Ionizing Radiation Measurements

Dosimetry of X Rays, Gamma Rays, and Electrons

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Technical Contacts:
Stephen M. Seltzer
All Services
Tel: 301/975-5552
E-mail: stephen.seltzer@nist.gov

Michael G. Mitch
46010C-47040S
Tel: 301/975-5491
E-mail: michael.mitch@nist.gov

C. Michelle O'Brien
46010C-46050S
Tel: 301/975-2014
E-mail: michelle.obrien@nist.gov

Ronaldo Minniti
46010C-46110C
Tel: 301/975-5586
E-mail: ronaldo.minniti@nist.gov

Heather Chen-Mayer
46110C
Tel: 301/975-5595
E-mail: heather.chen-mayer@nist.gov

Christopher G. Soares
47030C-47040S
Tel: 301/975-5589
E-mail: christopher.soares@nist.gov

Please contact the technical staff before shipping instruments or standards to the address listed below.

Mailing Address:
National Institute of Standards and Technology
100 Bureau Drive, Stop 8460
Gaithersburg, MD 20899-8460
Fax: 301/869-7682

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Special Instructions for Using Electron and Photon Dosimetry Services (46010C-47050S )

The NIST dosimetry calibration and test services for x-rays, gamma-rays, beta particles, and electrons are performed in NIST's laboratories at Gaithersburg, Maryland. Inquiries should be addressed to the appropriate technical contacts listed at the beginning of this section. The inquirer must provide the name and telephone number of an individual who can answer technical questions that may arise in any inquiry, order, or shipment.

Upon receipt of a purchase order, a report number is assigned. Calibrations are generally performed in the sequence established by those numbers, except when greater efficiency can be achieved by combining similar calibrations, or when work for a calibration laboratory is given a higher priority. Arrangements for calibration must be made in advance by letter, fax, e-mail or telephone, so that the instrument or source to be calibrated will not be shipped to NIST until the time of its scheduled calibration approaches. Inquiry should be made as to scheduling and turn-around time.

Except in the event of negligence by its personnel, NIST assumes no responsibility for loss of or damage to the instruments or sources while in its possession. The risk should be covered by insurance.

The report of calibration or test will carry a DG number (e.g., DG 9603/95). Subsequent reference to that calibration or test should cite the DG number.

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X-Ray and Gamma-Ray Measuring Instruments (46010C-46110C)

NIST calibrates x-ray measuring instruments are calibrated in terms of air kerma or exposure by a substitution method in an x-ray beam at a point where the rate has been determined by means of a free-air ionization chamber standard. In order to provide instrument calibrations over a wide range of x-ray beam qualities, many combinations of generating potential and filtration are available. Tungsten (W) anode, x-ray beams with U.S. established beam qualities are listed in Table 6 as lightly (L), moderately (M), and heavily (H) filtered beams. Two beam qualities that do not fit into these categories are considered as special (S) qualities. Cobalt-60 and cesium-137 gamma-ray beams are also listed in Table 6. New W-anode, ISO x-ray beam qualities, listed in Table 7 have been installed. Molybdenum (Mo) and rhodium (Rh) anode x-ray beam qualities, with application to mammography, are listed in Table 8. Beam qualities are identified by beam codes given in the first column. The calibration beam qualities requested should be appropriate to the instrument submitted.

Gamma-ray measuring instruments are calibrated in terms of air kerma or absorbed dose at points in the collimated cobalt-60 and cesium-137 gamma-ray beams that have been standardized by means of graphite cavity chambers or a water (or graphite) calorimeter. Rates at the time of calibration are computed from the original beam standardization data and appropriate decay corrections. Ionization chambers submitted for an air -kerma calibration should have sufficient wall thickness to provide electron equilibrium for the highest energy selected. Ionization chambers submitted for an absorbed-dose calibration must be suitable for calibration in a phantom.

An ionization chamber and electrometer combination, with the electrometer scale in units of air kerma, exposure, or absorbed dose, is calibrated by providing a dimensionless calibration factor for the electrometer scale. An ionization chamber and electrometer combination marked in electrical units is calibrated as follows: (1) the chamber is calibrated in terms of air kerma or absorbed dose per unit charge using an NIST electrometer; (2) the customer's electrometer is checked for linearity and charge measurement accuracy; and (3) the combination of chamber and electrometer is checked for consistency. An ionization chamber submitted without an electrometer is calibrated in terms of air kerma or absorbed dose per unit charge. Calibration can be based on measurements for positive or negative polarizing potential, or on the mean of measurements for both potentials, as requested. The ratio of ionization currents for full and half polarizing potentials and the corresponding ionization current will be stated in the calibration certificate, based on pre-calibration measurements.

