Dosimeters using plastic scintillators and fibre optics.
Thesis DisciplineMedical Physics
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A plastic scintillation detector exhibiting minimum interference using two different optical fibre light guides have been designed, constructed and evaluated for dosimetry (potentially in-vivo) of the high energy beams used in radiotherapy practice. One detector system contains the radiation resistant SiO2 optical fibre, while the other contains PMMA fibre, which has less resistance to radiation. Each fibre is connected to an independent photo diode. Also, each fibre is connected to a cylindrical water equivalent scintillator measuring 3mm in diameter and 10mm in length. The scintillator is coated with a thin, optically reflective coating. The light arising from the scintillator is transmitted by the fibre optic light guide and is detected at the photodiode. Each fibre has its own focusing and optical filtering techniques based on the fibre acceptance angle, so the photodiode for SiO2 and PMMA are the same. The photodiodes are connected to an electronics box with a digital integrator and counting system. The interference radiation could be minimised using inherent optical filtration and digital integration consistent with pulses from the linear accelerator, thereby avoiding the use of a second optical fibre to compensate for background signals. The SiO2 and PMMA fibre light guides have different properties especially with respect to flexibility. The dose distribution in water of each fibre light guide coupled scintillator is measured and shows good agreement with ionisation chamber results. Spatial resolution and water equivalence are the most important properties of miniature scintillator detectors. It is shown that these systems are not energy dependent, they do not disturb the main radiation beam, are independent of beam angle and have very good linearity with dose. The current study may stimulate the use of SiO2 and PMMA fibre coupled plastic scintillation detectors in medical dosimetry applications. The results of the PMMA scintillation detector are good and it is a promising detector for in vivo measurements due to its flexibility and low cost.