Examination of secondary electrons generated by encapsulated gamma sources to improve contact dosimetry estimates
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Secondary electron generation on the surface of encapsulated gamma sources can play a large role in the dose measured near the surface of the encapsulation. The National Council on Radiation Protection and Measurements (NCRP) Report - 40 contains contact dose rate conversion factors for encapsulated gamma sources, along with recommended secondary electron correction factors. However, the secondary electron correction factors were based on experiments done in the 1930s and 1940s with encapsulated radium sources, and the correction factors for the other sources in NCRP-40 were estimated based on these radium measurements. Simulations have been 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 rate conversion factors for all of the encapsulated gamma sources presented in NCRP-40, taking into account the dose from both gamma rays and secondary electrons. These simulations show that the contact dose rate conversion factors are actually much lower than those presented in NCRP-40. The simulations also demonstrate that the relative contribution of secondary electrons to the contact dose is significantly higher than what is predicted by NCRP-40. In addition to this computational work, the experiment that was used to determine the secondary electron correction factors for NCRP-40 was investigated. This experiment involved measuring the radiation field near an encapsulated radium source with an ion chamber, and using a magnetic field to separate the secondary electrons from the gamma radiation. A modernized version of this experiment was designed, and used to show that secondary electron correction factors measured with this type of setup were not applicable to the geometry of tissue in direct contact with the encapsulation.