advertised in HBS Special

Publication 250, 1986-1988 and earlier editions) is described in

detail. After a brief historical introduction and description

of the service, the theoretical basis (including what quantities

are measured, how, and why) and the philosophy of internal

quality checks are discussed in some detail. This is followed

by a description of the physical setup and of the step-by-step

operating and reporting procedures. Throughout this section,

there is reference to sample records of past performance, in

order to facilitate continuity of operation in the case of

personnel changes. The document concludes with a discussion of

the uncertainties involved in this measurement quality assurance

service, safety considerations, and an appendix containing

samples of all form letters and of the final report mailed to

the participants.

Key words: absorbed dose to water; bi-annual service; chemical dosimetry;

dose interpretation; dosimeter preparation; electron therapy; ferrous-sul f ate

(Fricke) solution; internal quality check; measurement quality assurance;

plastic phantom; spectrophotometry

i V
TAiiLE OF COIMTENTS

Abstract iv

1. Description of Service 1 1.1 History 1 1.2 The Service 1

2. Theory and Design Philosophy 3

2.1 Theory 3 2.2 Design 4

2.3 System Operation 5 2.3.1 Statistical distribution of absorbance growth in the NBS

unsealed system 5 2.3.2 Internal quality check on performance of individual

dosimeters 6

2.3.3 Internal quality checks on spectrophotometry 6

2.4 Upgrading the Service t- 7

3. Description of System and Operating Procedure 7 3.1 The Physical Setup 7 3.2 Preparation of Dosimeters and Check on Their Performance 9

3.2.1 Preparing the solutions 11

3.2.2 Filling the spectrophotometer cells 14

3.2.3 Pre-i rradiation with cobalt-60 gamma rays 17

3.2.4 Results of pre-i rradi ations and their application to

internal quality control 19

3.3 Spectrophotometry: Ensuring Long- and Short-Term Consistency

and Reproducibility 22

3.4 Dose Interpretation and Report to Participants 26

3.5 Uncertainties in the Dose Interpretation 29

4. Safety Considerations 29

References 32

Appendix 1. Forms Used in Written Exchanges with Participants 33

Appendix 2. Sample of Report to Participants 41

V
TABLE OF CONTENTS, CONTINUED

LIST OF TABLES

Page

Table 1. Sample, Absorbances of Cells Before Pre-i rradi ation 17

Table 2. Sample of Spectrophotometry Record of All Cells After Prei rradi ation with 50 Gy of Cobalt-60 Gamma Radiation, Prior to

Shipment 20

Table. 3 Sample of Distribution of Position-Corrected Absorbances 21

Table 4. Statistics, from Sample Results Shown in Table 3 22

Table 5. Check on G-Value and Product e G for the Particular Sample

Batch of Fricke Solution 23

Table 6. Sample, Absorbances of 1 g% Potassium Nitrate Solution 25

Table 7. Constancy and Linearity of Cary Spectrophotometer, Model 16,

between April 1971 and April 1974 25

Table 8. Sample of Spectrophotometry Record After Shipment and

Associated Absorbed-Dose Interpretation 27

Table 9. Uncertainties in the Dose Interpretations 30

vi
TABLE OF CONTENTS, CONTINUED

LIST OF FIGURES

Figure 1. Dosimeter Assembly [3] 2

Figure 2. Mapping the Cross Section of the Cobalt-60 Gamma-Ray Beam Used

for Pre-i rradi ati on 10

Figure 3, Weighing the Sulfuric Acid 12

Figure 4. Weighing the Salts 13

Figure 5. Aerating the Solution 15

Figure 6. Filling the Spectrophotometer Cells with Fricke Solution 16

Figure 7. Pre-Irradi ati on with Cobalt-60 Gamma Radiation 18

Figure 8. Spectrophotometry 24

Figure 9. Sample Plot, Automatic Scan of Ferric-Ion Absorbance-vsWavelength 24

Figure 10. Sample, Analysis of Spurious Absorbance Growth AA^p, of

Shipping and Home Controls 28

vi i
iii1iII IiIJi>IjI
1. DESCRIPTION OF SERVICE

1.1 History

Since 1967, the National Bureau of Standards (NBS) has been providing a measurement-assurance service for testing the status of dosimetry for electron

beams in the energy range from about 5 to 50 MeV. (For details see Chapter

VI, Ionizing Radiation fleasurements , test numbers 48010M and 48011M — formerly

8.5B and 8.5C - of NBS Special Publication 250, The NBS Calibration Ser vices

User s Guide 19_86-1988.) The service, which is directed mainly toward

radi ati on-tTerapy applications in the United States, was established upon the

request of the American Association of Physicists in Medicine (AAPM) at the

time of the electron-therapy clinical trials, for which uniform dosimetry

among all the participants was essential. Initially, it was thought that the

service would be discontinued as soon as NBS could offer in-phantom calibrations for radiation-therapy instruments in terms of absorbed dose to water, or when reliable methods would be available for arriving at absorbed dose to

water from exposure measurements in air [1]^ with suitably calibrated instruments. NBS is now in a position to perform these calibrations, and reliable

methods for deriving absorbed dose to water from exposure have been developed

[2], Nevertheless, the service is being continued since there is a continuing

demand for the "measurement-quality assurance" it provides for the daily

operation of radiation-therapy departments. During its first year, the

service was subsidized by the Bureau of Radiological Health and thus could be

offered quarterly and at a fraction of actual cost. Since the end of the

subsidy, it has been offered semi-annually, and the charges cover the actual

cost. During the past eighteen years, a total of roughly 75 groups have

participated, some annually, most of them semi-annually. Most of these groups

use electron beams for therapy. Occasionally we accept requests from groups

desiring checks on their cobalt-60 gamma-ray dosimetry. Requests from groups

desiring checks on their high-energy bremsstrahl ung dosimetry are accepted

only with the understanding that the NBS dose interpretation from high-energy

bremsstrahl ung is associated with larger uncertainties.

Over the years, descriptions of this service and reviews of results have

been published. Reprints of these and other pertinent publications are filed

with this document and are available to the public upon request. The

following highlights are partially extracted from the publications.

1.2 The Service

Ferrous-sul f ate (Fricke) dosimeters, prepared at NBS and checked by means

of ^°Co gamma pre-i rradi ation , are shipped to the participating institutions

with the request to irradiate them in a polystyrene phantom, using the

dosimeter depth and the phantom and field sizes specified by the AAPM [2].

Figure 1 shows the dosimeter assembly, consisting of the ferrous-sul fate

(Fricke) solution in a ground-glass stoppered, radiation-resistant, quartz

spectrophotometer cell with quartz windows. The cell is cradled in a polystyrene block machined to fit into a recess of the participant's polystyrene

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

paper.

1
phantom. The dosimeters are shipped pre-paid by a commercial delivery

service. Since they contain a corrosive liquid in unsealed containers they

cannot be sent by mail, and, if air transport at high altitudes is involved,

the packages must be marked for transportation only in the pressurized

compartment of tne plane. (See also Section 4.)

Figure 1. Dosimeter Assembly [3]. The stoppered spectrophotometer cells are cradled in a 5.1 cm x 7.6 cm x 2.0 cm (high) polystyrene block,

with a styrofoam plug (not shown) wedged in above the stopper. The

3-mm polystyrene lid is presently screwed on with two rather than

with the six screws shown.

The participants irradiate all but one of the furnished dosimeters to

between 50 and 80 Gy (5000 and 8000 rad) to water, usually at electron

energies between 5 and 50 MeV, employing the specified irradiation geometry.

After irradiation, the participants return the dosimeters to NBS for

spectrophotometric evaluation of the ferric-ion concentration in terms of

absorbed dose to water, using a value for the radiation-chemical yield compatible with the most recent ICRU recommendations [4]. Return shipment is

prepaid by participants, and the restrictions on delivery service and mode of

transportation are the same as for shipment from NBS. The participants are asked to return the dosimeters within three working days from their receipt,

if at all possible. Once the evaluation is completed, NBS informs each

participant by mail of his own test results and of the results of all other

participants, but without divulging their identities. In addition, regular

participants actively engaged in radiation therapy are alerted by telephone if

their dose interpretations are badly out of line, say, by 20 percent or more.

All participants are free to discuss their results and possible problems with

NBS, either by mail or by telephone.
Appendix 1 contains all forms used in written exchanges with

participants. A sample of a final report is shown in Appendix 2.

2. THEORY AND DESIGN PHILOSOPHY

2.1 Theory

The ferrous-sul fate (Fricke) dosimetry system has been discussed extensively in the 1 i terature.[5] The chemical changes it undergoes upon exposure

to ionizing radiation are well understood and lead to changes in optical

properties that are readily and accurately measurable by relatively simple

optical means. Because its net optical absorbance is proportional to the

absorbed dose to the system over a wide range of irradiation levels, and the

system consists mainly of water, it lends itself well to a determination of

absorbed dose to water, which is the quantity of interest in radiation

therapy.

NBS uses the following conventional dosimeter ingredients2:

0.001 M Fe(NH^)2(S0^)2-6H20, dissolved in

0.8 N H2S0j^, well aerated, and

0.001 N NaCl .

The NaCl desensitizes the system against organic impurities but causes an increase in the rate dependence of its radiation-chemical yield. Therefore,

participants using pulsed electron beams are asked to irradiate at dose rates

not exceeding 10 Gy per microsecond pulse or a total of 350 Gy for any train

of microsecond pulses [5]. A simplified reaction mechanism (i.e., one that

does not consider the modifications caused by NaCl) is shown below; it demonstrates how aeration (i.e., the production and maintenance of a sufficient

amount of O2 at all times) enters into the reaction and determines the Fe"*""^^ yield:

Fe++ + OH Fe'-^--'- + OH"

H + O2 HO2

2Meaning of the symbols "M" and "N":

Th e symbol M : The mole is the molecular weight of a substance expressed

in grams . TFTe molarity of a solute is the number of moles of the solute

contained in 1000 cm^ of solution. Therefore, a one-molar (1 H) solution of

sulfuric acid contains 98.08 g of sulfuric acid per 1000 cm^ of solution.

The symbol N : The normality of a solution is its equivalent grammolecular weight concentration. The equivalent gram-molecular weight of an acid is its molecular weight expressed in grams, divided by the number of

hydrogen atoms per molecule. Therefore, a normal (1 N) solution of sulfuric

acid contains 49.04 g of sulfuric acid per 1000 cm^ of solution.

