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

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.

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.is not therefore possible to tum this on or off: or to optimise the energy. Scattering tomography using cosmic ray muons may not provide a complete answer to the problem of illicit trafficking of RN materials, but could well form part of the solution. II. COSMIC RAY MUONSHigh energy cosmic rays -comprising roughly 90% protons, 9% a-particles and the remainder heavier nuclei -strike the upper reaches of the Earth's atmosphere at a rate of about 1000 1m 2 Isec [2]. These undergo deep inelastic collisions with molecules in the Earth's atmosphere to produce cascades of lighter particles. These include short-lived pions, which decay into muons.Muons have a rest energy of 105.7 MeV/c 2 , around 200 times that of an electron, and can be either positively or negatively charged. Although they have a short lifetime of --2.2 micro seconds, their near-relativistic speeds mean they form a significant fraction of the earth's cosmic radiation at sea level. The muon flux at sea level is approximately 10,000 m-2 min-1 [3], which is roughly equivalent to one through the surface of the hand every second. This is, however, dependent on a number of factors, including latitude, longitude, altitude, time in the solar cycle and angle. It is often claimed [4] that the intensity falls otT as cos 2 {) (where {) is the angle from the zenith) though a number of papers suggest that such statements may be rather sweeping [5][6][7]. The energy spectrum of cosmic ray muons at sea level varies over several orders of magnitude, from approximately lOMeV to 10 GeV. The graph below was generated using EXPACS [8] version 2.14, and assumes latitude of 52°. Integrating this curve with respect to energy suggests a total muon flux of 1.56 x 10-2 cm-2 sec-I, which is in agreement with the often quoted value of 10,000 m-2 min-l mentioned previously. Figure 1 also gives a mean muon energy of 4.01 GeV, which is the commonly used mean value.

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.

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