Electron yields from irradiated targets versus photon incident angle and quantum energy

This paper describes the application of a three-dimensional (3D) adjoint Mo nte Carlo radiation transport code, NOVICE, to the calculation of photo-Com pton and pair electrons emitted from targets, consisting of elements with a tomic numbers ranging from Z = 4 to Z = 82, health physics materials, organ ic insulators; scintillators, and semiconductor materials. Due to space res trictions, only the results obtained for lead and lucite will be discussed here. Calculations of electron yields were carried out for photon incident angles of 0 degrees, 1.15 degrees, 1.72 degrees, 5.74 degrees, 30 degrees, 45 degrees, and 90 degrees relative to the plane of the targets and in the forward and backward directions. The thicknesses of the targets considered in the calculations were varied to cover a range of applications, four thic knesses per material, and 20 quantum energies ranging from 0.001 to 1000 Me V. The results show that for the shallow angles of incidence the yield of e lectrons from a target sharply increases for small increases in the inciden t angle relative to 0 degrees for all the materials and photon energies tha t were considered. This rapid rise in electron yield with incident angle im pacts the properties of directional radiation sensors and explains their re sponses to gamma and X-ray radiation. Comparisons of NOVICE calculations to experiments where total yields of electrons from targets of Al, Cu, Ta, an d Au were measured, support and provide credence to NOVICE simulations of d irectional sensor responses in locating radiation sources. (C) 1999 Elsevie r Science Ltd. All rights reserved.