Thorium-uranium fission radiography

Abstract

The high energy secondary neutron flux, produced by stopping the 650 MeV proton beam of the Los Alamos Meson Physics Facility (LAMPF), has been tested for use in measurements of the microscale distribution of Th in heterogeneous samples, particularly rocks. Polished samples are covered with mica which serves as a detector for neutron-induced fission tracks. The sample is also irradiated in a nuclear reactor in order to measure the U distribution and to determine the correction required for U fission in the high energy exposure. The relative Th/U fission rates range from 0.14 to 0.31, depending on the amount on internal neutron moderation within the samples. The relative 232Th/ 238U fission rate, as calculated from the theoretical beam stop spectrum is somewhat higher (0.5), probably reflecting a contribution from low energy 235U fission in the observed ratio. The measured relative 209Bi/ 232Th fission rates are higher by a factor of 2–7 than those calculated from the theoretical neutron spectrum indicating that the flux of high energy (200 MeV) neutrons has been underestimated in the theoretical spectrum. Th fission track production rates of (0.5–2) × 10 4/cm 2·mA·h·ppm Th were measured, with the lower values being more in accord with those calculated from the theoretical spectrum. The technique is limited by background from recoil tracks in the mica. The relative rate of fission to recoil is ∼3 × 10 −6 fission/recoil·ppm Th. However, by differential annealing of the recoil tracks, we are able to count fission tracks in the presence of a large background of recoil tracks. In order to measure the Th concentration in individual grains to ±10% in 10 h exposures, the product of the Th concentration in ppm and the area of the grain in cm 2 is ∼2 × 10 −3, i.e., for 1 ppm Th roughly 400 μm grains are required.