Scintillator based coded-aperture imaging has proven to be effective when applied for X- and gamma-ray detection. Adaptation of the same method for neutron imaging has resulted in a number of propitious systems, which could be potentially employed for neutron detection in security and nuclear decommissioning applications. Recently developed scintillator based coded-aperture imagers reveal that localisation of neutron sources using this technique may be feasible, since pulse shape discrimination algorithms implemented in the digital domain can reliably separate gamma-rays from fast neutron interactions occurring within an organic scintillator. Moreover, recent advancements in the development of solid organic scintillators make them a viable solution for nuclear decommissioning applications as they present less hazardous characteristics than currently dominating liquid scintillation detectors. In this paper existing applications of coded-apertures for radiation detection are critically reviewed, highlighting potential improvements for coded-aperture based neutron source localisation. Further, the suitability of coded-apertures for neutron imaging in nuclear decommissioning is also assessed using Monte-Carlo modelling

There exists an ongoing need to develop and improve methods of detecting radioactive materials. As each radioactive isotope leaves a unique mark in a form of the particles it emits, new materials capable of detecting and measuring these particles are constantly sought. Neutrons and their detectors play a significant role in areas such as nuclear power generation, nuclear decommissioning and decontamination, border security, nuclear proliferation and nuclear medicine. Owing to the complexity of their detection, as well as scarcity of ³He, which has historically been the preferred choice for neutron detection in many application fields, new sensitive materials are sought. Organic and inorganic scintillating crystals have been recognised as particularly good alternatives, and as such systems that utilise them are increasingly common. As they allow investigation of the neutron energy spectra, greater information about the radioactive source can be inferred. Therefore, in this article, an extensive review of scintillating crystals used for neutron detection is presented. By describing the history of scintillating crystals and discussing changes that occurred in their use and development of methods for radiation detection, the authors present a comprehensive overview of the current situation. Supported by a practical example, possible future directions of the research area are also presented.

Pulse shape discrimination performances of single stilbene crystal, pure plastic and 6 Li loaded plastic scintillators have been compared. Three pulse shape discrimination algorithms have been tested for each scintillator sample, assessing their quality of neutron/gamma separation. Additionally, the digital implementation feasibility of each algorithm in a real-time embedded system was evaluated. Considering the pixelated architecture of the coded-aperture imaging system, a reliable method of simultaneous multi-channel neutron/gamma discrimination was sought, accounting for the short data analysis window available for each individual channel. In this study, each scintillator sample was irradiated with a 252 Cf neutron source and a bespoke digitiser system was used to collect the data allowing detailed offline examination of the sampled pulses. The figure-of-merit was utilised to compare the discrimination quality of the collected events with respect to various discrimination algorithms. Single stilbene crystal presents superior neutron/gamma separation performance when compared to the plastic scintillator samples.

The pulse shape discrimination performance of a pixelated organic plastic scintillator has been investigated. The scintillator has been built using 169 plastic scintillator blocks (arranged into a 13 × 13 square array) of 2.8 × 2.8 × 15 mm³each. The scintillator was coupled with a single-channel photomultiplier tube. The scintillator was exposed to a mixed-field environment provided by²⁵²Cf and its pulse shape discrimination capabilities are presented in this paper. Initial results revealed that a 150 MS/s digitising system was insufficient to separate neutrons from gamma-ray photons. Therefore, the experiment was repeated with a 500 MS/s system, which provided improved pulse shape discrimination performance. In order to validate the performance of the pixelated plastic scintillator, it was compared to that of a cylindrical plastic sample. Tests were also carried out in moderated neutron and gamma-ray fields of²⁵²Cf. The results indicate that acceptable levels of pulse shape discrimination are obtained for the case of a pixelated scintillator, when the higher sampling rate digitiser was used.

An investigation into the gamma-ray modulation properties of a tungsten coded aperture, whose design is based on the mathematical principles of Modified Uniformly Redundant Arrays (MURA), has been performed. Due to the small size of the individual cells, the aperture was built using additive manufacturing methods. The gamma-ray field was produced by a ¹³⁷Cs radioactive isotope at Lancaster University, U.K. An organic plastic scintillator sample, which is capable of pulse shape discrimination, has been used to detect the gamma-ray field modulated by a tungsten aperture. Prior to the investigation of the aperture modulation properties, energy calibration of the scintillator was performed. Its pulse shape discrimination capabilities were verified using a ²⁵²Cf fission source. In this study, each of 169 coded aperture cells was investigated by collimating the modulated gamma-ray field of ¹³⁷Cs through a 25.4 mm thick lead supporting plate. The supporting plate has one opening in the centre, of the same dimensions as the single aperture cell, i.e. 2.5 mm × 2.5 mm. The number of pulses detected for every aperture location were recorded in an array. The array was subsequently used to create a two-dimensional image of the source, which was encoded through the coded aperture pattern. Finally, the image was decoded using deconvolution techniques to reveal the actual source location. The new results obtained in this study indicate that sufficient gamma-ray modulation properties of the aperture can be determined, despite the relatively small footprint and thickness of the coded aperture.