Etude dosimétrique d'un faisceau collimé de neutrons thermiques destine à l' analyse par radioactivation “in vivo”

Abstract

In order to evaluate the absorbed dose in an individual locally irradiated by thermal neutrons with the intention of in vivo activation analysis, the study of the dosimetry of a collimated thermal neutron beam from the EL3 reactor at Saclay has been undertaken. Thermal flux measurements were done at several depths in tissue-equivalent phantoms representing a human neck as well as a human leg. According to these measurements, the absorbed dose due to the two main mechanisms of interaction of the thermal neutrons with the biological matter was evaluated theoretically as follows: (a) the absorbed dose due to the gamma rays from thermal neutron capture in Hydrogen and every other element in tissue was computed by the Mercure 3, IBM-360 FORTRAN IV, program. (b) the absorbed dose due to the 14N( n, p) 14C reaction was calculated by the use of the first collision dose per neutron per square centimeter formula. On the other hand, direct measurements of the absorbed dose were made with conventional ionisation chambers. The absorbed dose due to the gamma rays coming directly from the reactor core, as well as to the fast neutron fraction of the beam, was evaluated experimentally by straight measurement with an ionisation chamber of the dose rate behind a Li 6F shield. The dose equivalent thus evaluated was at the surface of the phantom for a 3 cm diameter thermal neutron beam and flux of 1.25 × 10 8 n cm −2 s −1 1. (a) Gamma capture dose: 180 mrem/min 2. (b) 14N( n, p) 14C reaction dose: 1190 mrem/min 3. (c) direct gamma and fast neutron dose: 94 mrem/min