NEUTRON AND GAMMA-RAY DOSIMETRY IN AN ANIMAL EXPOSURE VOLUME AT A PULSED TRIGA REACTOR.

Abstract

The distribution of steady-state and pulsed neutron and gamma-ray dose was measured in an animal exposure volume at the TRIGA reactor at the General Atomic facility. The usable volume was restricted to the lower 20 in. of an aluminum irradiation tube having an i.d. of 9 1/2 in. The tube was inserted into the pool adjacent to the reactor void tank, 134 cm from the core. Dosimetry films were used to determine gamma-ray dose while Sievert ionization chambers were used to determine neutron dose. Gold foils were used to measure the thermal neutron flux. Neutron doses were referenced to sulfur foil monitors. The neutron spectrum was examined with a system of threshold detectors and compared with spectra from Godiva II and the biological port of the Argonne CP-5 reactor. The vertical variation of dose along a lucite board, designed for retaining mice, was measured at the midplane of a lucite canister lowered into the tube. The dose in the lower 16 in. was found to vary less than 5 per cent. The ratio of neutron to gamma dose was approximately 6 to 1. A tissue-equivalent dog phantom, 20 in. long and 72 in. in diameter, was designed to determine the absorbed dose in depth. The fast neutron dose was about equal to the gamma-ray dose at the midline of the phantom having dropped from 2.3 times the gamma-ray dose in front of the phantom. The neutron plus gamma-ray dose at the midline was 44 per cent of the dose at the front surface of the phantom. Bilateral exposure doses were computed from the unilateral exposures.