NGN PhD Studentship Proposal

Transcription

1 NGN PhD Studentship Proposal Note that proposals will be assessed against both the quality of the scientific content and of the proposed training experience. Proposed supervisors (lead first) Dr Laura Harkness-Brennan, Dr Helen Boston, University of Liverpool Industrial supervisor/partner Potential partners: (1) NDA Beth Ripper National Waste Inventory Manager, NDA, (2) NNL Project Title Enhanced nuclear waste assay Host Institution(s) and Department(s)/School(s) University of Liverpool, Department of Physics Summary (250 words max; this will be used in recruitment publicity) Assay of nuclear waste is a problem for both legacy waste sites (decommissioning) and for newly developing monitoring solutions for existing facilities and new build. A common assay method is to use gamma-ray spectroscopy to both identify and quantify the waste. This project aims to increase the sensitivity of these measurements, using advanced algorithms and novel detector technology. The expected outcomes are significantly reduced counting times and improved isotope identification in the presence of large backgrounds. The student will lead the optimisation and evaluation of algorithms aimed at improving the spectral response of BEGe (Broad Energy Germanium) detectors. The technique is based upon distinguishing low energy gamma rays that are absorbed near the outer surface of the detector, from Compton events arising from higher-energy gamma rays, which interact throughout the whole volume of the detector. The digital Compton suppression technique relies on a detailed characterisation of the position dependent response of the detector to gamma irradiation, for which the University of Liverpool has well-established facilities. The systems being considered will be modelled using simulation codes such as MCNP or GEANT. The student will conduct realistic industrial performance tests using the developed algorithms to evaluate the proposed solutions. These will take place at Canberra (Harwell) using their facilities that include waste drum analogues. The student will also have the opportunity to perform measurements at the National Nuclear Laboratory (NNL) Central Laboratory, through support from the Nuclear Decommissioning Authority (NDA). Introduction (250 words max) High-resolution germanium gamma-ray detectors are an important tool in nuclear waste assay. The detectors can be coupled with digital electronics to extract with a high degree of accuracy the total energy deposited by gamma-rays in the detector, thus allowing radionuclides to be identified and quantified. However, in nuclear waste assay, the relative activities or location distributions of radionuclides is not necessarily known and the radionuclide of interest may be masked by the presence of others. Long measurement times can therefore be required to quantify the relative activities. The student will optimise and evaluate digital Compton suppression algorithms aimed at reducing the measurement time in nuclear waste assay, by improving the spectral response of BEGe (Broad Energy Germanium) detectors. This technique is not only applicable to assay solutions, but can also offer significantly improved spectroscopic performance for environmental measurements. This work will derive from the measurement of the relationship between the gamma-ray interaction position in the crystal and the shape of the charge pulse. Simulations will be used to model the electric field within the bulk of the detector to understand the charge collection process. The models will be validated by comparison with an experimental database of pulse shapes for known positions of single gamma-ray interactions in BEGe detectors of differing geometries.

