Proton-Recoil Counter Technique for Measurement of Fast Neutron Spectrum

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Fast neutron spectra were measured at the core centers of two plutonium-fuelled as-semblies of FCA by the proton-recoil counter technique. Small cylindrical counters with field tubes made by metallizing a ceramic-seal surface were constructed. The filling gases, hydrogen and methane, were purified to reduce the concentration of electronegative impurites. The highest resolutions measured by 3He(n, p)3T reaction were 3.8 and 5.5% (FWHM) for the hydrogen-filled and methane-filled counters respectively. Gamma-induced background was rejected by the pulse-shape discrimination technique, using a digital processing arithmetic unit and a two-dimensional pluse height analyzer. The spectrum measurement over a wide energy range was carried out by different settings of the main amplifier gain and of the gas multiplication of the counters ; its energy ranged from 2.5 keV to 2 MeV. Measurements were made with the energy resolution about 10% (FWHW) or less at energies down to 5 keV. The measured neutron spectra are compared with calculated spectra, obtained by the cell program SP-2000 with the fine group cross section library AGLI. The comparisons indicate that the measured spectra are in good agreement with the calculated spectra except near the large iron and oxygen resonances.
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