Neutron yield monitors for the fusion neutronics source (FNS) for 800 beam line

A powerful D-T neutron source named FNS (Fusion Neutronics Source) has been constructed in Japan Atomic Energy Research Institute. Measurement of absolute neutron yield is essential for fusion neutronics study. The associated α-particle method is the most suitable method for the determination of absolute yield of neutrons produced by 3T(d,n)4He reaction. For the 800 beam line of the FNS, an α-monitor using a silicon surface-barrier detector (SSD) has been installed as the main neutron yield monitor. A long counter and a 232Th fission counter are adopted as the sub-monitors. A thin aluminum foil is set in front of the SSD to reduce the intensity of back-scattered deuterons. The peaks in the pulse height spectrum detected by the SSD are separated by the aluminum foil and by adjusting the bias voltage of the SSD. The peaks are assigned by means of the time-of-flight method for the associated charged-particles. A method to eliminate the contribution of d-3He and d-D reactions has been developed. The absolute d-T neutron yield can be estimated accurately to within less than 2 % in the best case condition. The ratios of counts of the sub-monitors to the α-monitor have been observed to be stable within their statistical errors during the same experimental period. (author)