This investigation studied the growth and characterisation of thin film phosphors to examine the feasibility of enhancing a solid-state neutron scintillation detector system for increased detection resolution and detection rate. Undertaken in collaboration with the neutron facility ISIS at the Rutherford Appleton Laboratory, it was to examine whether an improved radioluminescent detector could be designed, based on the application of thin film phosphors. Optimisation for the detector was in the areas of increased luminescent emission from each neutron captured, increasing the rate of capture for each measurement and improved spatial resolution. The current ZnS:Ag,Cl phosphor used at ISIS was characterised in terms of its luminescent emission properties and comparisons were made to thin films of potential phosphor materials and the source material used to create the thin films. The phosphors were characterised via photo and radio-luminescent excitation to examine the spectral emission and the luminescent decay properties. Thin films were grown onto a silicon substrate from a ZnS:Ag,Cl phosphor source using RF magnetron sputtering and were characterised in terms of morphology, crystallinity and luminescent properties. Thin films grown using this technique were found to not produce visible emission when excited with UV excitation nor alpha excitation. Following further literature research and using the analysis technique of Rutherford Backscattering it was concluded that the chlorine charge compensation dopant was lost during the growth process resulting in incomplete incorporation of the luminescent silver dopant into the lattice. A range of chlorine doping methods were examined in order to promote charge compensation within the thin films, with the most controlled method using ion implantation, carried out at the Ion Beam Facility at Surrey University. Three samples of ZnS were grown to 800nm, followed by implantation of 0.01at% of silver and either 0.01at%, 0.05at% or 0.20at% of chlorine. Following implantation the thin films were thermally annealed to activate the samples and characterised by their luminescent emission via photo- and radioluminescent excitation. The result of the study was that although a luminescent emission was created using laser excitation, the emission using a 241Am source was of such low intensity they were uncharacterisable using the detection setup. The emission spectrum created by ZnS:Ag,Cl using the PL setup was not the standard broadband emission peak centred around 450nm, but was distorted so that it preferentially enhanced the emission around 485nm, hence create a shoulder peak. This is not significant to the optimisation of the detector, but important to know for characterisation purposes. The emission and decay characteristics of the wafer used for neutron detection by ISIS and the ZnS:Ag,Cl powder purchased from Phosphor Technology are the same. Combining this with extensive experience within the research group of growing ZnS and ZnS:Mn thin films made this powder a suitable as a potential target material for thin film growth. Independent of the calibration problem, the PL emission spectrum from thin films grown using RF magnetron sputtering should have been the same as the PL emission spectrum of the target material used in this technique. However, once a thin film had been grown there was no resultant emission observed. Testing the thin films using Rutherford Back Scattering found that although there were relative proportions of zinc, sulphur and silver there was no chlorine detected. Alternative method of forming a thin film of ZnS:Ag,Cl were investigated, with the most significant method using ion implantation. This technique can accurately implant the small concentrations of dopant required into a thin film with good distribution, hence requiring minimal thermal processing for the dopants to be incorporated into the lattice to activate it. The result was a thin film that when annealed in a furnace in air it rapidly oxidised, where as when annealed under vacuum this did not occur. After a period of eight hours annealing at 600°C a PL emission was observed from the implanted thin films. When the emission spectrum was analysed there was a series of emission peaks. There was potential that one of these peaks related to the ZnS:Ag,Cl emission, however there was a prominent peak at 575nm which when further analysed with RBS and PIXE confirmed there had been contamination of manganese in the thin films.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:629164 |
| Date | January 2010 |
| Creators | Miller, T. |
| Publisher | Nottingham Trent University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://irep.ntu.ac.uk/id/eprint/276/ |
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