Optimisation of the time-of-flight geometry for fast neutrons

It is usually assumed that the conventional ring geometry is the optimum configuration for time of flight measurements. It has been shown in this paper that this is not always so and that a surface of revolution geometry is often superior to a ring geometry. A geometrical method has been developed to estimate the widths of lines in the time-of-flight spectrum due to scatters of different shapes and sizes. Assuming a paraxiality spread from 2° to 8°, the shape of lines of elastically scattered neutrons from primary fast neutrons in the 14 MeV range has been calculated. Elastic scattering off hydrogenous converters has been considered for discussing the energy spread of secondary 'monoenergetic' neutrons in the 8-13 MeV range.