Monte Carlo simulations of a back scatter time-of-flight neutron spectrometer for the purpose of concept testing

The work focuses on Monte Carlo simulations for finding the optimal back scatter time-of-flight spectrometer design for concept testing at the NESSA facility at Uppsala University. The spectrometer consists of two scintillator detectors, D1 (placed in a neutron beam) and D2 (placed in front of D1), at some distance from each other. A fraction of the neutrons that impinge on D1 back scatter into D2 and information on the neutron energy distribution is acquired using the time-of-flight method. For the given constraints on geometry, resolution and efficiency a best resolution was found to be 6.6% with a corresponding efficiency of 10−4 which gives a sufficient count rate for a neutron generator producing 1011 neutrons/s. In order to achieve a minimum of 10 000 counts/h with the same setup a D2 with an area of at least 7 cm2 is required.