An analytic theory of the point-source nonuniformity correction factor for thick-walled ionisation chambers in photon beams

Abstract

In exposure and air kerma measurements, axial and radial nonuniformity perturbation correction factors are used to account for the nonuniformity of the incident photon field in the vicinity of the ionisation chamber cavity. An analytic theory for calculating the point source nonuniformity correction factor for thick-walled ionisation chambers is developed. The theory developed herein is compared with Monte Carlo calculations for chambers with pancake, cylindrical and spherical geometries similar in size to the instruments employed by Standards laboratories. The perturbations, which differ from those calculated or measured by some Standards laboratories, suggest that corrections of the order of 0.3% should be applied to typical pancake geometries, smaller corrections of the order of 0.05% or less for typical Farmer-type chambers, and no correction for spherical chambers.

A Fano cavity test for Monte Carlo proton transport algorithms

“…Our methodology is applicable to any kind of charged particle, provided that the considered MC code is able to track the charged particle considered. It is a generalization of the regeneration technique of Bielajew 23,24 to all charged particles. One does not need to modify the neutron transport algorithm and the interaction rate (defined by the macroscopic cross section ) can be completely controlled by the user.…”

“…One common method in photon beams is the regeneration technique, introduced by Bielajew. 23,24 In the regeneration technique, the photon keeps its initial momentum and energy at the interaction site, which effectively eliminates photon attenuation and scatter. 5 Secondary electrons are created according to the cross sections.…”

“…This is also an efficient way of achieving CPE no matter what the angular distribution of the electrons is being used, as long as it is uniform. While this version of the Fano test 6 has the advantage of being independent of photon cross sections, as opposed to the regeneration technique, 23,24 both tests are somewhat equivalent in their efficiency to achieve CPE.…”

“…In the present paper, a hybrid Fano test is defined based on both known techniques (i.e., Bielajew 23,24 as well as Sempau et al 6 ) and applied to the proton transport algorithm in an extension of PENELOPE to protons (PENH) and in Geant4. PENH is benchmarked with Geant4 in a recent publication.…”

Nouvelles chambres d’ionisation primaires du LNE-LNHB pour la détermination du kerma dans l’air dans un faisceau de cobalt-60

“…Ce facteur correctif est factorisé en deux termes, pour les non-uniformités axiale (k an ) et radiale (k rn ). Le facteur de correction de non-uniformité axiale est pris égal à l'unité [9,10] sans aucune incertitude pour le moment. Le facteur de correction de nonuniformité radiale est déterminé à partir de mesures faites à l'aide d'une petite chambre d'ionisation dans le plan de référence.…”