TlCl:Be,I: A High Sensitivity Scintillation and Cherenkov Radiator for TOF-PET

The material requirements for gamma-ray detectors for medical imaging applications are multifold and sensitivity is often overlooked. High effective atomic number (Z $_{\text {eff}}$ ) Cherenkov radiators have raised the attention in the community due to their potential for harvesting prompt photons. A material with one of the highest Zeff and thus short gamma-ray attenuation length is thallium chloride (TlCl). By doping TlCl with beryllium (Be) or iodine (I), it becomes a scintillator and therefore produces scintillation photons upon gamma-ray interaction on the top of the prompt Cherenkov luminescence. The scintillation response of TlCl:Be,I is investigated in terms of intensity, energy resolution, kinetics, and timing capability with and without energy discrimination. The ratio of prompt to slow scintillation photons is used to derive the intrinsic number of produced Cherenkov photons and compared with analytic calculations avoiding complex Monte Carlo simulations. The experimentally determined number of Cherenkov photons upon 511 keV gamma excitation of $17.9~\pm ~4.6$ photons is in line with our simple calculations yielding 14.5 photons. We observe three scintillation decay time components with an effective decay time of 60 ns. The scintillation light yield of 0.9 ph/keV is sufficient to discriminate events with low energy deposition in the crystal which is used to improve the measured coincidence time resolution from 360-ps FWHM without energy selection down to 235-ps after energy discrimination and time walk correction for 2.8-mm thick TlCl:Be,I crystals, and from 580 to 402 ps for 15.2-mm thick ones. Already with the first generation of doped TlCl encouraging timing capability close to other materials with lower effective atomic number has been achieved.