Distributed radiation dosimeter for a biologically-equivalent x-ray phantom

Abstract

Radiation dosimeters are used to measure ionising radiation exposures in many medical applications. The verification of dose delivered to patients is fundamental when conducting treatment for medical imaging. Biological equivalent phantoms are used as reference models to humans to measure the absorbed dose given at particular localised point. The actual absorbed dose given in real time is not available for radiographers hence they rely on diagnostic reference levels (DRLs) which are recommended by the International Commission on Radiological Protection (ICRP). In medical imaging, RADFETs are commonly used as radiation detectors for their enhanced sensitivity with respect to radiation dose. This sensor is not applicable for study as they are quite expensive to be used for numerous testing evaluations. This thesis proposes to use p-channel MOSFETs to measure the radiation dose applied by the x-ray machine. The MOSFET is a miniature device that has the facility to be placed inside the phantom. Also, it provides an instant readout technique which is suitable for instant measurement of the absorbed dose. This thesis presents a series of radiation experiments studying MOSFETs as a sensor and dosimeter of radiation dose. A thorough study was conducted on how the electrical parameter threshold voltage was used as a sensing parameter for radiation. The functionality of this sensor was based upon radiation-induced ionization in silicon oxide interface which leads to change in threshold voltage of the device. Radiation tests were conducted at the Electronics Lab at the Department of Electronics, to observe the shift in threshold voltage utilising constant radiation exposure. After analysing the radiation effects on different MOSFETs, the sensor most prone to changes was chosen to determine the absorption rate inflicted on the device. Furthermore, the MOSFET was tested using the medical x-ray machine found at the Department of Radiology at Mater Dei. On the contrary, this x-ray machine provides pulsed radiation shots where further testing was needed to understand the behaviour of the MOSFET at these conditions. A data acquisition system was designed to measure the most expected changes of radiation dose exerted by the medical x-ray machine.