NAT'L INST. OF STAND & TECH R.I.C,

AlllDS MQSflS?

isurement Services: REFERENCE

NIST Special Publication 250-44

Radiation Processing Dosimetry

Calibration Services and

Measurement Assurance Program

Jimmy C. Humphreys

James M. Puhl

Stephen M. Seltzer

William L. McLaughlin

Marc F. Desrosiers

Debra L. Bensen

Marlon L. Walker

IC

00

157

iO. 250-44

998

U.S. Department of Commerce

Technology Administration

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

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'At Boulder, CO 80303. ^Some elements at Boulder, CO.
NIST Special Publication 250-44

NIST MEASUREMENT SERVICES:

Radiation Processing Dosimetry Calibration

Services and IVIeasurement Assurance Program

Jimmy C. Humphreys, James M. Puhl, Stephen M. Seltzer,

William L. McLaughlin, Marc F. Desrosiers, Debra L. Bensen,

and Marlon L. Walker

Ionizing Radiation Division

Physics Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-0001

U.S. Department of Commerce

William M. Daley, Secretary

Technology Administration

Gary R. Bachula, Acting Under Secretary for Technology

National Institute of Standards and Technology

Raymond G. Kammer, Director

March 1998
National Institute of Standards and Technology Special Publication 250-44

Natl. Inst. Stand. Technol. Spec. Publ. 250-44, 40 pages (Mar. 1998)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1998

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

Page

PREFACE iii

SCOPE 1 REFERENCED DOCUMENTS 1 TERMINOLOGY 1

SIGNIFICANCE AND USE 3

GENERAL CRITERIA 4

SPECIFIC CRITERIA FOR IONIZING RADIATION 14

SPECIFIC CRITERIA FOR CALIBRATIONS

USING PHOTONS AND ELECTRONS 21

KEYWORDS 36

REFERENCES 36

ii

PREFACE

The basis for this document was developed within the American Society for Testing and

Materials (ASTM) subcommittee El 0.01 on Dosimetry for Radiation Processing. The version

that is published as an ASTM standard is designated as El 400 entitled "Standard Practice for

Characterization and Performance of a High-Dose Radiation Dosimetry Calibration Laboratory,"

with the latest revision dated 1995. This document differs substantially from the ASTM standard

in that it addresses calibration services operated under the direction of NIST staff, whereas the

ASTM standard deals with the accreditation of secondary calibration laboratories.

iii

1. Scope

1 . 1 This document contains descriptions of the characteristics of the high-dose radiation

dosimetry calibration services at the National Institute of Standards and Technology (NIST).

2. Referenced Documents

2.1 ASTMStandards:

E 1 70 Terminology Relating to Radiation Measurements and Dosimetry

E 1249 Practice for Minimizing Dosimetry Errors in Radiation Hardness Testing of Silicon

Electronic Devices using Co-60 Sources

E 1250 Method for Application of Ionization Chambers to Assess the Low Energy Gamma

Component of Cobalt-60 Irradiators Used in Radiation-Hardness Testing of Silicon Electronic

Devices

E 1261 Guide for Selection and Calibration of Dosimetry Systems for Radiation Processing

E 1 607 Practice for Use of the Alanine-EPR Dosimetry System

E 1 707 Guide for Estimating Uncertainties in Dosimetry for Radiation Processing

2.2 International Organizationfor Standards:

ISO/IEC Guide 25 (1990) General requirements for the competence of calibration and testing

laboratories

2.3 American National Standards Institute:

ANSI/NCSL Z540-1-1994 Calibration Laboratories and Measuring and Test Equipment - General Requirements

2.4 National Institute ofStandards and Technology:

NIST Special Publication 250-45 High-Dose Radiation Dosimetry Calibration Services: Manual

of Calibration Procedures

3. Terminology

3.1 Descriptions ofTerms Specific to This Document:

3.1.1 calibration - the process whereby the response of a dosimeter or measuring instrument is characterized through comparison with an appropriate standard that is traceable to, and consistent

with, a national standard.

3. 1 .2 dosimetry system - a system used for determining absorbed dose, consisting of

1
dosimeters, measurement instruments and their associated reference standards, and procedures

for the system's use. NOTE 1 - The types of dosimeters include reference standard dosimeters, transfer

standard dosimeters, and routine dosimeters. See Guide E 1261 for guidance on the

selection and calibration of the various dosimetry systems.

3.1.3 measurement assurance program - a documented program for the measurement process

that quantifies on a continuing basis the overall uncertainty of the measurements. This program

requires traceability to and consistency with national or international standards, and shall ensure

that the overall uncertainty meets the requirements of the specific application.

3. 1 .4 measurement traceability - the ability to demonstrate and document periodically that the

measurement results from a particular measurement system are in agreement with comparable

measurement results obtained with a national standard (or some identifiable and accepted

standard) to a specified uncertainty.

3.1.5 primary standard dosimeter - dosimeter, of the highest metrological quality, established

and maintained as an absorbed dose standard by a national or international standards

organization.

3. 1 .6 quality assurance - all systematic actions necessary to provide adequate confidence that a

calibration or measurement is performed to a predefined level of quality.

3.1.7 quality control - the operational techniques and procedures that are employed routinely to achieve and sustain a predefined level of quality.

3.1.8 quality manual - document stating the quality policy, quality system, and quality practices

of an organization.

3. 1 .9 quality system - organizational structure, responsibilities, procedures, processes, and

resources for implementing quality management.

3.1.10 radiation processing - the intentional irradiation of products or materials to preserve,

modify, or improve their characteristics.

3.1.11 reference standard dosimeter - a dosimeter, of high metrological quality, used as a standard to provide measurements traceable to, and consistent with, measurements made using

primary standard dosimeters.

3.1.12 routine dosimeter - dosimeter calibrated against a primary-, reference-, or transferstandard dosimeter and used for routine absorbed dose measurement.

3.1.13 transfer standard dosimeter - a dosimeter, often a reference standard dosimeter, suitable

2
for transport between different locations, used to compare absorbed dose measurements.

3.1.14 verification - confirmation by examination of objective evidence that specified

requirements have been met.

NOTE 2 - In the case of measuring equipment, the result of verification leads to a

decision either to restore to service or to perform adjustments, or to repair, or to downgrade, or to declare obsolete. In all cases it is required that a written trace of the

verification performed be kept on the instrument's individual record.

3.1.15 working standard - a standard, usually calibrated against a reference standard, used

routinely to calibrate or check measuring instruments or devices.

3.2 Also see Terminology E 170.

4. Significance and Use

4. 1 The radiation industry needs a source of reliable, prompt, dosimeter calibration services to support accurate measurements of absorbed dose during radiation processing. Those

measurements, made routinely in industrial facilities, should be consistent with and traceable to the physical measurement standards maintained by an appropriate national or international

standards laboratory.

4.2 To ensure the provision of adequate services, a calibration laboratory should be operating

with a full measurement assurance program (MAP). The fundamental requirements for such a program include: (1) compliance with operational requirements of this document; (2)

documented procedures and in-house quality assurance (QA) program specific to the calibration

services provided; and (3) periodic performance evaluations, such as internal review by NIST

staff and measurement intercomparisons between NIST and other national laboratories (1,2)\

4.3 In addition to standard calibration services such as irradiation of dosimeters and supplying

transfer dosimeters for client in-house source calibration, the NIST calibration laboratory MAP

can provide technical information, troubleshooting (at the client's facility if necessary), and serve

as an independent group for resolution of disputes. NIST can also conduct long term studies of

dosimetry system characteristics, such as environmental effects, and evaluate new, emerging

dosimetry systems to assist in technology transfer to industry.

4.4 Section 5 sets forth general criteria that shall be satisfied by the NIST calibration laboratory

These general criteria are completely consistent with ISO/IEC Guide 25 and its U.S. equivalent

ANSI/NCSL Z540-1. Laboratories that meet these general requirements comply, for calibration

activities, with Guide 25/Z540 and the relevant requirements of the ISO 9000 series of standards,

including those of the model described in ISO 9002 when they are acting as suppliers producing

'The bold numbers in parenthesis indicate references at the end of the text.

3
calibration results.