Ionization chambers are tested, prior to calibration, for connection to the atmosphere. Chambers found unsuitable for calibration will be returned with a statement of the reason for rejection. A charge may be made for time incurred on the tests.

Each instrument submitted to NIST for dosimetry calibration or test must be uniquely identified, usually by the manufacturer's name, model number, and instrument serial number. When the serial number is lacking, an alternative identifying mark should be provided. If none is found, NIST will mark the piece with an identification number. If the apparatus submitted has been calibrated previously by NIST, the serial number or identifying mark should be given on the new order so that a continuing record of stability can be maintained.

All shipments to NIST of instruments for dosimetry calibration must be in reusable containers. Even if properly packed, there can be no assurance that a calibrated instrument has maintained its calibration during shipment unless a method of verifying instrument stability has been established. Measurement should be made of the instrument response both before and after shipment, using a long-lived radioactive source and a highly reproducible measurement procedure. A long-term record of instrument stability using a suitable constancy check procedure is the most effective method for assuring the validity of the instrument calibration.

Irradiation of passive dosimeters, for readout by the customer, is available for most of the beam qualities listed in Table 6. These irradiations are generally in terms of air kerma; for passive dosimeters suitable for insertion in a phantom, irradiation in terms of absorbed dose can be provided by in-phantom irradiation using cobalt-60 gamma rays.

Calibrations of x-ray and gamma-ray measuring instruments and of passive dosimeters, described above, have a relative expanded uncertainty of 1%.

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Table 6. Tungsten-Anode X-Ray and Gamma-Ray Beam-Quality Parameters

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Table 7. Tungsten-Anode ISO X-Ray Beam-Quality Parameters

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Table 8. Mammography X-Ray Beam-Quality Parameters

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Gamma-Ray Sources, Beta-Particle Sources, and Measuring Instruments (47010C-47040S)

Sources submitted to NIST for dosimetry calibration are subject to the following conditions:

A. Preparation: Sources submitted for calibration must be sealed so that there can be no escape of any radioactive material, including any gaseous decay products. The sources, shielding, and packaging must be free of contamination. Contaminated or leaking sources cannot be measured and may cause considerable loss of time and damage to laboratory facilities. Sources must have been sealed for a sufficient time to be substantially in radioactive equilibrium with their decay products when these contribute to the emitted radiation.

B. Packaging for shipment: Packages must be in compliance with the regulations of the Department of Transportation as specified in DOT 49CFR173.401-173.476. Radionuclides must be packaged as Limited Quantities (DOT 49CFR173.421-173.422) or in Type A packages (DOT 49CFR173.412 and 173.433). Type A packages must bear the appropriate radioactive-hazard labels (DOT 49CFR172.403). If the source is considered by the shipper to be in DOT Special Form, a Special Form certificate must be furnished to NIST in strict compliance with DOT 49CFR173.476. Copies of the codes are available from the Government Printing Office, Washington, DC 20402.

All shipments to NIST of gamma-ray and beta-particle sources should be in reusable containers. A drawing showing the source container and a description of the method of source removal should be provided before the shipment is received at NIST.

If the nature of the shipment requires a Type B container subject to an NRC quality assurance program, documentation should be supplied to NIST certifying that the use of the container by NIST is part of the program of the shipper.

C. Possession of licensed materials: In submitting a source for calibration, it is necessary for the submitter to certify that he is duly authorized to possess the source under license by the applicable authority. In the case of individuals residing in a State that has entered into agreement with the Nuclear Regulatory Commission, State regulations are applicable to all sources. In the case of other individuals, NRC regulations are applicable. This certification may be by letter, by a suitable statement on the purchase order covering the calibration fee, or by a clear copy of the submitter's Possession License for the source.

Calibration in terms of air-kerma strength (air-kerma rate in free space times the square of the distance of the calibration point from the source center along the perpendicular bisector) is provided for gamma-ray sources of cobalt-60, cesium-137, iridium-192, iodine-125, and palladium-103. Calibration in terms of absorbed-dose rate is provided for suitable encapsulated beta-particle sources; the dose rate to a low-atomic-number material (graphite or plastic) is determined by measurement with an extrapolation chamber. The beta-particle sources may be either small-area sources such as ophthalmic applicators, or large-area plaques, and will be calibrated for absorbed dose rate to water either at the source surface or at a specified distance.

Ionization chambers to be calibrated with beta-particle sources must be parallel-plate chambers with thin walls. They can be calibrated with the radionuclides ⁹⁰Sr + ⁹⁰Y, or ²⁰⁴Tl, or ⁸⁵Kr, or ¹⁴⁷Pm.

