Beta particle energy spectra shift due to self-attenuation effects in environmental sources

In order to predict and control the environmental and health impacts of ionising radiation in environmental sources such as groundwater, it is necessary to identify the radionuclides present. Beta-emitting radionuclides are frequently identified by measuring their characteristic energy spectra. The present work shows that self-attenuation effects from volume sources result in a geometry-dependent shift in the characteristic spectra which needs to be taken into account in order to correctly identify the radionuclides present. These effects are shown to be compounded due to the subsequent shift in the photon spectra produced by the detector, in this case an inorganic solid scintillator (CaF2:Eu) monitored using a Silicon Photomultiplier (SiPM). Using tritiated water as an environmentally relevant, and notoriously difficult to monitor case study, analytical predictions for the shift in the energy spectra as a function of depth of source have been derived. These predictions have been validated using Geant4 simulations and experimental results measured using bespoke instrumentation

Design and Optimisation of a Three Layers Thermal Neutron, Fast Neutron and Gamma-Ray Imaging System

The design and configuration of a multi-layered imaging system with the ability to detect thermal neutrons, fast neutrons and gamma rays has been developed and its efficacy demonstrated. The work presented here numerically determines the systems efficiency and spatial resolution, using 252Cf and 137Cs as a case study. The novelty of this detection system lies in the use of small form factor detectors in a three-layer design, which utilises neutron elastic scattering and Compton scattering simultaneously. The current configuration consists of 10 mm thick natural lithium glass (GS10) scintillator integrated with a 20 mm thick plastic scintillator (EJ-204) in the first layer, a 15 mm thick lithium glass (GS10) scintillator in the second and a 30 mm thick CsI(Tl) scintillator forming the final layer. Each of these layers is backed with an 8 x 8 silicon photomultiplier diode (SiPM) array. The overall size of the imaging system is 27 mm x 27 mm x 135 mm. MCNPv6.1 and Geant4-10.04 were alternatively used to optimise the overall configuration and to investigate detection modalities. Results show promising performance with high precision source localisation and characterization abilities. Measurements were virtually obtained of two gamma-ray sources within steel enclosures at angles of 15o, 30o and 50o separation in order to test spatial resolution ability of the system. With the current active size of the system and the 8x8 SiPM configuration, the results estimate the spatial resolution to be close to 30o. The ability of the system to characterise and identify sources based on the type and energy of the radiation emitted, has been investigated and results show that for all radiation types the system can identify the source energy within the energy range of typical reported sources in literature

Neutron/Gamma Pulse discrimination analysis of GS10 Lithium glass and EJ-204 plastic scintillators

Two radiation sensitive scintillators known for their dual sensitivity to neutron and gamma-ray fields are investigated for their pulse discrimination abilities; a lithium glass GS10 inorganic scintillator and a fast organic plastic scintillator EJ-204. Each of these scintillators are optically coupled with an 8x8-silicon photomultiplier array to act as a photodetector. Pulse height analysis, the charge comparison method and pulse gradient analysis have all been applied here on neutron and gamma-ray events generated by a Cf-252 source. The three discrimination methods were evaluated based on the figure of merit of the probability density plots generated. Within a GS10 crystal, it has been deduced that pulse height analysis and pulse gradient analysis possess greater abilities to discriminate between the two radiation fields compared to the charge comparison method with both showing a figure of merit of over one. The charge comparison method indicated a lower discrimination ability with a figure of merit around 0.3. When the EJ-204 detector was used, it was deduced that only pulse height analysis exhibits discrimination abilities with a figure of merit around 0.6, while the other two discrimination methods presented no distinction between the two radiation fields

Development of an optimized converter layer for silicon carbide based neutron sensor for the detection of fissionable materials

Here, we describe the early stage design, construction and testing of a miniature silicon carbide diode neutron sensing instrument. It is intended that a more mature version of this instrument will be used as part of a robotic manipulator to investigate various parts of the stricken Fukushima nuclear power plant. Here, three such silicon carbide based proto-type sensors have been created, two of which have differing thicknesses of boron-10 deposited on, with the final one left bare. The thicknesses and materials chosen have been informed via Monte Carlo software (MCNP 6.2) which was also used to assess the suitability of two other potential converter materials – Lithium-6 and gadolinium-157. The work goes on to describe the design, construction and testing of the prototype device at two sites around the UK. The project is part of a UK/Japanese collaboration between Lancaster University and Kyoto University and is supported by an EPSRC grant via the UK Japan Civil nuclear research program

Signal Discrimination in Thinned Silicon Neutron Detectors using Machine learning

