Build-up factors for water and soft tissue by MCNP using a co-centric multilayer model: comparative study

Abstract

Build-up factor (BF) is one of the radiation interaction parameters, which is important in radiology and power sector to mitigate the radiation risks as low as possible. In this paper, the BF values for water and soft tissue have been computed for gamma rays within the energy range 0.2–2 MeV for penetration depths up to 10 mean free path (mfp), using MCNP simulation code by introducing a co-centric multilayer model. The results were compared with previously published data and standards. The employed model improved prediction of the BFs at 7 mfp and 10 mfp for 0.2 and 2 MeV compared to slab geometry model of MCNP. Besides, for the most part, the BFs calculated by simulation had a good agreement (<10%) with those of the ANSI/ANS-6.4.3 standard. There was a maximum difference at 10 mfp and 0.5 MeV and the minimum difference was in exact agreement. The uncertainties for the simulation were always below 5%, which is a partial explanation of the differences between the BFs given by the ANS standard and simulation. There were some improvements in prediction of BF values with reference to the mentioned standard, mostly for lower energies. The maximum improvement was 2%. It was also observed that the soft tissue (4 component) and water behave as the equivalent materials compared to soft tissue (ICRU-44) and A-150 plastic. It is concluded that the proposed model has such a potency for further extension of investigation for higher photon energies and penetration depths, to be used for calculation of the BFs.