Premium
An EGS nrc investigation of cavity theory for ion chambers measuring air kerma
Author(s) -
Buckley Lesley A.,
Kawrakow I.,
Rogers D. W. O.
Publication year - 2003
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1573891
Subject(s) - ionization chamber , monte carlo method , dosimetry , kerma , physics , stopping power , graphite , photon , nuclear physics , nuclear medicine , ion , atomic physics , materials science , computational physics , optics , mathematics , detector , composite material , medicine , statistics , quantum mechanics , ionization
The EGS nrc system is used to compare the response of an aluminum‐walled thimble chamber to that of a graphite‐walled thimble chamber for a60 Co beam. When compared to previous experimental results, the EGS nrc values of the ratios of chamber response differ by as much as 0.7% from the experiment. However, it is shown that this difference can be more than accounted for by switching from using the graphite mean excitation energy of 78 eV used in dosimetry protocols to the value of 86.8 eV suggested by more recent stopping‐power experiments. This suggests that the uncertainty analysis of Monte Carlo results must be done more carefully, by taking into account uncertainties in the underlying basic data such as the electron and photon cross sections. In comparison to Spencer–Attix cavity theory for a thick‐walled ion chamber, the Monte Carlo calculated values of the chamber response differ from the expected ones by 0.15% and 0.01% for the graphite and aluminum chambers, respectively, which are comparable to previously reported values for the Spencer–Attix correction factors. EGS nrc is also used to investigate the effect on the chamber response of thin dag layers on the inside of the aluminum wall. There is good agreement between the calculated and measured changes in chamber response versus the thickness of the dag. The results are compared to the predictions of the Almond–Svensson extension of cavity theory and show that the theory does not correctly predict the chamber response in the presence of thin dag layers. This finding is in agreement with previously reported experimental results. It is demonstrated that the values of α , the fraction of ionizations in the gas arising from electrons generated in the dag layer, used in the theory, are not the source of the disagreement.