Development of epsilon: a gamma ray imaging system for the nuclear industry

In recent years, much effort has been focused on the need to characterise, stabilise, decontaminate and eventually decommission radioactively contaminated nuclear facilities. These tasks can be complex and hazardous and are often hampered by a lack of knowledge of the nature and distribution of the contamination. A gamma-ray imaging system, capable of remotely mapping a contaminated scene, could be a useful tool in many tasks related to the decommissioning of nuclear facilities. This thesis describes and discusses the design, development, commissioning and characterisation of EPSILON, a multipurpose gamma-ray imaging system designed to meet the needs of a range of applications in the nuclear industry. Following a review of the range of potential applications and a critical assessment of the imaging systems available at the time, a set of design criteria for a multi-purpose gamma-ray imaging system are identified. The design and development of an imaging system to meet these criteria is described in detail. Suitable image-formation techniques are discussed and a pinhole collimator selected as the most appropriate. Consideration is given to the use of a scintillator-photodiode array as the imager detection plane but this option is rejected in favour of a position-sensitive photomultiplier tube (PSPMT). The development of two data-acquisition systems is described and comparisons made between the performance of these two techniques. The factors affecting the performance and image quality of this pinhole imaging system are discussed and a prototype design presented for a well-shielded, high-sensitivity imager. Experimental results obtained with this prototype are presented. These results demonstrate a good angular resolution of 1.6° across a 12.8° FOV and a factor of 7 improvement in sensitivity over a comparable resolution, single-pixel scanning-system. The spectral resolution of this prototype imager did not meet the design criteria. A modified design is presented which offers 2° angular resolution and a factor of 30 improvement in sensitivity over the scanning system. However, the spectral resolution of this second design was also poor. The causes of this poor resolution are identified and attributed largely to the significant variations in photocathode-response of the PSPMT. The coded-aperture imaging technique is considered with a view to reducing the mass of the instrument. The relative performance of a pinhole and a coded aperture imager for applications in the nuclear industry are compared experimentally. The conclusion drawn is that the pinhole collimator represents the better choice for this application. The use of an M-HPD as an alternative to the PSPMT for the EPSILON detector plane is discussed and a final design presented for an imaging system expected to meet the original design criteria.