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Experiments have been performed with the view to applying the 3He proportional counter to differential measurements of the continuous neutron spectrum in a fast reactor. The results of measurements in the JAERI Fast Critical Facility (FCA) proved the apparatus to be useful for the measurements of the main part of the fast reactor spectrum.A specially devised electronic circuit was employed to eliminate the effect of differences in the rise-times of pulses; the energy resolution of the spectrometer was 25 keV (F.W.H.M.).The modified inverse matrix method was used to obtain the neutron spectrum from the measured pulse-height disLribution, resulting in an improvement of the effective energy resolution in the resultant spectrum to about 10 keV. The applicable energy range of the spectrometer is for the time being from 20keV to 2MeV.

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