SIMULATION OF THE NEUTRON AND PROTON TRANSPORT IN THE 14 MEV NEUTRON TIME-OF-FLIGHT SPECTROMETER, TANSY

A Monte Carlo code simulating the neutron and proton transport in the spectrometer TANSY has been written in order to obtain more accurate knowledge of the response functions. TANSY is a combined proton-recoil and neutron time-of-flight spectrometer for 14 MeV neutrons to be used for fusion plasma diagnostics at the Joint European Torus (JET). In the code developed, the simulation starts with a sampled neutron scattering point in a polyethylene foil followed by a uniformly sampled position on the surface of one of 16 neutron detectors. In the foil an (n, p) scattering occurs and the transport of the recoil proton and the scattered neutron is simulated. For the recoil proton, passing through the foil, effects of stopping power and energy straggling are considered. The neutron transport in the scintillator detector is simulated until either a signal has occurred or the remaining neutron energy is less than 100 keV. Multiply scattered neutrons in the neutron detector are allowed to "build up" a signal until a bias level is exceeded. The results from Monte Carlo calculations have been compared with measurements using 14 MeV neutrons from a neutron generator, and a good agreement was obtained. Comparing Monte Carlo calculations and measurements it has also been possible to estimate the energy spread of the neutron generator to 173 keV at a beam energy of 300 keV and a reaction angle of 90-degrees. For a foil thickness of 0.95 mg/cm2 the Monte Carlo calculations gave as result the efficiency (1.38 +/- 0.01)X10(-5) cm2 and the resolution 155.1 +/- 0.2 keV for the complete spectrometer.