SU-E-T-158: Neutron Damage of Power Electronics Used during Image Guidance in Proton Therapy

A series of measurements were performed in a clinical proton therapy beam to assess the sensitivity of silicon-based electronics in commercial x-ray generators to single event burnout from the secondary neutron background in proton therapy treatments.

Failure rates were nondestructively measured in various metal oxide semiconductor field-effect transistors (MOSFETs) as a function of applied voltage using a dedicated test circuit board. Neutrons were produced by 230 MeV protons stopping in a brass beam target and high beam current was used to accelerate testing. Neutron fluences were measured by activation analysis of carbon and aluminum in both the test setup and in situ at the generator. Failure rates were determined by scaling results based on beam monitor output to the relevant neutron fluence rate.

Current pulses from the test board clearly indicated the onset of single event burnout without destroying the MOSFET. The neutron fluence measured on the test board was 4.3 ± 0.8×106 n cm-2 MU-1 and this is consistent with previous measurements. The MOSFET failure rate decreased rapidly with a reduction in the applied voltage and is 20-30 times lower in higher-rated components at the same voltage. Under nominal operating conditions the estimated failure rate is tens of failures per year for a generator 6m from the treatment position.

The sensitivity of x-ray generator power electronics to neutron-induced single-event burnout is significant and can affect the implementation of image-guided techniques for proton therapy. Strategies and system designs to mitigate this phenomenon are being investigated to help enable x-ray generators withstand the proton therapy environment. This research was supported by the NIH/NCI under grant number 6-PO1 CA 21239.