Niobium-aluminium foils as monitors for routine measurement of fast neutrons

Niobium monitors have to be very thin in order to limit self-absorption of the KX-radiation and conversion electrons emitted by the Nb93m. 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 Nb95 activities of thick enough Nb plates (to determine the mass) and Nb/Al monitors irradiated in direct contact with each other to 'see' the same thermal neutron flux. (Auth.)