Results from Monte Carlo simulations of the neutron transport for the new 2.45 MeV neutron time-of-flight spectrometer for the JT-60U tokamak

A 2.45 MeV neutron time-of-flight spectrometer is under design and construction for the measurements of neutron energy spectra from the JT-60U Tokamak. This paper describes the results from the Monte Carlo code, McTOF, which was written for the simulations of the response functions of the spectrometer. The spectrometer consists of two fast plastic scintillators (50 and 1800 cm2; thickness, 2 cm) located on a constant time-of-flight sphere. By introducing a second time-of-flight sphere, it was found that if the first scattering scintillator was tilted an angle of 70° (utilizing the properties of the second sphere), the resolution of the instrument improved by more than 30%. For a given standard set-up of the spectrometer, the resolution was obtained as 105 keV (4.3%) with an efficiency of 2.8×10−2 cm2. This means that for high JT-60U neutron yields (∼1016 n/s), five to 10 neutron spectra can be measured per plasma pulse. The simulated response functions showed a Gaussian distribution. The parameters of the Gaussian could then be expressed as third-degree polynomials in the source neutron energy interval 1.6–3.6 MeV. This implies that, to extract the measured neutron spectrum, a quick procedure for the folding of a guessed source neutron energy spectrum and an analytical value of the response function can be made.