4.5 For laboratories engaged in specific fields of calibration, the general requirements of

ISO/IEC Guide 25/ANSI Z540 need amplification and interpretation. Section 6 of this document

contains specific criteria which provide that amplification and interpretation for ionizing

radiation. Section 7 contain specific criteria for particular types of ionizing radiation, that is, gamma rays, electron beams and x-ray (bremsstrahlung) beams. It contains the theoretical basis

for the traceability of dosimetry measurements used in the calibration services. The detailed

procedures used in the handling of customer's calibration requests, for the generation of

calibration reports, and in the determination of the uncertainties for the various calibration

services are contained in the companion document NIST Special Publication 250-45 Manual of

Calibration Procedures.

4.6 For ease of use, all sections of this document after Section 5 employ the format established

in Section 5. It is therefore readily apparent how the subsequent Sections amplify and interpret

the general requirements contained in Section 5.

5. General Criteria

5.1 This section sets forth the general requirements that shall be satisfied by a laboratory. In

addition to satisfying the general criteria of this section, a laboratory shall also satisfy the specific

criteria contained in Section 6 and in those parts of Section 7 relevant to each calibration service

offered (see 4.4 and 4.5).

5.2 This section may also be used by calibration laboratories in the development and

implementation of their quality systems, and by others concerned with evaluating the competence

of laboratories.

5.3 Organization and Management

5.3.1 The laboratory shall be organized and shall operate in such a way that its facilities meet

the requirements of this section.

5.3.2 The laboratory shall:

a) have managerial staff with the authority and resources needed to discharge their duties;

b) have arrangements to ensure that its personnel are free from any commercial, financial,

and other conflicts which might adversely affect the quality of their work;

c) be organized in such a way that confidence in its independence ofjudgement and integrity

is maintained at all times;

4
d) specify and document the responsibility, authority, and interrelation of all personnel who

manage, perform, or verify work affecting the quality of calibrations;

e) provide adequate supervision by persons familiar with the calibration methods and

procedures, the objective of the calibration and the assessment of the results;

f) have a technical manager (however named) who has overall responsibility for the

technical operations;

g) have a quality manager (however named) who has responsibility for the quality system

and its implementation. The quality manager shall have direct access to the technical

manager and to the highest level of management at which decisions are made on calibration

laboratory policy or resources. In some laboratories, the quality manager may also be the

technical manager or deputy technical manager of the calibration laboratory;

h) nominate deputies in case of absence of the technical or quality manager;

i) where relevant, have documented policy and procedures to ensure the protection of

clients' confidential information and proprietary rights;

j) where appropriate, participate in interlaboratory comparisons.

5.4 Quality System and Review ofActivities

5.4.1 The laboratory shall establish and maintain a quality system appropriate to the type, range,

and volume of calibration activities it undertakes. The elements of this system shall be

documented. The quality documentation shall be available for use by the laboratory personnel.

The laboratory shall define and document its policies and objectives for, and its commitment to, good laboratory practice and quality of calibration services. The laboratory management shall ensure that these policies and objectives are documented in a quality manual and communicated

to, understood, and implemented by all laboratory personnel concerned. The quality manual shall

be maintained current under the responsibility of the quality manager.

5.4.2 The quality manual, and related quality documentation, shall state the laboratory's policies

and operational procedures established in order to meet the requirements of this section. The

quality manual and related quality documentation shall also contain:

a) a quality policy statement, including objectives and commitments, by top management;

b) the organization and management structure of the laboratory, its place in any parent

organization and relevant organizational charts;

c) the relations between management, technical operations, support services, and the quality

5
system;

d) procedures for control and maintenance of documentation;

e) job descriptions of key staff and reference to the job descriptions of other staff;

f) identification of the laboratory's approved signatories;

g) the laboratory's procedures for achieving traceability of measurements;

h) the laboratory's scope of calibrations;

i) arrangements for ensuring that the laboratory reviews all new work to ensure that it has

the appropriate facilities and resources before commencing such work;

j) reference to the calibration and verification procedures used;

k) procedures for handling calibration and test items;

1) reference to the major equipment and reference measurement standards used;

m) reference to procedures for calibration, verification, and maintenance of equipment;

n) reference to verification practices including interlaboratory comparisons, use of reference

materials, and internal quality control schemes;

o) procedures to be followed for consultation and corrective action whenever discrepancies

in interlaboratory comparisons are detected, or departures from documented policies and

procedures occur;

p) the laboratory management arrangements for exceptionally permitted departures from

documented policies and procedures or from standard specifications;

q) procedures for dealing with complaints;

r) procedures for protecting confidentiality and proprietary rights;

s) procedures for review of calibration program and measurements; and

t) statements of uncertainties of all offered services, including detailed breakdown of all components contributing to each uncertainty.

5.4.3 The laboratory shall arrange for review of its activities at appropriate intervals to verify

6
that its operations continue to comply with the requirements of the quahty system. Such reviews

shall be carried out by trained and qualified NIST staff. Where the review findings cast doubt on

the correctness or validity of the laboratory's calibration results, the laboratory shall take

corrective action and shall notify, in writing, as soon as practically possible, any client whose

work may have been affected.

5.4.4 The quality system adopted to satisfy the requirements of this section shall be reviewed

at least once a year by the management to ensure its continuing suitability and effectiveness and

to introduce any necessary changes or improvements. Any changes in the quality system shall be

approved by NIST oversight staff prior to implementation.

5.4.5 All review findings and any corrective actions that arise from them shall be documented.

The person responsible for quality shall ensure that these actions are discharged within the agreed

time scale.

5.4.6 In addition to periodic reviews, the laboratory shall ensure the quality of results provided

to clients by implementing and documenting checks. These checks shall be reviewed by

management and shall include, as appropriate, but not be limited to:

a) internal quality control schemes using, whenever practical, statistical techniques;

b) participation in interlaboratory comparisons;

c) replicate calibrations using the same or different methods; and

d) re-calibration of retained instruments and dosimeters.

5.5 Personnel

5.5.1 The laboratory shall have sufficient personnel with the necessary education, training,

technical knowledge, and experience to carry out their assigned functions.

5.5.2 The laboratory shall ensure that the training of its personnel is kept up-to-date (see Section

6).

5.5.3 Records on the relevant qualifications, training, skills, and experience of the technical

personnel shall be maintained by the laboratory.

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5 . 6 Facilities and Environment

5.6.1 Laboratory facilities including calibration areas, electrical power sources, lighting, heating,

and ventilation shall be adequate to facilitate proper performance of calibrations.

5.6.2 The environment in which calibrations and related activities are undertaken shall not

invalidate the results or compromise the specified uncertainty of measurement.

5.6.3 The laboratory shall provide facilities for the effective monitoring, control, and recording

of environmental conditions as appropriate. Due attention shall be paid, for example, to dust,

electromagnetic interference, humidity, electrical power stability, temperature, and sound and

vibration levels, as appropriate to the calibrations performed.

5.6.4 The laboratory design shall provide adequate protection between areas where the activities are incompatible.

5.6.5 Access to and use of all areas affecting the quality of calibration and related activities shall

be defined and controlled.

5.6.6 Adequate procedures shall be taken to ensure good housekeeping.

5.6.7 The laboratory shall comply with all relevant health and safety requirements.

5.7 Equipment

5.7.1 The laboratory shall be furnished with all items of equipment required for the correct

performance of calibrations. In those cases where the laboratory needs to use equipment outside

its permanent control it shall ensure that the relevant requirements of this section are met.

5.7.2 All equipment relevant to calibration processes shall be properly maintained. Maintenance

procedures shall be documented. Any item of equipment which has been subjected to overloading or mishandling, or which gives suspect results, or has been shown by verification or during calibration or use to be defective, shall be taken out of service, clearly identified, and

wherever possible stored at a specified place until it has been repaired and shown by calibration,

verification, or test to perform satisfactorily. The laboratory shall examine the effect of this

defect on previous calibrations.

5.7.3 Each item of equipment shall, when appropriate, be labeled, marked, or otherwise

identified to indicate its calibration status.

5.7.4 Records shall be maintained for each item of equipment significant to the calibrations

performed. The records shall include:

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a) the name of the item of equipment;

b) the manufacturer's name, type identification, and serial number or other unique

identification;

c) the date received and the date placed in service;

d) the current location, where appropriate;

e) the condition when received (for example, new, used, reconditioned);

f) a copy of the manufacturer's instructions, where available;

g) the dates and results of calibrations or verifications, or both, and the date of the next

calibration or verification or both;

h) the name and signature of the person who performed the calibrations or verifications, or

both;

I) the details of maintenance carried out to date and planned for the future; and

j) the history of any damage, malfunction, modification, or repair.

