An Ultrahigh-Sensitivity Threshold Neutron Detector for Plasma Diagnostics

An ultrahigh-sensitivity fast fission counter was developed to selectively detect high-energy neutrons (En>1 MeV) produced by D-D and D-T fusion reactions in thermonuclear plasmas. Discrimination against slow and epithermal neutrons produced by scattering or electrodissociation of deuterium was achieved by sensitizing the detector with 100 g of highly depleted 238UO2, which has a fission threshold for neutron energies En>1 MeV. The capacitive signal loading effects of the detector´s large electrode area (5 m2) were mitigated by using a transmission-line electrode configuration. Wideband, low-noise preamplifiers were developed to match the 25-¿ characteristic impedance of the transmission line. The neutron sensitivity of the detector was shown to be 0.07 counts·s-1·[neutron/(cm2·s)]-1 for En>2 MeV in initial laboratory testing.