(Thursday, 7/18/2019) 1:00 PM - 3:00 PM
Room: 304
Purpose: Update the Canadian air-kerma standards for x-rays, Co-60 and Cs-137 beams, as well as for HDR Ir-192 sources to adopt the recommendations of ICRU Report 90.
Methods: The spectrum averaged product kwkii is calculated for all NRC and BIPM x-ray beam qualities. Spectra are obtained using a BEAMnrc model of our low- and medium energy x-ray tubes as sources for cavity. The stopping power ratio graphite to air, sc,a, is calculated using SPRRZnrc. Perturbation corrections for a graphite-walled ionization chamber are recalculated using cavity. The factor F to obtain NK(Cs-137) from knowledge of NK(Co-60) is obtained from calculations for both beams using cavity. Using NK values of the 2S spherical chamber for Cs-137 and N250 x-ray beam quality, an estimate for HDR Ir-192 is obtained.
Results: At low energies kwkii values lie between 0.995 and 0.996. At medium energies values are closer to unity ranging between 0.998 and 0.999. An excellent agreement with BIPM kwkii values for their beam qualities is obtained. The air-kerma rate for Co-60 beams will decrease by about 0.66 %, mostly due to changes in sc,a. sc,a. The combined effect of changes in Kwall, Kstem , Kan, and Kcomp corrections cancels out. Using the F values for 6 chambers and their readings when exposed to the NRC Cs-137 beam, the air-kerma rate for the Cs-137 beam will decrease by about 0.65 %. The effect of these changes on the Ir-192 HDR calibration coefficient for the 2S spherical chamber is only a 0.2 % decrease.
Conclusion: Current ICRU recommended quantities will have a significant effect on the users’ chambers NK for Cs-137 and Co-60. The impact on x-ray beams is only significant a very low energies. At higher x-ray beam energies and for HDR Ir-192 sources the change is equal or less than 0.2 %.
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