5.8 Measurement Traceability and Calibration

5.8.1 All measuring equipment having an effect on the accuracy or validity of calibrations shall

be calibrated or verified, or both, before being put into service. The laboratory shall have an

established program for the calibration and verification of its measuring equipment (3).

5.8.2 The overall program of calibration or verification, or both, and validation of equipment

shall be designed and operated so as to ensure that, wherever applicable, measurements made by

the laboratory are traceable to national or international standards of measurement where

available. Calibration certificates shall, wherever applicable, indicate the traceability to national

standards of measurement, and shall provide the measurement results and associated uncertainty

of measurement or a statement of compliance, or both, with an identified metrological

specification.

5.8.3 Where traceability to national or international standards of measurement is either not

available or not applicable, the laboratory shall provide satisfactory evidence of correlation of

results, for example by participation in a suitable program of interlaboratory comparisons.

5.8.4 Reference standards of measurement held by the laboratory shall be used for calibration

only and for no other purpose, unless it can be demonstrated that use for other purposes will not

9
invalidate their performance as reference standards.

5.8.5 Reference standards of measurement shall be traceable to a national or international

standard of measurement. There shall be a program of calibration and verification for reference

standards.

5.8.6 Where relevant, reference standards and measuring equipment shall be subjected to inservice checks (constancy checks) between calibrations and verifications.

5.9 Calibration Methods

5.9.1 The laboratory shall have documented instructions for using and operating all relevant

equipment, for handling and preparing dosimeters, and for performing calibrations, where the

absence of such instructions could jeopardize the calibrations. All instructions, standards,

manuals, and reference data relevant to the work of the laboratory shall be maintained up-to-date

and be readily available to the staff.

5.9.2 The laboratory shall use appropriate methods and procedures for all calibrations and

related activities within its responsibility (including handling, transport, storage, and preparation

of dosimeters, estimation of uncertainty of measurement, and analysis of calibration data (4,5)).

The methods and procedures shall be consistent with the accuracy required, and with any

standard specifications relevant to the calibrations concerned.

5.9.3 Where methods and procedures are not specified, the laboratory shall, wherever possible, use those that have been published as international or national standards, published by reputable

technical organizations, or published in relevant scientific texts or journals.

5.9.4 Where it is necessary to employ methods and procedures that have not been established as standard, these shall be subject to agreement with the client, be fully validated and documented,

and be available to the client and other recipients of the relevant reports.

5.9.5 Calculations and data transfers shall be subject to appropriate checks.

5.9.6 Where computers or automated equipment are used to capture, process, manipulate,

record, report, store, or retrieve calibration data, the laboratory shall ensure that:

a) computer software is validated and documented where the software provides operational

control or contributes to a quality management decision process;

b) procedures are established and implemented for protecting the integrity of data; such

procedures shall include, but not be limited to, integrity of data entry or capture, data storage,

data transmission, and data processing;

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c) computer and automated equipment is maintained to ensure proper functioning and is provided with the environmental and operating conditions necessary to maintain the integrity

of calibration data; and

d) appropriate procedures for the maintenance of security of data including the prevention

of unauthorized access to, and the unauthorized amendment of, computer records are established and implemented.

5.9.7 Documented procedures shall exist for the purchase, receipt, and storage of consumable

materials used for the technical operations of the laboratory.

5.10. Handling ofClient's Dosimeters

5.10.1 The laboratory shall have a documented system for uniquely identifying the dosimeters, to ensure that there can be no confusion regarding the identity of such dosimeters at any time.

5.10.2 Upon receipt, the condition of the dosimeters, including any abnormalities or departures

from standard condition as prescribed in the relevant calibration method, shall be recorded.

Where there is any doubt as to the dosimeter's suitability for calibration, where the dosimeter

does not conform to the description provided, or where the calibration required is not fully

specified, the laboratory shall consult the client for further instruction before proceeding. The

laboratory shall establish whether the dosimeter has received all necessary preparation, or whether the client requires the laboratory to perform or arrange for such preparations.

5.10.3 The laboratory shall have documented procedures and appropriate facilities to avoid

deterioration or damage to the dosimeters during storage, handling, preparation, and calibration;

any relevant instructions provided with the dosimeter shall be followed. Where dosimeters have

to be stored or conditioned under specific environmental conditions, these conditions shall be

maintained, monitored, and recorded where necessary. Where dosimeters are to be held secure

(for example, for reasons of record, safety, or value, or to enable check calibrations to be

performed later), the laboratory shall have storage and security arrangements that protect the

condition and integrity of the secured dosimeters.

5.10.4 The laboratory shall have documented procedures for the receipt, retention, or safe

disposal of dosimeters, including all provisions necessary to protect the integrity of the

laboratory.

5.11 Records

5.1 1.1 The laboratory shall maintain a record system to suit its particular circumstances and

comply with any applicable regulations. It shall retain on record all original observations,

calculations and derived data, calibration records, and a copy of the calibration certificate or

report, for an appropriate period. The records for each calibration shall contain sufficient

11
information to permit their repetition. The records shall include the identity of personnel

involved in calibration.

5.1 1.2 All records (including those listed in 5.7.4 pertaining to calibration equipment),

certificates, and reports shall be safely stored, held secure, and in confidence to the client.

5.12 Certificates and Reports

5.12.1 The results of each calibration or series of calibrations carried out by the laboratory shall

be reported accurately, clearly, unambiguously, and objectively in accordance with any

instructions in the calibration methods or procedures. The results should normally be reported in a calibration certificate or report, and should include all the information necessar}' for the

interpretation of the calibration results and all information required by the method used.

5.12.2 Each certificate or report shall include at least the following information:

. a) a title, for example, "Calibration Certificate", or "Calibration Report";

b) name and address of the laboratory, and location where the calibration was carried out if

different from the address of the laboratory;

c) unique identification of the certificate or report (such as serial number) and of each page,

and the total number of pages;

d) name and address of client, where appropriate;

e) description and unambiguous identification of the dosimeters calibrated (supplier, type,

and batch number);

f) characterization and condition of the dosimeters;

g) date of receipt of dosimeters and date(s) of performance of calibration, where appropriate;

h) identification of the calibration method used, or unambiguous description of any nonstandard method used;

i) any deviations from, additions to, or exclusions from the calibration method, and any

other information relevant to a specific calibration, such as environmental conditions;

j) measurements, examinations, and derived results, supported by tables, graphs, sketches,

and photographs as appropriate, and any failures identified;

k) a statement of the estimated overall uncertainty of the calibration result (where relevant);

12
1) a signature and title, or an equivalent identification of the person(s) accepting

responsibility for the content of the certificate or report (however produced), and date of

issue;

m) where relevant, a statement to the effect that the results relate only to the dosimeters

calibrated; and

n) a statement that the certificate or report shall not be reproduced except in full, without the

written approval of the laboratory.

5.12.3 Where the certificate or report contains results of calibrations performed by subcontractors, these results shall be clearly identified.

5.12.4 Particular care and attention shall be paid to the arrangement of the certificate or report,

especially with regard to presentation of the calibration data and ease of assimilation by the

reader. The format shall be carefully and specifically designed for each type of calibration carried

out, but the headings shall be standardized as far as possible.

5.12.5 Material amendments to a calibration certificate or report after issue shall be made only in

the form of a further document, or data transfer including the statement "Supplement to Calibration Certificate (or Calibration Report), serial number... (or as otherwise identified)", or equivalent form of wording. Such amendments shall meet all the relevant requirements of 5.12.

5.12.6 The laboratory shall notify clients immediately, orally and in writing, of any event such as the identification of defective measuring equipment that casts doubt on the validity of results

given in any calibration certificate or report, or amendment to a report or certificate.

5.12.7 The laboratory shall ensure that, where clients require transmission of calibration results

by telephone, telex, facsimile, or other electronic or electromagnetic means, staff will follow

documented procedures that ensure the requirements of this Section are met and that

confidentiality is preserved.

