Study of radiation damage of 28 MeV protons to the Lassena Monolithic Active Pixel Sensor

We are developing an upstream, thin, real-time, X-ray radiotherapy verification device based on a large area Monolithic Active Pixel Sensor (MAPS), the Lassena. The operational radiation exposure will eventually damage the device and affect its performance. Here we present results where two Lassena sensors were irradiated to doses of up to 50 kGy with 28 MeV protons using the MC40 cyclotron at the University of Birmingham. The radiation mainly led to an increased leakage current, and hence a smaller effective dynamic range and higher noise. Nevertheless, the performance of the sensor remains more than sufficient for excellent clinical operation until 20 kGy without (external) cooling. After irradiation the MLC leaf position resolution remains below 60 μm for areas that received less than 10 kGy, rising to ∼130 μm for regions irradiated to between 16 and 20 kGy. As there was no external cooling available during these measurements, regions that received a dose larger than 20 kGy saturated. However, cooling the sensor would ensure it can be operated even after receiving a dose up to 50 kGy. The results show that the sensor can operate well for ∼2 years of normal clinical operation with cooling and about 1 year without cooling. In proton therapy, the annual expected dose is around 15 kGy. Hence, the sensor also shows excellent performance without cooling in proton beam therapy monitoring for ∼2 years of clinical operation.