Optimisation of silicon detectors for detection of thermal neutrons

Detection of thermal neutrons is necessary in a wide range of contexts from nuclear energy and safeguarding to monitoring of accelerators and radiotherapy treatments. The most popular method of measuring thermal neutron fluxes has historically been the He-3 gas proportional counter. However, due to the global shortage of He-3 there is a need for lower cost, large area thermal neutron detectors which still maintain high efficiency (https://doi.org/10.2172/956899). In addition to this, there is also a need to have detectors with intrinsic spatial resolution that could be used for neutron imaging. Application of converter layers to silicon (e.g. 10B, 6Li) and other solid state detectors in order to make them sensitive to thermal neutrons has long been investigated as an alternative to He-3 detectors. Use of a stable, enriched Lithium fluoride layer (6LiF) uniformly deposited to a thickness of a few microns on silicon has been carried out by Micron Semiconductor Ltd and the characterisation of these devices carried out at Liverpool using an AmBe neutron source located on campus. The benefit of this (Si-6LiF) configuration, is not only the large cross section for thermal neutron detection on 6Li (940b) but the production of two charged fragments with >2 MeV of kinetic energy in the resulting fission reaction after neutron capture (n + 6Li ? a(2.05MeV ) + 3H(2.73MeV)). Since these two charged fragments are produced with sufficient kinetic energy to exit a thin converter layer and reach the neighbouring silicon layer(s), this gives a greater opportunity to register the presence of a neutron. This is an advantage compared with converter layers that only produce a single charged particle with sufficient range (kinetic energy) for detection e.g 10B. Moreover, detecting a coincidence between the alpha and the triton reaction products from 6Li allows suppression of backgrounds, in particular the high fluxes of gamma rays often present with neutron production that cause false events to be registered as neutrons. The aims of the CASE award proposal outlined here will be to build on previous work (doi: 10.1117/12.2236752) carried out with Micron sensors and converters to deliver the following results:

Optimisation of the converter deposition process on silicon to obtain a thermal neutron converter layer that maximises sensitivity to thermal neutrons
Application of the above converter layer to segmented detectors such as pixels and strip detectors.
Characterisation of these detectors in the lab and also in neutron sources and neutron beams
Investigation of how temporal and spatial resolution can improve detection efficiency for single and for multiple layers to better compete with He-3
Investigation, (via simulation and/or measurement) into the application of this technology to neutron monitoring in clinical radiotherapy beams