5.13 Sub-contracting ofCalibration

5.13.1 Where a laboratory sub-contracts any part of the calibration, this work shall be placed

with a laboratory complying with these requirements. The laboratory shall ensure and be able to demonstrate that its sub-contractor is competent to perform the activities in question and

complies with the same criteria of competence as the laboratory in respect to the work being

subcontracted. The laboratory shall advise the client in writing of its intention to sub-contract any

portion of the calibration to another party.

5.13.2 The laboratory shall record and retain details of its investigation of the competence and

compliance of its subcontractors and maintain a register of all sub-contracting.

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5.14 Outside Support Services and Supplies

5. 14. 1 Where the laboratory procures outside services and supplies other than those referred to

in this section, in support of calibrations, the laboratory shall use only those outside support

services and supplies that are of adequate quality to sustain confidence in the laboratory's

calibrations.

5.14.2 Where no independent assurance of the quality of outside support services or supplies is available, the laboratory shall have procedures to ensure that purchased equipment, materials,

and services comply with specified requirements. The laboratory should, wherever possible, ensure that purchased equipment and consumable materials are not used until they have been

inspected, calibrated, or otherwise verified as complying with any standard specifications

relevant to the calibrations concerned.

5.14.3 The laboratory shall maintain records of all suppliers from whom it obtains support

services or supplies required for calibrations.

5.15 Complaints

5.15.1 The laboratory shall have documented policy and procedures for the resolution of

complaints received from clients or other parties about the laboratory's activities. A record shall

be maintained of all complaints and of the actions taken by the laboratory.

5.15.2 When a complaint, or any other circumstance, raises doubt concerning the laboratory's

compliance with the laboratory's policies or procedures, or with the requirements of this section or otherwise concerning the quality of the laboratory's calibrations, the laboratory shall ensure

that those areas of activity and responsibility involved are promptly reviewed in accordance with

5.4.3.

6. Specific Criteria for Ionizing Radiation

6. 1 This section sets specific requirements to which a laboratory shall adhere if it is to perform

calibrations using ionizing radiation. This section amplifies and interprets the general

requirements set forth in Section 5 (see 4.4 and 4.5).

6.2 This section may also be used as a guide by ionizing radiation calibration laboratories in the

development and implementation of their quality systems.

6.3 Organization and Management: The relationship among the various technical groups, the

quality manager, deputy quality manager, and the calibration services of the Ionizing Radiation

Division are shown in Fig. 1 . This chart includes all calibration activities within the Division, not

just those relating to high-dose level dosimetry. However, only those subjects relating to highdose calibration activities are specifically addressed in this document.

14
Ionizing Radiation

Divison Cliief

Radiation

Interactions &

Dosimetry

Quality

Manager

Neutron

Interactions &

Dosimetry

Deputy Quality

Manager

Radioactivity

High-Dose

Dosimetry

X-ray &

Gamma-ray

Instruments

Beta-particle DosimetryNeutron

Instruments

Neutron

Sources

Radioactivity Sources

Standard

Reference

Materials

Figure 1 - Organizational Chart for Ionizing Radiation Division Calibration Services

6.4 Quality System and Review ofActivities

6.4.1 The laboratory shall perform interlaboratory comparisons at least annually for those

types of services offered. If the test results indicate that corrective action is required, the

laboratory shall take action to achieve the overall uncertainty stated in the appropriate section of

this document.

6.4.2 The interlaboratory comparisons shall be performed with a nationally or internationally

recognized standards laboratory.

6.4.3 If necessary, the interval between interlaboratory comparisons may be increased. Under

no circumstances shall more than 15 months pass between such comparisons.

6.4.4 The quality manual and related quality documentation shall contain:

15
a) a statement of the scope of the calibration services to be performed by the laboratory,

including the radiation types, energies, and dose rates;

b) documentation of the model and serial numbers of each critical piece of equipment used

in a particular calibration;

c) a fully documented generic (or representative specific) procedure for each type of

calibration performed (for example, passive radiochromic dosimeters calibrated with gamma

radiation). The procedure shall provide the appropriate operational steps to permit a knowledgeable person to reproduce a particular calibration technique with a precision

consistent with the specified uncertainty of the laboratory. Each calibration procedure shall

give the following information where relevant:

• concise but complete account of the procedure;

• range and limitations of the procedure;

• equipment and standards to be used;

• environmental constraints to be met in addition to those in 6.6;

• sequence of the procedure, drawing attention to special precautions;

• an example of a completed data sheet; and

• an example of a calibration report or certificate.

d) a tabulated assessment of the various components of uncertainty and their associated

confidence levels, the method used in combining the components, and the calculated overall

uncertainty associated with determination of the reference field for each generic calibration;

and

e) the procedure or reference for auditing calibration data and approving reports.

6.4.5 Each page of the quality manual shall indicate its date of initiation or revision.

6.4.6 A copy of the latest revision of the laboratory's quality manual shall be kept on file and be

readily available to the staff.

6.5 Personnel

6.5.1 The technical manager (however named) shall understand the quality system and

calibration procedures, and ensure that they are being followed.

16
6.5.2 The technical manager shall have a minimum of a bachelor's degree and should have at least three (3) years of experience in radiological physics or a closely related scientific field.

6.5.3 The supervisor of the calibration laboratory shall have at least three (3) years of

experience in radiological physics or a closely related scientific field.

6.5.4 NIST oversight staff may consider an alternative organizational structure utilizing

existing expertise, provided a clear line of responsibility exists and the organization clearly

provides for continuing control.

6.6 Facilities and Environment

6.6.1 Suitable storage facilities shall be provided for reference standards, equipment,

documented instructions, manuals, and calibration certificates and reports.

6.6.2 Environmental monitoring equipment shall be provided for recording temperature and

relative humidity within the laboratory.

6.6.3 Although strict temperature control is not essential, it is desirable that the laboratory be

kept at a reasonably uniform temperature so that the accuracy of equipment is not adversely

affected, and so that an adequate stability is achieved before the start of calibration

measurements. It is recommended that the laboratory temperature be maintained within the

range of20°C to 24°C.

6.6.4 It is recommended that the relative humidity should be maintained within the range of 1

to 65 percent for laboratory operation unless the calibration of specific dosimeter types require a

different range.

6.6.7 A closely controlled environment is not normally necessary in a storage area, but wide

temperature and humidity fluctuations should be avoided so as to protect instruments and

standards temporarily held there, and to minimize the time required for an instrument to reach

equilibrium when brought to the calibration laboratory fi-om the storage area.

6.6.8 The electrical power shall be appropriate to the equipment used, suitably stable, and free

of switching surges and significant line noise. When necessary, local auxiliary voltage stabilizers

and filters shall be provided. For critical control and data acquisition equipment, battery backup

uninterruptable power supplies may be required.

6.6.9 The laboratory shall be provided with an adequate grounding system. Where there is a

possibility of interference arising from equipment connected to a single grounding system,

separate grounding systems shall be provided and adequate precautions taken against any

possible interconnection between systems.

17
6.6.10 The radiation room shall be of sufficient size and design such that scattered radiation at the positions where dosimeters or instruments are normally placed for calibration does not

compromise the specified accuracy goals. If necessary, appropriate scatter corrections shall be

applied. In uncollimated free-air calibration facilities, the radiation room should be used

exclusively for calibrations to avoid variable scatter conditions. The contribution to absorbed

dose by scattered radiation should be known.

6.6.1 1 If compressed air is used, a pressure regulator and means for removing moisture, dust,

and oil from the compressed air shall be provided.

6.7 Equipment - The laboratory shall have reference standards and transfer standards that cover the range of calibrations performed.

6.8 Measurement Traceability and Calibration

6.8. 1 The reference standards used by the laboratory shall be traceable to national or

international standards.

6.8.2 The use of a working standard instead of a reference standard is acceptable for

calibrations.

6.8.3 The standards or equipment originally calibrated by comparison with a higher-level

standard shall be recalibrated when the need is demonstrated by the results of interlaboratory

comparisons or routine quality control.

6.9 Calibration Methods - All new or amended calibration procedures that could have

significant impact on the accuracy of a calibration shall receive approval from the NIST

oversight staff before being adopted for routine use.

6. 1 0 Handling ofDosimeters and Associated Equipment - All handling, unpacking, and

packing of dosimeters, instruments, and reference standards, shall be done by trained staff who

are familiar with the equipment.