3
Fe++ + Fe+++ + HO"

ho; + H+ > H2O2

Fe++ + H2O2 > Fe+++ + OH + OH"

Ferric-ion yield is defined as the number of Fe'*"'"'' ions formed in the

Fricke solution per unit of absorbed dose to the solution. It may be determined spectrophotometri cal ly with high accuracy by the change in absorbance

of the solution, which is the logarithm to the base ten of the ratio of

incident to transmitted radiant energy Flux • Absorbance is proportional to

the molar concentration of the Fe"''"*'^' ions and the optical pathlength, with the

proportionality constant referred to as the molar linear absorption coefficient, or the molar extinction coefficient (Beer's law). The change in

optical density measured with a spectrophotometer with good optical properties

is equal to the change in absorbance. In general, absorbance depends on the

wavelength and the temperature at which the measurements are made. At a given

t_emperature and wavelength, the average absorbed dose to the Fricke solution.

Dp, then is obtained from the change in absorbance, aA, as:

^F ^ "gW"^)" '

where aA is the change in absorbance, p is the density of the Fricke solution,

£ is the optical pathlength, e is the molar linear absorption coefficient, and

G(Fe+++) is the Fe+++ ion yield.

The average absorbed dose to water, D^^^, is obtained as [4]

= 1.004 Dp . (2)

For measurements of absorbances at 25°C, at the 304-nm absorption peak,

at an optical pathlength of 0.01 m, and assuming a Fricke-solution density of

1024 kg m-3 and a product of e G(Fe+++) = 352 x lO'^ m2 kg-i Gyi [4], the

expressions reduce to

1024 X 0.01 X 352 X 10 — aA ^ = WA-T-n m-TWJTn-T Gy = 277 aA Gy (3)

and

2.2 Design

D^^ = Dp X 1.004 = 278 aA Gy . (4)

At the time the service was first implemented as a stop-gap procedure,

simplicity of design and operation was of the essence. Making the expensive

but re-usable stoppered quartz spectrophotometer cells an integral part of the

Fricke dosimeters (see fig. 1) reduces handling time and possible sources of

contamination of the Fricke solution.
An alternative v^ould be to transfer the solution first into specially

cleaned, disposable borosilicate ampoules; flame-seal the ampoules without

damage to the solution from contact with gases escaping from the heated glass;

and keep the solution in the sealed ampoules until ready for final readout.

The solution would then be transferred once more, this time into a spectrophotometer cell that would remain in the spectrophotometer and would be filled,

rinsed and re-filled until the contents of all the ampoules was read out.

Such a procedure seemed too elaborate for a stop-gap service, but the adopted

system incorporating unsealed dosimeters also has its draw-backs. Because the

Fricke solution is corrosive, the unsealed dosimeters have to be shipped by

carriers other than the U.S. Mail and flown either at low altitude or in

pressurized compartments. Fricke solution in unsealed cells also is more

prone to succumb to influences of the environment than solution in sealed

ampoules, causing a larger dependence of spurious absorbance growth^ on dosimeter history. The resulting additional uncertainty in the dose interpretation would be unacceptable in a NBS calibration service, but is acceptable

in a service with the primary function of having the participants compare

their ability to measure absorbed dose to water with that of their peers.

The service was designed to be run on a small scale (offered twice a year, not over forty participants at a time), with manual dosimeter

preparation and automation only in the spectrophotometric-readout and datahandling stages. A discussion of a possible upgrading and expansion of the

service is given in section 2.4.

2.3 System Operation

2.3.1 Statistical d istribution of absorbance growth in the NBS unsealed

system .

Prior to the start of the service, an experiment was performed to

ascertain the influence of irradiation level and storage conditions on absorbance growth during a period of about four weeks between preparation and

final readout of the Fricke solution kept in the stoppered spectrophotometer

cells. The cells were irradiated at NBS to different exposure levels, and

one-half of them were then shipped cross-country while the other half remained

at NBS. There was no significant dependence of spurious absorbance growth on exposure level (and thus on absorbance level), and there was no significant

difference between the average absorbance growth of shipped cells and those

remaining at NBS, the average absorbance growth amounting to between 0.003 and

0.004, which corresponds to between 0.8 and 1.1 Gy to the solution. Over the

years, a record has been kept of the standard deviation of the average

absorbance growth on identically pre-i rradi ated Fricke-dosimetry control

cells, some of which were shipped and the rest remained at NBS. The

distribution of the absorbance growths has a tendency to be more symmetrical

for the shipped batch than for the batch remaining at NBS, which shows a "tail" on the side of larger absorbance growth. [3]

3ln this paper, the term "spurious absorbance growth" is used to mean absorbance growth not caused by ionizing radiation but, e.g., by autooxidation.

5
2.3.2 Internal qua l ity chec k on performance of Individual dosimeters. I

In order to check on the performance of the Fricke dosimeters prior to

j their use, it was decided to pre-i rradi ate all the freshly-filled spectroj photometric cells, about 50 or 60 at a time, with soqo gamma radiation to a I level of between 40 and 50 Gy to water. This procedure was adopted after it

had been established [3] that the response of the dosimeters to ^^Co gamma : radiation and to high-energy electrons vvas additive. Irradiation and readout ; of each dosimeter before actual use permits (a) a statistical study of the

random variations from dosimeter to dosimeter in the response to a given | irradiation level, as measured spectrophotometrical ly ; (b) the detection and 1 elimination from further use of dosimeters with responses far removed from the ; mean for the particular batch; and (c) the evaluation of the absorbed doses : administered by the participants from higher (and therefore more precise) I absorbance levels. If a system employing sealed ampoules were adopted in the i future, the advantages of individual dosimeter readouts prior to shipment

j would be lost since the ampoules would have to be re-opened and the solution i transferred to spectrophotometer cells for readout. Pre-i rradi ati on of all

dosimeters could continue if raising the absorbance level for the readout were I still found to be of advantage, but a portion of each solution batch would ] have to be set aside for pre-i rradi ation as an internal quality check. ^

Since the dosimeters can be used for determining doses up to more than \ 300 Gy while participants are being asked to deliver only about 50 Gy, it is

in principle possible to use the same set of dosimeters up to six times, as long as linearity between optical absorbance and absorbed dose has been

established to hold for the particular batch over the range of absorbances I employed. In practice, the dosimeters so far have never been used for more j than one test series. But, since 1983, the number of dosimeters requested for

| any one series has exceeded the number of dosimeters available. Therefore, I two consecutive mailings, about six to eight weeks apart, were made per j series, using some of the dosimeters for a second round of irradiations, with

| satisfactory results. However, there are some indications that the reproduci- J bility of the dose interpretation decreases slightly with consecutive

j dosimeter use because of an increase in spurious absorbance growth and growth

j variability. For this reason, the number of available spectrophotometer cells ! and accessories was increased during 1986.

2.3.3 Internal quality checks on spectrophotometry [6]

j Wavelength scale . Because of spectrophotometer variability it is recom- j mended that the spectrophotometer wavelength scale be periodically checked. ; It was decided to perform this check prior to each series of spectrophotometer 1 readouts by determining the wavelength setting near 304 nm for which the 1 ferric-ion absorbance shows a maximum, and to make all readouts at this i wavelength. ]

i Molar linear absorption coefficient for ferric ions . It is further

| recommended that the molar linear absorption coefficient for ferric ions for

the particular spectrophotometer in use be determined. This procedure has | been followed throughout the years the Fricke-dosimetry service has been in \ operation, re-determinations having been performed whenever there was a change

in spectrophotometer, spectrophotometer components or spectrophotometer ] adjustment. The preparation of the ferric-ion solution is described in detail I in Appendix S2 to the ASTM Standard [6].

6
ICRU recently recommended the use of a fixed product of the molar linear

absorption coefficient and the ferric-ion yield, [5] The value recommended

for the product agrees to within 1 percent with the value based on our measured molar linear absorption coefficient and the ferric-ion yield used by

us through 1985 [7]. The recent ICRU recommendations have now been incorporated in our procedure and are reflected in the constants given in eqs. (3)

and (4) (section 2.1). However, we plan to continue the practice of

re-determining the molar linear absorption coefficient experimentally when the

need arises.

Constancy and Linearity . As a check on the constancy and linearity of

the spectrophotometer over the one- to two-month period of any given test

series, it was suggested that an independent check be kept on these parameters

by means of absorbance measurements on several dilutions of a "1 g%" solution

of potassium nitrate (i.e., an aqueous solution containing 1 g of potassium

nitrate per 100 cm^), which has an absorption peak at a wavelength of about

302 nm, i.e., close to the ferric-ion absorption peak used for Fricke

dosimetry .[8] Since then. Standard Reference Material SRM 2031, consisting of

metal -on-quartz filters for the verification of spectrophotometric absorbance

scales in the ultraviolet, has become avai 1 abl e.[9] This material may replace

the potassium-nitrate solution in the future.

2.4 Upgrading the Service

In preparation for a possible upgrading of the Fricke-dosimetry measurement assurance service to a calibration service, switching to a system

employing sealed dosimeters is contemplated if additional experiments prove

that such a system would lead to a more uniform spurious absorbance growth and

therefore to a higher reproducibility in dose interpretation from absorbance

differences. For this purpose, a semi-automatic ampoule-sealing unit was constructed. Its design is patterned after a unit in use by the National

Physical Laboratory in Teddington, England. Also, thought is being given to

automating the rinsing and filling operations of the spectrophotometer cells

or glass ampoules.

Further preparations for upgrading the service would entail studies for

obtaining the corrections required to produce adequate agreement with the

results of calibrations in terms of absorbed dose to water by other methods.

Such corrections may be needed because of the physical inhomogeneities

introduced in the dosimeter-phantom system by the presence of glass and of an air space. Upgrading the service may also require at least a temporary

increase in the staff assigned to the project during the preparation period,

and probably some permanent staff reassi gnments or additional chemistry

training of the present staff.

3. DESCRIPTION OF SYSTEM AND OPERATING PROCEDURE

3.1 The Physical Setup, (Most of the items enumerated are shown in the

figures of sections 3.2 and 3.3.)

7
1

I

At the time this document is being prepared, the setup consists of:

tt around 150 cylindrical 10-mm pathlength quartz spectrophotometer cells

with quartz windows and yround-glass stoppers;

® Lucite racks, built to order in the NBS Shops, each holding 10 spectrophotometer cells in two rows of five, each row being identified by a letter

and each cell location by a number;

o the chemicals: concentrated (96%) sulfuric acid, sodium chloride,

ferrous-ammonium sulfate, and potassium nitrate, all of reagent grade;

o two glass-stoppered ZOOO-cm^ glass volumetric flasks for preparing and

storing the Fricke solution;

9 one glass-stoppered 500-cm3 glass volumetric flask for preparing and

storing the potassium nitrate solution;

« two 500-cm3 glass wash bottles, each with plastic tubing and a rubber

balloon for pressure filling the cells with either specially purified

distilled water** or Fricke solution through a glass tube drawn into a narrow

nozzl e

;

© a suction pump with plastic tubing connected to a small glass bottle with

dropping pipette for emptying the cells;

® access to a water-purifying setup;

o access to two electronic scales, one having a 0.1 kg capacity and 1-mg

scale divisions, and the other having an - 1-kg capacity;

® an acrylic platform with a surface ruled in 2.5-cm squares for supporting

the spectrophotometer cells horizontally in a vertical ^^Co gamma-ray beam;

a access to an ultrasonic cleaner equipped with a stainless-steel liner;

« access to an up-to-date, computer-compatible spectrophotometer with good

optical properties, equipped with temperature-control for the cell

compartment

;

o access to a micro-computer, for data acquisition and data handling; and

9 various accessories, e.g., three dust-free cabinets, a graduated glass

cylinder, beakers, a weighing dish, a spatula, tweezers with flexible tips, a funnel

.

