Reasons why Plutonium 242 is the best fission chamber deposit to monitor the fast component of a high neutron flux

The FNDS project aims at developing fission chambers to measure on-line the fast component of a high neutron flux ( ∼ 10 14 n cm - 2 s - 1 or more) with a significant thermal component. We identify with simulations the deposits of fission chambers that are best suited to this goal. We address the question of the evolution of the deposit by radiative capture and decay. A deposit of 242Pu appears as the best choice, with a high initial sensitivity to fast neutrons only slowly degrading under irradiation. The effect of unavoidable impurities was assessed: small concentrations of 241Pu and 239Pu can be tolerated.