Relative Enhancement of a 3 He Ion Chamber Response to Fast Neutrons by TOF Rejection of Slow Neutrons with a Pulsed Neutron Source

As was found in our previous study, the presence of the thermal peak in neutron spectra measured by the 3 He ionization chamber makes it difficult to use its complete response function in an unfolding process. In order to significantly reduce the thermal peak counting rate, an active method of rejecting low energy neutrons (<180 eV) based on their time-of-flight has been developed and tested. The method required a modification of the proton accelerator beam line to produce short pulses of protons. The ratio of the number of detection events above thermal peak to the number of events scored in the thermal peak has been improved by a factor of 5.