Non-Interceptive Beam Current and Position Monitors for a Cyclotron Based Proton Therapy Facility

Abstract

In PSI’s dedicated proton therapy facility PROSCAN a pulsed 250 MeV proton beam is delivered by a superconducting cyclotron. During the proton-irradiation treatments, there is a need to accurately measure beam current, in the range of 0.1-10 nA, and beam position (required accuracy 0.5 mm). The beam current is directly associated with the dose-rate in the treatment and the beam position with the quality of the dose distribution in the patient. However, the presently used measurements compromise the beam quality. Nevertheless, it is a necessity to perform these measurements online and with minimal beam disturbance. This thesis reports on the development of two types of cavity resonators to perform non-interceptive measurements of these beam parameters, within the required accuracy. For beam current measurements, a single cavity resonator has been built. For the beam position measurements, a cavity resonator consisting of four separate segments has been built. Both cavity resonators have been tuned to the second harmonic of the beam pulse rate, i.e., 145.7 MHz. In test bench experiments and with proton beams, a good agreement between the expected and measured sensitivity of these resonators has been found. The cavity used to measure beam current can measure currents down to 0.15 nA with a resolution of 0.05 nA. The cavity for measuring beam position delivers position information with the required accuracy and resolution demands of 0.5 mm. The design, tests and performance in the beam as well as special applications, future improvements and limitations are discussed.