Special Instructions for Using Electron and Photon Dosimetry Services (46010C-48020S)

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. 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 an instrument, 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 for 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.

X-Ray and Gamma-Ray Measuring Instruments (46010C-46050S)

• X-Ray Calibrations

  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 VIII.5 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 available. New W-anode, ISO x-ray beam qualities, listed in Table VIII.6 are being installed; check for availability. Molybdenum (Mo) and rhodium (Rh) anode x-ray beam qualities, with application to mammography, are listed in Table VIII.7. 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 Calibrations

  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 graphite (or water) 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.

• Calibration Coefficient vs. Calibration Factor

  The calibration quantity is defined as the conventional true value of the quantity the instrument is intended to measure, divided by the instrument's reading; this calibration ratio is termed a coefficient if it has dimensions or a factor if it is dimensionless. 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.

• Calibration - Quality Chambers

  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. A method of verifying instrument stability should be established before shipment to NIST. 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.

• Proficiency Tests

  In an effort to meet the needs of calibration facilities which maintain traceability with NIST for air kerma measuring instruments, NIST offers proficiency tests upon request. NIST will conduct a proficiency test as part of an accreditation program or on an individual basis with a calibration facility, such as the biennial mammography proficiency test requirement of the FDA/MQSA. Inquiries should be addressed to the appropriate technical contacts. The inquirer must provide the name and email or telephone number of an individual who can answer technical questions that may arise in any inquiry, order, or shipment. No NIST calibration service ID number is associated with proficiency testing. The cost of proficiency testing is the cost of x-ray and gamma-ray calibrations plus 15 percent.

• Irradiations

  Irradiation of passive dosimeters, for readout by the customer, is available for most of the beam qualities listed in Table VIII.5. 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 %, based on a coverage factor of k = 2 to define an interval having a level of confidence of approximately 95 %.

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

1. 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.

2. 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.

3. 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, and iodine-125. 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 ¹⁴⁷Pm.

   Measurement services in this series have uncertainties listed in Table VIII.8 and Table VIII.9.

Dosimetry of High-Energy Electron Beams (48010M-48020S)

Alanine dosimeters are provided twice a year to users requesting assistance with absorbed-dose measurements in high-energy electron beams. The user irradiates all but one of the three furnished dosimeters to between 50 Gy and 80 Gy (5000 rad and 8000 rad) to water at electron energies between 5 MeV and 50 MeV, employing the irradiation geometry (field size, phantom, position of dosimeter in phantom) given in the "Protocol for Dosimetry of High-Energy Electrons," Phys. Med. Biol. 11, 505 (1966).

After irradiation, the dosimeters are returned to NIST for evaluation terms of absorbed dose in the phantom, using appropriate methods. These dose interpretations ignore certain corrections for the effects of spectral perturbations, and so represent a measurement quality assurance service rather than a calibration service. The measurement procedure with alanine dosimeters is undergoing refinement; for the current procedure, the estimate of the relative expanded uncertainty is 2.3 %, based on a coverage factor of k = 2 to define an interval having a level of confidence of approximately 95 %.

Inquiries or comments: s.seltzer@nist.gov
Online: June 1996   -   Last update: March 2005