Calibrations of Radiation Measuring Instruments in Cs-137 and Co-60 Gamma-ray Beams

The Dosimetry Group maintains and disseminates the national measurement standards for air-kerma (exposure) from ¹³⁷Cs and ⁶⁰Co gamma-ray beams  and for absorbed dose to water from ⁶⁰Co gamma-ray beams. 

The dissemination of the primary standard is performed via calibration of gamma-ray measuring instruments using oneof seven gamma-ray irradiator sources.  These calibration facilities provide a range of air-kerma and absorbed dose to water rates that help to serve a large number of users: two ¹³⁷Cs sources provide air-kerma rates from 4 mGy/h to 90 mGy/h; a third ¹³⁷Cs source provides air-kerma rates of 2 Gy/h and 3 Gy/h; two ⁶⁰Co sources provide air-kerma rates from 0.1 mGy/h to 1 mGy/h; and  two ⁶⁰Co tele-therapy-level sources provide absorbed dose to water and air-kerma rates of  26 Gy/h and 6 Gy/h, respectively.  The NIST standards are disseminated routinely through a number of U.S. secondary standard instrument calibration laboratories. These include the three Accredited Dosimetry Calibration Laboratories (ADCLs) that are accredited by the American Association of Physicists in Medicine (AAPM), military secondary calibration facilities, the U.S. DOE, state laboratories, instrument manufacturers, nuclear power plants and U.S. customs.

Proficiency tests of the AAPM ADCLs for the measurement of air-kerma and absorbed dose to water from ⁶⁰Co gamma-ray beams were conducted during the first part of 2009. An A12 chamber was used. Excellent agreement, within less than 0.3 % for the air kerma tests and 0.2 % for the absorbed dose to water tests, was obtained between the NIST and all the ADCLs. This is the third round of proficiency tests conducted for air-kerma from ⁶⁰Co gamma-ray beams between the NIST and the ADCLs since the re-characterization of the NIST ⁶⁰Co gamma-ray beams in terms of air-kerma that was completed in 2003. The first proficiency test was conducted in early 2004, which resulted in a similar level of agreement to that obtained in 2009.