Monte Carlo calculated correction factors for primary standards of air kerma

Abstract

Many laboratories with cavity chambers as primary standards for air kerma are considering using additional Monte Carlo calculated correction factors, in particular the correction for attenuation and scatter in the walls, Kwall, and possibly the correction for point of measurement, Kan. Standards labs also use Monte Carlo calculated Spencer-Attix stopping-power ratios for graphite to air. The purpose of this article is to investigate the sensitivity of these calculations to their details and to assign uncertainties to the calculated values. We also investigate the correction needed for the Canadian primary standard to account for a polystyrene insulator, Kcomp and find that it is quite large (1.0046 +/- 0.0017). The article shows that the values of correction factors are very robust and insensitive to most details of the calculations except the values of the underlying electron stopping powers which have a significant effect on the stopping-power ratio and on Kcomp. The 1% uncertainties on the photon cross-sections have a negligible effect on these correction factors except for Kcomp. As a result of these investigations, with no change in the stopping power data used, the Canadian primary standard of air kerma in a 60Co beam needs to be increased by 0.54%.

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

“…15 However, the choice of cross sections is unimportant since the analysis that follows depends upon the ratios of responses to theory which, for graphite chambers, has been shown to be independent of the cross sections if a completely consistent set of cross-section data is used. 3,5 We have confirmed that the same holds true for high-Z chambers within the statistical precision considered here. Under the simulation conditions described above, the K h , K an , and K comp corrections are unity by definition.…”

“…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. Figure 9 shows calculations of K fl for the plane-parallel graphite chamber as a function of the density of the graphite wall.…”

“…3,10 Using Monte Carlo simulations, it has been shown that this prescription for L is a particularly good approximation for plane-parallel and Baldwin-Farmer-type chambers in a 60 Co beam. 3 Using the 4V/S approach to determine ⌬, SA predicted values of the response can be obtained using Eq. ͑4͒ for a variety of chamber wall materials and cavity dimensions.…”

“…3,4 Under these simulation conditions, the accuracy of the air kerma formalism given by Eq. ͑1͒ is limited by the accuracy of the SA cavity theory.…”

Comparison of air‐kerma strength determinations for HDR 192Ir sources

“…Monte Carlo simulation using the EGSnrc user code cavity.cpp (version 1.41) (Ref. 12) was chosen for this task, since it has been shown by Rogers and Kawrakow 13 that EGSnrc is well suited for making…”