NIST calibrated 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. Tungsten-anode beam qualities used in international comparisons are listed in Table 9. 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 submitted without an electrometer is calibrated in terms of air kerma or absorbed dose per unit charge, normalized to reference conditions of 295.15 K and 101.325 kPa. 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. 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. 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.

The calibration of well-type ionization chambers, instruments that measure x-rays from electronic brachytherapy sources, is performed in terms of the physical quantity air-kerma in units of Gy. Calibrations are performed by comparing the well-type ionization chamber response to air kerma realized by a NIST primary x-ray standard. The measurement process results in a NIST calibration coefficient for the well chamber for each source in units of the NIST air-kerma rate (Gy/s) at 50 cm per ampere, Gy/(A s) normalized to 295.15 K (22 oC) and 101.325 kPa (1 Atm). Only well-ionization chambers known to be stable and reproducible are accepted for calibration in this program. Facilities submitting well-ionization chambers for calibration are strongly urged to perform stability checks involving redundant measurements in highly reproducible radiation fields before sending their instruments to NIST, and to repeat those checks after NIST calibration, and again at suitable intervals. The appropriate chamber insert must be provided by the customer.

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 %.

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