Obtaining Directionality from Scattering and Capture in Fast Neutron Detectors

Abstract

Neutron detection with directional sensitivity is an increasingly sought-after technology in nuclear safety and security. To keep the nuclear material secure, an accurate assessment of its material composition, quantity, and location is needed. One approach to achieve position-sensitive neutron detection is the use of recoil-capture composite scintillators comprising of a neutron recoil and neutron capture component coupled with an array of SiPM pixels. This dissertation encompasses work to advance the development of directional recoil-capture neutron scatter cameras, which share much of the underlying technology with highly segmented reactor antineutrino detectors. To discriminate neutrons from gamma radiation, differences in pulse shapes are exploited in a process known as pulse shape discrimination (PSD). This work extends previous work on heterogeneous composite detectors that comprise a PVT recoil component and a 6Li glass capture component. In such detectors, it is found that charged particles depositing energy through two different materials can produce pulse shapes with characteristics dependent in proportion of energy deposited in each material. The nature these pulses are investigated through experimentation with a representative composite detector and in a Geant4 simulation to find energy deposition fractions. An estimate for the resulting PSP is made by combining PVT and glass waveforms from averaged experimental waveforms and the simulated energy deposition fraction. From this, the gamma rejection was reconstructed. Estimates for various 6Li glass loadings were obtained. Discrepancies are identified between the simulation and experiment, and simulation of the effect of uneven shard distribution suggests a possible explanation. To gain insight into the potential contribution that capture location information can have on improving directionality information, a Bayesian updating approach is applied to an idealized spherical detector. Lookup tables describing the likelihood of the interaction types and locations were constructed with Geant4 simulations. Bayesian updating was then performed on a sequence of simulated events to find the improvement of the angular estimate with each additional incident neutron. The recoil-only case was compared to the case with recoil with capture. In cases where the neutron/gamma discrimination threshold is high, neutron capture was able to improve an angular estimate of incoming neutron direction by 20 degrees for 100 incident neutrons due to the recovered recoil events that would fall below the discrimination threshold. In practice, arrays with tens to hundreds of channels are needed to attain acceptable event position resolution. The feasibility of using an ASIC was investigated for compact data acquisition. The TOFPET2 ASIC was adapted for PSD data acquisition. This was done by splitting an input signal from a SiPM into two channels to be integrated over different periods. PSD was shown for a stilbene cube exposed to a 252Cf source mounted on a SiPM, with a figure of merit of 1.190. An LED-based calibration system was developed to correct for nonlinearities in the SiPM/ASIC system. To obtain the calibration curve, an LED powered by a square pulser with varying pulse lengths and amplitudes produced light pulses and was measured by the SiPM/ASIC system, compared against a photodiode with full waveform digitization. A dependence of the calibration curve on pulse shape was observed at intense light levels. An operational range was established where the calibration curve is independent of pulse shape, and spectroscopy using stilbene attached to a SiPM was performed on gamma sources, a DD neutron generator source and a 252Cf source.