The Use of Surface-Barrier Diodes for Fast-Neutron Spectroscopy

A neutron-sensitive semiconductor counter has ben constructed by depositing a thin layer of Li6F between two silicon surface-barrier counters. Neutrons are detected by observing the α + T pair resulting from the Li6 (n, α)T reaction; pulses from the two counters are added and the sum pulse is amplified and recorded on a multi-channel analyzer. Since the sandwich geometry permits simultaneous detection of both reaction products, the magnitude of the sum pulse is proportional to the energy of the incoming neutron. A coincidence circuit was used to reduce background counts. Pulse-height spectra from slow neutrons and from monoenergetic fast neutrons, in the energy region 0.6 to 15 Mev, have been recorded from several counters of this type using sengitive areas of about 0.7 to 0.8 cm2, with a Li6F layer of order 150 μ gm/cm2 thick. In all cases a well defined neutron peak was observed in the pulse-height spectra. The full width at half maximum of the fast-neutron peak was about 300 kev approximately independent of neutron energy. [Counters of this type seem relatively insensitive to gamma-ray background but at the higher energies (8 Mev and up) background from (n, charged particle) reactions in materials other than the Li6 is quite high.]