A scintillating fibre based beam profile monitor for ion therapy beams

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This project has been developed as a result of discussions with the Heidelberg Ion-beam Therapy Clinic (HIT) and aims to further develop and study a beam profile monitor for use at ion therapy clinics or other accelerator facilities. At HIT protons, helium, carbon, and oxygen ion beams can be scanned over a 20 cm by 20 cm area with multiple energy and intensity settings for patient treatment. The tracking system monitors the beam position online and provides feedback.The proposed detector is based on 0.25 mm plastic scintillating fibres bonded as a ribbon with photodiode array readout (0.8 mm channels) providing 1-dimensional profile information from each detector plane. The current system is a commercial Multi-Wire Proportional Chambers (MWPC) with 2 mm channel pitch and 4-8 kHz readout. Ions from gas ionisation have drift times of ~150 µs and create a dead time. The MWPC will reach their life span soon and a faster, more precise system is desired for its replacement. A faster readout rate (~10kHz), would allow for higher dose rates and faster patient treatment times. A fibre detector with photodiode arrays should perform intrinsically better than the current system due to their speed and channel size. Preliminary development began with a proof-of-concept detector using available materials and electronics to determine the feasibility of the project. An existing 5-layer fibre ribbon was used in a small detector with one 64 ch photodiode read out with a desired signal integration period (100 microseconds) but only sampled at 1 kHz due to the available electronics. The results were positive and have been published in JINST (https://doi.org/10.1088/1748-0221/13/05/P05030).The next phase (in progress) attempts to achieve all the specifications required for an ion therapy beam monitor. It consist of 2/5 of the final detector (by area) using the first version of custom electronics, and uses the first 2-layer fibre ribbons (to minimise the material in the beam) using two different types of scintillator. The unprocessed data is output for offline analysis at the target readout rate of 10 kHz. Two fibre ribbons have been irradiated with the equivalent of half a year of dose to study radiation damage effects. The response of plastic scintillating fibre to different ion types and energies is studied in detail. Other facilities for testing with other ions are being investigated. The 2nd phase (future) would involve development of on-board processing in addition to offline output of a full size prototype. Additionally, lighter fibre mats would be developed. A successful 2nd phase would lead to transferring the knowledge and technology to HIT for further development and use in a medical environment. The hiring of a doctoral researcher is foreseen to develop the new detectors, processing algorithms, and detector simulations, and analyse the data collected together with the primary investigator and electronics developer.