PMT System for Prompt Gamma-ray Measurements during Proton Therapy Treatments

Measuring the prompt gamma-rays generated as a by-product of the irradiation in proton therapy treatments constitutes a way for real-time range verification. A new measurement strategy based on a single scintillation detector positioned coaxially to the beam and behind the irradiated area is being developed. Clinical conditions require the detection system to be able to operate under count rates of about 10 million gamma-rays per second. We report on initial developments of a demonstrator detection system consisting of a cerium bromide scintillation detector coupled to a photomultiplier tube and perform an exhaustive study of the voltage divider response. A 1.5 inch diameter R13408-100 photomultiplier tube from Hamamatsu Photonics is being characterized with the help of controlled light sources. Three different voltage dividers (passive, hybrid and active) are being evaluated. First experimental tests at the extreme count rate of ten million events per second show that the active voltage divider is the most promising one, thanks to its ability to resolve incoming pulses in microseconds-long tests, and also showing a promising behavior in seconds-long tests.