THE JET NEUTRON TIME-OF-FLIGHT SPECTROMETER

An instrument for measuring neutron energy spectra over the interval 1 to 20 MeV has been developed and tested. It is based on time-of-flight measurements in between correlated events in two spatially separated sets of plastic scintillators. This instrument has been installed at the Joint European Tours (JET). We describe here the required operating conditions, performance tests and results of three years of operation during which neutron energy spectra in the 2-3 MeV range from D(d, n)He-3 reactions in JET were studied. Some technical details are given and the results from Monte Carlo and analytical model calculations of the spectrometer energy resolution and response function are presented. The efficiency of the system is almost-equal-to 1 x 10(-2) cm2 counted at the position of the first detector. Together with the geometry conditions at JET this yields 6 X 10(2) counts per 10(15) neutrons emitted. The energy resolution is in the interval from 125 to 133 keV (FWHM) depending on conditions and is known to an accuracy of +/- 5 keV. Correction for the inevitable random background is dealt with in detail and a reduction procedure valid for fast variations in neutron count-rates is provided. Plasma ion temperatures deduced from the neutron spectra agree within statistical limits with the results from other diagnostic techniques in use at JET. Stable behaviour up to useful count-rates of 3 x 10(3) counts/s have been obtained, making possible the study of neutron spectra on the short time-scales typical of fusion plasmas.