Time-of-flight measurements of the neutron spectrum in a sub-critical fast reactor assembly

Abstract

The neutron energy spectrum at the centre of a sub-critical vERA 7A assembly has been determined by the time-of-flight method in the range 100 eV 4·8 MeV using the Harwell 45 MeV electron linear accelerator to provide a pulsed source of fast neutrons. A determination of the spectrum in a closely similar assembly at Aldermaston has been reported previously. The Aldermaston determination used time-of-flight measurements on a sub-critical system for energies below 40 keV and proton recoil measurements in a critical system for the higher energies. Comparison with the shape of the present spectrum shows good agreement below 200 keV but if both spectra are normalized in this region the Harwell measurement shows fewer high-energy neutrons, the discrepancy being about 25 per cent in the region of 1 MeV. Part of this difference is attributed to the different criticality conditions of the two measurements but there still remains an unexplained discrepancy at high energies. A diffusion theory calculation of the spectrum in a spherical equivalent of the assembly appears to be in rather better agreement with the present measurement than with the Aldermaston measurement.