Gamma Residual Radioactivity Measurements on Rats and Mice Irradiated in the Thermal Column of a Triga Mark II Reactor for BNCT

The current Boron Neutron Capture Therapy (BNCT) experiments performed at the University of Pavia, Italy, are focusing on the in vivo irradiations of small animals (rats and mice) in order to evaluate the effectiveness of BNCT in the treatment of diffused lung tumors. After the irradiation, the animals are manipulated, which requires an evaluation of the residual radioactivity induced by neutron activation and the relative radiological risk assessment to guarantee the radiation protection of the workers. The induced activity in the irradiated animals was measured by high-resolution open geometry gamma spectroscopy and compared with values obtained by Monte Carlo simulation. After an irradiation time of 15 min in a position where the in-air thermal flux is about 1.2 x 10(10) cm(-2) s(-1), the specific activity induced in the body of the animal is mainly due to Na-24, Cl-38, K-42, Mn-56, Mg-27 and Ca-49; it is approximately 540 Bq g(-1) in the rat and around 2,050 Bq g(-1) in the mouse. During the irradiation, the animal body (except the lung region) is housed in a 95% enriched Li-6 shield; the primary radioisotopes produced inside the shield by the neutron irradiation are H-3 by the Li-6 capture reaction and F-18 by the reaction sequence Li-6(n,alpha)H-3 -> O-16(t,n)F-18. The specific activities of these products are 3.3 kBq g(-1) and 880 Bq g(-1), respectively