Development and experimental validation of response modelling for time-of-flight neutron detection and imaging systems

Development and experimental validation of response modelling for time-of-flight neutron detection and imaging systems

Development and experimental validation of response modelling for time-of-flight neutron detection and imaging systems 150 150 UKAEA Opendata
UKAEA-CCFE-CP(23)08

Development and experimental validation of response modelling for time-of-flight neutron detection and imaging systems

Conventional means of fast neutron detection typically involves moderation and subsequent detection of thermal neutrons via gas filled detectors such as He-3, or alternatively indirect neutron detection via gamma activation systems. Whilst these are often the most conclusive systems for neutron detection, inherent timing and energy information associated each neutron is lost. Parallelised developments in organic plastic scintillators and fast digitiser technology, capable of resolving incident pulses on nanosecond timescales, have facilitated an alternative practical means of direct fast neutron detection. Plastic scintillators such as EJ-309 are sensitive to both photon and neutron interactions. Photon induced secondary electron generation and neutron induced secondary recoil proton generation form distinct, characteristic pulse shapes in the pre-amplification signal. Real-time pulse shape discrimination algorithms incorporated onto fast digitiser firmware enable real-time signal type attribution to either photons or gammas; this unique capability of a single detection system has a novel application: time-of-flight neutron detection. The application and feasibility of a time-of-flight neutron detection system is explored for sources with time correlated gamma and neutron emissions, such as the spontaneous fission emitter, Cf-252. For the emission of multiple gammas and neutrons from a single spontaneous fission, a near instantaneous gamma detection, followed by a later neutron detection on a multi-detector array, allows for an associated time-of-flight to be determined for that event. High fidelity Monte-Carlo modelling is used to simulate the response of a real system, capturing the fission signature of the neutron source, in order to predict the response of a multi-detector array and assess its feasibility as an imaging system. This technique could have multiple applications in the nuclear industry, such as portal monitoring, spent nuclear fuel monitoring, and diagnostic applications in both fission and fusion devices. Three main areas have been extensively researched in this paper. Firstly, an assessment of imaging capability has been determined for many pseudo-random detector arrays, attributing a figure of merit to each configuration. The findings of this study have been used in conjunction with a UKAEA developed computational toolkit, ‘pytrac’, to model and optimise real time-of-flight detection systems, enabling event-by-event visualisation, detector response characterisation and the reconstruction of neutron emission maps using a choice of 3D image construction algorithms. The final aspect of our work involves modelling of the water moderated Cf-252 source facility at Lancaster University, replicating multiple experiments to provide validation of the modelling approach. Initial findings indicate solid agreement between the modelling and experimental approach.

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PHYSOR 2020, University of Cambridge, United Kingdom, 29 March - 2 April 2020