6.11 Records

6.1 1.1 The laboratory's permanent records shall include:

a) the date, client, description of the dosimeters calibrated, the batch number or serial

number, details of the service provided, calibration report or certificate number, and invoice

or other accounting number; •

b) documentation of routine quality control actions and any resultant control charts;

18
c) the results of all interlaboratory comparisons.

6.12 Certificates and Reports

6.12.1 Calibration certificates or reports shall include an appropriate statement clearly specifying

the conditions under which the calibrations or measurements were performed. These conditions

could include the type of radiation (gamma ray, x ray, or electron beam), the dose rate(s),

temperature, and for electron beams, the electron energy, pulse width, dose rate within the pulse,

and repetition rate.

6.12.2 Certificates or reports should state that application of the calibration results to an

individual measurement is the responsibility of the user, and that care must be exercised in

interpolation of the calibration results.

6.12.3 The laboratory shall indicate whether the calibration was performed using either an

accredited or non-accredited procedure. The use of non-accredited procedures shall be justified

and those procedures completely explained and documented.

6.12.4 If the calibration laboratory discovers a mistake in a calibration report, the person or

institution that received the report shall be immediately notified. The mistake shall be corrected

as soon as possible, either by sending a corrected report to the client or by recalibrating a new

group of dosimeters, as applicable. The laboratory shall determine the reason for the mistake and

take action necessary to prevent recurrences.

6.13 Sub-contracting ofCalibration - There are no additional requirements beyond the general

requirements set forth in Section 5.

6. 14 Outside Support Services and Supplies - There are no additional requirements beyond the

general requirements set forth in Section 5.

6.15 Complaints

6.15.1 The policy of the Ionizing Radiation Division on complaints is:

All complaints receivedfrom customers or other parties on calibration services are important and every effort shall be made to resolve the complaints in a prompt and

professional manner. A record shall be maintained ofall complaints and ofall

actions taken by the Division to resolve them.

6.15.2 A customer complaint is an expression of dissatisfaction with a requested calibration

service. Complaints can be broken, damaged, or missing dosimeters or equipment, excessive

delay from the estimated completion date without a reasonable explanation, or missing data in

the report. A complaint can be caused by the action (or inaction) of Division persormel or

19
associated equipment involved with the caHbration services.

NOTE 3 - A complaint is not that an estimated completion date does not meet the customer's

schedule. The Division will make every effort to help a customer, but the Division's service

schedule can not be dictated by a customer's needs. Also, complaints are not that the

calibration fees are too costly, that the type of calibration services offered do not meet the

customer's requirements, or that the reported uncertainties are too large.

6.15.3 A discrepancy is a customer expression of disagreement or divergence (an

inconsistency) in measurement results from a calibration service. This is different than a complaint but should be handled in essentially the same manner, since this could indicate a problem with the calibration service's data handling or quality control.

NOTE 4 - Many times discrepancies are the result of the customer's inappropriate use of the

calibrated dosimeters or misunderstanding how to apply the calibration data. Division

personnel should determine if the calibration service is operating properly or if the customer

is having a problem at their end. The latter situation is often impossible to determine

definitively, but a reasonable effort should be made as resources allow (i.e., Division

personnel can not do the measurements for the customer unless they are willing to pay for it).

6. 1 5.4 Complaint Process Procedures

6. 1 5.4. 1 Any Division personnel receiving a complaint about calibration services shall follow

the procedures detailed in this section.

6.15.4.2 The complaint (whether received by phone, fax, mail, or in person) shall be entered in a Complaint Log. This Log shall contain the customer's (company) name and address, name of

the point-of-contact person, and telephone number, date received, date closed and the Group

Leader for that service. The complaints shall be numbered in chronological order.

6.15.4.3 A Complaint Response Form shall be filled out for each complaint showing the

person receiving the complaint, date received, calibration service complaint number (fi-om the

Complaint Log), the Group Leader for that service, the customer information from the Complaint

Log, a description of the complaint or a copy of a written complaint attached to the Response

Form, a description of the action taken to resolve the complaint, and the date of closure. Upon

closure, the Response Form shall be initialed by the Group Leader, the Technical Manager, and

the Quality Manager.

6. 1 5.4.4 Any and all immediate and reasonable action shall be taken to resolve the complaint.

Each action shall be recorded and dated in the Response From.

6. 1 5.4.5 If the complaint is received by a person other than the Group Leader, then the

complaint shall be forwarded to the Group Leader who is responsible for resolving the complain.

6. 1 5.4.6 Copies of the Complaint Response Form shall be distributed to Technical Manager,

20
the Quality Manager, and all Division personnel involved.

6.15.4.7 In most cases the complaint probably can not be resolved by the person receiving the

complaint. In those cases the Group Leader shall be responsible for resolving the complaint. If

necessary, the Group Leader and all Division personnel involved shall meet to decide on a plan

of action to resolve the complaint. That plan and the date shall be noted on the Complaint

Response Form and a copy of the plan attached,

6.15.4.8 The Quality Manager or Deputy shall follow up on all complaints to see that they are resolved in a timely maimer. When the complaint is resolved, the date shall be entered on the

Complaint Response Form and the Group Leader, Technical Manager, and Quality Manager shall

initial the Form. The date shall also be entered in the Compliant Log. A copy of the closed

(resolved) Complaint Response From shall be filed with the Complaint Log.

7. Specific Criteria for Calibrations using Photons and Electrons

7. 1 This section sets specific requirements to which the laboratory shall adhere if it is to provide calibrations using gamma rays, electron beams, or x-ray (bremsstrahlung) beams. This

section amplifies and interprets the general requirements set forth in Section 5, and expands upon

the specific requirements contained in Section 6 (see 4.4 and 4.5).

7.2 The criteria contained in this section apply to calibration of dosimeters at absorbed-dose

levels appropriate for radiation processing.

7.3 Organization and Management - There are no additional requirements beyond those set forth in Sections 5 and 6.

7.4 Quality System and Review ofActivities - There are no additional requirements beyond

those set forth in Sections 5 and 6.

7.5 Personnel - There are no additional requirements beyond those set forth in Sections 5 and 6.

7. 6 Facilities and Environment

7.6. 1 If interpretation of the response of a particular type of dosimeter requires a history of the

environmental conditions, the temperature and humidity shall be recorded.

7.6.2 Fluorescence lamps, sunlight, and other sources of ultraviolet light shall be filtered if the

dosimeters are adversely affected by ultraviolet radiation.

7.6.3 Any area used for storage of dosimeters shall have its temperature and relative humidity

controlled as required for the specific dosimetry system employed.

21
7.7 Equipment

1.1. \ Radiation Source(s)

1.1. 1 . 1 The laboratory shall have access to a source of gamma radiation, either ^°Co or '"Cs,

with a fluence rate sufficient to deliver an absorbed dose within the range of 10 to 10^ Gy (10^ to 10' rad) within a reasonable time.

7.7.1 .2 In addition, the laboratory may have an electron beam or x-ray beam (bremsstrahlung)

radiation source, or both, that can provide dose rates appropriate to radiation processing

conditions.

7.7.2 Characterization ofthe Radiation Field

7.7.2.1 Determine the absorbed-dose rate in each location in which dosimeters are irradiated

using reference standard dosimetry systems. Ensure that dosimeters are irradiated in the locations

where the dose rate is determined. At the time a given radiation facility is first calibrated and at intervals not to exceed three years thereafter, demonstrate that the dose rates are traceable to appropriate national standards by direct measurement intercomparisons.

7.7.2.2 Ensure that the absorbed-dose rate over the volume in which dosimeters are irradiated

does not vary more than ± 1% at a 95% confidence level from its average value.

7.7.2.3 If the dosimeters are irradiated in open air (for example, with a beam-port or panoramic

irradiator), ensure that the room is of sufficient size and design such that scattered radiation at each position where dosimeters are placed for irradiation does not compromise the specified

overall accuracy goals.

7.7.2.4 Monitor and control the temperature of the irradiation volume during irradiation to the

degree required by the characteristics of the dosimeter. Measure this temperature during a simulated irradiation of dosimeters or in a manner that will not perturb the radiation field during

the irradiation of dosimeters.

7.7.2.5 Maintain information related to the photon or electron energy spectrum at each dosimeter

irradiation location.

