Optical filtering based neutron/γ-ray discrimination in a phoswich detector

Abstract

Abstract Neutron/ γ -ray ( n ∕ γ ) discrimination is a crucial issue in neutron measurements and is usually accomplished employing detectors transparent to γ -rays (3He proportional counters), or techniques for γ rejection (such as Pulse Shape Discrimination). The aim of this work is to test a system able to perform a quasi-digital n ∕ γ discrimination without using techniques based on pulse shape analysis. The system is based on a phoswich consisting of a LiCAF:Eu crystal sensitive to neutrons and two LuAG:Ce ceramics sensitive to γ -rays. Optical filtering allows obtaining two separate acquisition channels, which selectively collect the light emitted by only one of the two scintillators. Photon induced events are then discarded through an anti-coincidence algorithm. The results show that the proposed technique is actually capable of discarding γ -induced events, but the implemented setup downgrades the LiCAF:Eu light collection because of the light attenuation of glasses serving as filters. In some situations, it is preferable to employ the LiCAF:Eu crystal alone, discarding the γ -induced events on the basis of the signal amplitude. Therefore, the discrimination technique proposed with this setup is intended for situations in which the energy of the incoming photons or the pile-up effects related to the high fluence rate can generate pulses whose amplitude is comparable to a neutron event. In addition, it was noted that some optical filters contain impurities (10B in our case) which capture neutrons lowering the system detection efficiency. A Monte Carlo model was also implemented to replicate the system response and predict an optimal configuration for further development.