| Popis: |
This paper describes the results from calculations with a Monte Carlo code, McKM3, which treats the neutron transport of an upgraded 2.45 MeV neutron time-of-flight spectrometer recently installed at the JET Joint Laboratory. The old instrument has been successfully operated at JET for several years and measured the energy spectrum of the neutron emission from hot deuterium plasmas. The upgraded version of this instrument, with the same resolution and with circa six times the efficiency, consists of two identical and cuboid organic scintillator start detectors and 30 identical organic scintillator stop detectors. The cylindrically shaped stop detectors are evenly distributed on the periphery of a constant time-of-flight sphere and positioned at a 30° scattering angle with respect to the start detectors. In this work, the response function for different thicknesses and areas of the start detectors has been investigated. Apart from finding a suitable compromise between efficiency and resolution, it is also important to find such a geometry that keeps the tails of the response function, arising from multiply scattered neutrons in the start detectors, at a low level. A suitable geometry of the start detectors was calculated to possess an area of 32 cm 2 and a thickness of 2 cm. This geometry gave resolution 116.8±0.5 keV and efficiency (6.25±0.10)×10 −2 cm 2 for 2.45 MeV source neutrons. For this geometry, the fraction of singly scattered neutrons in the start detectors was ⋍ 93% and the ratio between the amplitudes of the low energy tail and the peak of the response function was ⋍ 0.7%. |