'^Unless otherwise noted, the distilled water referred to later always is the

specially purified distilled water produced in the water-purifying setup,

that was used to prepare all the solutions.

8 i

I
Initidlly, all glassware to be used for the Fricke solution was cleaned

with hot concentrated sulfuric acid, the bottles were filled with the acid and

then heated on a hot-plate until the acid almost boiled; the other items were immersed in almost boiling sulfuric acid and rinsed several times in distilled

water before and after the sul f uri c-acid cleaning procedure. The spectrophotometer cells and their stoppers were initially rinsed several times with

distilled water. After being placed in their racks, they were filled several

times with distilled water and emptied between fillings alternately by suction

and shaking. The cells then were filled with 0.8-normal sulfuric acid and

stoppered; racks and cells were immersed up to the cell necks in a bath of

singly-distilled water for 15 minutes of ultrasonic cleaning.

Also, the cross section of the ^^Co gamma-ray beam in room 6034 used for

the pre-i rradi ati on of all Fricke dosimeters was initially mapped at the

irradiation distance of 147.9 cm from the source by means of readings of a laboratory-standard ionization chamber placed on the support platform and

moved across the beam cross section in 2.5-cm steps to obtain a grid of

absorbance-correction factors for all Fricke-dosimeter locations. The

resulting array of correction factors is shown in figure 2 for the source and

distance used through 1985. At the time of preparation of this document, the source is being replaced by a source of higher activity in room B035, in order

to reduce the irradiation time.

The procedures discussed above are not repeated prior to each subsequent

use. The stoppered cells (still filled with Fricke solution, and sitting in

their racks), the wash bottles, the beaker for washing the stoppers, and

utensils such as weighing dish, spatula, tweezers, suction bottle, are stored

in the dust-free cabinets whose doors had been replaced by cheese cloth to

prevent condensation of water on the stored articles. Prior to the preparation of a new set of dosimeters, all glassware is thoroughly rinsed with

distilled water.

3.2 Preparation of Dosimeters and Check on their Performance

Preparing the Fricke solution, filling the spectrophotometer cells and

starting the performance check (pre-i rradiation of the first batch of between

50 and 60 cells) takes at least one day. Prior to starting, at least six one- gallon well -cleaned brown glass bottles equipped with screw caps lined with

aluminum foil are filled with the special distilled water from the waterpurification system maintained by the Dosimetry Group of the Ionizing

Radiation Division. The system presently consists of one high-capacity Raven

still and the Milli-Ro 60 system of the Millipore Corporation, equipped with

one organic absorption stage, two deionizing stages and one filtration stage.

Instead of giving a lengthy description of the procedure, copies are shown in

the following of typical pages from data books, giving the step-by-step record

of the procedures for preparing the Fricke dosimeters and carrying out the

internal quality checks on dosimeter performance.

9
)

i

COLUMN NUMBER

Figure 2. Mapping the Cross Section of the Cobdlt-60 Gamma-Ray Beam in

Room B034, Used for Pre-Irradiation; scale: 1 cm = 2.5 cm

encircled numbers:

other numbers:

solid-line envelope:

dashed line:

correction factors from data points

interpolated correction factors

52 locations with corr. factors < 1.015

extending envelope to include 10 further

locations, with corr. factors < 1.019

10
3.2.1 Preparin g the solutions

( a ) Weighing the ingredients (sample from Data Book 837, pp . 160 and 163)

Sulfuric Add, Baker reagent grade, 96. 3 percent, fresh (see fig. 3)

^

graduate cylinder in beaker, tared on Mettler balance model H 1200

poured ~ 46 cm^ of H2S0^ into cylinder

weight of acid 84.45 g delivered 84.05 g into 2000-cm3 flask A

graduate cylinder in beaker, tared again

poured ~ 46 cm^ of l^2S0^ into cylinder

weight of acid 83.80 g delivered 83.80 g, into 2000-cm3 flask B

5 Ferrous-ammonium s ulfate, same bottle as for last test series, _ wei ghed

oh Mettler balance, model H 16 (see Tfg. 4)

dish alone 4.13160 g the salt 0.78430 g should be 4.91590 g 5 is 4.91585 g, into 2000-cm3 flask B

dish alone 4.13162 g the salt 0.78430 g should be 4.91592 g 5 is 4.91596 g into 2000-cm3 f^^^k A

Sodium chlori de , same bottle as for last test serie s, ^ weighed o n Mettler

balance H 16 (see fig. 4")

dish alone .13162 g .11690 g should .24852 g .24855 g. into 2000-cm3 flask B

dish alone .13163 g .11690 g should .24853 g .24894 g. into 2000-cm3 flask A

dish alone .13156 g

5

5A11 reagents are periodically replaced.

^Prior to adding the weighed ingredients, all flasks had been filled partially

with water. Note that, throughout the text, the term "water" unless

otherwise stated is used to mean the specially distilled water from the water

purification system described in section 3.2 (p. 14).

11
12
Figure 4. Weighing the Salts.

The weighed portions are transferred quantitatively from the

weighing dish into the volumetric flasks through a clear, dry

funnel, and washed down with water from the wash bottle.

13
Potassi urn n itrat e , same bott le as for last test series , weighed oil Mettler balance H 16~^

dish alone 4.13376 g g the salt 5.00000 g should be 9.13376 g Is 9.13366 g Into 500-cm3 flask

dish alone 4.13350 g

All flasks filled to neck junction with water at about 11:00 a.m.

(b) Finishing the stock solution s

After having allowed enough time for all solid Ingredients to dissolve

completely and for the solution to return to room temperature:

• All volumetric flasks are filled with water to their marks and

stoppered

;

o The Fricke-sol ution flasks are shaken about 50 times, by turning them

upside down and back again with a rotary motion (see fig. 5); during this

procedure, stoppers are loosened periodically to permit air pressure to

equal 1 ze,

3.2.2 Filling the spectrophotometer cells (see fig. 6)

Sample of Cell-Rinsing and -Filling Record (from Data Book 837, p . 161)

(a) Rinsing with Water

cells sucked out and filled with water, finished 11 a.m.; 10 min. rest

cells shaken out and filled with water, finished 11:20 a.m.; 10 min. rest

cells sucked out and filled with water, finished 1:30 p.m.; 10 min. rest

cells shaken out and filled with water, finished 1:46 p.m.; 10 min. rest.

"^In 1986, the function of this solution was taken over by a standard reference

material. (See section 3.3)

8A close to 1.00000 g% solution was required at the time, because the solution

was used for a long-term constancy check of the spectrophotometer's absorbance scale. To prepare such a solution, both the weighing dish and the

salt have to be in temperature and relative-humidity equilibrium with the

balance (and the room). Repeated weighings, about one minute apart, have to

be made until weight constancy is reached. This procedure is not required

when the potassium nitrate solution is used simply to establish the location

of the absorbance peak and of absorbance-scal e linearity.

^Prior to adding the weighed ingredients, the flask was filled partially with

water.

14
Figure 5. Aerating the Solution.

Aerating prior to filling of individual spectrophotometer cells

ensures the same degree of initial aeration of the dosimeters.

Normally, rubber aprons are worn during this process.

15
1

1

I

Figure 6. Filling the Spectrophotometer Cells with Fricke Solution. ^ Singly distilled water bathes the racks and the outside of the cells. ;

i

16

i
(b) Ferrous-Su1 f ate Rinses

cells sucked out and filled, finished 2:08 p.m.; 10 inin. rest

cells sucked out and filled, finished 2:32 p.m.; 10 min. rest

cells sucked out and filled, finished 3:00 p.m.; 10 min. rest

last (final) fill, completed at about 3:40 p.m.

All cells stoppered by about 4 p.m.; racks and cells washed with singly

distilled water; let dry overnight.

3.2.3 Pre-i rradi ati on with cobalt-SO gamma rays

Prior to pre-i rradiation, spectrophotometry is done on the cells of one

rack. (Spectrophotometry is discussed in more detail in section 3.3.) This

furnishes the average background absorbance required for obtaining the net

absorbance of the pre-i rradi ated cells for a check on the G-value of the

particular batch of Fricke solution.

Figure 7 shows the pre-i rradi ation setup in room 6 034, used through

1985; figure 2 (page 10) the associated bean cross section; and table 1 a sample of the absorbance record in one rack, obtained prior to irradiation.

Table 1. Sample, Absorbances of Cells Before Pre-i rradi ati on , Required for

Check on G-Value and e G Product, from Data Book 837, page 99.^

Cell Absorbance

(Average of 15 readings, and standard devi ati on)

Designation Side a Side b

U-1 0.0417 ± 0.0001 0.0419 + 0.0001

U-2 0.0415 ± 0.0001 0.0413 + 0.0001

U-3 0.0416 ± 0.0001 0.0418 ± 0.0001

U-4 0.0389 ± 0.0001 0.0399 + 0.0001

U-5 0.0416 ± 0.0001 0.0431 + 0.0001

V-1 0.0397 ± 0.0001 0.0379 + 0.0001

V-2 0.0400 ± 0.0001 0.0414 + 0.0001

V-3 0.0424 t 0.0001 0.0431 + 0.0001

V-4 0.0397 ± 0.0001 0.0404 ± 0.0001

V-5 0.0444 ± 0.0001 0.0435 t 0.0000

^Al 1 readings done with a slit width corresponding to 2 nm in wavelength, at a wavelength setting of 301 nm and a temperature of 25.00 ± 0.05°C. The

designations, "side a" and "side b," identify the cell windows facing the

ultraviolet source. We routinely reverse the cells for the second reading

since specks of dirt on the cell windows may influence the reading to a different extent in the two geometries.

i^If desired, irradiation of first batch of 50 to 60 cells may be started

immediately after drying the cells individually with a lint-free cloth.

17
Fiyure 7. Pre-i rradi ati on with Cobalt-60 Gamma Radiation in Room B034.

The surface of the support is ruled, with rows and columns marked

as in figure 2 (page 15). The fixed-distance support visible at

shoulder height is not in the beam.

