Abstracts Category : Other

Examination of secondary electrons generated by encapsulated gamma sources to improve contact dosimetry estimates

by Eric Heritage

Secondary electron generation on the surface of encapsulated gamma sources canplay a large role in the dose measured near the surface of the encapsulation. TheNational Council on Radiation Protection and Measurements (NCRP) Report - 40contains contact dose rate conversion factors for encapsulated gamma sources, alongwith recommended secondary electron correction factors. However, the secondaryelectron correction factors were based on experiments done in the 1930s and 1940swith encapsulated radium sources, and the correction factors for the other sourcesin NCRP-40 were estimated based on these radium measurements. Simulations havebeen done using the state-of-the-art Monte Carlo radiation transport code PHITS(Particle and Heavy Ion Transport code System), to calculate the contact dose rateconversion factors for all of the encapsulated gamma sources presented in NCRP-40,taking into account the dose from both gamma rays and secondary electrons. Thesesimulations show that the contact dose rate conversion factors are actually muchlower than those presented in NCRP-40. The simulations also demonstrate that therelative contribution of secondary electrons to the contact dose is significantly higherthan what is predicted by NCRP-40. In addition to this computational work, theexperiment that was used to determine the secondary electron correction factors forNCRP-40 was investigated. This experiment involved measuring the radiation fieldnear an encapsulated radium source with an ion chamber, and using a magnetic fieldto separate the secondary electrons from the gamma radiation. A modernized versionof this experiment was designed, and used to show that secondary electron correctionfactors measured with this type of setup were not applicable to the geometry of tissuein direct contact with the encapsulation.Advisors/Committee Members: Waller, Edward.