7.7.2.6 Minimize low-energy components of the photon source spectrum through the use of a

filter box when dosimeters used for radiation hardness testing are irradiated. (For additional

information see Practice E 1249 and Method E 1250.)

22
7.7.3 Descriptions ofNIST Irradiation Facilities

IRAWCR CAP

tUNCSTCN ALLOY

mONT HAHCPLATC

THUMI SCREW

nxCD SHUTTCR BLOCK

TUNGSTEN ALLOY

SIttL SHELL

SOURCE DRAWCn

SHIELDING SHUTTER BLOCK

FULLY OPKN SIlUTTEIt POSITION

LIGHT STSTCM

7.7.3.1 Vertical Beam Co Source - This source consists of a single capsule containing about 7

kilocuries (kCi), housed in a shielding head with a shutter

and collimator assembly, as shown in Figure 2. The

dosimeters to be irradiated are placed at a measured distance

from the source, the shutter

assembly is opened, and the

gamma rays emerge through

the collimator unit into the

shielded room. The irradiation

time is controlled by an

automatic preset timer on the

shutter assembly. The absorbed

dose rate available from this

unit varies as the inverse square

of the distance from the source. At a distance of 146 cm (105 cm scale distance), for December 1995, the absorbed dose rate in

water was 0.43 Gy/min. This absorbed dose rate has been determined with an NIST water

calorimeter (6). The estimated overall uncertainty in this rate is about ± 0.8% at a 95%

confidence level. See the companion document SP250-45 Manual ofCalibration Procedures for

detailed discussion of uncertainties in absorbed dose values for dosimeters irradiated with this

unit. This source is located in Room B036 of Building 245. There is a similar source with a much

lower dose rate located in Room B034; it is generally used only for low dose applications and has

not been calibrated with the NIST water calorimeter.

Figure 2 - Vertical Beam Co Source

23
Figure 3 - Pool ^°Co Source

7.7.3.2 Pool ^°Co Source - The pool-type source consists of a stationary annular array of twelve

source rods of about 1 kilocurie (kCi) total activity. It is situated at the bottom of a twelve-foot

deep tank filled with water for shielding and personnel protection. The source array is shown in

Figure 3. Irradiation of dosimeters is done manually by inserting into the source array a

watertight stainless-steel can with an inside diameter of 8 cm. The volume of uniform absorbed

dose rate is about 4 cm in diameter by 4 cm high, centered at 6.5 cm above the inside bottom of

the can on the central axis. The absorbed dose rate of this source has been previously determined

by an adiabatic graphite spherical calorimeter and a graphite ionization chamber of identical

dimensions to an overall uncertainty estimated to be about ±0.4% at a 99% confidence level (7).

Periodic verification checks of the absorbed dose rate is done annually by means of reference

transfer dosimeters in collaboration with the National Physical Laboratory (NPL) of the U.K. The

absorbed dose rate in water in December 1995 was 22 Gy/min. A complete discussion is given in

reference (7) of the sources of the uncertainties in the absorbed dose rate of this ^°Co facility. The

effect of these uncertainties on absorbed dose values assigned to irradiations performed in this

facility is discussed in the SP250-45 Manual ofCalibration Procedures..

1.133 Gammacell 220 ^"Co Irradiators - Two Gammacell 220 irradiators ^ are employed in the

irradiation of dosimeters and other materials.

^The mention of commercial products throughout this document does not imply

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

does it imply that the products identified are necessarily the best available for the purpose.

24
ACCESS TUBE

3.5 cm 11.375 inl Dm

MICHOSWITCH

(Top Shielding Plugl

COLLAR &SAMPLE CHAMBER INTERLOCKS -

3 MICROSWITCHES

SAMPLE CHAMBER DOOR -

TOP SHIELDING PLUG

- SAFETY COLUMN

. LEAD SHIELDING COLLAR

(Opened (or loadingi MICROSWITCH

(Door Inlerlockl

SAMPLE CHAMBER

SHIELDING PLUG -

RADIOACTIVE SOURCE "

RADIATION SHIELD

DRAWER. UP " (Loading Positrc

2 MICROSWITCHES

(Drawer Down)

These units, illustrated in Figure 4, are manufactured by Nordion, Intl. of Canada. Each unit

contains 24 ^°Co source rods in a stationary annular array, one with a total activity of about 1

kCi (serial number GC-232) and the other with about 5 kCi (serial number GC-45). The source

rods are located inside a massive lead-steel shielding structure that provides personnel protection.

A motorized "drawer"

assembly, consisting of an open irradiation chamber

and shielding plugs above

and below, can be moved up

and down through the

cylindrical source array. Dosimeters axe placed in the

irradiation chamber (with

the drawer up), then the

drawer is lowered so that

the chamber is positioned

reproducibly in the

geometrical center of the source array. Control of the

drawer is by means of a

preset timer so that

irradiations may be

performed automatically.

The irradiation chamber has

inside dimensions of 1 5 cm

diameter by 20 cm high.

The absorbed-dose rate in

water in December 1 995

was 1 99 Gy/min for GC-232

and 62 Gy/min for GC-45. These rates were determined by means of alairine reference transfer

dosimeters referenced to the dose rate of the Vertical Beam source described in 7.7.3.1. These

rates, in turn, are verified on an annual basis by means of interlaboratory comparisons using

transfer dosimeters from the National Physical Laboratory (NPL) in the U.K. and with NIST

transfer dosimeters. The estimated overall uncertainty in this rate is about ± 2% at a 95%

confidence level. Discussion of uncertainties associated with absorbed dose values for

dosimeters irradiated with these units is given in the SP250-45 Manual ofCalibration

Procedures. Significant radiation heating of dosimeters can occur during irradiation in GC-232,

so an active cooling system consisting of a flow of chilled dry compressed air is provided to maintain the sample temperature at 23 °C.

BACK-UP MICROSWITCHES

(For Orawer-up & Drawer-downl

Figure 4 - Gammacell 220 ^°Co Irradiator

25
7.8 Measurement Traceability and Calibration

7.8.1 The Vertical Beam 60,

ALANINE

RADICAL

CATION

RADICAL

ANION

no measurable

paramagnetic species

NH3 +

CO,

long-lived paramagnetic

species

Figure 5 - Production Mechanism of Paramagnetic Species

Co source described in 7.7.3.1 is the reference source to which all the other source

facilities are compared.

The comparisons are performed using

reference transfer

standard dosimeters, in

this case, NIST

electron paramagnetic

resonance (EPR)

dosimetry with alanine

as the sensor. Alanine, an amino acid of

composition

CH3CH(NH2)C02H,

has been identified (8) as an excellent material

for use as a radiation

dosimeter. Upon absorption of ionizing radiation by dry crystalline alanine, deamination of the

radical anion (Fig. 5) produces a free-radical center that is exceptionally stable (9). The attributes

of the alanine system have been summarized elsewhere (see ASTM Practice E 1607).

7.8.2 The alanine dosimeters used in this work were 90 percent L-a-alanine (Aldrich, 99.9 %)

by weight and prepared as described below. The alanine was ground in a centrifugal mill

(Brinkmann) fixed with a 0.5 mm ring sieve. The ground crystals of alanine were transferred to a

vibrating siever (Brinkmann) from which a particle size distribution in the range 53 \x.m to 125 |am was selected. Appropriate weights of sieved alanine and polyethylene (Polysciences,

mw = 700, 60 |Lim) were blended in a powder mixer (Paterson-Kelly). The mixture was loaded

into a Manesty hand-tableting press to produce alanine dosimeters 4.9 mm in diameter and

approximately 2.7 mm thick. The dosimeters were then placed in a 130 °C oven for 30 min,

followed by 5 min in an 85 °C oven. They were then stored in the dark at ambient temperature

under controlled humidity conditions (45 ± 5 % r.h.).

7.8.3 The NIST EPR dosimetry facility consists of a Bruker ESP 300E and an ECS 1 06

electron paramagnetic resonance spectrometers. The alanine dosimeters were measured in a Bruker TMH microwave resonator. A specially designed clear-fused quartz holder (Wilmad

Glass) held the dosimeter in the center of the resonator reproducibly. The spectrometer settings were: microwave frequency, 9.8 GHz; center magnetic field strength, 0.3485 T; magnetic field

sweep width, 8.0 mT; microwave power, 10 mW; modulation frequency, 100 kHz; modulation

amplitude, 1.0 mT; conversion time, 41 ms; sweep-time, 41 s; time constant, 655 ms. The EPR

spectrum for an alanine dosimeter irradiated with gamma radiation to 100 Gy is shown in Fig. 6.