18
Sample of Pre-i rradiation Record (from Data Book 856, p. 39)

Exposure rate on day of last calibration (April 24, 1985): 46.98 R/h,

Exposure rates in air, at point of cell irradiations:

On November 6, 1985, 9. = 46.98 g''^'^^^^^^^ = 43.79 R/h.

On November 10, 1985, % = 46.98 g"^*^^^^^'^^ = 43.73 R/h.

The number 0.0003601 is the decay constant for the cobalt-60 gamma ray source,

and 195 and 199, respectively, is the number of days from the last calibration

to rouyhly the middle of the planned irradiations.

Corresponding absorbed-dose rates to the Fricke solution:

On November 6, 1985, Op = 43.79 x 0.961 = 42.09 rad/h (0.4209 Gy/h).

On November 10, 1985, 5p = 43.73 x 0.961 = 42.03 rad/h (0.4203 Gy/h).

The factor, 0.961, was obtained empirically as the ratio of Dp/X,ii averaged

over sixteen determinations during the period of 1967 through 1974. (See Data

Book 767, page 34.) Dp was computed from measured net absorbance, using the

recommended G-value for cobalt-60 gamma rays and the measured value of the

molar linear absorption coefficient (see table 5, page 23).

For each of the two sets of irradiations, a record is kept of the

following data: Time set and time delivered; date and time when timer was started and date when timer stopped; timer reading at beginning and end of

irradiation; total dose delivered to the Fricke solution, as computed from

rate and time.

3.2.4 Results of pre-i rradi ati ons and their application to internal quality

control

A sample of the spectrophotometry record of all cells after preirradiation and prior to shipment is shown in table 2. The distribution of

position-corrected absorbances 12 corresponding to the data in table 2 is shown

in table 3 and its statistical analysis is given in table 4. Studies of this

type are used to eliminate cells with very high and very low absorbances,

since it cannot be ruled out that their absorbance growth with time does not

show characteristics different from that of other cells.

^Note that, for cobalt-60 gamma rays,

Dc = 1.004 D - 1.004 X 0.869 X = 0.970 X

F w en,w

in air, where Dr- and D are the absorbed doses to the Fricke solution and to

' F w water, respectively, and is the energy-absorption coefficient of water

for cobalt-60 gamma rays. Tlius, the effect of the quartz window of the cell

amounts to a decrease in absorbed dose of roughly one percent.

i^For details on position corrections, see section 3.1, figure 2.

19
Table 2. Sample of Spectrophotometry Record of All Cells After Preirradiation with 50 Gy of Cobalt-60 Gamma Radiation, Prior to

Shipment.^ (From Data Book 837, p. 104.)

Readoul, Before Shi pment

Position d J 1 Uc b Average

Cel 1 duri ng Cnrrpct i on Absorbance

,

No. Pre- Irrad. Factor to Absorb- Temp.

°C

Absorb- Temp

.

°C

beomeL ryRow, Col umn Absorbance*^ ance ance rnrrprtprl^

0-1 10 , 12 1.007 0.2172 24.92 0.2173 24.92 0.2188

-2 11 , 14 1.014 0.2149 24.93 0.2129 24.90 0.2169

-3 10 5 1.011 0.2178 24.93 0.2166 24.94 0.2196

-4 10 7 1.006 0.2231 24.96 0.2219 24.93 0.2238

-5 10 9 1.004 0.2265 24.94 0.2226 24.93 0.2255

P-1 10 , 11 1.003 0.2231 24.94 0.2232 24.90 0.2238

-2 10 , 13 1.008 0.2188 24.92 0.2178 24.93 0.2201

-3 10 , 15 1.017 0.2189 24.95 0.2184 24.94 0.2224

-4 9 4 1.014 0.2205 24.94 0.2140 24.95 0.2203

-5 9 6 1.007 0.2188 24.94 0.2193 24.93 0.2206

Q-1 9 8 1.003 0.2308 24.99 0.2306 24.97 0.2314

-2 9 , 10 1.000 0.2213 24.97 0.2221 24.97 0.2217

-3 9 , 12 1.004 0.2238 24.97 0.2239 24.98 0.2248

-4 9 , 14 1.010 0.2253 24.99 0.2247 24.98 0.2273

-5 8 , 3 1.018 0.2209 25.01 0.2220 24.96 0.2254

R-1 8 5 1.010 0.2188 24.99 0.2189 24.97 0.2210

-2 8 7 1.004 0.2207 24.99 0.2243 24.97 0.2234

-3 8 9 1.000 0.2208 24.98 0.2203 24.98 0.2206

-4 8 , 11 1.002 0.2224 25.00 0.2231 25.00 0.2232

-5 8 , 13 1.006 0.2217 24.97 0.2211 25.00 0.2227

S-1 8 , 15 1.012 0.2163 25.03 0.2179 25.02 0.2197

-2 7 4 1.014 0.2159 25.03 0.2170 CO no m uu

-3 7 6 1.007 0.2232 25.04 0.2227 25.04 0.2245

-4 7 8 1.004 0.2188 25.02 0.2208 25.03 0.2207

-5 7 , 10 1.003 0.2192 25.01 0.2192 25.04 0.2199

etc.

•

•

etc

•

•

. etc.

•

•

etc.

•

•

etc.

•

•

etc.

•

•

etc.

•

etc.

•

•

etc.

•

•

i ^Al 1 readings done with a slit width corresponding to 2 mm in wavelength, at a

^ wavelength setting of 301 nm. The designations, "side a" and "side b,"

identify the cell windows facing the ultraviolet source. ^

This IS a correction to the exposure during pre-i rradiation, which is

proportional to absorbance.

c

' The small temperature fluctuations recorded did not warrant a correction. |

20
Table. 3 Sample of Distribution of Position-Corrected Absorbances

(From table 2; see Data Book 837, p. 109.)

Absorbance

Interval

0 216-0 217 M-3 X- 1 0- 2 D- 3 0 217-0 218 W-1 A- 5 B- 4 F- 2 0 218-0 219 A-3 B- 1 X- 5 H- 5 0 219-0 220 M-4 A- 1 A- 2 E- 3 0 220-0 221 G-5 P- 2 K- 5 C- 4 0 221-0 222 T-6 H- 4 R- 1 W- 4 0 222-0 223 E-1 I- 3 V- 4 I- 1 0 223-0 224 L-4 T- 2 R- 4 1- 2 0 224-0 225 F-5 S- 3 D- 1 Q- 3 0 225-0 226 E-4 Q- 5 0- 5 N- 4 0 226-0 227

0 227-0 228 Q-4 I - 5 0 228-0 229 X-4

0 229-0 230 V-5

0 230-0 231

0 231-0 232 Q-1

0 216-0 217 0-5

0 217-0 218 A-5 B- 4 F- 2 D- 4 0 218-0 219 A-3 B- 1 H- 5 B- 5 0 219-0 220 A-1 A- 2 E- 3 J- 5 0 220-0 221 G-5 K- 5 C- 4 L- 1 0 221-0 222 H-4 F- 1 E- 5 0- 3 0 222-0 223 E-1 I - 3 I -

1 J- 1 0 223-0 224 L-4 I - 2 C- 1 0 224-0 225 F-5 D- 1 0 225-0 226 E-4

0 225-0 227

0 227-0 228 1-5

0 216-0 217 M-3 X- 1 0- 2 0 217-0 .218 W-1 V- 1 0 218-0 219 X-5 T- 3 0- 1 0 219-0 .220 M-4 S- 2 0- 3 M- 1 0 220-0 .221 P-2 P- 4 M- 5 U- 1 0 221-0 .222 T-5 R- 1 W- 4 W- 3 0 .222-0 .223 V-4 M- 2 V- 2 P- 3 0 .223-0 .224 T-2 R- 4 R- 2 N- 2 0 .224-0 .225 S-3 Q- 3 T- 1 0 .225-0 .226 Q-5 0- 5 N- 4

0 .226-0 .227

n .227-0 .228 Q-4

0 .228-0 .229 X-4

0 .229-0 .230 V-5

0 .230-0 .231

0 .231-0 .232 0-1

V-1 B-5

J-5

L-1 W-3

J-1 C-1

T-1

L-3

C-5

0-2

C-3

L-5

S-5 V-3 W-5

R-5

P-1

D-4

T-3

S-2

D-2 W-5

L-5

R-2

R-3

U-4

X-2 0-4

L-3

C-5

F-3

U-4 M-2

N-2

Cell Designation

B-3

K-2

U-1 Q-2

1-4

All

J-4 0-1 0-3 S-1

P-4 M-5

U-2 F-1 V-2 P-3

P-1 0-4

A-4 H-3 G-2 C-2

H-1 J-3 M-1 K-4 J-2 G-1 S-5 K-1 F-4 V-3 R-3 P-5 H-2 L-2 S-4 N-1 N-3 N-5

X-3 E-5 U-3 T-4 0-3 C-3

K-3 R-5 X-2 G-3 U-5 G-4

£-2 B-2 W-2

First Readout Session

B-3

H-1 H-2

A-4 J-3

L-2

G-2

J-2

B-2

C-2

G-1 K-1 F-4

1-4 K-3 G-3 G-4

P-5

U-2

U-5

Second Readout Session

S-4 N-1 N-3 N-5 W-2 Q-2 X-3 U-3 T-4

NOTE: The cells designated Q-4, 1-5, X-4, V-5, and Q-1 were eliminated.

21
Table 4. Statistics, from Sample Results Shown in Table 3. (See also Data

Book 837, p. 108.)

Quantity All First Half Second Half

120 Cells (60 Cells) (60 Cells)

Average Absorbance 0.2212 0.2205 0.2218

Standard Deviation

absolute 0.0026 0.0021 0.0030

percent 1.19 0.95 1.33

Maximum Reading

all cells 0.2314 0.2275 0.2314

after el imination

of outliers^ 0.2296 0.2251 0.2296

Minimum Reading

all cells° 0.2164 0.2170 0.2164

^See Note on table 3

^None eliminated

The results shown in tables 1 and 3 are further used to obtain a check on the G-value of the particular batch of Fricke solution. The details are shown

in table 5. The first quantity, the average position-corrected absorbance

growth due to pre-i rradi ati on , is calculated as the average absorbance from

table 4 minus the average absorbance of the ten unirradiated cells from

table 1. The third quantity, the G-value, is obtained with the aid of the

first and second quantities and the experimentally determined molar linear

absorption coefficient (in both SI and conventional units); it is compared

with the recommended G-value [7]. The last quantity is the product of the

G-value obtained for the particular Fricke solution and the linear molar

absorption coefficient (see section 2.3.3) measured for our spectrophotometer,

again given in both SI and conventional units; it is compared with the

recommended value for this product [4J. Both the G-value obtained for our

solution, our geometry, and the product e G are seen to be, on the average,

within 0.3 percent of the respective recommended values. Therefore, the

switch taken in 1986 from interpreting the absorbed dose administered by the

participants by means of the measured value of e, the recommended G-value, and

our empirical geometry correction (see footnote 11, page 19), to interpreting

the absorbed dose by means of the now recommended product, e G, did not affect

our results significantly.

