title: Development and testing of multi-radiation systems for the characterisation of nuclear facilities
Creators: name: Gray, Richard J.
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: Radioactivity monitoring, nuclear site characterisation, scintillating materials, photon detectors, prototype development.

description: The Nuclear Decommissioning Authority’s mission to clean up the United Kingdom’s nuclear legacy provided the funding and inspiration for this work. The identification of technology and knowledge gaps in the field of pipeline characterisation for post operational nuclear facilities has become an active field of research in recent years. Many kilometres of pipes and ducting are located behind walls, underground and are often shielded by concrete. Surface contamination can only be assessed by a detector small enough for internal deployment. This thesis describes the development of a miniature detector system capable of gamma spectrometry and limited β − particle detection for pipeline characterisation. A scintillation detector has been selected as it is inexpensive to produce and is a well studied method of radioactive waste measurement. Novel inorganic scintillators and photon detectors that have been developed for fundamental physics research have not thus far been adapted for use in the nuclear industry. Such materials and technology are being applied to large scale detectors at particle accelerators but they can also enable miniaturisation to a greater degree than has been possible in low resolution spectrometry until now. Gadolinium Aluminium Gallium Garnet doped with Cerium (GAGG:Ce) has been chosen as the scintillation material and a detailed characterisation of GAGG:Ce and GAGG:Ce:Mg has been undertaken. Scintillator size was determined through GEANT4 optical transport simulations in order to produce a miniature detector system without impacting on detection performance. Scintillation counting is traditionally performed by the photomultiplier tube (PMT). The silicon photomultiplier (SiPM) is being proposed to replace the PMT for future photon counting applications as it is durable, compact and can operate at low voltage. Several silicon photomultipliers were investigated and the ArrayJ-60035 was selected and characterised. This device was found to have a room temperature breakdown voltage (Vbr) of 24.26 ± 0.04 V and a linear relationship between Vbr and temperature with a gradient of 20.27 ± 0.12 mV/◦C. Photoluminescence and absorption studies on GAGG:Ce:Mg uncovered suppression of the Gd3+ absorption band at 275 nm which was measured in the GAGG:Ce samples and has been previously documented. It has been reported that a broad charge transfer absorption band indicating the presence of Ce4+ ions can be found in the vicinity of this Gd3+ band. This however was not identified so the suppression is as yet unexplained. An additional absorption band was observed overlapping with the emission band between 560 and 700 nm. This has not been documented previously and requires explanation. In order to confirm that a GAGG:Ce detector would function in a high radiation environment, studies were carried out at the Dalton Cumbrian Facility with a high activity 60Co irradiator. Samples received an absorbed dose of up to 100 kGy. The material showed a 16 % degradation of its light output. The novel approach of using photoluminescence spectra comparisons revealed that no damage to the scintillation mechanism had occurred as a result of extreme exposure. Induced absorption coefficients were calculated showing GAGG:Ce to be extremely radiation tolerant compared to other scintillators currently in use. In the vicinity of its emission band the induced absorption coefficient was 0.02 cm−1. A prototype was developed with custom readout circuitry and a miniature MCA. Its performance was tested and compared with a benchmark detector. The prototype described in this document delivered comparable performance to the benchmark detector at a fraction of the size, cost and power consumption. Energy resolution of 7.10 % and a peak to total ratio of 33.9% for the 662 keV photopeak of 137Cs has been measured. The detector system was also shown to be capable of peak identification in a mixture of radionuclides equalling the performance of the benchmark detector and out performing any other documented scintillation spectrometer of these dimensions in the literature. This robust and extremely small scale detector can be applied to any detection scenario that requires a robust and radiation hard radionuclide identification or confirmatory monitoring in tight or convoluted geometries. This project began as a proof of concept for the proposed detector system leading to system development and technology demonstration with an estimated technology readiness level of 6 or 7. A feasibility study was performed on a novel β − counter . This is a promising new development that may have potential in identifying low energy β − emitters like 90Sr in the future.