Flux Measurements of Medium-Energy Neutrons by Means of Stilbene Scintillators and 6Li Glasses

Earlier investigations on flux measurements for 14-MeV neutrons with inorganic (stilbene) scintillators (1 in. x 1 in. ) delivering an accuracy of about 4 % were extended to 2. 5-MeV neutrons from D(D, n) 3He. Effects of gamma radiation were excluded by applying pulse-shape discrimination. Edge effects and multiple(n,p) scatteringcanbe accounted for. The results were checked by comparative measurements with a 6Lil(Eu) scintillator, and by counting the beta radiation of 32P from 32S(n, p) in 2 π-geometry with a methane-flow proportional counter. The stilbene scintillator counter can therefore be regarded as an accurate instrument for measuring the absolute flux of medium-range energy neutrons paired with a good energy resolution. Low-cost instruments with good long-term stability are often needed for monitoring the neutron flux in fixed local points in neutron laboratories. As we feel that 6Li-glass scintillators are very appropriate tools for this purpose, some further investigations were made with NE 902, NE 905, NE 901 and NE 908 glasses. The results with monoenergetic neutrons were that flux measurements, including some energy resolution, can be made up to energies of about 2 MeV, with lower accuracy at the higher energies. The best results were found with NE 905. As experiments with definite γ-rays showed that pulses from slow neutrons can well be discriminated from γ-pulses, further experiments were started with counters where the 6Li- glasses on top of a lucite light guide and a 150-AVP multiplier were surrounded with hydrogen-containing materials of appropriate thickness ((CH2)n ≈12 cm). All neutrons are now counted within a small thermal-neutron peak in the pulse spectrum. The efficiency of these counters is about 10-2 depending on glass size, somewhat decreasing with neutron energies (up to 20 MeV). An accuracy of better than 10% of flux destination is claimed after checking the counters with stilbene counters and Lil-counters. 6Li-glass counters proved to keep their properties much more stable for long periods than stilbene crystal of 6Li-crystal counters. The counting efficiency is sufficiently high and the discrimination with respect to γ-pulses is much less critical than with BF3(long) counters. So these counters can be established for neutron - flux monitoring with good reliability. (author)