3.3 Spectrophotometry: Ensuring Long- and Short-Term Consistency and

Reproducibility.

Preceding absorbance measurements, the Fri cke-dosimeter cells are cleaned

with distilled water, using lint-free cloths ("Optic Cloth" made by Texwipe).

Currently, their absorbance is measured with a Cary, Model 219, spectrophotometer, as shown in figure 8. During the measurements ("readouts"), the

temperature in the cell compartment is kept close to 25°C by means of a

22
relatively large-volume (0.02 m^) thermostatically controlled circulating

water bath. In order to bring the temperature of the dosimeter cells close to

compartment temperature prior to readout, the loaded dosimeter racks are stored for about 20 minutes in the bath compartment, above the water level.

One cell at a time is then placed manually in the cell compartment for readout

on both sides. The timing of the readout is computer-controlled. After a fifteen-second delay, there is a thirty-second readout period, in which

fifteen absorbance readings are alternated with fifteen temperature readings.

The readings are averaged automatically and stored for off-line analysis. The

cell is then manually turned for readout on the other side, and the procedure

is repeated, but without the initial delay. Controlling the readout schedule

in this way ensures consistent absorbance readings even though complete

temperature equilibrium may not have been reached.

In order to check spectrophotometer performance, each readout session is

preceded by a number of spectrophotometer tests. Figure 9 and tables 6 and 7 give sample records of such tests.

Figure 9 shows the record of a determination of the wavelength

corresponding to the ferric-ion absorption peak, at which the absorbances were

to be read during the particular session. Shifts of a few nanometers are not

unusual. (Note that the absorption peak for ferric ions given in the

literature is 304 nm.)

Table 5. Check on G-Value and Product e G for the Particular Sample Batch of

Fricke Solution (from sample data shown in table 4 and figure 10,

and from eq. (3) of section 2.1).

Quantities All 120 Cells First Series

of 60 Cells

Second Series

of 60 Cells

Average position-corrected

absorbance growth aA1 rr 0.2212-0.0413=0.1799 0.2205-0.0413=0.1792 0.2218-0.0413=0.1805

Absorbed dose to Fricke solution (Op) 50 Gy 50 Gy 50 Gy

6-va1ue = 0.0977 iJK. /e 0_

1 rr r in units of mol J"^ (using measured value of e) Value

1.615 X 10-6 mol j-i

or 15.55/100 eV recommended by ICRU:

1.608 X 10-6 niol J-i or 15.49/100 eV

1.61 X 10-6 mol J-i

1.620 X 10-6 ^ol J-i or 15.60/100 eV or 15.5/100 ev'' [7]

Product e G = 0.0977 aA. /dr.

1 rr r In units of kg-i Sy-i

Value

351.5xlO-6m2kg-lGy-i

recommended by ICRU:

350. 2x10- 6m2kg-lGy-l

352 X 10-6 ni2 kg-i (

352.7xl0-6m2kg-lGy-l

iy-i [4]

*From ASTM Standard E 1026-84 [6].

''For both high-energy electrons and cobalt-60 garana rays.

23
Figure 8. Spectrophotometry

Attendant is inserting spectrophotometer cell into temperaturecontrolled readout compartment of spectrophotometer. The circulating water bath is at right, the computer display in the center.

290 294 298 302 306 310

Wave Length, nm

Figure 9. Sample Plot, Automatic Scan of Ferric-Ion Absorbance-vs-Wavel ength

Setting, Required to Establish Wavelength Setting for Absorption

Peak. Spectrophotometry was done at a setting of 301 nm. (From

Data Book 837, page 108.) Scan speed corresponded to 0.1 nm/s,

slit width to 0.2 nm.

24
Table 6. Sample, Absorbances of 1 g% Potassium Nitrate Solution, Obtained as

a Check on the Reproducibility of Absorbance Readings Throughout the

Roughly Six Weeks Required to Complete One Test.^ (From Data

Book 337, page 163.)

Readout Session Absor

Z- side a

'bance 1 1

side b

'or Cell

Z- side a

No.

-2side b

Average

Absorbance

first session, start

first session, end

Before Shipment

.7383

.7389

.7406

.7413

.7421

.7421

.7380

.7384

.7400

second session, start

second session, end

.7371

.7375

.7390

.7396

.7438

.7464

.7405

.7425

.7408

session start

After Shipment

session end

.7367

.7361

.7388

.7383

.7453

.7433

.7414

.7399

.7406

°A11 readings done with a slit width corresponding to 2 nm in wavelength, at a ! wavelength setting of 301 nm. The designations, "side a" and "side b,"

identify the cell windows facing the ultraviolet source. —- I

Table 7. Constancy and Linearity of Cary Spectrophotometer, Model 16, between

April 1971 and April 1974. The study was carried out by measuring

absorbance at two levels of concentration of potassium nitrate at

the absorption peak.

Date

Peak

Location

nm

Net

absorbance

1 g% KNO3

Twice net

absorbance

0.5 g% KNO

April 1971 302.5 0.7071^ 0.7073^

November 1971 302.5 0.7082^ 0.7164

April 1972 302.0 0.7067^ 0.7067

November 1972 302.5 0.7060^ 0.7014

April 1973 302.5 0.7089^ 0.7094

November 1973 302.5 0.7073^ 0.7058

April 1974 302.0 0.7069^ 0.7024^

3

^From fitted graphs

25

I
Table 6 gives a sample record of the absorbances obtained on two cells

filled with 1 g% potassium-nitrate solution measured before and after the two

readout sessions prior to shipment of the Fricke dosimeters and before and

after the readout session some six weeks later, after dosimeter return. The

averages of the absorbances are seen to have remained essentially constant,

indicating spectrophotometer reproducibility. Known dilutions provide a check

on absorbance-scal e linearity. Table 7 shows the net (background-corrected)

absorbance values for a 1 g% solution and twice the corresponding values for a 0.5 g% solution, for an earlier spectrophotometer model, followed over a period of three years. Such a long-term absorbance-constancy check is

facilitated if exactly the same concentration of potassium nitrate is used

each time. Since the spectrophotometer in use since 1980 (Gary, Model 213) is even more reliable than the Gary, Model 16, potassium-nitrate readings later

were taken only for the 1 g% solution (see table 6).

Starting in 1986, system linearity and constancy tests similar to those

performed with potassium nitrate up to then, will be performed with metal-onquartz filters, SRM 2031 [9]. Use of these filters will save time. But while

they have been available since 1980, they still require further periodic

checks of long-term absorbance constancy. These checks are carried out by the

NBS Center for Analytic Ghemistry.

3.4 Dose Interpretation and Report to Participants

The average absorbance growth for the Fricke solution contained in the

spectrophotometer cells measured before and after shipment of the cells is

used to compute the absorbed dose to water, (in grays), administered by the

participants. Using eq (4) (section 2.1), one obtains:

D„= 278 (M,^^ - AA^p) (5)

where aA^. is the difference between the absorbance before and after shipment

of the cells irradiated by the parti ci pants ^ ^ and AAgp is the average

difference between the absorbance before and after shipment of the shipped and

unshipped ("shipping" and "home") control cells. Sample values of

^'^^irr

" ^SQ? ^^"^"^""^^ cells and of individual values of aA^ for not

further irradiated cells are shown in column 6 of table S.^** Fi§ure 10 shows

the distribution of the individual values of from table 8, and gives the

i^Here, the absorbances before shipment are not geometry-corrected (as are

those in table 2) since the absorbances of the quartz windows of the

spectrophotometer cells and of the Fricke solution due to pre-i rradi ation

drop out when the differences are formed.

I'+The sample record shown in table 8 pre-dates the switch to SI units for

absorbed dose. It also pre-dates ICRU Report 35, recommending the use of

a fixed value for the product of e G in the computation of absorbed dose

from optical absorbance. The absorbed-dose values shown were computed

with a G-value of 15.5/100 eV (non-SI) and a measured value for e of

217.7 m2 mol"i. The complete report included in the appendix incorporates

the same data.

26
Table 8. Sample of Spectrophotometry Record After Shipment and Associated

Absorbed-Dose Interpretation.^ (From Data Book 837, p. 104 and 105.)

Cell

No.

0-1

0-2

0-3

0-4

0-5

P-1

P-2

P-3

P-4

P-5

Q-1

Q-2

Q-3

Q-4

Q-5

R-1

R-2

R-3

R-4

R-5

S-1

S-2

S-3

S-4

S-5

Readout After Shipment

Side a Absorb- Temp, ance °C

0.2216

0.4152

25.13

25.12

0.2223 25.05

0.4079 25.05

0.4081 24.89

0.4275

0.4342

0.4344

0.4396

0.2194

0.2329

0.2257

24.86

24.82

25.06

25.14

25.14

25.12

25.10

0.2277

0.4062

0.4107

0.4012

0.4074

0.4094

0.2258

25.11

25.15

25.16

25.14

25.17

25.17

25.15

0.2234

0.2197

0.4366

0.4510

0.4445

0.4487

25.18

25.21

25.19

25.18

25.19

25.22

Side b

Absorb- Temp, ance °C

0.2214

0.4132

25.14

25.10

0.2209 25.08

0.4067 24.95

0.4039 24.85

0.4278

0.4331

0.4338

0.4322

0.2197

0.2325

0.2262

0.2277

0.4052

0.4117

0.4015

0.4115

0.4088

0.2266

0.2224

0.2208

0.4378

0.4505

0.4463

0.4487

24.88

24.94

25.12

25.16

25.15

25.12

25.13

25.16

25.16

25.18

25.16

25.16

25.19

25.18

25.18

25.23

25.19

25.24

25.22

25.21

Di f ference.

or aA

- aA rr ^sp

sp

+ 0

+ 0

+ 0 + 0

+ 0

+ 0 + 0 + 0 + 0 + 0 + 0 + 0

0043

1963

0044

1808

1774

2005

2113

2114

2146

0005

0020

0042

0038

1767

1857

1785

1829

1845

0034

0015

0032

2167

2238

2216

2255

Absorbed Dose

to Water, rad

NBS Particip.

