S. Lumsden

The OLYMPUS experiment

OLYMPUS is an experiment mounted by an international collaboration at DESY, Hamburg, Germany to provide a ±1% measurement of the cross section ratio of positron-proton to electron-proton elastic scattering in the range 0.6 < Q2 < 2.2 (GeV/c)2. The goal is to provide a definitive experimental verification of the generally accepted explanation of the discrepancy between cross-section and recoil polarization techniques in determination of the form factor ratio GEp(Q2)/GMp(Q2).

GEANT4 Simulation of a Scintillating-Fibre Tracker for the Cosmic-ray Muon Tomography of Legacy Nuclear Waste Containers

Cosmic-ray muons are highly penetrative charged particles that are observed at the sea level with a flux of approximately one per square centimetre per minute. They interact with matter primarily through Coulomb scattering, which is exploited in the field of muon tomography to image shielded objects in a wide range of applications. In this paper, simulation studies are presented that assess the feasibility of a scintillating-fibre tracker system for use in the identification and characterisation of nuclear materials stored within industrial legacy waste containers. A system consisting of a pair of tracking modules above and a pair below the volume to be assayed is simulated within the GEANT4 framework using a range of potential fibre pitches and module separations. Each module comprises two orthogonal planes of fibres that allow the reconstruction of the initial and Coulomb-scattered muon trajectories. A likelihood-based image reconstruction algorithm has been developed that allows the container content to be determined with respect to the scattering density λ, a parameter which is related to the atomic number Z of the scattering material. Images reconstructed from this simulation are presented for a range of anticipated scenarios that highlight the expected image resolution and the potential of this system for the identification of high-Z materials within a shielded, concrete-filled container. First results from a constructed prototype system are presented in comparison with those from a detailed simulation. Excellent agreement between experimental data and simulation is observed showing clear discrimination between the different materials assayed throughout.

The design and performance of a scintillating-fibre tracker for the cosmic-ray muon tomography of legacy nuclear waste containers

Tomographic imaging techniques using the Coulomb scattering of cosmic-ray muons are increasingly being exploited for the non-destructive assay of shielded containers in a wide range of applications. One such application is the characterisation of legacy nuclear waste materials stored within industrial containers. The design, assembly and performance of a prototype muon tomography system developed for this purpose are detailed in this work. This muon tracker comprises four detection modules, each containing orthogonal layers of Saint-Gobain BCF-10 2 mm-pitch plastic scintillating fibres. Identification of the two struck fibres per module allows the reconstruction of a space point, and subsequently, the incoming and Coulomb-scattered muon trajectories. These allow the container content, with respect to the atomic number Z of the scattering material, to be determined through reconstruction of the scattering location and magnitude. On each detection layer, the light emitted by the fibre is detected by a single Hamamatsu H8500 MAPMT with two fibres coupled to each pixel via dedicated pairing schemes developed to ensure the identification of the struck fibre. The PMT signals are read out to standard charge-to-digital converters and interpreted via custom data acquisition and analysis software. The design and assembly of the detector system are detailed and presented alongside results from performance studies with data collected after construction. These results reveal high stability during extended collection periods with detection efficiencies in the region of 80% per layer. Minor misalignments of millimetre order have been identified and corrected in software. A first image reconstructed from a test configuration of materials has been obtained using software based on the Maximum Likelihood Expectation Maximisation algorithm. The results highlight the high spatial resolution provided by the detector system. Clear discrimination between the low, medium and high-Z materials assayed is also observed.

Characterising encapsulated nuclear waste using cosmic-ray muon tomography

A prototype scintillating-fibre detector system has been developed at the University of Glasgow in collaboration with the UK National Nuclear Laboratory (NNL) for the non-destructive assay of UK legacy nuclear waste containers. This system consists of four tracking modules, two above and two below the container under interrogation. Each module consists of two orthogonal planes of 2 mm-pitch fibres yielding one space point. Per plane, 128 fibres are read out by a single Hamamatsu H8500 64-channel MAPMT with two fibres multiplexed onto each pixel. The configuration allows the reconstruction of the incoming and scattered muon trajectories, thus enabling the container content, with respect to atomic number Z, to be determined. Results are shown from experimental data collected for high-Z objects within an air matrix and within a shielded, concrete-filled container. These reconstructed images show clear discrimination between the low, medium and high-Z materials present, with dimensions and positions determined with sub-centimetre precision.

The HERMES recoil detector

The HERMES recoil detector is an exciting addition to the HERMES spectrometer, specifically designed to make one of the first exclusive measurements of deeply virtual Compton scattering (DVCS). DVCS is the experimentally cleanest way to access generalised parton distributions - a theoretical framework that describes the structure of the nucleon. The recoil detector utilises a silicon detector with a large dynamic range capable of reconstructing the momenta of protons in the range of 135 MeV/c to 450 MeV/c, placed directly into the HERA beam vacuum (around the HERMES target) to make both position and energy deposition measurements (for the purposes of momentum reconstruction) of the recoil protons from the process. In addition there is a scintillating fibre tracking (SET) detector placed directly outside the beam vacuum that provides both tracking information and momentum reconstruction data for protons at higher momenta. The third sub-detector is a photon detector that lies concentrically outside the SET and provides useful information on other processes for the purposes of background subtraction. Leptons involved in the interaction will be detected in the existing parts of the HERMES spectrometer. The recoil detectors silicon sub-detector was the subject of a presentation at the IEEE NSS in 2003 by Mathias Reinecke. This presentation is intended as an update on the successful development of the silicon sub-detector as well as providing more information on the impending installation of the detector into the HERMES spectrometer in November 2005

Characterising encapsulated nuclear waste using cosmic-ray Muon Tomography (MT)

A prototype scintillating-fibre detector system has been developed at the University of Glasgow in collaboration with the UK National Nuclear Laboratory (NNL) for the non-destructive assay of UK legacy nuclear waste containers. This system consists of four tracking modules, two above and two below the container under interrogation. Each module consists of two orthogonal planes of 2 mm-pitch fibres yielding one space point. Per plane, 128 fibres are read out by a single Hamamatsu H8500 64-channel MAPMT with two fibres multiplexed onto each pixel. The configuration allows the reconstruction of the incoming and scattered muon trajectories, thus enabling the container content, with respect to atomic number Z, to be determined. Results are shown from experimental data collected for high-Z objects within an air matrix and within a shielded, concrete-filled container. These reconstructed images show clear discrimination between the low, medium and high-Z materials present, with dimensions and positions determined with sub-centimetre precision.

Application of muon tomography to encapsulated nuclear waste

Muon tomography is a non-invasive imaging technique which can be used to visualize the internal structures of an object using the Coulomb scattering of cosmic-ray muons. Because of their high penetrative power of high energy muons, this technique can potentially be used to image large and dense objects. It has been demonstrated in our previous study that this technique is capable of identifying and characterising high-Z materials within an air matrix using a prototype scintillating-fibre tracker system. In the present work, we extended our study to image a more complex object where two high atomic number materials and a medium atomic number material are sealed in a concrete filled stainless steel container. The same tracking detector was used to collect the muon scattering data, and images were reconstructed by using dedicated statistical image reconstruction software. This work serves as a feasibility study to determine if it is possible to use this technology to characterise the potential high-atomic number contents of legacy nuclear waste containers for the UK Nuclear Industry. Our results showed clear discrimination between the steel barrel wall, the concrete matrix and the high atomic number materials in the container. The possible application of this technique for future industrial scenarios is discussed.