Fast neutron spectroscopy can provide valuable information about the form of the nuclear material – metal or oxide. This is possible since the α-n and fission reactions produce very distinctive neutron energy spectra. While (α,n) neutrons are produced with energy about 2 MeV, the fission neutrons cover larger range of possible energies. Also the burn-up of the nuclear fuel (Pu or U) can be estimated from the ratio of the alpha reaction peak and fusion part of the neutron energy spectra above 3 MeV. Pu is very favorable to this analysis due to the high neutron production of the Pu isotopes. This can be applied to U as well by using a neutron generator. Hydrogenous materials (liquid, plastic or organic crystalline scintillators) are mainly used for these measurements. The main limitation of these materials is that they do not provide direct fast neutron spectroscopy. Since the detection of fast neutrons relies on the scattering process, the energy transfer from the neutron to a hydrogen atom is random and a typical spectrum has Compton-like distribution of gamma rays. When multiple energies are present, the spectrum becomes complicated and requires post measurement unfolding to determine the incident energies. Recently, it has been shown that Cs_2LiYCl_6:Ce (CLYC) scintillator is capable of providing fast neutron energy directly. This is possible due to the ~(35)Cl(n,p) reaction. The protons produced during this capture reactions generate signals that are directly proportional to the energy of incident neutrons, and thus allow for fast neutron spectroscopy. However the thermal neutron interaction with ~6Li produces a peak at about 3 MeV which interferes with the neutron spectrum. Therefore we investigated a version of CLYC with depleted ~6Li. We denoted the composition as C~7LYC. While the standard CLYC version is optimized for thermal neutron detection, the new version is dedicated to fast neutron spectroscopy. To build fast neutron spectrometer neutron-gamma discrimination, neutron energy resolution and linearity of the response are of primary importance. The paper presents the first results with C~7LYC scintillator.
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