The Spatial Response of a Multiplying System to a Periodic Neutron Source

Abstract

The source transfer function has been measured in a natural uranium-light water moderated sub-critical assembly as a function of source frequency and position. A square wave input of fast neutrons from the D-T reaction was supplied by a SAMES type J accelerator suitably modified for the purpose. Thermal neutrons inside the assembly were detected with lithium glass scintillation counters and the time variation of the detector outputs was recorded on a multi-channel analyser used in the time sequence storage mode and synchronised with the source pulsing frequency. By Fourier analysing the recorded waveform the source transfer function was found. An attempt was also made to determine the source transfer function by detecting the prompt gamma radiation from fission. Two theoretical models were used to determine the spatially dependent source transfer function for a system similar to the experimental one. The first was a multigroup, multi-region two dimensional approach based on diffusion theory and Telegrapher's equation which used a standard static computer code (SNAP) to obtain the transfer function at chosen frequencies. The other was based on the time-dependent Fermi Age and diffusion theory and was limited to a single region description of the system. Both methods gave good agreement with each other and with experiment but the first method showed an upper frequency limit of 400 Hz due to solution methods used in the code.