Calculation of exposure buildup factors for point isotropic gamma ray sources in stratified spherical shields of water surrounded by lead and optimization of water-lead combination

Exposure buildup factors have been calculated by Monte Carlo method for point isotropic gamma ray sources, penetrating a two-layer spherical shield of water surrounded by lead, and the effect of bremsstrahlung radiation on buildup factors has, in particular, been investigated, the results of which are in good agreement with previous works. The buildup factors were then calculated for various combinations of water and lead layers at some gamma ray energy points in the range from 0.04 MeV to 10 MeV and for shield thicknesses from 1 to 10 mean free paths (mfp). From the results obtained, one can select the proper (or optimum) water and lead combination which results in minimum value of buildup factor for a two-layer water-lead shield of a given thickness (in mfp) at each energy point. Here the optimization analysis has been performed for a shield of 10 mfp thick at gamma ray energies from 0.04 MeV to 10 MeV.