This research was focused on the thermoluminescence (TL) response of commercially produced single-mode telecommunication optical fibre manufactured by INOCORP (Canada). The fibres were either in the form of pure silica (SiO2) or as SiO2 doped with Ge or Al at concentrations appropriate for total internal reflection, as required for telecommunication purposes. Each of these INOCORP fibres had a core diameter of 125 ± 0.1 μm. It was noted that dopant concentration was not included among the data provided in the accompanying product data sheet. A particularly important parameter for obtaining the highest TL yield in this study was the dopant concentration of the SiO2 fibre. The dopants tended to diffuse during the production of the optical fibre. To obtain this parameter, proton induced X-ray emission (PIXE) analysis was utilised. PIXE while having limited depth resolution could unambiguously identify elements and analyse trace elements with a detection limit approaching μg g–1. For Al-doped fibres, dopant concentrations in the range of 0.98 – 2.93 mol% had been estimated, the equivalent range for Ge-doped fibres was 0.53 – 0.71 mol%. A linear dose response was observed following 2.5 MeV proton irradiation for Ge- and Al-doped fibres for up to 7 min exposure.
Modern linear accelerators, the predominant teletherapy machine in major radiotherapy centres worldwide, provide multiple electron and photon beam energies. To obtain reasonable treatment times, intense electron beam currents are achievable. In association with this capability, there is considerable demand to validate patient dose using systems of dosimetry offering characteristics that include good spatial resolution, high precision and accuracy. Present interest is in the thermoluminescence response and dosimetric utility of commercially available doped optical fibres. The important parameter for obtaining the highest TL yield during this study is to know the dopant concentration of the SiO2 fibre because during the production of the optical fibres, the dopants tend to diffuse. To achieve this aim, proton-induced X-ray emission (PIXE), which has no depth resolution but can unambiguously identify elements and analyse for trace elements with detection limits approaching microg/g, was used. For Al-doped fibres, the dopant concentration in the range 0.98-2.93 mol% have been estimated, with equivalent range for Ge-doped fibres being 0.53-0.71 mol%. In making central-axis irradiation measurements a solid water phantom was used. For 6-MV photons and electron energies in the range 6, 9 and 12 MeV, a source to surface distance of 100 cm was used, with a dose rate of 400 cGy/min for photons and electrons. The TL measurements show a linear dose-response over the delivered range of absorbed dose from 1 to 4 Gy. Fading was found to be minimal, less than 10% over five days subsequent to irradiation. The minimum detectable dose for 6-MV photons was found to be 4, 30 and 900 microGy for TLD-100 chips, Ge- and Al-doped fibres, respectively. For 6-, 9- and 12-MeV electron energies, the minimum detectable dose were in the range 3-5, 30-50 and 800-1400 microGy for TLD-100 chip, Ge-doped and Al-doped fibres, respectively.