Niobium-Aluminium Foils as Monitors for Routine Measurement of Fast Neutrons

Abstract

Niobium is an excellent monitor for fast neutrons. The reasons are the long half-life of about 16 years of the Nb93m and the low neutron energy threshold of about 0.1 MeV. Nb93m is produced by inelastic neutron scattering. The low energy of the emitted KX-radiation and conversion electrons between 15 and 27 keV does not call for great care when handling. On the other hand the monitors have to be very thin in order to limit self-absorption, and if the content of tantalum is small -less than 50 ppm- the influence of Tal82 can also be neglected. Thin monitors have an additional advantages small amounts of radioactivity compared with monitors produced from parts of a large piece of irradiated niobium. Thin niobium foils however suffer at least some loss of mass, or may be completely powdered by radiation embrittlement. To overcome this problem, a thin (3 micron) Nb layer was stabilized using 0.1mm Al foil. The niobium layer did not show any loss of mass due to irradiation up to 1021 /cm2. The problem of Nb~mass determination in Nb/Al foils can be solved by exact weighing before and after Nb deposition, or by comparing Rb95 activities of thick enough Nb plates (to determine th© mass) and Nb/Al monitors irradiated in direct contact with each other to “see” the same thermal neutron flux.