5483

5050

4956

5600

5903

5906

5995

4935

5188

4985

5110

5154

6054

6251

6189

6298

5440

5000

5000

6000

6000

6000

6000

4826

4990

4840

5045

5067

5830

6020

6030

6120

Abs. D. Differ- ence

%

0.78

- 0.99

+ 0.89

+ 7.14

+ 1.64

+ 1.59

+ 0.08

- 2.21

- 3.82

- 2.91

- 1.27

- 1.69

3.70

3.70

2.57

2.83

Partici pant

No.

17

18

19

20

21

^All readings done with a slit width corresponding to 2 nm in wavelength, at a wavelength setting of 301 nm. The designations, "side a" and "side b,"

identify the cell windows facing the ultraviolet source. Note that this

table was prepared prior to the switch to SI units and the use of the product

e G for computing absorbed dose.

27
Shipped Controls ( 22 )

X

X

X

X

X

X X X X X

X

X

XX XXXXXXX

Cut -off for Average

.0310-

-.002 .002 .004 .006 .008 .010 .012 Spurious Absorbance Growth

.014 .016 .018 .020

Home Controls (22 ) XXX

X X X X

X X X X

X X X X

Cut-off for Average

X

X X

X X X X

.260-^

-.002 ,002 .004 ,006 .008 .010 .012 Spurious Absorbance Growth

.014 .016 .016 .020

Obtaining the Average Spurious Growth:

Average, aA^^

22 shipping controls 0.0038 ± 0.0026 (stand. dev .

)

22 home controls 0.0043 ± 0.0027 (stand. dev.

)

al 1 44 control

s

0.0040 ± 0.0038 (stand. dev.

)

corresponding to 1.1 t 1.1 Gy

ure 10. Sample, Analysis of Spurious Absorbance Growth AA^p, of Shipping

and Home Controls. (From Data Book 837, p. 107.) For the assessment of the uncertainty in the correction for spurious absorbance

growth (table 9, p. 42), absorbance growth was averaged over ten

test series.

28
averages of the plotted data. The average of the growth of both shipping and

home controls combined was used to compute the absorbed-dose values shown in

column 7 of table 8, which — after rounding to three significant figures — is

incorporated into the report sent to the participants. The report also

contains plots of the distributions of the differences between assigned doses

and doses reported by the participants. A sample of a complete report to the

participants is included in the appendix.

3.5 Uncertainties in the Dose Interpretation

In a measurement-assurance program of the nature of the electrondosimetry uniformity service, the uncertainties in the individual dose interpretations are of major interest. Two independent methods were used to arrive

at values for the overall uncertainty. One involved a set of measurements of

the differences between dose interpretations and actually delivered doses, and

the other involved estimates of the uncertainties associated with the

individual steps leading to a dose interpretation. The results are shown in

table 9.

To arrive at the total uncertainty experimentally (first method), Fricke

dosimeters were exposed at two levels of cobalt-60 gamma radiation and shipped

to a number of participants who kept the packages for several days and then

returned them to NBS for evaluation in the usual manner. [4] The standard

deviation of the difference between the dose computed from exposure and that

determined from spectrophotometry of the individual Fricke dosimeters was about 1.5 percent, corresponding to an overall uncertainty of three times this

standard deviation, or ± 4.5 percent. (The "overall" uncertainty is con- sidered to have approximately the character of a 99% confidence limit.)

In the second method, the overall uncertainty in the dose interpretation was estimated from the component uncertainties involved in the individual procedural steps. In table 9, the component uncertainties not based on repeated

measurements are estimated at a level of approximately one standard deviation,

so that they can be treated like standard deviations. They are added in

quadrature to obtain a "combined" uncertainty, which is multiplied by three to

obtain the overall uncertainty. The results of the two methods are seen to be

comparabl e.

The assessment of the uncertainties in the dose interpretation excludes

the uncertainties introduced by the spectral perturbations arising from the

use of quartz cells and a polystyrene phantom rather than a homogeneous water

medium. This exclusion is justified in a measurement quality assurance

service. If the service were ever upgraded into a calibration service,

consideration would have to be given to whether or not a correction for the

effect of the spectral perturbations was required, and the associated

uncertainties would have to be assessed. It is expected that these

uncertainties would be small compared to the uncertainty in the spurious

absorbance growth between readouts, which probably could be decreased

materially by the use of a sealed system. (See section 2.4.)

4. SAFETY CONSIDERATIONS

The precautions taken during the preparation, handling, and shipping of

the Fricke dosimeters are those usually taken in a chemistry laboratory that

handles corrosive liquids such as sulfuric acid: Old clothes and rubber

29
Table 9. Uncertainties in the Dose Interpretations

Procedure

First Method

Uncertainty in dose interpretation derived from repeated

measurements in a specially designed experiment^

Overall uncertainty

Second Method

Uncertainty estimated for each individual step involved

in the dose interpretations:

Calibration of spectrophotometer^

Measurement of absorbance*^

Correction for spurious absorbance growth between

first and second readout

Choice of reference value, e G = 352 x 10-^ m^ kg-i Gyi

(d) Combined uncertainty

(e) Overall uncertainty^ '

Estimated

Uncertai nty^

,

percent

1.5

4.5

0.2

0.05

1.5

0.6

1.6

4.8

^At the one standard deviation level.

^Based on table 5.4, ICRU Report 35 [4].

^Based on the measurement of the percent spontaneous growth for all control

dosimeters in ten test series, at an average electron-dose level

corresponding to an absorbance growth of 0.20 (i.e., about 56 Gy).

'^Individual uncertainties added in quadrature.

^Three times the uncertainty for Method 1 or three times the "combined"

uncertainty for Method 2.

30
aprons are worn by the staff; the concentrated sulfuric acid is transported

and stored in a rubber "carrying sling," and face shields are worn by the

staff during pouring and weighing of the acid; acid-proof composites, glass or stainless steel, are used as supports of all containers of the Fricke

solution; all liquid waste is poured into stainless-steel sinks equipped with

glass-lined waste pipes; the polystyrene blocks containing the stoppered

Fricke-dosiineter cells are sealed in plastic bags and packed professionally in

card-board boxes for transmittal to the participants by private carrier rather

than the U.S. Mail; the box labels are marked with a note cautioning the

shipper to store the boxes in the pressurized compartment of high-flying

aircraft. Also, the persons handling the potassium nitrate are cautioned to

handle it with care because of its potentially explosive nature.

31
REFERENCES*

[1] For definitions of the radi ati on-dosimetry quantities and units (both SI

and special) used in this document, see Radiation Quantities and Units;

ICRU Report 19, International Commission of Radiation Units and

Measurements, Bethesda, MD, 1971.

[2] A Protocol for the Determination of Absorbed Dose from High-Energy Photon

and Electron Beams, Task Group 21, Radiation Therapy Committee, AAPM;

Med. Phys. 10(6): 741-771, 1983.

[3] Ehrlich, M. and Lamperti , P. J. Uniformity of High-Energy Electron-Beam

Calibrations, Phys. Med. Biol. 14(2), 305-314, 1969.

[4] Radiation Dosimetry: Electron Beams with Energies Between 1 and 50 MeV;

ICRU Report 35, International Commission of Radiation Units and

Measurements, Bethesda, MD, 1984.

[5] See, e.g., section 5.3.2 of ICRU Report 35 (preceding reference).

[6] Standard Method for Using the Fricke Dosimeter to Measure Absorbed Dose

in Water, ASTM Designation E 1026-84, 1985 Book of ASTM Standards,

Section 12, American Society for Testing and Materials, Philadelphia,

Pennsylvania, 1985.

[7] Radiation Dosimetry: X Rays and Gamma Rays with Maximum Photon Energies

Between 0.6 and 50 MeV; ICRU Report 14, International Commission on Radiation Units and Measurements, Bethesda, MD, 1969.

[8] Mavrodineanu, R., NBS Center for Analytical Chemistry, private

communi cati on.

[9] Mavrodineanu, R. and Baldwin, J. R., Standard Reference Materials:

Metal -On Quartz Filters as a Standard Reference Material for

Spectrophotometry - SRM 2031; NBS Special Publication 260-68; U.S.

Government Printing Office, Washington, D.C, 1980.

Addition a l References to the NBS Fricke Dosimetry Service

Ehrlich, M. and Lamperti, P. .]., Electron-Therapy Dosimetry, pp. 365-370,

NBS Special Publication 456, Proceedings of a Symposium on the

Measuretnents for the Safe Use of Radiation, held at NBS in March, 1976;

S. P. Fivozinsky, editor; U.S. Department of Commerce, National Bureau of

Standards, 1976.

Soares, C. G. and Ehrlich, M. , NBS Traceability Program for Radiation

Therapy, pp. 89-97, Proceedings of a Meeting on Traceability for Ionizing

Radiation Measurements, held at NBS in May, 1980; H. T. Heaton II,

editor; U.S. Department of Commerce, National Bureau of Standards,

1982.

* Referenced literature is available on request from the Dosimetry Group,

Ionization Radiation Division, Center for Radiation Research.

32
Appendix 1

Forms Used in Written Exchanges with Participants

33
NATIONAL BUREAU OF STANDARDS

UNIFORMITY CHECK, DOSIMETRY OF HIGH-ENERGY ELECTRON-BEAMS

To All Participants:

If you plan to participate in the service, you will need a

polystyrene phantom. Directions for its construction are given

in the "Protocol for the Dosimetry of High Energy Electrons",

SCRAD, Physics in Medicine and Biology, Vol. 11, No. 4, p. 505,

October 1966, with the exception that the recess required for

the dosimeter units supplied by NBS is of different dimensions

from the one described in the Protocol. The enclosed figure,

from NBS SP 250-4 (1986), is a picture of the unit that will be

sent to you for exposure. It consists of a stoppered

spectrophotometer cell filled with f errous-sulfate solution,

held firmly in place in a polystyrene parallelepiped of outer

dimensions

+0.00 cm +0.00 cm +0.00 cm

7.62 cm x5.08cm x2.01cm

-0.03 cm -0.03 cm -0.03 cm

The dosimeter unit should fit snugly into the phantom

recess. (It is suggested that the airspace on all sides be kept

to a width of less than 0.5 mm.) The electron beam should be

perpendicular to the 7.62 cm x 5.08 cm planes of the dosimeter

unit. The center of the spectrophotometer cell should be in the

center of the 10 cm x 10 cm field. The dosimeter may be

irradiated at any phantom depth where the dose rate across the

thickness of the dosimeter is known. Because the dosimeter

averages absorbed dose over a depth of 1 cm, you should take

this into account in your dose computation, especially below

10 MeV, since absorbed dose changes significantly over this

depth, even near the depth-dose peak.

