Development of a Double Scatter Time-of-Flight Technique for Fast Neutron Spectroscopy

To fulfill the demand of applications utilizing accelerator-based neutron sources, techniques for characterizing neutrons with energies up to tens of MeV are essential. The time-of-flight (TOF) represents a straightforward method for measuring fast neutron spectra. However, the necessity of a long flight distance in conventional TOF to achieve a high time resolution for measuring neutrons with energies > 10 MeV tends to limit the use of TOF in compact spaces. Hence, we are motivated to develop a double scatter TOF technique suitable for installation in confined spaces to characterize fast neutrons. A double scatter TOF spectrometer, composed of a trigger and a record organic scintillation detector, was implemented to evaluate the accuracy of a measured spectrum for neutrons emitting from an AmBe source. The energy of an incident neutron, En, is the summation of the energy of recoil proton, Ep′, in the trigger detector and the energy of scattered neutron, En′, determined by the TOF between the trigger and the record detector. As Ep′ is estimated from the light yield in the trigger detector, our simulations indicate that events related to neutron multiple-scattering in the trigger detector can cause significant underestimation of neutron energy. Setting appropriate discrimination thresholds for signals from the two detectors can effectively remove multiple-scattering events in a measurement and improve the accuracy of a retrieved spectrum. When using discrimination thresholds of 0.125 and 0.5 MeVee for the trigger and the record detector, respectively, the experimentally measured spectrum for neutrons from the AmBe source exhibits good agreement with the ISO reference spectrum in the range of 5.5 MeV to 11 MeV. Our results emphasize the significance of setting appropriate discrimination thresholds for detectors in a double scatter TOF spectrometer, which is anticipated to facilitate the measurement of fast neutrons with energies exceeding 5 MeV in confined spaces.