Development of a novel beam position monitor for proton therapy facilities

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Cut plane of the cavity BPM prototype. Dimensions are in mm. Credit: S. Srinivasan et al 2022 JINST 17 P09013 CC BY 4.0

Former OMA fellow Sud Srinivasan and colleagues from the Paul Scherrer Institute (Switzerland) and the University of Groningen (Netherlands) have presented a novel type of non-intercepting beam position monitor (BPM) for the Paul Scherrer Institute’s superconducting cyclotron “COMET”, which delivers 250 MeV proton beams for radiation therapy in pulses of 1ns at the cyclotron frequency of 72.85 MHz.

The report, published in the latest issue of the Journal of Instrumentation, discusses the fundamental characteristics of the BPM, is based on the detection of the transverse magnetic dipole mode of the EM field generated by the beam in a cavity resonator tuned to the second harmonic of the cyclotron’s RF. This mode is only excited for off-center beam positions and can be measured with the help of four floating cavities within a common grounded cylinder.

The authors estimated the expected signals from the prototype BPM for position offsets from simulations and compared them with test-bench measurements and beam measurements with the prototype and an improvised BPM design.

This new system can achieve a position precision (1𝜎) better than 0.5 mm at proton beam currents of 0.1–10 nA in the beam transport line downstream of the degrader, which is essential for efficient operation of the cyclotron and a safe beam delivery for treatment.

This is the first non-interceptive beam position monitor to have demonstrated position measurements in a cyclotron-based proton therapy facility. Such a cavity BPM could also have advantages compared to the typically used ionization chambers for the purpose of daily quality checks in irradiations using higher beam intensities.

Full article:

S. Srinivasan, S. Brandenburg, J.M. Schippers and P.A. Duperrex, “Development of a fourfold dielectric-filled reentrant cavity as a beam position monitor (BPM) in a proton therapy facility”, Journal of Instrumentation 17, P09013 (2022).