An EGSnrc investigation of cavity theory for ion chambers measuring air kerma

Abstract

The EGSnrc system is used to compare the response of an aluminum-walled thimble chamber to that of a graphite-walled thimble chamber for a 60Co beam. When compared to previous experimental results, the EGSnrc 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. EGSnrc 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 alpha, 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.

Wall correction factors, , for thimble ionization chambers

“…Similarly, stoppingpower ratios computed using the user-code SPRRZnrc, agree to within 0.1% of TG-51 values. A previous study 31 has shown that values of ␣ used in the TG-51 protocol agree well with the EGSnrc values for the wall thicknesses considered here. Since EGSnrc values for the dosimetry parameters used in Eq.…”

CSnrc: Correlated sampling Monte Carlo calculations using EGSnrc

“…A calculation using CSnrc results in a value of D gr / D Al = 0.9927± 0.0001. This compares with, but shows much lower uncertainty than, a previous CAVRZnrc calculation 24 and with an EGS4/PRESTA correlated sampling calculation 7 that gave values of 0.9937± 0.0010 and 0.9942± 0.0013, respectively. It also agrees with the measured result of 0.992± 0.002 from Palm and Mattsson.…”

Accuracy of Spencer‐Attix cavity theory and calculations of fluence correction factors for the air kerma formalism

“…In previous investigations, it was shown that, combined, the density effect and I value of graphite can have a large effect ͑i.e., 1.5% or more͒ on the calculated response of graphite chambers 6 and on calculations of the SA graphiteto-air stopping-power ratio. 3 Although these large effects likely cancel when computing values of K fl , the effects of both the density and I value on K fl were examined at the level of statistical uncertainty considered here.…”

“…Their calculated values of K SA ranged between 0.9970 and 1.0005 for a 60 Co beam depending on the chamber and the value of low-energy cutoff ⌬ used in the calculation of stopping-power ratio. Buckley et al 6 later showed that K SA is unity for one particular graphite thimble chamber at 60 Co energies but is a 0.5% correction ͑1.0050Ϯ 0.0003͒ for the same chamber configured with an aluminum thimble. However, neither study determined whether the K SA correction was due to fluence perturbations by the cavity or due to the wrong choice of ⌬ in the calculation of the SA stopping-power ratio since, as we discuss in the following sections, this requires an indepth knowledge of how K SA is affected by changes in chamber wall material and cavity dimension.…”