Fibre Optics Approach to Dosimetry
Thesis DisciplineMedical Physics
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Dosimetry is the methodology of determining the amount of radiation energy imparted in matter and volume. Although several techniques and devices are available for use in both laboratory and clinical settings, most rely on certain conditions, assumptions and approximations to convert the energy into radiation dose. The many uncertainties from current techniques are introduced due to the material differences between the sensitive detector volume and the phantom material, typically water. The aim of this thesis is to use the water sample itself to detect the amount of radiation energy that has been imparted upon it. Radiation energy absorbed by the sample is ultimately converted into heat, raising the temperature of the sample and changing the refractive index property. The refractive index change results in a shortening of the optical path length and as a result, light passing through the sample experiences a phase change. Phase information cannot be directly measured, as it is merely a property of light wave propagation, thus another technique must be used. Digital holographic interferometry was employed to capture snapshots of the sample’s changing state over time, and when compared with a reference, the interference phase information was extracted and used to calculate the refractive index change, which can then be related to radiation absorbed dose. The aim of this research was to design and build interferometry setups using holographic interferometry to determine the refractive index change induced by radiation and to explore the possibilities of using fibre optics. Experiments were conducted on the setups to determine the validity of the method and the accuracy of the system. With external heating sources in the forms of an open flame and infrared laser, we could see distinct heating patterns formed in the phase images. The phase allowed the calculation of the temperature and therefore energy from the change in refractive index, but was limited to phase differences within 2π between the images, due to wrapped phases. In the stability tests, we demonstrated the accuracy of the system and found it was heavily influenced by the amount of vibration in the vicinity. In the short term, a standard deviation of 0.015 degrees was recorded but a larger standard deviation of 0.078 degrees was measured in the longer term. We can be confident of the temperature measurements to within 0.1 of a degree, equal to hundreds of Grays in radiation dose, however this is not sufficiently accurate for dosimetry. Future work may include improving accuracy by reducing the vibration in the system.