Streaming Calculations using the Point-Kernel Code RANKERN

Abstract

RANKERN solves the gamma-ray transport equation in generalised geometry using the point-kernel method, with build-up factors accounting for the contribution from scattered radiation. Developments to RANKERN have overcome the main limitation of the point-kernel method and the associated build-up data; i.e. that they are based on the calculation of the flux arising from radiation which has passed in a direct line from the source to the dose-point. This assumption is adequate for a wide range of systems that are comprised of bulk shielding. However, it is inaccurate in situations where the dominant contribution to the result arises from radiation which has undergone a number of changes in direction between source and dose-point in order to travel along weaknesses in the shield. This is the phenomenon that has to be modelled in streaming calculations.When the direct flight from source to the detectors is at an angle to the shield, the build-up data can be altered to take account of the obliquity. Such oblique penetration build-up factors are an approximation, which work for small angles but break down when the angle of obliquity becomes large. When this occurs, intermediate points of scatter between the source and dose-point need to be included in the calculation to provide a more accurate calculation. In many cases, the mechanism of scatter in a volume can be approximated by reflection at a surface, with appropriate albedo data giving the probability of reflection.This paper describes how RANKERN may be used to perform streaming calculations. Validation evidence is presented for the use of the code in such situations, including streaming along multi-legged ducts.