34
ANNOUNCEMENT TO ALL PAST PARTICIPANTS

UrJITEO STATES DEPARTMENT OF COMMERCE

National Bureau of Scandarda

GaiChersburg, Marylena SOBgS

<date> Announcement of the Next Uniformity Study of

Dosimetry of High-Energy Electron Beams

We plan to ship the next batch of f errous-sulf ate dosimeters

between <dates>. Please let us know immediately whether or not you wish to participate, and if so, send us your purchase order

before <date> and tell us how many dosimeters you require.

Currently the fees are <cost> for the basic set of three

dosimeters (two for irradiation and one unirradiated control),

and <cost> for each additional dosimeter.

When you receive the dosimeters, please expose them according to the directions that were given to you previously. Administer

between 50 and 80 Gy to water using a 10 cm x 10 cm field size. Return the dosimeters to us as soon as possible, preferably

within three working days.

Please do not tamper with the dosimeter units. After exposure, place them in the zip-lock bag and seal the bag for safety

reasons. Use the same packing material, or material similar to that used by us. Send the package by the same carrier as received, and mark the box, "Please ship in pressurized

compartment or at low altitude". Please send return shipment pre- paid to:

Dr. Christopher Soares

C-210 Rad Phys Building

National Bureau of Standards

Route 270 & Quince Orchard Rd. Gaithersburg, MD 20899

NoteIt would be appreciated if you returned the dosimeters

within three working days from the day of receipt. We are reserving the right to refuse participation in the current study

if the dosimeters are not returned within ten working days after

receipt

,

Sincerely

,

Christopher Soares Dosimetry Group

35
ANNOUNCEMENT TO GROUPS THAT INDICATED THEIR INTENTION TO PARTICIPATE,

BUT DID NOT SUBMIT A PURCHASE ORDER

UNITED STATES DEPARTMENT OF COMMERCE

Nat:ional Bureau of Scandards

GaitfTersburg Mar /land 2QB99

<date> Announcement of the Next Uniformity Study of Dosimetry of High-Energy Electron Beams

We plan to ship the next batch of f errous-sulf ate dosimeters

between <dates>. Since you have already indicated your intention

to participate, all that is required now is your purchase order.

Currently the fees are <cost> for the basic set of three

dosimeters (two for irradiation and one unirradiated control),

and <cost> for each additional dosimeter.

When you receive the dosimeters, please expose them according to the directions that were given to you previously. Administer

between 50 and 80 Gy to water using a 10 cm x 10 cm field size. Return the dosimeters to us as soon as possible, preferably

within three working days.

Please do not tamper with the dosimeter units. After exposure, place them in the zip-lock bag and seal the bag for safety

reasons. Use the same packing material, or material similar to that used by us. Send the package by the same carrier as received, and mark the box, "Please ship in pressurized

compartment or at low altitude". Please send return shipment pre- paid to:

Dr. Christopher Scares

C-210 Rad Phys Building

National Bureau of Standards

Route 270 & Quince Orchard Rd. Gaithersburg, MD 20899

NoteIt would be appreciated if you returned the dosimeters

within three working days from the day of receipt. We are reserving the right to refuse participation in the current study

if the dosimeters are not returned within ten working days after

receipt

.

Sincerely

,

Christopher Scares Dosimetry Group

36
ANNOUNCEMENT TO GROUPS THAT INDICATED THEIR INTENTION TO PARTICIPATE,

AND SUBMITTED A PURCHASE ORDER

UNITED STATES DEPARTMENT OF COMMERCE

Naclonal Bureau of Stsndarda

Gaittersbury MarylaniJ 20B99

<date> Announcement of the Next Uniformity Study of Dosimetry of High-Energy Electron Beams

We plan to ship the next batch of f errous-sulf ate dosimeters

between <dates>. When you receive the dosimeters, please expose

them according to the directions that were given to you previously. Administer between 50 and 80 Gy to water using a 10 cm X 10 cm field size. Return the dosimeters to us as soon as possible, preferably within three working days .

Please do not tamper with the dosimeter units. After exposure, place them in the zip-lock bag and seal the bag for safety

reasons. Use the same packing material, ot material similar to that used by us. Send the package by the same carrier as received, and mark the box, "Please ship in pressurized

compartment or at low altitude". Please send return shipment pre- paid to:

Dr. Christopher Scares

C-210 Rad Phys Building

National Bureau of Standards

Route 270 & Quince Orchard Rd. Gaithersburg, MD 20899

Sincerely,

Christopher Scares Dosimetry Group

NoteIt would be appreciated if you returned the dosimeters

within three working days from the day of receipt. We are reserving the right to refuse participation in the current study

if the dosimeters are not returned within ten working days after

receipt

.

37
FERROUS- SULFATE DOSIMETER IRRADIATION FORM

Pleasse complete this form and ship it back with the exposed

dosimeters

Dosimeter Dose* Electron Energy

Number ( Gy ) ( MeV

)

control

After irradiation, place the dosimeters in the zip-lock bag and

reseal the bag for safety reasons. Use the same pacliing material

or material similar to that used by us. Return package by the

same type of carrier as you received it, Parted:

" FRAGILE. IF SENT BY AIR AT HIGH ALTITUDES, KEEP IN PRESSURIZED

CABIN. "

If at all possible, return the package within three working days

after receipt. Please send return shipment prepaid.

"For energies : 10 MeV , ple.^fise indicate whether or not y'ou

averaged over the depth of the dosimeter.

38
U.S. Department of Commerce

NATIONAL BUREAU OF STANDARDS

GAITHERSBURG. MARYLAND 20899

REPORT OF TEST

DG <dq «/yr>

< d a t e >

To : All F a r t i c i o a n t s i n t h e < s t l i d y name ' - U n i f

q i tti i t y 5 b u d / o f

Dos i me try _ i_n Hi qh - Ener gy Electro n Beams

The enclosed tables and figures cental the results- of the

current study for all participants. A group number was as'^tgned

to r?ach participant. The dosimeters are listed by group and not

necessarily in alphabetical order. Thi i s procedure, wh i 1 1? not

giving away a participant's i d e ri t i t y to t he t" e s t of t he pa r t i c i

- pants? eriables each participant to check his standing amonc^ his

peers with regat d to accuracy as well as consistency of

per f or ma,"\ce' . """he red arrow in the table points to the grt-uj..'

number of ttie pai" t i c i pant to whom the common i t: a t i o ri addr e'i-sed . The' figures cri vt- a statistical survey of ttie re^S'alts. Gur evaluation procedur"e anri estimated erro' are those discus--.ed i ri "Tt;e

Fr 1 c f-i.?- Do s ! me t r y F.ervice of the- IMational Bureau of G I a nd a r s " . M. Ehrlicti. NBS Special Pu!)lication c?50-'^. An pF prfidurh of

355 m' ' kq Gy" ' i-jas '.j'=-.!;:-d for- both ele'' tr oris and ccbdtl t- 6F

ciamma rays. Application o"*^ the oroduct of the formerly used

V <;i 1 1 e s f o r e 3 n d G w o u I d can 'f-. r

^ N B F — a s s i g n e d a h s o r t. e d ~ d o s e ^ a 1 u e s to t>e larger by a factor of ] . Ci05 ,

Y o u r sine e 1 e ] y .

Christopher G . S n a " e s 1 h y s 1 c i s t

I o n i 1 no, Rad 1 a t i o n D i ^ 1 s i o n C e n t e r f o . R a d 1 a t i o n R e s e a <" c i

E r'c 1 osur es

F.5. This is to re-'ovind you t^iat ou'; dov-.. i mt^ t er- s ave' ago at- so r l)!.--d dose over a dcjoth of i '.:m. At electi^on ener' c! 1 e 3. less than 1*' Me''.-' yoLi should tale this ird;o account in your^ dosi.o c o nip u +. a t 1 o n , ->ince

absorbed dose changes s 1 gn i f i c arrt 1 v' o /er this dcp th , ev^-n nea-"

the d e f.) t h -- d o s e 1 j e <V! t: .

39
GrouD Dosimeter Absorbed Dose Dose Difference Irradiated

No. Designation NBS Participant (Pet. of with:

(6y to water) (6y) NBS Dose)

40
Appendix 2. Sample of Report to Participants

41
U.S. Department of Commerce

NATIONAL BUREAU OF STANDARDS

GAITHERSBURG, MARYLAND 20899

DG 8561/86

REPORT OF TEST

June 26, 1986

To : All Participants in the Spring 1986 Uniformity Study of

Dosimetry in High-Energy Electron Beams

The enclosed tables and figures contain the results of the

current study for all participants. A group number was assigned

to each participant. The dosimeters are listed by group and not

necessarily in alphabetical order. This procedure, while not

giving away a participant's identity to the rest of the participants, enables each participant to check his standing among his

peers with regard to accuracy as well as consistency of

performance. The red arrow in the table points to the group

number of the participant to whom the communication is addressed.

The figures give a statistical survey of the results. Our evaluation procedure and estimated error are those discussed in "The

Fricke-Dosimetry Service of the National Bureau of Standards",

M. Ehrlich, NBS Special Publication 250-4 (1986). An eG product

of 352 X 10"^ m^ kg"-'- Gy"-'- was used for both electrons and

cobalt-60 gamma rays. Application of the product of the formerly

used values for e and G would cause NBS-assigned absorbed-dose

values to be larger by a factor of 1.005.

Yours sincerely, ^

r-- / y

ij Christopher G. Scares, Physicist

Ionizing Radiation Division

Center for Radiation Research

Enclosures

P.S. This is to remind you that our dosimeters average absorbed

dose over a depth of 1 cm. At electron energies less than 10 MeV

you should take this into account in your dose computation, since

absorbed dose changes significantly over this depth, even near

the depth-dose peak.

