Characterisation of Organic Semiconductors for Ionising Radiation Dosimetry in Medical Applications

Extensive use of ionising radiation has increased dramatically with advances in medical diagnostic and radiation therapy, space exploration, security and defence, personal wearable dosimeters, and nuclear science. Many of these applications require the safe delivery of ionising radiation to the target volume while minimising toxicity or exposure to the patient or safety personnel. Therefore, dosimetry systems need to be in place to monitor and measure the dose delivered to the human body, with instantaneous feedback. In radiotherapy treatments, the target volume is often a cancerous cell accumulation, surrounded by healthy tissues. In this case, margins of error to the target volume during exposure to ionising radiation become crucial in order to spare surrounding vital organs when large doses have the potential to permanent damaging. Furthermore, advancements of radiotherapy modalities that make use of high dose rates, smaller fields and x-ray imaging for image guided dose delivery modalities have demonstrated that higher absorbed doses in shorter times increase tumour control while minimising treatment times and toxicity to surrounding tissue. Additionally, safety organisations have introduced stricter quality assurance measures that require the implementation of live monitoring systems positioned between the source and target volume as a wearable monitor, named in-vivo dosimetry. Therefore, the dosimetry systems must not only exhibit a tissue equivalence response with high spatial and temporal resolution, and high radiation tolerance, but also allow for mechanical flexibility and radiation transmissibility for conformability on the body without perturbing the beam. The requirements of in-vivo dosimetry make quality assurance measures difficult to carry out with the current dosimeters available. The current detectors utilise inorganic solar-state semiconductors that are expensive or unable to be fabricated as large-area flexible sensors and are composed of heavier elements that perturb the beam. Recent innovations in material sciences have presented novel semiconductors comprised of organic compounds capable of tailored electronic performances and low-cost fabrication onto flexible substrates. This thesis presents the application of organic semiconducting detectors as the first flexible and fully tissue equivalent ionising radiation detector capable of operating with low operational bias. Commercially available organic semiconducting detectors, used for solar cell applications, were used as a feasibility study to verify their capabilities and material advantages in radiotherapy dose verification. The dosimeter performance of the detectors was extensively characterised under medical radiation beams, demonstrating stable and reproducible responses when coupled with a plastic scintillator. The ability of organic semiconductors to inherently mimic the response of human tissue due to their similar chemical composition was demonstrated with experimental evidence in terms of their energy and dose-rate dependence. The radiation hardness of the organic semiconductors was investigated in this thesis up to a total irradiation dose of 40 kGy to advocate their long shelf-life in these harsh environments