Study of exposure buildup factors with detailed physics for cobalt-60 gamma source in water, iron, and lead using the MCNPX code

Abstract.

In the past, the exposure buildup factors have been calculated with simple physics (free-electron Compton scattering, no coherent scattering, and only K X-ray) such as ANS-6.4.3, etc.. However, in this paper, we calculated the exposure buildup factors for cobalt-60 gamma source with detailed physics (bound-electron Compton scattering, coherent scattering, bremsstrahlung photons and K and L X-rays, pair production) using the MCNPX code. We also showed that there is a significant difference between simple physics and detailed physics for calculating the exposure buildup factors for high atomic number elements. In this work, we used the MCPNX code to perform Monte Carlo simulations as this methodology can provide a better description of the complex chain of microscopic processes involved in the whole volume of material. Therefore, considering the various applications of cobalt-60 both in the industry and in medicine, calculating the exposure buildup factors with detailed physics is very important. In this work, we calculated the exposure buildup factors of cobalt-60 gamma source up to depths of 20 mean free path (mfp) of water, iron and lead with detailed physics through Monte Carlo method by the MCNPX 2.6 code (with particle splitting as an appropriate variance reduction technique). To validate the results, we calculated the exposure buildup factors (with simple physics and detailed physics by the MCNPX code) for a 1 MeV monoenergetic gamma source up to depths of 20 mfp of water, iron and lead and our results showed a good agreement with the literature. Our paper shows that the MCNPX code can be used to calculate the exposure buildup factors of monoenergetic and multi-energy gamma sources with simple physics and detailed physics and to provide an estimate of the difference in the factors between choosing the former or the latter.