Methodology and optimisation of neutron-gamma pulse shape discrimination through specifically engineered scintillators coupled to Silicon Photomultipliers

Nuclear security has strict requirements on hand-held instrumentation. The detectors have to be neutron sensitive, capable to discriminate between neutrons and gamma rays, possibly providing the gamma emitter identification. Furthermore, the detecting unit has to be compact, low power consuming and integrated into systems to perform online or near-realtime data analysis and assess the alarm level according to international standards. The thesis reports a detailed study on factors contributing to achieving required performances with compact detectors based on neutron sensitive scintillators coupled to Silicon Photomultipliers. Those scintillators light time development varies with the ionising density of the interacting particle, making possible a neutron-gamma discrimination via Pulse Shape Analysis. However, SiPM has a sizeable intrinsic capacitance that may tamper the neutron-gamma discrimination capability of the scintillator detector, making necessary the definition of algorithms to guarantee the required discrimination capabilities. Moreover, power consumption and enhanced real-time functionalities push the development and use of Application Specific Integration Circuits (ASIC) and digital systems based on FPGA in the abovementioned tools. In the context of two research projects with AWE and Kromek LTD, the definition and qualification of a new approach to pulse shape discrimination with largearea Silicon Photomultipliers was applied to two newly engineered instruments.