The identification and characterisation of nuclear material is important for a number of security, safeguards and verification purposes. Nuclides susceptible to nuclear fission produce a characteristic energy distribution of fission neutrons, known as the Prompt Fission Neutron Spectrum (PFNS). The energy distribution of the PFNS can be indicative of the material under observation; the measurement of the energy spectrum of neutrons however is notoriously difficult, due to the stochastic manner in which they interact with radiation instrumentation. Time of flight (TOF) spectrometry however is one way of deterministically measuring the neutron energy.
This research has used a variety of the associated particle TOF technique, to perform real time TOF spectrometry of fission nuclides with a small-scale portable set up, triggering on the prompt fission gamma emission. In order to conduct this, the low energy threshold of EJ-309 has been determined experimentally via monoenergetic neutron irradiations to be 0.75 MeV, and efficiency curves have been fitted to these data to ensure selfconsistency.
The TOF spectrometry system has been developed and used to measure the prompt neutron energy spectrum of 252Cf and 244Cm, the latter of which has never been explored in this way. The Maxwellian fission spectrum has been fitted to the PFNS of 244Cm with a parameter T= 1.33 MeV (±0.7) and the Watt spectrum has been fitted with parameters of a =0.918 and b =1.151. On the basis of these measurements it is thought that this small scale, real time TOF spectrometer is able to discriminate between certain nuclides.