Study of Čerenkov Light Emission in the Semiconductors TlBr and TlCl for TOF-PET

Thallium bromide (TlBr) and thallium chloride (TlCl) are semiconductor materials with high transparency to visible light, a high index of refraction, and high detection efficiency for gamma rays and annihilation photons. This manuscript reports on measurements of the light intensity and timing response of Čerenkov light emitted in one 3 mm ×3 mm ×5 mm slab of each of these materials operated in coincidence with a lutetium fine silicate (LFS) crystal with dimensions of 3 mm ×3 mm ×20 mm. A
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Na radioactive source was used. The measured average number of detected photons per event was 1.5 photons for TlBr and 2.8 photons for TlCl when these materials were coupled to a silicon photomultiplier. The simulation predicts these results with an overestimation of 12%. The best coincidence time resolution (CTR) for events in TlBr and TlCl was 329 ± 9 and 316 ± 9 ps, respectively, when events with four photons and >7 photons were selected. The simulation showed the CTR degraded from 120 to 405 ps in TlCl, and from 160 to 700 ps in TlBr when the first or second Čerenkov photon was selected. Results of this work show that TlCl has a stronger Čerenkov light emission compared to TlBr and a greater potential to obtain the best timing measurements. Results also stress the importance of improving detection efficiency and transport of light to capture the first Čerenkov photon in timing measurements.