Versatile Passive Dosimetry at Radiotherapy and Greater Levels using Borosilicate Glass

Abstract

In line with the rapid advances that are being made in medical and industrial applications of radiation at elevated doses, a need is seen for convenient detectors that also offer a favourable range of characteristics. Particular utilisations herein largely depend upon the sensitivity of the dosimetric medium and also upon spatial resolution. The work herein is novel, encompassing characterisation of defects and dosimetric studies of commercial low-cost borosilicate (Pyrex) microscope glass slides and glass-fibre filters, with a view to making use of these as thermoluminescent dosimeters (TLDs). The particular glass forms studied herein have a number of potentially favourable physical characteristics, including being small in thickness (from a few mm down to sub mm diameters), inexpensive, readily available, resistant to moisture/wetting and reusable. Having established the utility of both the glass slides and filter forms towards photon irradiations, the work makes a more detailed investigation of the microscope slide (coverslips), the high degree of optical performance of these being indicative of homogeneity. Dosimetric characterisation of borosilicate glass has been obtained by irradiating them in various radiotherapy and product-irradiator beams, including kilovoltage and megavoltage photons, and megavoltage electrons. The borosilicate glass is found to exhibit minimal fading, competitive with commercial LiF thermoluminescent (TLD-100) dosimeters, also demonstrating a large dynamic dose range over which they remain linear, from sub-Gy through to 250 kGy. Via computerised glow curve deconvolution, the broad overarching peak has been analysed, being best fit deconvolved at into five peaks, associated with silicon oxygen defect centres (Si-ODC), B-O centres and B structural defects. More definitively, the first, second and third peaks, with activation energies of less than 1.2 eV, can be related to the existence of O+ and O- in the silica network. Also shown is that the borosilicate glass is capable of neutron detection, arising predominantly from the order of 4 % boron content, use being made of linacs operating in the photon mode. In line with conventional wisdom, no evidence is found at 6 MV for photo-neutron contamination. Conversely, at 10 MV and in particular 15 MV, appreciable neutron contribution to the signal is seen. The evident dose-dependent non-linearity is suggested to arise from the 10B (n, ) 7Li reaction, the alpha particles delivering dose dependent damage to the glass insulating material, forming additional defects.