Measurement services in this series have uncertainties listed in Tables 9 and 10.

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Table 9. Uncertainties for Gamma-Ray Source Calibrations

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Table 10. Uncertainties for Beta-Particle Source and Instrument Calibrations

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References-Dosimetry of X-Rays,Gamma-Rays, and Electrons

X-Ray and Gamma-Ray Measuring Instruments

NIST Measurement Services: Calibration of X-Ray and Gamma-Ray Measuring Instruments , P. J. Lamperti, and M. O'Brien, Natl. Inst. Stand. Technol. Spec. Publ. 250-58 (Apr. 2001).

The photon-fluence scaling theorem for Compton-scattered radiation , J. S. Pruitt and R. Loevinger, Med. Phys. 9, 176 (1982).

The Graphite Calorimeter as a Standard of Absorbed Dose for Cobalt-60 Gamma Radiation , J. S. Pruitt, S. R. Domen, and R. Loevinger, J. Res. Natl. Bur. Stand. (U.S.) 86 (5), 495-502 (1981).

Medical Dosimetry Standards Program of the National Bureau of Standards , R. Loevinger, Proc. Symp. on Natl. and Intl. Standardization in Rad. Dosimetry, Atlanta, GA, Dec. 5-9, 1977, Intl. Atomic Energy Agency, Vienna (1978). (This article provides references for earlier publications on NBS exposure and absorbed-dose standards.)

Uncertainty in the Delivery of Absorbed Dose, R. Loevinger and T. P. Loftus, Ionizing Radiation Metrology, International Course at Varenna, Italy, 1974 (E. Casnati, Ed.) G-6, 459, Editrice Compositori, Bologna (1977).

Exposure Spectra from the NBS Vertical-Beam ⁶⁰Co Gamma-Ray Source , M. Ehrlich and C. G. Soares, Natl. Bur. Stand. (U.S.) NBSIR 76-1117 (1976).

Spectrometry of a ⁶⁰Co Gamma-Ray Beam Used for Instrument Calibration , M. Ehrlich, S. M. Seltzer, M. J. Bielefeld, and J. I. Trombka, Metrologia 12, 169 (1976).

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Gamma-Ray Sources, Beta-Particle Sources, and Measuring Instruments

A Method for the Calibration of Concave ⁹⁰SR + ⁹⁰Y Ophthalmic Applicators , C. G. Soares, Phys. Med. Biol. 37, 1005 (1992).

Calibration of ophthalmic applicators at NIST - A revised approach , C. G. Soares, Med. Phys. 18, 787 (1991).

NBS Measurement Services: Calibration of Gamma-Ray-Emitting Brachytherapy Sources , J. T. Weaver, T. P. Loftus, and R. Loevinger, Natl. Bur. Stand. (U.S.) Spec. Publ. 250-19 (1988).

NBS Measurement Services: Calibration of Beta-Particle Radiation Instrumentation and Sources , J. S. Pruitt, C. G. Soares, and M. Ehrlich, Natl. Bur. Stand. (U.S.), Spec. Publ. 250-21 (Apr. 1988).

NBS Measurement Services: Calibration of Beta-Particle-Emitting Ophthalmic Applicators , J. S. Pruitt, Natl. Bur. Stand. (U.S.), Spec. Publ. 250-9 (July 1987).

Calibration of Beta-Particle Ophthalmic Applicators at the National Bureau of Standards, J. S. Pruitt, J. Res. Natl. Bur. Stand. (U.S.) 91, 165 (1986).

The Effect of Altitude on Beta-Ray Source Calibrations , J. S. Pruitt, Radial. Protec. Dosim. 11, 151 (1984).

Exposure Standardization of Iodine-125 Seeds Used for Brachytherapy, T. P. Loftus, J. Res. Natl. Bur. Stand. (U.S.) 89, 295 (1984).

Standardization of Iridium-192 Gamma-Ray Sources in Terms of Exposure, T. P. Loftus, J. Res. Natl. Bur. Stand. (U.S.) 85, 19 (1980).

Medical Dosimetry Standards Program of the National Bureau of Standards, R. Loevinger, Proc. Symp. on Natl. and Intl. Standardization in Rad. Dosimetry, Atlanta, GA, Dec. 5-9, 1977, Intl. Atomic Energy Agency, Vienna (1978). (This article provides references for earlier publications on NBS exposure and absorbed-dose standards.)

Standardization of Cesium-137 Gamma-Ray Sources in Terms of Exposure Units (Roentgens), T. P. Loftus, J. Res. Natl. Bur. Stand. (U.S.), 74A, 1 (1970).

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