Browsing by Author "Abdul, Sitifairus"

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    Characterisation of Defects and Thermoluminescence Yield of Novel Tailor-Made Doped Optical Fibres for Dosimetry
    (Addis Ababa University, 2015-05-05) Abdul, Sitifairus; Bradley, David (Professor)
    This work encompasses characterisation of defects and dosimetric studies of novel tailor made doped SiO2 fibres. Present studies have been carried out seeking to improve upon the thermoluminescence (TL) yield of commercially produced small diameter telecommunication optical fibres. Using the modified chemical vapour deposition (MCVD) process, the optical fibres have been fabricated to a range of dopant concentrations of nominal value 6- 8- and 10 wt%. In this study, three different types of optical fibres have been utilised, made using the same doped preform. The doped fibres are cylindrical fibres (CF), flat fibres (FF) and photonic crystal fibres (PCF). It should be noted that the process of fibre drawing has been found to produce defect centres, influencing characteristics of optical fibre and TL response. To seek support of this, an X-ray Photoelectron Spectroscopy (XPS) study of a Gedoped SiO2 fibres sample has been undertaken to determine the oxidation state of Ge. Results from this have confirmed the efficiency of the surface analysis technique, leading to understanding of the Ge structure. Following on from this, facilities supporting characterization of the fibres are outlined, including an ion beam facility used for Particle Induced X-ray Emission (PIXE)/Rutherford Back Scattering (RBS) analysis to localize and determine the concentration of Ge dopants. Building upon these characterisations, thermoluminescence studies were carried out. For the first of the experiment, undoped flat fibres were used, comparison of response being made with that of conventional TLD-100 and commercial Ge-doped silica fibres. The undoped flat fibres provide competitive TL yield to that of TLD-100, being some 100 times that of the Ge-doped fibres. Pt-coated flat fibres have then been used to increase the photoelectron production and hence local dose deposition, obtaining significant increase in dose sensitivity over that of undoped flat fibres. Using 250 kVp X-ray beams, the TL yield reveals a progressive linear increase in dose for Pt thicknesses from 20 nm up to 80 nm. Finally, to illustrate the potential of novel tailor-made doped SiO2 optical fibres, the dosimetric characteristics that have been investigated include, dose response, glow curves and energy dependence. Taking TLD-100 as a benchmark, results are presented for Ge-doped, Ge-B-doped and Ge-Br-doped optical fibres. The dose response of doped silica fibres was found to be linear over the range 2 cGy up to 50 Gy, also showing good dosimetric response for low photon energies. Additional investigation of the same doped SiO2 optical fibres have been conducted for measurement of TL yield from the high linear energy transfer (LET) radiation offered by a liquid 223Ra alpha particle source.