C. Steer

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.

Electronic phase transitions and magnetoresistance in a new bilayer manganate, Ca2.5Sr0.5GaMn2O8

The crystal structure of the anion-deficient perovskite Ca2.5Sr0.5GaMn2O8 has been studied at 290 and 5 K by neutron diffraction (290 K; space group Pcm21, a = 5.4294(1), b = 11.3722(3), c = 5.2983(1)A). The vacant oxide sites order to create a structure in which perovskite bilayers consisting of MnO6 octahedra are isolated from each other along [010] by a single layer of GaO4 tetrahedra. At 5 K the material is antiferromagnetic with an ordered magnetic moment of 3.09(1) μB per Mn cation. Magnetic susceptibility measurements suggest that short-range magnetic ordering within the bilayers occurs above 200 K, and muon spin relaxation data show that the transition to long-range magnetic order occurs between 150 and 125 K. The resistivity of Ca2.5Sr0.5GaMn2O8 decreases by an order of magnitude at 125 K, and ~50% magnetoresistance is seen in a field of 80 kOe at 110 K.

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.

μSR studies of organic and molecular magnets

Muon-spin rotation and relaxation (μSR) experiments have been performed on a variety of novel organic and molecular magnetic systems. In these experiments, implanted muons are used to study the magnitude, distribution and dynamics of the local field at the muon site. Calculations of the spatial dependence of the dipole-field inside the unit cell are used to interpret the data and determine the muon site in certain cases. We describe and review muon experiments on nitronyl nitroxide organic ferromagnets and antiferromagnets. We discuss a muon study of the spin crossover phenomenon which has been studied in Fe(PM-PEA)2(NCS)2, and which shows Gaussian and root-exponential muon relaxation in the high-spin and low-spin phases, respectively. The effects of high temperature annealing on TDAE-C60 have also been studied with μSR. Experiments on a disc-shaped molecular complex containing Fe19 (with spin 31/2) reveal the effects of fluctuations of magnetization and allow an estimate of the fluctuation rate. These experiments demonstrate the wide range of problems which can be tackled using the μSR technique.

μSR in polymers

μSR can be applied to the study of various dynamical processes in polymers. These processes may relate to carrier motion, as in studies of conducting polymers which make use of muon generated polarons to measure carrier diffusion rates. Alternatively the processes of interest may be related to the structural dynamics of the polymer, which can show dramatic changes around the glass transition temperature. We report here examples of the use of μSR to study the muon states and muon mobility in the polymers polyethylene and polytetrafluoroethylene, where coherent FμF precession signals have been observed. In the case of polystyrene, muon radical states formed on the phenyl ring have been used to make a detailed study of the dynamical freezing and onset of static disorder that accompanies the glass transition. Finally, we report a study of polaron diffusion in two polyphenylenevinylene conducting polymers.

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

μSR study of organic systems: ferromagnetism, antiferromagnetism, the spin-crossover effect, and fluctuations in magnetic nanodiscs

Abstract We present the results of recent μSR experiments on a variety of novel organic and molecular magnetic systems. Muons are sensitive to local static fields and magnetic fluctuations, but can probe much more than just the onset of long-range magnetic order. We review our work on nitronyl nitroxide organic ferromagnets and antiferromagnets. We describe a muon study of the spin-crossover phenomenon which has been studied in Fe(PM-PEA)2(NCS)2, and which shows Gaussian and root-exponential muon relaxation in the high-spin and low-spin phases, respectively. Experiments on a disc-shaped molecular complex containing Fe19 (with spin 31 2 ) reveal the effects of quantum tunneling of magnetization and allow an estimate of the quantum tunneling rate.

Muon-spin relaxation study of anisotropic charge carrier motion in polyphenylene vinylene-based polymers

Muon-spin relaxation (μ SR) experiments on the conducting polymers poly(2, 3-dibutoxy-1, 4-phenylene vinylene) and poly(2, 5-bis(dimethyloctylsilyl)-1, 4-phenylene vinylene) probe the dynamics of the highly mobile polarons created by the muon-implantation process in which muonium reacts with the polymer forming a radical state. The fluctuating spin density induced by the electronic spin defect rapidly diffusing up and down the chain leads to a characteristic relaxation, the temperature and field dependences of which permit the extraction of intrachain and interchain diffusion rates. The intrachain diffusion rate decreases with temperature and can be fitted to a model of phonon-limited transport. The interchain diffusion rate increases with temperature and can be fitted to an activated temperature dependence.

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.