The results are given of theoretical and experimental studies on the correlation amplitude in the Rossi-a experiment, which does not follow Orndoff's theory in the usual procedure of measurement. It is shown that the correlation amplitude depends on the condition of the trigger count, and that Orndoff's amplitude is the average of those triggered by all pulses from the detector. Experimental results substantiating this theory are presented.The two-fork approximation is proved from experiment to be a good approximation near the critical state; which is a revelation of practical significance.A new method is proposed for obtaining the correlation amplitude by measuring the distribution of the number of family counts, and its application promises to be of great practical usefulness.

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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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