Novel concept of time-of-flight neutron spectrometer for measurement of the D/T burning ratio in the ITER

Abstract

A time-of-flight (TOF) neutron spectrometer is a candidate for the measurement of the D/T burning ratio in the International Thermonuclear Experimental Reactor (ITER). In ITER high-power experiments, the TOF system suffers from a high event rate or accidental counts due to high radiation intensities, which is one of several background sources in DD neutron measurement. We herein propose a new neutron spectrometer to apply to the measurement of the D/T burning ratio in the ITER high-power operation region. This system is based on the conventional double-crystal TOF method and consists of a water cell and several pairs of scintillators. A water cell is inserted before the first scintillator of the TOF system and acts as a radiator or neutron scattering material. Because DD neutrons have a larger cross section of elastic scattering with hydrogen than DT neutrons, the elastic scattering in the radiator enhances the relative ratio of DD/DT intensity by approximately three times before entering the TOF system. The enhancement of the relative intensity of DD neutrons makes the detection of DD neutrons easier. The feasibility of this method as a neutron spectrometer has been verified through a preliminary experiment using a DT neutron beam (20mm ϕ) at the Fusion Neutronics Source, Japan Atomic Energy Agency. The present article describes the basic performance of the prototype system.