S. Quillin

Detector requirements for a cosmic ray muon scattering tomography system

Cosmic ray muon scattering tomography is one of four techniques currently being investigated at AWE for the detection of special nuclear material (SNM). In order to develop a prototype muon detection system, it is necessary to consider the requirements of the radiation detectors with respect to; coincidence timing for system triggering; tracking of the muon trajectory; and determination of muon energy. The detector requirements for a prototype muon scattering tomography system are presented and a variety of detector types considered and assessed against these requirements. The advantages, disadvantages, potential compromises and compatibility with other complementary detection techniques are discussed. Future plans are outlined for an initial prototype and future, long-term development of a muon scattering tomography system for detection of SNM.

A high resolution resistive plate chamber tracking system developed for cosmic ray muon tomography

This work describes the performance of a muon tracker built with high resolution glass resistive plate chambers. The tracker is the result of a collaboration between University of Bristol and the Atomic Weapon Establishment to develop a reliable and cost effective system to scan shipping containers in search of special nuclear materials. The current setup consists of 12 detection layers, each comprised of a resistive plate chamber read out by 1.5 mm pitch strips. For most of the layers we achieved an efficiency better than 95%, a purity above 95% and a signal-to-noise ratio better than 300. A spatial resolution better than 500μm was obtained for most layers, thus satisfying the main requirements to apply resistive plate chambers to cosmic ray tomography.

A binned clustering algorithm to detect high-Z material using cosmic muons

We present a novel approach to the detection of special nuclear material using cosmic rays. Muon Scattering Tomography (MST) is a method for using cosmic muons to scan cargo containers and vehicles for special nuclear material. Cosmic muons are abundant, highly penetrating, not harmful for organic tissue, cannot be screened against, and can easily be detected, which makes them highly suited to the use of cargo scanning. Muons undergo multiple Coulomb scattering when passing through material, and the amount of scattering is roughly proportional to the square of the atomic number Z of the material. By reconstructing incoming and outgoing tracks, we can obtain variables to identify high-Z material. In a real life application, this has to happen on a timescale of 1 min and thus with small numbers of muons. We have built a detector system using resistive plate chambers (RPCs): 12 layers of RPCs allow for the readout of 6 x and 6 y positions, by which we can reconstruct incoming and outgoing tracks. In this work we detail the performance of an algorithm by which we separate high-Z targets from low-Z background, both for real data from our prototype setup and for MC simulation of a cargo container-sized setup. (c) British Crown Owned Copyright 2013/AWE

A readout system for a cosmic ray telescope using Resistive Plate Chambers

Resistive Plate Chambers (RPCs) are widely used in high energy physics for both tracking and triggering purposes. They have good time resolution and with finely segmented readout can also give a spatial resolution of better than 1 mm. RPCs can be produced cost-effectively on large scales, are of rugged build, and have excellent detection efficiency for charged particles. Our group has successfully built a Muon Scattering Tomography (MST) prototype, using 12 RPCs to obtain tracking information of muons going through a target volume of ~ 50 cm × 50 cm × 70 cm, reconstructing both the incoming and outgoing muon tracks. We describe a readout system for fine-pitch RPCs using MAROC3 readout chips capable of scaling to a large system.

A drift chamber tracking system for muon scattering tomography applications

Muon scattering tomography (MST) allows the identification of shielded high atomic number (high-Z) materials by measuring the scattering angle of cosmic ray muons passing through an inspection region. Cosmic ray muons scatter to a greater degree due to multiple Coulomb scattering in high-Z materials than low-Z materials, which can be measured as the angular difference between the incoming and outgoing trajectories of each muon. Measurements of trajectory are achieved by placing position sensitive particle tracking detectors above and below the inspection volume. By localising scattering information, the point at which a series of muons scatter can be used to reconstruct an image, differentiating high, medium and low density objects. MST is particularly useful for differentiating between materials of varying density in volumes that are difficult to inspect visually or by other means. This paper will outline the experimental work undertaken to develop a prototype MST system based on drift chamber technology. The planar drift chambers used in this prototype measure the longitudinal interaction position of an ionising particle from the time taken for elections, liberated in the argon (92.5%), carbon dioxide (5%), methane (2.5%) gas mixture, to reach a central anode wire. Such a system could be used to enhance the detection of shielded radiological material hidden within regular shipping cargo.

