'Hot particle' skin dosimetry calculations are commonly performed using homogeneous dose-point kernels (DPK) in conjunction with scaling and backscatter models to account for non-homogeneous geometries. A new scaling model for determining the actual DPK for beta-particles transmitted by a high-Z source material has been developed. The model is based on a determination of the amount of mono-energetic electron absorption that occurs in a given source thickness through the use of EGSnrc (Electron Gamma Shower) Monte Carlo simulations. Integration over a particular beta spectrum provides the beta-particle DPK following self-absorption as a function of source thickness and radial depth in water, thereby accounting for spectral hardening that may occur in higher-Z materials. Beta spectra of varying spectral shapes and endpoint energies were used to test our model for select source materials with 7.42 < Z ��� 94. A new volumetric backscatter model has also been developed. This model corrects for beta-particle backscattering that occurs both in the source medium and in the atmosphere surrounding the source. Hot particle backscatter factors are constructed iteratively through selective integration of point-source backscatter factors over a given source geometry. Selection criteria are based on individual source-point positions within the source and determine which, if any, backscatter factors are used. The new scaling model and backscatter model were implemented into the DPK-based code VARSKIN 4 for extensive dose testing and verification. Verification results were compared to equivalent Monte Carlo simulations. The results demonstrate that significant improvements can be made to DPK-based models when dealing with high-Z volumetric sources in non-homogeneous geometries. / Graduation date: 2013

The problem with which we shall concern ourselves in this paper is the self-scattering and self-absorption of beta particles by the source.

Counting efficiencies were determined by empirical measurement and Monte Carlo simulation for carbon-14, strontium-89, strontium-90, and yttrium-90 standards counted by low-background gas flow proportional counter for strontium carbonate precipitates in the range from 3 to 33 mg cm⁻². The maximum beta particle energies range from 0.156 MeV for carbon-14 to 2.28 MeV for yttrium-90. The parameters for estimating the counting efficiency are summarized for sources with areal thickness of 14 mg cm⁻² and over the range in strontium carbonate areal thickness from 0.1 mg cm⁻² to 33 mg cm⁻². Uncertainty budgets providing estimates of the uncertainty, sources of variability in the calibration process, and the total expanded uncertainty are presented. Information is presented for the Monte Carlo simulation regarding the composition of the detector window, the energy excluded by the amplifier discriminator of the counting system, and the physical density of materials for this analytical process. The histogram normalization routine implemented within MCNP is described and found to bias the probability distribution for beta-particle energy spectra. The difference in the specification of the probability distribution for beta-particle energy spectra in ICRU 56 Appendix D and MCNP requirements are described and a correction for the bias introduced during the normalization process for beta spectra is provided. Counting efficiencies determined by empirical measurement and Monte Carlo simulations agree within the total expanded uncertainties of the measurements and the uncertainties of the Monte Carlo simulations.