Calculation of Low-Energy Neutron Dose Indices and Depth Doses in the ICRU Tissue Sphere

Detailed neutron dose distributions were calculated for the ICRU tissue sphere for broad, parallel beams of incident neutrons with 10 different energies ranging from thermal to 0.3 MeV. The calculation was carried out by the Monte Carlo method on the CYBER 175 computer of the University of Illinois. From these calculated data, a set of fluence-to-dose-index conversion factors, needed to establish the allowable limits for exposure to external neutron beams, was obtained. Normalized depth doses along the principal axis at depths of 0.007, 0.3, and 1.0 cm are also provided. The fluence-to-dose-index conversion factors are compared to previous conversion factors based on cylindrical phantoms, and any differences are explained.