Estimation of the single event upset rate in the dose delivery system at MedAustron

Abstract

MedAustron is a particle accelerator facility that uses hadron therapy for cancer treatment in humans. The gantry, an array of magnets used for beam guidance around the patient, is now integrated into the regular treatment process. New beam angles in the direction of sensitive electronics are to be commissioned for even more effective patient treatment, making it necessary to investigate the effect of the radiation on these sensitive electronics. The main concern is Single Event Upset (SEU) rates in the components of the Dose Delivery System (DDS). An SEU can occur when high energy hadrons (> 20 MeV), intermediate energy neutrons (0.2 MeV to 20 MeV) or thermal neutrons (≈ 25 meV) deposit enough energy inside a random access memory (RAM), a field programmable gate array (FPGA) or a processor to generate a charge Q_dep over a threshold value Q_crit . This results in a bit flip, which can cause information corruption, which in turn can lead to disruptions in the beam line operation. The DDS is responsible for steering the particle beam and cross-checking the delivered dose in real time. Although errors in this system are not critical for patient safety, they could lead to downtime of the beam line, which in turn would result in fewer treatments. For a gantry angle of 60◦ a radiation field of secondary particles is expected at the position of the electronics rack where the sensitive components of the DDS are located. This mixed field results from scattering of the particle beam in a steel plate above the rack, which is used as a floor of the irradiation room. To get an estimate of the risk for an SEU, the fluence of thermal neutrons (THNs) Φ_THN, high energy hadrons and Weibull weighted intermediate energy neutrons, grouped under the expression high energy hadron equivalent (HEHeq), Φ_HEHeq, is investigated at the positions of the sensitive components using FLUKA (FLUktuierende KAskade), a MonteCarlo (MC) code environment used for simulation of particle and matter interactions.The fluences can then be used to calculate cross sections and subsequently SEU rates. The goal is to get an understanding of the magnitude of the SEUs happening. In order to obtain the fluences, the geometry of the DDS rack is implemented with silicon region of interests (ROIs) in the places of the sensitive electronics. Literature values for SEU cross sections are used to get an estimation of the amount of SEUs occurring. Analysis of the simulation results shows that there is a non-negligible risk of SEUs when using the gantry with energies greater than 150 MeV at 60◦ .