Development and Evaluation of a Fast Calculation Method for Gamma Ray Skyshine Dose Using Data Libraries

Abstract

A fast calculation method for gamma ray skyshine dose was developed. Four data libraries including anisotropy factors and buildup factors in the gamma ray transport calculation in air and the buildup flux energy spectrum and edge correction factor in the calculation of the precise energy spectrum of gamma ray flux penetrating through the upper shields were used. The anisotropy factors were generated on the basis of line beam response functions obtained by Hirayama et al. using the EGS4 code. The other three data libraries were generated by the invariant embedding method. Multilayer shields and angular distributions of gamma rays from sources were assumed to have a rotational symmetry with respect to the center axis of the shields along which the sources were located. Accuracy of the method was evaluated through comparison with the measured dose in the skyshine benchmark experiment and doses calculated by other dependable methods. It was found that the present method could simulate the measured dose with a discrepancy of 50% or less, and that transport of gamma rays in air due to a conical beam could be solved within an error of about 10–20%, except for the cases in which the beam angle lies in the range < 10 deg. The present method has two advantageous features: the skyshine dose can be obtained very quickly and very easily by general users.