High gamma backgrounds can pose a significant source of interference in solid-state neutron detectors making the neutron flux approximation inaccurate. This work focuses on optimizing a thin sensor thickness to enhance the neutron capture rate and reject gammas, and analysis of multiple input source through the differentiation of signals using pattern recognition. Gamma isotopes and neutron spectrums have been simulated using GEANT4 + Electronic noise estimation. Different machine learning tools have been considered to discriminate different gamma and neutron sources, including PCA, RNN, SVM, KNN, ResNet and others

Issues in Vision, Semi-autonomous Control, Haptics and Manipulation in Robotics for Nuclear Decommissioning

Traditionally, the nuclear industry has preferred the use of tele-operated control within robotic applications such as decommissioning. This is due to obvious safety reasons, along with other less apparent motivations such as the safeguarding of industry jobs, and a lack of coding expertise in the industry. However, problems with the use of such techniques have been evident within the past few years, mostly associated with operator fatigue leading to errors. A typical modern autonomous robotic system will utilise some sort of stereoscopic 3D vision system (often based on LIDAR) to aid recognition. However, this information can be hard to relay to a human tele-operator not used to such information. Further, tele-operation of a modern robot is a truly complex and specialised skill, and there are a lack of people with in the nuclear industry (and indeed industry as a whole) with these skills. A potential solution to alleviate these problems may be in the use of semi-autonomous control where the robotic artificial intelligence may be used for low-level tasks while the human operator would handle the higher-level decisions. Instead of the operator having to directly control the robot via two joysticks, the operator would be more likely to be confronted with a large touchscreen complete with a list of tasks and highlighted objects on which the robot can perform them upon

Development of an Optimized Converter Layer for a Silicon-Carbide-Based Neutron Sensor for the Detection of Fissionable Materials

We describe the early stage development of a miniature silicon carbide neutron sensor, for applications including robotic monitoring at the Fukushima Daiichi nuclear power plant, specifically, within the primary containment vessel for fuel debris detection and retrieval. Monte Carlo simulations using MCNP 6.2 and Geant4 10.05.01 are used to investigate and optimize converter layers for thermal neutron detection. Performance of a 10B4C:SiC detector system is investigated in detail and a neutron detection efficiency ∼4% is predicted, with a gamma discrimination ratio of the order of 105

Heterogeneous Scintillator Geometries to Maximise Energy Deposition for Waterborne Beta Particle Detection

Here the geometries that maximise detection efficiency of heterogeneous scintillators used to detect beta particles in aqueous solutions by maximising energy deposition are described. The determination of the geometry was achieved with the Monte Carlo code Geant4 using CaF2:Eu scintillator as a pertinent case study, and validated with experimental data using single crystal CaF2:Eu and heterogeneous CaF2:Eu scintillators. Both 2D and 3D structures composed of arrays of primitive unit cells of packed spheres were examined to find the optimal geometry to maximise detection of volumetric sources of tritium and aqueous Carbon 14 and Lead 210. The 2D structures were evaluated relative to a single crystal scintillator and results show the detection efficiency of the 2D structures is maximised when the sphere radius is c.a. 0.46x the maximum track length of the beta particle in the scintillator. Data for the 3D structures show that the efficiency is maximised when the sphere radius is minimised, but it is further shown that practical issues limit the minimum radius that can be used for transient radiological contamination monitoring

Feedback control-based inverse kinematics solvers for a nuclear decommissioning robot

The article develops two novel feedback control-based Inverse Kinematics (IK) solvers. They are evaluated for a dual-manipulator mobile robotic system with application to nuclear decommissioning. The first algorithm has similarities to other feedback control based solvers, and borrows ideas from the Cyclic Coordinate Decent and the Jacobian Transpose methods. This yields a particularly straightforward algorithm with tunable Proportional-Integral-Derivative (PID) gains to determine performance. The second approach utilises a discrete-time state space modelling framework to solve the IK problem. Although the second solver is more complex to implement, preliminary simulation results for the case study example, show that it can converge quicker, and has improved immunity to the kinematic singularities that can occur in Jacobian based methods

Invisible design: exploring insights and ideas through ambiguous film scenarios

Invisible Design is a technique for generating insights and ideas with workshop participants in the early stages of concept development. It involves the creation of ambiguous films in which characters discuss a technology that is not directly shown. The technique builds on previous work in HCI on scenarios, persona, theatre, film and ambiguity. The Invisible Design approach is illustrated with three examples from unrelated projects; Biometric Daemon, Panini and Smart Money. The paper presents a qualitative analysis of data from a series of workshops where these Invisible Designs were discussed. The analysis outlines responses to the films in terms of; existing problems, concerns with imagined technologies and design speculation. It is argued that Invisible Design can help to create a space for critical and creative dialogue during participatory concept development