Development of ³He solid state detectors epithermal neutron spectrometer

A neutron spectrometer was developed for measuring the neutron energy in the range from 1-300 keV. Its fabrication was based on the ³He(n,p)³H reaction, with two Si solid state detectors used for measuring the energy of the two charged particles. The sum of the energies of the two particles is the neutron energy E_n plus the reaction Q-value (764 keV). The FWHM detector resolution is ≈3%. The detector efficiency, ≈3% for thermal neutrons, is proportional to 1 E (i.e., to the detection reaction cross section). Cooling improves the resolution. To avoid obscuring of the low keV energy range by thermal neutrons, the detector was covered with a 1 mm layer of Cd. Background due to neutron interaction with the Si detector, and with the detector construction materials and gamma rays, was removed by repeating the foreground measurement with no ³He in the detector. Neutron spectra were measured both with a moderated ²⁵²Cf source and with an epithermal beam at the Brookhaven Medical Research Reactor (BMRR). Results were in good agreement with Monte Carlo calculations using the MCNP code.