Christian Thomay

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.

Muon scattering tomography with resistive plate chambers

Muon Scattering Tomography is a scanning technique which uses cosmic muons as probes to gather information on the content of the inspected volume. Because the scattering angle depends on the Z2 of the materials in the volume, it is possible to obtain a 3D image of the volume content by carefully tracking the muon paths. This has very interesting potential applications in several fields, from engineering to homeland security, where MST could be used to inspect shipping containers and trucks in search of special nuclear materials (SNM). For such applications to be feasible it is necessary to have large area detectors (10-100 square meters) while maintaining the efficiency and angular resolution necessary to discriminate between low-Z and high-Z materials. Resistive Plate Chambers (RPCs) are very good candidates. RPCs are widely adopted in high energy physics experiments thanks to their excellent performance in terms of time resolution, charged particle detection efficiency and low cost per unit area. In collaboration with the UK Atomic Weapon Establishment we have built and tested a prototype based on 12 RPCs (50 cm × 50 cm). To obtain the required spatial resolution we adopted a novel approach for the RPC readout, coupling each detector with 300 fine pitch strips (1.5 mm) and using multiplexing analog readout chips to reduce the amount of readout channels. The prototype performs very well: all the chambers have efficiency above 99% and purity above 95%. The signal-to-noise for the electronic readout ranges from 25 to 90. The spatial resolution for the layers is better than 1 mm and we show that this is sufficient to successfully image a block of lead of 10 cm × 10 cm × 15 cm. We are now in the process of upgrading the electronics with new ASICs which feature built-in ADCs. The first test

Toward a RPC-based muon tomography system for cargo containers

A large area scanner for cosmic muon tomography is currently being developed at University of Bristol. Thanks to their abundance and penetrating power, cosmic muons have been suggested as ideal candidates to scan large containers in search of special nuclear materials, which are characterized by high-Z and high density. The feasibility of such a scanner heavily depends on the detectors used to track the muons: for a typical container, the minimum required sensitive area is of the order of 100 2. The spatial resolution required depends on the geometrical configuration of the detectors. For practical purposes, a resolution of the order of 1 mm or better is desirable. A good time resolution can be exploited to provide momentum information: a resolution of the order of nanoseconds can be used to separate sub-GeV muons from muons with higher energies. Resistive plate chambers have a low cost per unit area and good spatial and time resolution; these features make them an excellent choice as detectors for muon tomography. In order to instrument a large area demonstrator we have produced 25 new readout boards and 30 glass RPCs. The RPCs measure 1800 mm× 600 mm and are read out using 1.68 mm pitch copper strips. The chambers were tested with a standardized procedure, i.e. without optimizing the working parameters to take into account differences in the manufacturing process, and the results show that the RPCs have an efficiency between 87% and 95%. The readout electronics show a signal to noise ratio greater than 20 for minimum ionizing particles. Spatial resolution better than 500 μm can easily be achieved using commercial read out ASICs. These results are better than the original minimum requirements to pass the tests and we are now ready to install the detectors.

Passive 3D imaging of nuclear waste containers with Muon Scattering Tomography

The non-invasive imaging of dense objects is of particular interest in the context of nuclear waste management, where it is important to know the contents of waste containers without opening them. Using Muon Scattering Tomography (MST), it is possible to obtain a detailed 3D image of the contents of a waste container on reasonable timescales, showing both the high and low density materials inside. We show the performance of such a method on a Monte Carlo simulation of a dummy waste drum object containing objects of different shapes and materials. The simulation has been tuned with our MST prototype detector performance. In particular, we show that both a tungsten penny of 2 cm radius and 1 cm thickness, and a uranium sheet of 0.5 cm thickness can be clearly identified. We also show the performance of a novel edge finding technique, by which the edges of embedded objects can be identified more precisely than by solely using the imaging method.

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 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.

Discrimination of high-Z materials in concrete-filled containers using Muon Scattering Tomography

An analysis method of identifying materials using muon scattering tomography is presented, which uses previous knowledge of the position of high-Z objects inside a container and distinguishes them from similar materials. In particular, simulations were performed in order to distinguish a block of Uranium from blocks of Lead and Tungsten of the same size, inside a concrete-filled drum. The results show that, knowing the shape and position from previous analysis, it is possible to distinguish 5 × 5 × 5 cm3 blocks of these materials with about 4h of muon exposure, down to 2 × 2 × 2 cm3 blocks with 70h of data using multivariate analysis (MVA). MVA uses several variables, but it does not benefit the discrimination over a simpler method using only the scatter angles. This indicates that the majority of discrimination is provided by the angular information. Momentum information is shown to provide no benefits in material discrimination.

High-resolution imaging of nuclear waste containers with muon scattering tomography

Muon scattering tomography is a non-invasive technique that can be used to scan large containers and search for high-Z materials. This is of particular interest to characterise large volume legacy nuclear waste. Here we detail a method that allows to measure the size of high-Z objects embedded in concrete using muon scattering tomography. The resolution obtained was of 0.9 ± 0.2 mm.

Discrimination of high-Z materials using muon scattering tomography

Legacy nuclear waste can contain anything from parts of fuel rods to coveralls worn by the workers, stored inside large concrete containers. It is important to identify the materials present in these containers with techniques that are non-invasive and scalable, in order to be applied for a considerable amount of large volumes. Using muon scattering tomography, we show that it is possible to discriminate uranium blocks from lead, tungsten and plutonium, inside a concrete filled cylinder, by selecting the muon tracks that pass through the volumes of interest. There is very good discrimination between uranium and lead or tungsten for block sizes upwards of 2 × 2 × 2 cm3. We even show that we can discriminate between lumps of uranium and plutonium.