(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose:
Investigate accuracy of Monte Carlo (MC) calculated key dosimetric quantities for kilovoltage air kerma standards when using renormalized photoelectric cross sections. Assess whether this renormalization improves the agreement with experimental measurements of the HVL and the free-air chamber (FAC) attenuation correction, A(att).
Methods:
Comparison of measured values of the HVL in terms of mm of Al and A(att), with MC calculations is performed for 111 beam qualities (combination of tube voltage and filter thickness). A model of the NRC low-energy x-ray tube is modeled with BEAMnrc to estimate the spectrum near the exit window for 10 kV to 80 kV x-ray beams and different Al filtrations. The EGSnrc application egs_fac is used to calculate Aatt, while HVL values are determined using in-house software. Results are obtained for different electron impact ionization (EII) and photoelectric models and compared to the experimental values.
Results:
Calculated HVL and A(att) values are insensitive to the choice of EII cross sections for the 10 kV beam. The agreement with measured Aatt values is better than 0.3 % in the whole range of beam qualities investigated. In the case of HVL calculations the agreement is not as good, with differences decreasing for harder beams to better than 4 %. On average, renormalized photoelectric cross sections results agree slightly better with experimental HVL values.
Conclusion:
Renormalized photo-electric cross sections improve marginally the agreement between measured and calculated HVL and FAC attenuation corrections. Calculated A(att) values could be used as part of a consistent set of MC calculated corrections for x-ray beams air kerma primary standards.
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