Graduate Thesis Or Dissertation
 

Characterization of the onset of charged particle equilibrium for photon dose to skin

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/zg64tp079

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  • In the U.S., the Nuclear Regulatory Commission (NRC) is responsible for setting annual dose limits. For cases of hot particle contamination these limits are set at depths in skin of 7 mg/cm², 300 mg/cm², and 1000 mg/cm². However, at such shallow depths, the lack of charged particle equilibrium (CPE) precludes the use of traditional fluence-to-dose conversion methods. In this work, an enhanced photon dosimetry model is constructed based on simulations of photon point sources using MCNP5 (Monte Carlo N Particle version 5) transport code. An empirical relationship between KERMA and absorbed dose was established, and used to develop a correction factor, f[subscript CPE], accounting for the lack of charged particle equilibrium (CPE) at shallow depths. This correction factor, in conjunction with traditional point-kernel fluence-to-dose conversion, provides a more accurate prediction of photon dose. The photon model is implemented such that empirical mathematical formulations, rather than look-up tables, drive the estimation of integrated dose over a disk up to 10 cm². In addition to fCPE, the creation of a secondary off-axis correction (s[subscript C]) was necessary in order to accurately calculate the integrated dose to a disc at shallow depths. When calculating the integrated dose to a disc of 10 cm² at a depth of 0.007 cm for photon energies of 0.662, 0.835, and 1.333 MeV the new model is within 2.0%, 3.7%, and 2.5%, respectively, of the dose calculated by MCNP5. This corresponds to improvements of 134.4%, 165.2%, and 275.5% over VARSKIN 3's results for the same shallow-depth calculations. The photon dosimetry model presented here also incorporates the parameters of energy, attenuation, and dose-averaging area, thereby addressing deficiencies in previous models.
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