Measured response of bubble neutron detectors and prospects for alpha knock-on diagnostics

Measurement of the neutron energy spectrum above similar to 16 MeV will yie ld information on the spatial and energy distributions of confined fast alp has in deuterium-tritium (DT) tokamaks (Fisher, Nucl. Fusion; Gorini Rev. S ci. Instrum.). The energetic neutrons result from fusion reactions involvin g the energetic ions created by alpha-fuel ion knock-on collisions. Standar d two-gas bubble neutron detectors, designed to only detect neutrons with e nergies above a selectable threshold determined by the gas mixture, were us ed in preliminary attempts to measure the knock-on neutrons from DT plasmas in the Tokamak Fusion Test Reactor and Joint European Torus (JET). Subsequ ent measurements at accelerator neutron sources showed an unexpected below- threshold detector response that prevented observations of the alpha-induce d neutron tails. Spontaneous bubble nucleation measurements show that the m ajority of this below-threshold response is due to slight variations in the gas mixture, and is not present in single-gas detectors. Single-gas detect ors will be tested at the University of California Berkeley to determine th e neutron energy threshold as a function of detector operating temperature and to confirm their suitability for alpha knock-on tail measurements. An a rray of single-gas detectors operating at different temperatures should all ow measurements of the alpha knock-on neutron tail during planned DT experi ments on JET. (C) 2001 American Institute of Physics.