3He Neutron Spectrometer Using Pulse Risetime Discrimination

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Abstract
A 3He filled proportional counter can be used for neutron detection in the range 100 keV to 8 MeV by using the 3He(n,p)T reaction. The major limitation as a neutron spectrometer is that the 3He recoil distribution, arising from the elastic scattering of the higher energy neutrons present, masks the 3He(n,p)T peaks due to lower energy neutron groups. Since a 3He recoil and a proton of equal energies have difference specific ionizations and, therefore, different ranges in the counter filling, one thus has a means to distinguish between pulses from the 3He(n,p)T and 3He(n,n)3He reactions. For appropriate operating conditions the risetime of the pulses for these two events will be different. By converting this risetime to a pulse height and only accepting pulses with long risetimes [which correspond to protons from the 3He(n,p)T reaction], one has a neutron spectrometer useful in an energy interval between the maximum neutron energy present and that energy for which the proton range equals the range of the maximum energy 3He recoil. Spectra of monoenergetic neutrons obtained with a 3He filled proportional counter and the pulse risetime discrimination are presented to illustrate the energy range over which this detector may be used as a neutron spectrometer. Under typical operating conditions, spectra of the 24Mg(d,n)25Al, 28Si(d,n)29P, 32S(d,n)33Cl reactions were obtained to illustrate the performance of the spectrometer.

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