26
7.8.4 The EPR signal intensity of each alanine pellet was measured in two orientations, one at an initial orientation (0°) and then

with the pellet rotated 90°. As can be

seen from Fig. 6, the signal is the

vertical distance between the

minimum and maximum of the trace. A normalized signal, Sj^, is obtained

by taking the average of the signals at the two orientations and dividing by

the mass of the pellet. The mass of

each pellet is measured with a Denver Instrument Company model

1 60 electronic balance to the nearest

0.1 mg before measuring its EPR

signal. The masses of the pellets

average about 55 mg. The EPR

system sensitivity is checked at the

begirming and end of each measurement session by means of a stable paramagnetic species (a

pitch sample). The response of alanine has a small temperature dependence during irradiation of

about 0.18%/°C. The mass normalized signal, Sj^, from above is normalized to a reference

irradiation temperature by means of the following equation:

334.0 344.0 Magnetic Field (mT)

Figure 6 - Electron Paramagnetic Resonance

Spectrum of Alanine

+ [S^ (0.0018 (T^,-T,j,^))] (1)

where S^t ^ signal normalized for mass and irradiation temperature;

Tref ^ reference temperature (usually IS^C); and

TiRR = irradiation temperature.

7.8.5 Alanine dosimeters were used to measure ratios of the dose rates of each ^°Co facility,

i.e., the Pool source and the two Gammacells, in comparison to the vertical beam source. This

was accomplished by irradiating several alanine pellets in a polystyrene holder to the same

absorbed dose in each facility and calculating the ratios of responses.

7.8.6 The dose rate of the reference vertical beam source was measured with an NIST water

calorimeter at a source-to-detector distance of 100 cm at a depth of 5 cm in a water phantom (6).

To transfer those measurements to alanine dosimeters, it was decided that a polystyrene phantom

was more practical than one of water. In order to determine the equivalent distance and thickness

in polystyrene that corresponds to those for the water phantom, a scaling theorem was employed

(10). This theorem has the following assumptions:

• Radiation interacts with the media only by pure Compton scattering;

27
Two configurations, each consisting of regions of medium surrounded by vacuum:

System 1, medium has electron density of:

^1 = Pi<Z/A>iN^ (2)

System 2, medium has electron density of:

62 = P2<Z/A>2N^ (3)

where p = medium density;

<Z/A> = ratio of the number of electrons in a compound to the molecular weight;

and= Avogadro constant.

All distances and dimensions are scaled according to

X2 = ^x, (4)

where the quantity €1/62 is the scaling factor.

As a result of using the scaling theorem, the fluence at points P, and appropriately scaled

positions has identical energy and angular distributions, differing only by a constant given by the

square of the scaling factor, i.e..

/ \2

6.

cl>i (5)

The relationship of the various elements of the vertical beam irradiation geometry is shown in

Fig. 7. The calculation of x and z distances are shown in Table 1 .With the collimator opening s held constant, the field size f scales automatically.

28
SOURCE

Monte Carlo calculations were performed to verify the

validity of the scaling theorem. The measured incident

fluence rate spectra from the ^°Co source, averaged

over corresponding field sizes, were calculated for the

corresponding detector planes, i.e., at depths of 5 cm in

water, and 4.87 cm in polystyrene. Both fluence rate spectra were integrated over the mass energyabsorption coefficient for water to obtain the dose rate for a point water detector. For a point source, the

square of the scaling factor corresponds exactly to the

1/R^ correction factor. Comparisons of the dose rates from the two methods indicate agreement within 0.1%.

Independent experimental test measurements (10)

indicate agreement with the scaling theorem within

0.5% or better.

Figure 7 - Vertical ''"Co Beam Geometry

for Polystyrene Phantom

Table 1 Medium p (g/cm^) <Z/A> e/N, X (cm) z (cm)

water 1.00 0.555087 0.555087 5.00 100.00

polystyrene 1.06 0.537680 0.569941 4.87 97.39

7.8.7 Take the normalized EPR signal, S^x, from 7.8.4, and divide by the irradiation time, tj^, to get a response proportional to dose rate:

R = ^

(6)

'^IRR

29
For an irradiation in the Vertical Beam source in a polystyrene phantom, the scaling theorem

gives:

For the Gammacells or Pool sources, the alanine is located inside a polystyrene cylinder with a

wall thickness of 3.7 mm. It has been experimentally determined that this wall thickness causes an attenuation of 0.980 to the incident gamma radiation, requiring a correction to the dose rate given by:

Rq = (8)

° 0.980 ^

^

The ratio of the response, R, for a particular gamma source to another is equal to the ratio of the

dose rates for those gamma sources:

R, Dj

R

= — (9)

For example, the ratio of responses of the Pool source, Ro^°°^, to that of the Vertical Beam

source, Rq^, is given by:

^„ POOL pool= ^

(10)

R„TM D

TM

Thus,

p POOL

POOL tS ^0 _

Do = Do • (11)

Experimentally determined ratios of dose rates for the Pool source and Gammacells, with serial

numbers GC-45 and GC-232, relative to the Vertical Beam source are shown in Table 2. The

chain of traceability is shown in Fig. 8.

30
Table 2

^POOL ^GC-45 ^GC-232

26.85 75.18 248.6

PRIMARY STANDARD

(Domen water calorimeter)

VERTICAL BEAM SOURCE

HIGH-DOSE

IRRADIATORS

ALANINE

ATA

TRANSFER

DOSIMETRY

GC'

45 .

DOSE RATE AT CENTER POSITION ^poa. ^GC-45 ^GC-232

FILM '^POOL ^QC-232

PERSPEX

iPRJSPBf fJ>€RSPiX

'-'gC-232 AMPOULE J^AMPOUt£

'-'GC-45 lYMKMf

'-'sC-232

Figure 8 - Traceability Chain for Vertical Beam

Source to High-Dose Rate Irradiators

Calculation of the dose rate for a given geometrical configuration, e.g., radiochromic film

dosimeters in the Pool source, is given by:

p POOL p. POOL

^FILM ^ ^FILM

r, POOL POOL ^

^ ^ ^0

and,

d;--^ = d;--^.-^ (13)

p POOL

POOL A POOL ^FILM

r> POOL

31
Now, using equation (11), gives:

D

POOL

FILM D

VB

POOL RPOOL

FILM POOL (14)

and,

D

POOL

FILM D

VB R

POOL

FILM VB (15)

Equivalent equations have been developed for other configurations and dosimeter types; these are discussed in detail in the next section.

Polycarbonate base,

80 & 152 mm dia.

6 mm thick

\

A

Single-hole cup

for alanine pellets,

5.1 mm hole dia.,

3.7 mm wall

Support

Rod

sxxxxxxxxxxx\x\xsxvx\xvxx^

7.8.8 The alanine ratio measurements

of the various sources described thus far were for a single geometrical

arrangement, namely, a polystyrene

single-hole cylindrical cup with a hole

diameter slightly larger than the alanine

pellets (5 mm) and a wall thickness of

3.7 mm. This cup was mounted on a

polystyrene rod which was, in turn,

mounted to a 6 mm thick polycarbonate

base disk to locate the alanine pellets in

the geometrical center of the radiation

field of each irradiator (see Fig. 9).

