The effect of scattered photons for x-ray air-kerma calibrations

The parallel-plate free-air ionization chambers are used for X-rays air-kerma rate standards at National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST). The diaphragms are used to free-air chambers to fix the air volume of ionizing region in chambers. It means that absolute air-kerma rates are defined for collimated x-ray beams, because most of scattered photons are collimated by diaphragms. On the other hand, calibrated chambers are measured with non-collimated conditions. The variation of calibration constants was observed at different calibration distances and different diaphragm sizes for A6 Exradion chambers calibrations. The difference of the calibration constants are about 1.5% for 250kV X-ray beams. We thought that these results could be explained by the effect of scattered photons from the filters. In this study, we evaluate the contribution of scattered photons for free-air chambers and cavity chambers by Monte Carlo simulations, and suggest a new correction factor for evaluation of x-ray standards. The difference of calibration constants decreases below 0.4%.