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[en] In this paper, characteristics of neutron source localisation using an EJ-426 detector with a polyethylene collimator are presented. The detector is placed inside of the polyethylene collimator, where the collimator has a constant internal diameter and external diameter gradually increases towards the back end. The combined detector-collimator unit was mounted onto an equatorial mount and, data were collected from an ultra-fast mixed-field analyser. Neutron images have been produced to characterise the area investigated in real-time. Radiation images are fundamentally related to the detector efficiency and to the collimator geometry. In order to characterise the neutron source localisation capability of the polyethylene collimator, with an EJ-426 detector, experiments have been carried out using three different neutron sources. Results indicate that the existing design of the polyethylene collimator is suitable for localisation of low energy neutron sources. The measurements were performed in the low-scatter facility of the Neutron Metrology, National Physical Laboratory, Teddington, UK
[en] A series of Monte Carlo simulations have been conducted, making use of the EJ-426 neutron scintillator detector, to investigate the potential of using hexagonal uniformly redundant arrays (HURAs) for scintillator based coded aperture neutron imaging. This type of scintillator material has a low sensitivity to gamma rays, therefore, is of particular use in a system with a source that emits both neutrons and gamma rays. The simulations used an AmBe source, neutron images have been produced using different coded-aperture materials (boron- 10, cadmium-113 and gadolinium-157) and location error has also been estimated. In each case the neutron image clearly shows the location of the source with a relatively small location error. Neutron images with high resolution can be easily used to identify and locate nuclear materials precisely in nuclear security and nuclear decommissioning applications. (authors)
[en] In this paper, a coded-aperture design for a scintillator-based neutron imaging system has been simulated using a series of Monte Carlo simulations. Using Monte Carlo simulations, work to optimise a system making use of the EJ-426 neutron scintillator detector has been conducted. This type of scintillator has a low sensitivity to gamma rays and is therefore particularly useful for neutron detection in a mixed radiation environment. Simulations have been conducted using varying coded-aperture materials and different coded-aperture thicknesses. From this, neutron images have been produced, compared qualitatively and quantitatively for each case to find the best material for the MURA (modified uniformly redundant array) pattern. The neutron images generated also allow observations on how differing thicknesses of coded-aperture impact the system. A system in which a neutron sensitive scintillator has been used in conjunction with a MURA coded aperture to detect and locate a neutron emitting point source centralised in the system has been simulated. A comparison between the results of the different coded-aperture thicknesses is discussed, via the calculation of system error between the reconstructed source location and the actual location. As the system is small scale with a relatively large step along the axis (0.5 cm), it is justifiable to say that the smaller error values provide satisfactory results, which correlate with only a few centimetres difference in the reconstructed source location to actual source location. A general trend of increasing error can be deduced both as the thickness of the coded-aperture material decreases and the capture cross section of the different materials reduces. (authors)
[en] A novel analytical approach is described that accounts for self-shielding of γ radiation in decommissioning scenarios. The approach is developed with plutonium-239, cobalt-60 and caesium-137 as examples; stainless steel and concrete have been chosen as the media for cobalt-60 and caesium-137, respectively. The analytical methods have been compared MCNPX 2.6.0 simulations. A simple, linear correction factor relates the analytical results and the simulated estimates. This has the potential to greatly simplify the estimation of self-shielding effects in decommissioning activities. - Highlights: → We describe a novel analytical approach for self-shielding of gamma radiation. → We examine the correlation between the analytical results with the Monte Carlo simulated results. → We introduce the simple linear correction factor relates the analytical results with the simulated results. → We provide the complete mathematical analysis, which can be easily programmed.
[en] Having been overlooked for many years, research is now starting to take into account the directional distribution of the neutron work place field. The impact of not taking this into account has led to overly conservative estimates of dose in neutron workplace fields. This paper provides a critical review of this existing research into directional survey meters which could improve these estimates of dose. Instruments which could be adapted for use as directional neutron survey meters are also considered within this review. Using Monte-Carlo techniques, two of the most promising existing designs are evaluated; a boron-doped liquid scintillator and a multi-detector directional spectrometer. As an outcome of these simulations, possible adaptations to these instruments are suggested with a view to improving the portability of the instrument. (authors)
[en] In this paper imaging of neutron sources and identification and separation of a neutron source from another neutron source is described. The system is based upon organic liquid scintillator detector, tungsten collimator, bespoke fast digitiser and adjustable equatorial mount. Three environments have been investigated with this setup corresponding to an AmBe neutron source, a "2"5"2Cf neutron source and both sources together separated in space. In each case, events are detected, digitised, discriminated and radiation images plotted corresponding to the area investigated. The visualised neutron count distributions clearly locate the neutron source and, relative gamma to neutron (or neutron to gamma) fraction images aid in discriminating AmBe sources from "2"5"2Cf source. The measurements were performed in the low scatter facility of the National Physical Laboratory, Teddington, UK
[en] Pulse shape discrimination performances of single stilbene crystal, pure plastic and "6Li 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 "2"5"2Cf 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.
[en] A novel technique for assay of thermal and fast neutrons in a "6Li-loaded plastic scintillator is presented. Existing capture-gated thermal neutron detection techniques were evaluated with the "6Li-loaded plastic scintillator studied in this work. Using simulations and experimental work, shortcomings in its performance were highlighted. As a result, it was proposed that by separating the combined fast and thermal neutron events from gamma events, using established pulse shape discrimination techniques, the thermal neutron events could then be assayed. Experiments were conducted at the National Physical Laboratory, Teddington, performing neutron assays with seven different neutron fields using the proposed technique. For each field, thermal and fast neutron content was estimated and were shown to corroborate with the seven synthesised fields
[en] A comparison of four algorithms for the digital discrimination of neutrons and γ-rays in organic scintillation detectors is described. The mixed fields from four different isotopic neutron sources have been investigated with a fast digital processing system. This system has been used to digitise the raw sampled data for each individual pulse collected for each of the mixed radiation fields. The discrimination methods considered are the digital Charge Comparison Method, Pulse Gradient Analysis, Neutron-γ Model Analysis and Simplified Digital Charge Collection. The ratio of the number of γ-ray counts to the number of neutron counts, often referred to as the R-factor, has been calculated for each mixed radiation field. These provide a basis for reference where future R-factor analysis of isotopic sources is to be carried out for the purposes of calibrating discrimination systems or when estimating the γ-ray rejection effectiveness of neutron detector systems based on fast liquid scintillators. The efficacy of each of these methods has been assessed in terms of the figure-of-merit (based on the separation of the event distributions). The SDCC technique is observed to provide the best discrimination performance in this research.
[en] We have performed combined digital imaging of neutrons and γ rays in mixed radiation fields. An organic liquid fast scintillation detector has been integrated into a system with a digitizer, a collimator and an equatorial mount to characterize two mixed-field environments. Images have been produced corresponding to the angular distribution of events and the three-dimensional Cartesian co-ordinate system for both γ rays and neutrons simultaneously. These data indicate that mixed fields can be readily imaged with a single detector in terms of their γ-ray and neutron components at the same time.