Prompt gamma timing for proton range verification with TlBr and TlCl as pure Cherenkov emitters

Objective. Prompt gamma timing (PGT) uses the detection time of prompt gammas emitted along the proton track in proton radiotherapy to verify Bragg peak positioning. Cherenkov detectors offer the possibility of enhanced signal-to-noise ratio (SNR) due to Cherenkov physics which enhances detection of high energy prompt gamma rays relative to other induced uncorrelated signals. In this work, the PGT technique was applied to 3 semiconductor materials that emit only Cherenkov light: a 3x3x20 mm³ TlBr, a 12x12x12 mm³ TlBr, and a 5x5x5 mm3 TlCl. Approach. A polymethyl methacrylate (PMMA) target was exposed to a 67.5 MeV, 0.5-nA proton beam and shifted in 3-mm increments at the Crocker Nuclear Laboratory (CNL) in Davis, CA, USA. A fast plastic scintillator coupled to a PMT provided the start reference for the proton time of flight (TOF). TOF distributions were generated using this reference and the gamma ray timestamp in the Cherenkov detector. Main results. The SNR of the proton correlated peaks relative to the background was 20, 29, and 30 for each of the three samples, respectively. The upper limit of the position resolutions with the TlCl sample were 2 mm, 3 mm, and 5 mm for 30k, 10k, and 5k detected events, respectively. The time distribution of events with respect to the reference reproduced with clarity the periodicity of the beam, implying a very high SNR of the Cherenkov crystals to detect prompt-gammas. Background presence from the neutron-induced continuum, prompt-gammas from deuterium, or positron activation were not observed. Material choice and crystal dimensions did not seem to affect significantly the outcome of the results. Significance. These results show the high SNR of the pure Cherenkov emitters TlBr and TlCl for the detection of prompt-gammas in a proton beam with current of clinical significance and their potential for verifying the proton range.