Diaphragm correction factors for free-air chamber standards for air kerma in x-rays

Abstract

At present, only a correction factor for photon transmission, k(l), is systematically applied for the entrance diaphragm of free-air chamber standards for air kerma. In the present work, the Monte Carlo code PENELOPE is used to re-evaluate k(l) for the BIPM standards and new correction factors are calculated for photon scatter and for fluorescence production in the diaphragm. An additional effect arising from electrons emitted from the diaphragm is shown to be significant at the highest photon energies. The results for the radiation qualities used for international comparisons give a combined diaphragm correction factor k(dia) = 0.9980(3) for the BIPM medium-energy standard at 250 kV. This is significantly different from the factor k(l) = 0.9996(1) in use at present and it might be concluded that differences are likely to exist for all free-air chamber standards. The effect of using a conical taper at the downstream edge of the diaphragm is shown to be negligible for these radiation qualities.

Key comparison BIPM.RI(I)-K3 of the air-kerma standards of the MKEH, Hungary and the BIPM in medium-energy x-rays

“…Initially, there was evidence of a slight trend in the results for the different radiation qualities. It was postulated that this might be due to the fact that a diaphragm correction factor, k dia , is applied to the BIPM standard (Table 5), based on the calculations described in [6], but no corresponding correction was applied at that time to the MKEH standard. Subsequently, an estimation of k dia for the MKEH standard was made, based on [6] and on calculations made for different source-to-chamber distances [9].…”

“…[4]. The diaphragm correction, described in [6], is evaluated by Monte Carlo calculation and includes the effect of photon transmission and scatter in the diaphragm as well as fluorescence and secondary electron production in the diaphragm.…”

Key comparison BIPM.RI(I)-K7 of the air-kerma standards of the ENEA-INMRI, Italy and the BIPM in mammography x-rays

“…The measuring volume V is defined by the diameter of the chamber aperture and the length of the collecting region. The BIPM air-kerma standard is described by Boutillon et al (1969); the correction factors for the mammography qualities are described by Kessler (2006) and the changes made to certain correction factors, by Burns et al (2009). A description of the ENEA-INMRI CAL02 standard can be found in the report of the direct comparison carried out at the BIPM in the W/Al low-energy x-ray beams (Burns et al 2011).…”

Key comparison BIPM.RI(I)-K3 of the air-kerma standards of the ARPANSA, Australia and the BIPM in medium-energy x-rays

“…[4]. The diaphragm correction, described in [7], is evaluated by Monte Carlo calculation and includes the effect of photon transmission and scatter in the diaphragm as well as fluorescence and secondary electron production in the diaphragm.…”