Application of the GEMPix Detector in Radiation Protection and Particle Therapy

The GEMPix is a small gaseous detector with a highly pixelated readout. It consists of a sensitive volume (2.8 × 2.8 × 0.3 cm^{3} or 2.8 × 2.8 × 1.2 cm^{3} ) to detect ionising radiation, three Gas Electron Multipliers (GEMs) for signal amplification and four Timepix Application-Specific Integrated Circuits (ASICs) with 262,144 pixels of 55 × 55 μm^{2} area each for readout. It is continuously flushed with Ar:CO_{2}:CF_{4} (45:15:40) gas. Before applications are investigated, a detailed detector characterisation is performed and results such as gain scans are reported. Also, a correction necessary for a stable detector response despite changes of temperature, pressure and humidity is developed. The GEMPix has found several applications two of which are investigated in this thesis. A proof-of-concept for measurements of the {55}^Fe content in activated steel samples of radioactive waste is reported. {55}^Fe is an important radionuclide in the elimination procedures of waste produced at facilities like the European Organization for Nuclear Research (CERN) housing accelerator based experiments. Forty-five samples are used to calibrate the GEMPix counts to specific activity. The detection limit and the sensitivity to the Swiss clearance limit are investigated. The GEMPix is potentially sensitive to the clearance limit but a detailed uncertainty study will be needed before a potential operational phase. Liquid radioactive sources could improve the calibration. 3D measurements of the energy deposited by a hadron beam in a water phantom represent an important step towards a possible use of the GEMPix for quality assurance in hadron therapy, one of the main cancer treatment modalities. An integrated system is developed and consists of a commercial water phantom with a motorised positioning system, a reference ionisation chamber and the GEMPix. Bragg curves obtained with this system match Monte Carlo simulations with the Fluctuating Cascade (FLUKA) code within 10% for most data points for carbon ions of three different clinical energies. For this application, a larger sensitive area matching the typical maximum radiation field size of 20 × 20 cm^{2} is necessary. Therefore solutions based on the Timepix ASIC and on other readout principles such as the detection of optical photons produced in the GEMs are currently under investigation.