A Fast Plastic Scintillator for Low-Intensity Proton Beam Monitoring

In the context of particle therapy monitoring, we are developing a gamma-ray detector to determine the ion range in vivo from the measurement of particle time of flight. For this application, a beam monitor capable to tag in time the incident ion with a time resolution below 235-ps full width at half maximum (FWHM) (100-ps rms) is required to provide a start signal for the acquisition. We have therefore developed a dedicated detector based on a fast organic scintillator (EJ-204) of $25\times 25\times $ 1 mm3 coupled to four silicon photomultiplier strips that allow measuring the particle incident position by scintillation light sharing. The prototype was characterized with single protons of energies between 63 and 225 MeV at the MEDICYC and ProteusONE facilities of the Antoine Lacassagne proton therapy center in Nice. We obtained a time resolution of 120-ps FWHM at 63 MeV, and a spatial resolution of ~2-mm rms for single particles. Two identical detectors also allowed to measure the MEDICYC proton energy with 0.3% accuracy.