A novel technique to detect special nuclear material using cosmic rays

We present a novel method to detect special nuclear material using cosmic rays. Muon Scattering Tomography (MST) is a method in homeland security for scanning cargo containers and vehicles for special nuclear material with cosmic muons. These are abundant, highly penetrating, not harmful against organic tissue, cannot be screened against, and can easily be detected. Muons undergo multiple Coulomb scattering when passing through material, and the amount of scattering is proportional to the ℤ2 of the material. By reconstructing incoming and outgoing tracks, we can obtain variables to determine the ℤ of the target material. In a real life application, this has to happen on a timescale of 1 min and thus with small numbers of muons. We have built a detector system using resistive plate chambers (RPCs). 12 layers of RPCs allow for the readout of 6 × and 6 y positions, by which we can reconstruct incoming and outgoing tracks. In this work we detail the performance of two algorithms by which we separate high-ℤ targets from low-ℤ background.

Angle Statistics Reconstruction: a robust reconstruction algorithm for Muon Scattering Tomography

Muon Scattering Tomography (MST) is a technique for using the scattering of cosmic ray muons to probe the contents of enclosed volumes. As a muon passes through material it undergoes multiple Coulomb scattering, where the amount of scattering is dependent on the density and atomic number of the material as well as the path length. Hence, MST has been proposed as a means of imaging dense materials, for instance to detect special nuclear material in cargo containers. Algorithms are required to generate an accurate reconstruction of the material density inside the volume from the muon scattering information and some have already been proposed, most notably the Point of Closest Approach (PoCA) and Maximum Likelihood/Expectation Maximisation (MLEM) algorithms. However, whilst PoCA-based algorithms are easy to implement, they perform rather poorly in practice. Conversely, MLEM is a complicated algorithm to implement and computationally intensive and there is currently no published, fast and easily-implementable algorithm that performs well in practice. In this paper, we first provide a detailed analysis of the source of inaccuracy in PoCA-based algorithms. We then motivate an alternative method, based on ideas first laid out by Morris et al, presenting and fully specifying an algorithm that performs well against simulations of realistic scenarios. We argue this new algorithm should be adopted by developers of Muon Scattering Tomography as an alternative to PoCA.

Current status of AWE programmes for remote detection of SNM

AWE has recently commenced a research programme to develop techniques for the remote detection of illicitly trafficked, special nuclear material. The result of a review of potential techniques to remotely detect special nuclear material (SNM) conducted by AWE has stimulated a programme at AWE to further investigate four remote detection techniques. These are a photonuclear based and a neutron based active interrogation technique, the use of nuclear resonance fluorescence and cosmic ray muon scattering tomography. The current status of work in the development of these techniques is presented with particular emphasis on the photonuclear and neutron techniques. Developments in physics modeling capabilities, active source technologies, detector technologies and data fusion and analysis approaches are summarised for each technique where applicable. Particular attention is given to the development of high current, pulsed power based, active interrogation sources for remote detection of SNM. Potential technologies for active source technology are discussed and requirements for further technology developments in this area are presented

Advanced active interrogation sources for remote detection of special nuclear material

AWE has recently commenced a research programme to develop techniques for the remote detection of illicitly trafficked, special nuclear material. Three techniques that are being investigated as part of this programme utilise an active interrogation source to stimulate a radiation signature within SNM. A photonuclear technique and nuclear resonance fluorescence require an active gamma source while a technique stimulating fission with low energy neutrons requires a low energy neutron source. The potential for consolidation of the photonuclear and neutron technique is discussed. Conceptual designs of intense, pulsed, ion beam diodes to generate suitable sources of 6-7MeV gamma photons for the photonuclear technique and ≪100keV neutron sources for the active neutron technique are presented. The sources required for remote detection via the 60keV neutron or photofission techniques are contrasted with the source requirements for nuclear resonance fluorescence.

Current status of AWE programmes into stand-off detection of special nuclear material (SNM)

A brief overview of an AWE survey of existing techniques to detect SNM will be presented. This survey has identified two techniques that incorporate active gamma sources for further study. These are a photonuclear based technique and the use of nuclear resonance fluorescence. The current status of work in the development of these techniques is presented. Developments in physics modelling capabilities, active source technologies, detector technologies and data fusion and analysis approaches are summarised. Particular attention will be given to the potential for high current, pulsed power based, active interrogation sources for remote detection of SNM.