42
Report of Test, Continued

roup

NO

Dosi meter'

De s 1 Q D a 1 1 On

Ab sorb

MISOP Qi_>

( Gv 1 n water —

ed Dose

F' ar 1 1 c. i p rA n t

) (Gv) — - •

Dose Difference

•i r C L . OT

NBS Dose)

I r r acli a ted

with:

1 A -2 55. 2 56. 42 8 MeV el

A-3 62 . 0 63. 27 + 2.2 13 MeV el

A-4 59. 1 60. 78 + 2,9 10 MeV el

B~l 49. 4 5 1 . 00 + 3. 4 30 MeV el

Y<-2 49. 4 50. 60 + 2,6 7.5 MeV el

B-3 48. 6 50 . 00 3 , C* i_* 1 1 tr V tr X B-5 49. 2 50. 00 + 1 . 7 12 MeV el

C-1 50 . 7 50 . 00 -1.3 22 MeV el

C-3 59. 1 50 . 40 -14.7 12 Mp>V f=>] C--4 51.9 50 . 5(."i -2.5 20 MeV el

4 D~2 5C) . 0 49. 95 + 0. 1 6 MeV el

D-3 52. 1 49. 95 -4. 1 12 MeV el

D-4 53. 0 49. 95 -5. 6 15 MeV el

cr E-1 40. 6 47 . 95 + 1 £.1 . 1 6 MeV el

E-2 45. 4 47. 95 +5. 6 9 MeV el

E-3 66. 9 49, 95 13 MeV el

E-4 50 . 3 49. 95 -0. 6 17 MeV el

F-1 51.4 49. 95 -2 . 6 20 MeV el

6 F-2 60 . 7 60 . 00 --1 . 1 10 MV >:rv F-Z 59 . 9 60 . 00 + C> . 3 6 MeV el

F-4 61.2 60 . 00 -1.9 9 MeV el

G-1 59 . 9 60. 00 +0. 3 12 MeV el

G-3 59. 7 60 00 + 0.5 15 MeV el

R —

4

AO A 60 ,

(')(") — ('1 Q

G-5 60. 4 60 . C>0 -0.6 Cobal t-60

7 =14 '"•'() 7/ MoVJ Mfcr V cal tr ^ H-4 52. 4 54 . 92 + 4.8 18 MeV el

H-5 57. 5 54, 05 -5. 9 12 MeV el

3 I -2 52. 4 50. 00 -4.4 9 MeV el

1-3 50 . 5 50 ^ 00 -0. 9 12 MeV el

9 J-1 60. 1 60 . 00 -0. 1 6 MeV el

J-2 59. S 60. 00 +0 . 5 15 MeV el

10 J-4 57. 0 57. 87 + 1 . 7 Cobal t-60

J -5 55 . 0 56. 89 + 3.5 Cabal t-60

1 1 K-2 49. 0 49. 00 +0. 2 8 MeV el

K-3 52 . 6 49. 04 -6.7 10 MeV el

43
Report of Test, Continued

Group Dosimeter Absorbed Dose Dose Difference Irradiated

No. Desianation NBS F^ar t i c.i piant (F'ct. of with:

(Gv in water) (Gv) NBS Dose)

13

14

V5

16

17

IS

19

L— 1 ,_' r . 4

\- 1 1 1

(")

1 I tr V t: .1 L-2 60. 9 60 . 00 10 MeV el

._'0 . uJ — 1 1 7 MeV el

L — uJ LT T cr ^•_• . U O . wJ 1 (

)

( 1 tr V Cr J. n— 1 — 1 1Mp>V-' 1 tr N' tnr J

M-3 6(1) . 5 63. 68 + 5. 3 9 MeV el

M A -0 - 5 1 2 MeV el

tvt <=. A A {

'\ A 1 9A -6 . 0 1 8 MeV el

N —

2

•=,()

_ (")(") — '? 4 7 Ne?V' el

N-3 50. 8 50 . 00 -1 . 4 10 MeV el

N-4 51 . 1 50 . 00 -2.0 15 MeV el

K 1 cr N—

J

. o 1 1 cr V Cr J.

0-1 66. 5 63. 49 -4 . 5 10 MV >; rv

0-3 56. 3 52. 26 -7. 1 9 NeV el

0-4 A '~* A DA . O

. ,_. ,.,

-»- 1 A 1 2 MeV el

LI — ^' A l\ '7 +' J.1 M 4" 1 5 MeV el

P-2 34 . 3 49. 64 +44. 9 6 MeV el

P-4 61.1 7 MeV el

p-5 'T fl

:• . S 63 . <->(.* i. m J- 1

(")

1 1 tr V fcr .1 Q-1 61 . 8 63. 00 +2 . 0 12 MeV el

Q-3 51.2 50 • ^ . 1 ifcf V tr X -4 49. 1 50. 00 -t-1 . 9 20 MV xrv

Q-5 51.4 50 . 00 -2.6 6 MV >:rv

R-2 60. 2 60. 70 +0.9 10 MV xrv

R-3 48. 4 50 . 00 + 3.5 12 MeV el

R-4 50 .5 5<.> . 00 OA 1 1 tr V tr X

S-3 56. a 50 . 00 -11.9 17 MeV el

S-4 49 . 6 50 . 00 + 1.1.' Q F 1 tr V cr 1

T-4 51.4 50.00 -2 . 7 1 2 MeV el

U-1 51 . 4 50.00 -2.6 14 MeV el

U-2 56 . 6 55. 97 -1.0 12 MeV el

U-4 56 . 0 57. 29 + 2.5 7 MeV el

V'-2 49.4 50. 00 + 1 . 4 10 MeV el

V-3 50. 4 50 . 00 -0.8 14 MeV el

V-4 49. 9 50. 00 +0. 3 9 MeV el

V-5 56. 1 50. 00 -10.9 20 MeV el

44
o

Report of Test, Continued —I—I—-f—1—I—I—

r

CO

00

CD

c

CL

00

J L I I I I

1—

r

I I I

O

O

o

c

CD

O

CD

CL

CD

_^

>

CD

CO

OQ

(/)

CD

O

O

c

o

-I-' o

*>

CD Q

O

CX)

LO O LO O

|DAJ9:).UJ J9Cl S^UOdlOI^JDJ J.0 J9qLUn[\|

4b
Report of Test, Continued

TZ 1 r \

\

\

o

a

a

\

Z5

00

o

c

\ a\

a \

a

CO

1

'

? 00

CD

c

JC

CO

\

\

\

\

\ \

\

\

o\ \

+ N

\

CMo

\

\

\

\

\

CD

O

\

\

c

CD

C

<

CD

CO

O

tn

CD

-1 1 1 L \ o

o

^s:].UDdioi:^jDd Aq pauBissy 9S0Q

46
NBS-n4A (REV. 2-6CI U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

'BS/SP-25ri /A

2. Performing Organ. Report No. 3. Publication Date

July 1987

4. TITLE AND SUBTITLE

NBS Measurement Services: Fricke Dosimetry in High-Energy

Electron Beams

5. AUTHOR(S)

Christooher G. Scares, Eric L. Brig'nt, and Margarete Ehrlich

6. PERFORMING ORGANIZATION (If joint or other than NBS. see in struction sj

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

Gaithersburg , MD 20B99

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Same as item #6 above.

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card T]arr>J:>er : 37-619834

Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less (actual summary of most si gnificant information. If document includes a si gnificant

bibliography or literature survey, mention it here)

The i^3S Fricke-Dosimetry Service (advertised in NBS Special

Puiolication 250, 1985-1988 and earlier editions) is describee^ in

detail. A,fter a brief historical introduction and description of the

service, the theoretical basis (including what quantities are

ioeasured, how, and why) and tJie philosophy of internal ;^jality checks

are discussed in some detail. This is followed, by a description of

the pliysical setup and of tlie step-by-step operating and reporting

procedures. Throughout this section, there is reference to sa.Tple

rea^rds of oast joerformance, in order to facilitate continuity of

operation in the case of personnel changes. The docuiT\ent concludes

with a discussion of the uncertainties involved in this measurei'ient

quality assurance service, safety considerations, and an appendix

o^ntaining samples of all forT, letters and of the final report vaailed

to the participants.

12. KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) absorbed dose to water; bi-annual service; chemical dos.imetry; dose interpretation;

dosiioster preparation; electron therapy; ferrous-sulfate (Fricke) solution; internal

quality check; .neasureinent quality assurance; plastic phantom; spectrophotometry.

13. AVAILABILITY

[X] Unlimited

[33! For Official Distribution. Do Not Release to N' IS [X] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

331 Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

54

15. Price

USCOMM-DC 6043-P80

PUBLICATIONS IN THE SP 250 SERIES FROM THE CENTER FOR RADIATION RESEARCH

P 250-1

SP 250-2

SP 250-3

SP 250-4

SP 250-5

SP 250-6

SP 250-7

SP 250-8

SP 250-9

SP 250-10

SP 250-11

NBS Measurement Services:

Spectral Radiance Calibrations

J.H. Walker, R.D. Saunders, and A.T. Hattenburg

NBS Measurement Services:

Far Ultraviolet Detector Standards

L.R. Canfield and N. Swanson

NBS Measurement Services:

Radiometric Standards in the Vacuum Ultraviolet

J.Z. Klose, J.M. Bridges, and W.R. Ott

NBS Measurement Services:

Fricke Dosimetry in HighEnergy Electron Beams

C.G. Scares, E.L. Bright and M. Ehrlich

NBS Measurement Services:

Alpha-Particle Calibrations

J.M.R. Hutchinson

NBS Measurement Services:

Regular Spectral Transmittance

K.L. Eckerle, J.J. Hsia, K.D. Mielenz,

and V.R. Weidner

NBS Measurement Services:

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W.R. Waters, J.H. Walker, and A.T. Hattenburg

NBS Measurement Services:

Spectral Reflectance

V.R. Weidner and J.J. Hsia

NBS Measurement Services:

Calibration of Beta-Particle-Emitting

Ophthalmic Applicators

J.S. Pruitt

NBS Measurement Services:

Radioactivity Calibrations with the "4tt" Gamma Ionization Chamber, and Other

Radioactivity Calibration Capabilities

J.M. Calhoun

NBS Measurement Services: ' Dosimetry for High-Dose Applications

J.C. Humphreys, D. Hocken, and W.L, McLaughlin

SP 250-12 NBS Measurement Services: Neutron Personnel Dosimetry

R.B. Schwartz

SP 250-13 NBS Measurement Services:

Activation Foil Irradiation

with Californiim Fission Sources

G.P. Lamaze and J. A, Grundl

SP 250-14 NBS Measurement Services:

Activation Foil Irradiation by Reactor Cavity Fission Sources

G.P. Lamaze and J. A. Grundl

SP 250-15 NBS Measurement Services:

Photometric Calibrations

R.L. Booker and O.A. McSparron

SP 250-16 NBS Measurement Services:

Calibration of X-Ray and Gamma-Ray

Measuring Instrunents

P.J. Lamperti, T.P. Loftus, and R. Loevinger

SP 250-17 NBS Measurement Services:

The NBS Photodetector Spectral Response

Calibration Transfer Program

E.F. Zalewski

SP 250-18 NBS Measurement Services: Neutron Source Strength Calibrations

E.D. McGarry and E.W. Boswell

SP 250-19 NBS Measurement Services:

Calibration of Gamma-Ray-Emitting

Brachytherapy Sources

J.T. Weaver, T.P. Loftus, and R. Loevinger

SP 250-20 NBS Measurement Services:

Spectral Irradiance Calibrations

J.H. Walker, R.D. Saunders, J.K. Jackson,

and D.A. McSparron

SP 250-21 NBS Measurement Services:

Calibration of Beta-Particle

Radiation Instrimentation and Sources

J.S. Pruitt, C.G. Scares, and M. Ehrlich
U.S. Department of Commerce

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