Summary Time-of-flight positron emission tomography is a very effective nuclear imaging modality for the diagnosis and staging of a range of pathologies such as cancer, cardiovascular diseases, or musculoskeletal disorders. Commercial TOF-PET scanners currently employ lutetium-(yttrium)-oxyorthosilicate (L(Y)SO) crystal detectors coupled to silicon photomultipliers (SiPMs) to achieve coincidence time resolutions (CTR) between 200-500 ps full width at half maximum (FWHM). High production costs of L(Y)SO crystals and their intrinsic radiation background are currently hindering the evolution and spread of very promising TOF-PET modalities such as long axial field-of-view (LA-FOV) scanners or studies involving very low doses such as cell tracking or imaging with theranostic agents. New scintillator materials with lower production cost, radiation background-free, and with TOF-level timing accuracy are needed. We propose to use thallium chloride (TlCl) as a scintillator material for TOF-PET. TlCl is a material with a simple cubic structure that allows for a relatively easy and flexible doping process. Preliminary data obtained with TlCl crystals doped with beryllium (Be) and indium (I) show a very fast scintillation component of ~10 ns that has a high potential for very accurate timing measurements. TlCl has a greater detection efficiency than LYSO or even bismuth germanate (BGO) for 511 keV gammas, is background radiation-free, and its estimated production cost is 1/3 of L(Y)SO based on its low melting point of 430C (compared to 2050C for L(Y)SO) and simple lattice structure. Moreover, unlike BGO, TlCl uniquely combines a very fast scintillation process with a high Cherenkov generation yield to further boost timing potential. We aim to prove the feasibility of using TlCl detectors for TOF-PET by combining expertise in crystal growth, simulation of light generation and detection, and benchtop characterization. First, will study the effects of Be and I as dopants in TlCl