In the European XFEL, electrons bunches are accelerated up to 20 GeV and thenenter undulators where coherent X-rays are produced which can be used for imaging atva molecular level. Electrons may stray from the path and hit the permanent magnets inthe undulators. It is well known that ionizing radiation can affect the magnetic characteristics of permanent magnets. The undulators are therefore equipped with a type of semiconductor dosimeters, RADFETs, so that the potential damage from ionizing radiation to the magnets can be measured and corrected for. It is also known that heat will be generated from air-coils in the accelerator which can change the ambient temperature around the dosimeters up to 25 K. All semiconductor technology is highly susceptible totemperature. This report investigates the fading characteristics of the RADFET under different temperatures and times after irradiation. It also investigates the dose responseunder dierent temperatures and estimates the magnitude of errors in measured dose which can be expected if temperatures are not accounted for. It is seen that a delta T of a few K can have a large impact on RADFETs' ability to both record and retain dose. A strong time dependence is also seen. The fading is the largest during irradiation andthen slows down exponentially, stabilizing after 1 to 2 months. An increase from 20 deg C to 26 deg C will increase the fading by 2 Gy/h during irradiation, and 0.015 Gy/h weeks afterirradiation. It is estimated that dose measurements at XFEL can have errors of up to 14% if long-term fading is not accounted for. A model for estimating long-term fading as a function of temperature is proposed.

During the last decades radiotherapy has made major improvements in accuracy and individualization of the treatment techniques. In this project the thickness of a tungsten filter has been optimized using both simulations and experiments in order to further reduce the uncertainty in the dose given to the patient. With the filter, a dosage with less uncertainty can be obtained and less electrons will strike the patient which means less skin damage. In the simulations a program called PENELOPE, has been used which uses Monte Carlo methods for electron and photons transports. The experiment has been done on real Linear Accelerators.