Figure 9 - Single-Hole Geometry for Alanine

Pellets in High-Dose Rate Irradiators

7.8.9 The Pool source and

Gammacells are used to calibrate a

variety of types of dosimeters including

liquid solutions sealed in glass

ampoules such as eerie cerous sulfate

and dichromate, thin radiochromic solid films, and thick solid slabs such as Red Perspex. Each

type of dosimeter requires a unique holder and geometry during irradiation. For the liquid in

ampoules type, a polystyrene holder was used as shown in Fig. 10. This holder is referred to as the "five-hole cup" since the five circumferential holes are normally used to hold the dosimeters

being calibrated and the center hole is used for quality control dosimetry. The hole size was

chosen so that a 2-mL ampoule would fit snugly. To determine the effects of scattering and

attenuation of the holder, alanine pellets were placed in polystyrene cylinders that fit snugly in

the holes in place of the ampoules. Red Perspex dosimeters are normally sealed in aluminum

pouches by the manufacturer and are not opened until after irradiation. The usual method for

32
handling these during caHbration is to fold the pouch tightly around the slab of each dosimeter

and place it into the hole of the five-hole cup normally occupied by an ampoule. To determine

the effects of this geometry, Red Perspex slabs were removed from their pouches, 5 mm holes were drilled in each in three places, and alanine pellets were inserted in these holes. The slabs were reinserted in the pouches, the pouches are refolded and inserted into the holes of the fivehole cup.

Dimensions \

in mm /.

Holes:

13 mm dia.

51.50

I

Support

Rod

Figure 10 - Five-Hole Cup

for Ampoule Irradiation

33
The radiochromic thin film dosimeters are normally irradiated in polystyrene two-piece square

slab holders as shown in Fig. 1 1 . The slab assembly, with several radiochromic films sealed in

the interior cavity, is held vertically during irradiation. To simulate this geometry with alanine

dosimeters, a 3 mm thick polystyrene slab was drilled through in five places as shown in Fig. 12.

After alanine pellets were placed in the five holes, slabs of 3 mm thick polystyrene were

sandwiched on each side of the alanine-loaded slab. The effects of the three main irradiation

configurations on the dose rates in the three calibration sources were determined by the alanine

measurements and compared to the single-hole cup alanine measurements and appropriate

correction factors calculated. The dose rates for all sources and configurations are given in Table

3. The conversion of absorbed dose values in water to dose in silicon was done with the use of

ratios of mass energy absorption coefficients.

Dimensions in mm

25.4

Figure 11 - Holder for Radiochromic Thin Film Irradiation

Dimensions in nun Cavities for alanine pellets

o o

_ 25.4 ^

Figure 12 - Holder for Alanine Pellets to Simulate Thin Film

Holder Geometry

34
Table 3

Dose Rates for ''"Co Sources for December 31, 1996

filename = B140 P5-90 c:\qpw\DR97c.WBl

Effective 3/1/97

Co-60 half-life value is adjusted from 1925.34 days to 1925.12 days

Domen Water Calorimeter Transfer Calibration Source in B036 under 5 cm water at 100 cm source

for 14.5 cm by 14.5 cm field size For 1/11/90 Dose rate = 1.8144 Gy/min

For 12/3 1/96 Elapsed time = 2546 days

Co-60 daily decay constant ( ln2 / 1925.12 ) = 3.60054e-04

Decay correction factor = 0.399837

Dose rate = 0.72546 Gy/min

or 0.012091 Gy/s

Absorbed Dose Rate in Water

P ofin rvallO Liy/s Gy/min Koy/n

3.5 mm Single-hole vial geometry 248.6 3.005 180.3 10.82

5.0 mm Polystyrene Film Block Geometry 0.9870 2.966 178.0 10.68

4.2 mm polystyrene 5-hole cup Perspex Geometry 0.9710 2.918 175.1 10.51

4.2 mm polystyrene 5-hole cup Ampoule Geometry 0.9560 2.873 172.4 10.34 GC232 in Stainless Steel Dewar

3.5 mm Single-hole vial geometry 239.9 2.901 174.0 10.44

5.0 mm Polystyrene Fihn Block Geometry 0.9913 2.875 172.5 10.35

4.2 mm polystyrene 5-hole cup Perspex Geometry 0.9692 2.811 168.7 10.12

4.2 mm polystyrene 5-hole cup Ampoule Geometry 0.9599 2.784 167.1 10.02 GC45

3.5 mm Single-hole vial geometry 75.18 0.9090 54.54 3.272

5.0 mm Polystyrene Film Block Geometry 0.9889 0.8989 53.93 3.236

4.2 mm polystyrene 5-hole cup Perspex Geometry 0.9730 0.8844 53.07 3.184

4.2 mm polystyrene 5-hole cud Amooule Geometry 0.9595 0.8722 52.33 3.140

FlOl Pool Source

3.5 mm Single-hole vial geometry 26.85 0.3246 19.48 1.169

5.0 mm Polystyrene Fikn Block Geometry 0.9941 0.3227 19.36 1.162

4.2 mm polystyrene 5-hole cup Perspex Geometry 0.9883 0.3208 19.25 1.155

4.2 mm polystyrene 5-hole cup Ampoule Geometry 0.9749 0.3164 18.99 1.139

for Gammacell and Pool, Combined Relative Expanded Uncertainty = 1.8%; dose rate (Silicon) = dose rate (Water) * 0.913

elapsed time = 366 days, decay correction factor = 0.876534

B036 Co-60 Vertical Beam Geometry Gy/s Gy/min Gy/h

B036 s.d.=30.0 cm, coll=20 x 20 6 mm ps plate 2.676e-02 1.605e+00 96.32

B036 s.d.=41.6 cm, coll=20 x 20 6 mm ps plate 1.944e-02 1.167e+00 70.00

B036 s.d.=105 cm, coll=20 x 20 6 mm ps plate 6.227e-03 3.736e-01 22.42

B034 Co-60 Vertical Beam

B034 s.d.= 15.0 cm, coll.= 25 x 25 6 mm ps plate 5.306e-03 3.184e-01 19.10 B034 s.d.= 100 cm, coll.= 25 x 25 6 mm ps plate 9.476e-04 5.685e-02 3.411 B034 s.d.=130 cm, coll.= 25 x 25 6 mm ps plate 6.568e-04 3.941e-02 2.364

for vertical beams. Combined Relative Expanded Uncertainty = 2.2%; dose rate (Silicon) = dose rate (Water) * 0.896

35
8, Keywords - Absorbed dose; calibration laboratory; dosimeter; dosimetry system; electron beam; gamma ray; ionizing radiation; radiation processing; reference standard dosimeter.

9, References

1 . Inn, K.G.W., Coursey, B.M., Eisenhower, E.H., Walker, M.D., Humphreys, J.C., Heaton, H.T., Duvall, K.C.,

"The role of the Office of Radiation Measurement in quality assurance," The Science ofthe Total Environment,

130/131, Elsevier Science Publishers B.V., Amsterdam, 1993, pp. 497-507.

2. McLaughlin, W.L., "Reference dosimetry and measurement quality assurance," Rad. Phys. Chem. 40, 1989, pp. 945-951.

3. Humphreys, J.C., Hocken, D., and McLaughlin, W.L., "Dosimetry for High Dose Applications," NBS Special

Publication 250-1 1, Natl. Bur. Stand., Gaithersburg, MD, 1988.

4. McLaughlin, W.L., "Standardization of high-dose measurement of electron and gamma-ray absorbed doses and

dose rates," High-Dose Dosimetry, Proceedings of International Symposium, Vienna, 1984, IAEA STI/PUB/671,

International Atomic Energy Agency, Vienna, 1985, pp. 357-371.

5. McLaughlin, W.L., Boyd, A.W., Chadwick, K.H., McDonald, J.C., and Miller, A., Dosimetry for Radiation

Processing, Taylor and Francis, London, 1989.

6. Domen, S.R., "A Sealed Water Calorimeter for Measuring Absorbed Dose," J. Res. Natl. Inst. Stand. Tech., 99,

1994, pp. 121-141.

7. Petree, B. and Lamperti, P., "A Comparison of Absorbed Dose Determinations in Graphite by Cavity Ionization

Measurements and by Calorimetry," J. Res. Natl. Bur. Stand., 71C, 1967, pp. 19-27.

8. Bradshaw, W.W., Cadena, C.G., Crawford, E.W., and Spetzler, H.A.W., "The Use of Alanine as a Solid

Dosimeter," Radiat. Res. 1 7, 1 962, pp. 1 1 -2 1

9. Sinclair, J.W. and Hanna, W., "Electron Paramagnetic Resonance Study of L-Alanine Irradiated at Low

Temperatures," J. Phys. Chem., 71(1), 1967. pp. 84-88.

10. Pruitt, J. S. and Loevinger, R., "The photon-fluence scaling theorem for Compton-scattered radiation," Med.

Phys., 9(2), 1 982, pp. 1 76- 1 79.

36


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