Air-kerma determination using a variable-volume cavity ionization chamber standard

Abstract

A graphite-walled cavity ionization chamber of modular design and variable volume has been used to determine the air-kerma rate in the reference 60Co field at the BIPM. The chamber can be configured in five sizes. High-accuracy mechanical measurements of the volume of the air cavity were made for each configuration using a co-ordinate measuring machine. Ionization current measurements were made for each configuration and corrected for the effects of ion recombination and diffusion, stem scatter and chamber orientation. Monte Carlo calculations of cavity dose were made to evaluate the correction factors kwall and kan. A reproducibility of the ionization current per mass of 1.5 parts in 10(4) was achieved on the repeated assembly of each configuration. The results show an air-kerma rate determination that increases with volume, the total change being around 8 parts in 10(4). When analysed differentially, the air-kerma rate relative to the BIPM standard is Kdiff/KBIPM = 1.0026(6). A detailed uncertainty budget is presented. Possible reasons for the observed behaviour are discussed that might have consequences for all existing standards for air-kerma.

Accuracy of EGSnrc calculations at energies for the response of ion chambers configured with various wall materials and cavity dimensions

“…Combined, these experiments provide a comprehensive set of 60 Co data covering various chamber geometries and wall materials. In a more recent paper, Burns et al 9 reported high-precision measurements but only for a graphite chamber. 9 Based on information in the original papers, schematic diagrams of the chambers used in the above-mentioned studies are shown in Fig.…”

“…In a more recent paper, Burns et al 9 reported high-precision measurements but only for a graphite chamber. 9 Based on information in the original papers, schematic diagrams of the chambers used in the above-mentioned studies are shown in Fig. 1 as they were modeled in this investigation using EGSnrc.…”

“…The most recent set of experiments investigated here was reported by Burns et al 9 in 2007. The goal of their experiment was to derive the air-kerma rate in a 60 Co beam from a differential measure of the ionization current with respect to cavity volume using a variable-volume graphite planeparallel chamber ͑Fig.…”

“…Unfortunately, since these early comparisons were not concerned with achieving a high level of precision, uncertainties on the measured values were rarely discussed. Yet despite the challenges involved with experiments of this type, 9 it is reasonable to assume that the measurements were, on average, reproducible to within a few percent or less. This is comparable to the level of uncertainty on the cross sections 10,11 and stopping powers 12 used in modern Monte Carlo codes such as EGSnrc, 13,14 which are estimated to be within 1%-2% at 60 Co energies.…”

“…These comparisons with experiment will, at best, only test the transport mechanics at the level of experimental precision, which is assumed to be Ϸ1% in most cases. However, recent high-precision measurements ͑standard uncertainty of Ϸ0.015%͒ by Burns et al 9 of the response to 60 Co as a function of cavity volume were also simulated for the opportunity to better confirm the accuracy of the transport algorithms for graphite wall materials.…”