Characterisation of Defects and Thermoluminescence Yield of Novel Tailor-Made Doped Optical Fibres for Dosimetry
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Date
2015-05-05
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Addis Ababa University
Abstract
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.
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Keywords
Characterisation, Defects, Thermoluminescence Yield, Novel Tailor-Made Doped Optical Fibres, Dosimetry