2 The student will attain many transferrable skills applicable to nuclear industry. They will develop expertise in nuclear instrumentation, Monte-Carlo simulation, computer programming, data acquisition, project management and will be encouraged to work independently and as part of a team. Hypotheses, aims and objectives. Hypotheses If digital Compton suppression algorithms based on known detector response can be used to suppress the detection of background radiation, then the counting times for radionuclides of interest in waste assay can be significantly reduced. Aims To optimise and evaluate the performance of digital Compton suppression algorithms, which aim to improve the sensitivity of BEGe detectors for nuclear waste assay. Objectives To experimentally generate a database of pulse shapes for known positions of single gamma-ray interactions in BEGe detectors of differing geometries (WP 1). To use the database to evaluate and validate an existing electric field simulation code (WP 2). To optimise the developed digital Compton suppression algorithms for BEGe detectors of differing geometry (WP 3). To conduct realistic industrial performance tests using the developed algorithms to evaluate the proposed solutions. These will take place at Canberra (Harwell) using their facilities that include waste drum analogues. The student will also have the opportunity to perform measurements at the NNL Central Laboratory, through support from the NDA (WP 4). Methodology and Approach (1000 words max) WP1: Generation of Experimental Databases It is well known that germanium detectors have variable pulse shapes due to the different electron and hole drift times that constitute the formation of a charge pulse signal. This drift differential means that energy deposited at different locations in a germanium detector will produce differently shaped pulses. Depending on the exact detector and electrode geometry, the weighting field will also affect the time dependence of the charge collection and therefore strongly influence the shape of the observed pulse. This is particularly important in point like contact detectors such as the BEGe detector. The advent of electronics systems with sufficient speed and dynamic range that the primary signal pulse can be directly digitised now allows detailed information to be extracted from the signals. As part of WP1, the student will experimentally generate databases for known positions of single gamma-ray interactions in BEGe detectors of differing geometries. The methodology for each detector will be to: setup experimental equipment for data acquisition acquire pulse shape data for single gamma-ray interactions at known 3-D positions in the detector analyse the data to produce an average pulse shape response at each 3-D position in the detector The experimental method is a coincidence scanning technique used previously at the University of Liverpool to characterise germanium detectors. In this technique, a collimated high-energy source ( 137 Cs) is scanned across the top of the detector, using an automated x-y scanning table. Secondary collimators surround the detector sides so that only gamma rays which Compton scatter by 90 in the detector can pass through the secondary collimator apertures. The well-defined kinematics of Compton scattering states that for these types of events, the original gamma-ray energy of 662 kev is shared between the 374 kev deposited in the BEGe detector and the 288 kev carried away by the scattered gamma-ray. By placing auxiliary detectors near the secondary collimator apertures, the gamma-rays with 288 kev energy can be absorbed. These detectors provide information on the interaction depth (z) in the BEGe detector, as each collimator aperture corresponds to a particular z position. In the experiment, a coincidence window is setup, which ensures that data is recorded when the BEGe detector and auxiliary detector collect energy of 374 kev and 288 kev, respectively, within a

3 small time window (approximately 500 ns). By examining the pulse shapes readout from the BEGe detector for these types of events, the student will be able to create an average response at each (x,y,z) position. This will be done using custom software at the University of Liverpool, although packages such as MATLAB can also be used to analyse data. For WP1, the student will develop skills in project management, nuclear instrumentation, data acquisition and computer programming. WP2: Electric Field Model Evaluation As part of this work package, the student will use existing electric field models using an existing toolkit. This has been validated for large-volume, segmented detectors but not yet for BEGe detectors. This toolkit will allow the student to simulate the pulse shape response at specific (x,y,z) positions. The information required by the model includes details that are only available through the collaboration with our industrial partner, Canberra. The student will therefore liaise with experts at Canberra to ensure the models represent the experimental response as accurately as possible. The student will develop methods to compare the similarity of the experimentally produced and simulated pulse shape databases. One typical method is to use statistical minimisation techniques and the student will have access to existing methods used at Liverpool. Data will be generated for two BEGe detectors, which will allow the model to be adapted, where necessary, so that the output is comparable to the experimental data. For WP2, the student will develop programming skills in MATLAB and C++ and will significantly develop their understanding of detector response. WP3: Optimise Algorithms The student will work alongside a postdoctoral researcher who will be the lead developer of the digital Compton suppression algorithms. The student will use the results from WP1 and WP2 to optimise the algorithm parameters. This will be done by extracting information from the experimental and simulated databases and inputting it to the existing algorithm. The student will perform measurements using standard laboratory sources, to test and further optimise the performance of the algorithm. In particular, the improvement in Minimum Detectable Activity (MDA) will be quantified and mixed sources of radionuclides will be examined. The optimised algorithm will then be input to a FPGA for the Canberra LYNX system, so measurements can be undertaken using a complete standard system. The student will use their expertise in nuclear instrumentation to allow them to perform these spectroscopic measurements and will develop their teamwork skills as they collaborate regularly with the postdoctoral researcher. WP4: Performance Evaluation The student will undertake realistic industrial performance tests using the developed algorithms to evaluate the efficacy of the proposed solutions. These will take place at Canberra (Harwell) using their facilities that include waste drum analogues and at the NNL Central Laboratory. The student will initially visit Harwell for on-site training and to acquire preliminary data. The student will return to Liverpool for a month to analyse the data and adjust the parameters of the algorithm, where necessary. During this time the student will collaborate with experts at Canberra. Following the final optimisation of the algorithm, the student will return to Harwell to complete the industrial performance tests. Various tests will be carried out, to assess the performance of the algorithm. It is possible that the student will be able to compare the performance of the solution with the existing solutions they investigated during the training period. The student will then take the finalised solution to the NNL to perform final measurements of realistic waste assay. During this work package, the student will perform experimental measurements, analysis of data and will interpret the results, to allow them to form conclusions and recommendations for their thesis.

4 Programme of Work Student Specific Training In addition to safety inductions at the University of Liverpool and the industrial partners, the student will undergo the following (provisional) specific training: 2 weeks at Canberra Harwell, followed by 2 weeks at the NNL Central Laboratory, learning about existing waste assay solutions. The student will prepare a poster presentation, to showcase the various solutions. Approximately 5 weeks of academic training on the following MSc Modules: o NTEC N02: Nuclear Fuel Cycle o NTEC N04: Decommissioning/Waste/Environmental Management o NTEC N10: Processing, Storage and Disposal of Nuclear Waste o Radiometrics: High Resolution Gamma-Ray spectroscopy

5 o Radiometrics: Gamma-ray Detection and Modelling 6 weeks of practical training in using BEGe detectors and associated readout electronics. The student will prepare a short report detailing the operation and performance of such a system. 4 weeks of literature survey of existing pulse shape characterisation methods. The student will determine the best experimental method to be used in the project, and deliver a short oral presentation to describe the different methods. 5 weeks of learning how to use the University of Liverpool digital data acquisition and analysis systems, including the custom software. An introduction to C++ programming course, delivered at the University of Liverpool (approximately 1 week). 6 weeks of training in using the chosen Monte-Carlo toolkit, e.g. MCNP or GAMOS. The student will write a report on how they plan to use the chosen toolkit for their project. Track record (500 words max) The proposed supervisors Dr Laura Harkness-Brennan and Dr Helen Boston have both the supervisory experience and requisite scientific expertise to fully support this PhD studentship. Laura is a world-leading expert on the characterisation of germanium detectors and has collaborated successfully with the sole manufacturer of BEGe detectors, Canberra. Modelling detector response by Monte-Carlo simulation is a key component in this PhD project and Laura has experience in supervising students who use such techniques. She is the leading UK expert on the GAMOS Monte-Carlo toolkit and has significant expertise in GEANT4. She has published work on experimental measurements of semiconductor detectors and the Monte-Carlo toolkits GEANT4 and GAMOS. Dr Helen Boston has a wealth of experience with germanium detectors, and has published work on their use in applied fields. Helen has significant project management experience and is currently the principal investigator of a project that aims to use a semiconductor detector based imaging device for nuclear decommissioning. Helen thus has excellent relationships within the UK nuclear industry and has recognised the industrial requirements for this PhD project. In addition to direct support from the supervisory team, the student will receive support from academic and research staff in the nuclear physics group at the University of Liverpool. The group has a strong track record in (PhD) studentships that utilise and improve the performance of germanium detectors and is internationally leading in this field. There have been many successful collaborative projects with industrial partners, such as Canberra, NNL, AWE and BAE systems. The group has recently secured a 3-year STFC-IPS grant to support the technical development aimed to improve the performance of BEGe detectors, in collaboration with Canberra. This grant will provide funding for a postdoctoral researcher for 3 years, in parallel with the PhD student who will carry out the work outlined in this proposal. The student will therefore receive direct support from experienced researcher, academic staff and industrial partners. The project brings together the internationally leading expertise of the University of Liverpool Nuclear Physics group, with the world s largest germanium detector supplier, Canberra. The student will be part of a Liverpool team to gain access to key knowledge on the requirements of the UK nuclear industry customer base, access to large scale manufacturing facilities and the associate commercial expertise.