A novel metascintillator approach for ultra-fast timing in Positron Emission Tomography

Fast timing in gamma-ray detectors is pivotal to enable accurate time-of-flight (TOF) in Positron Emission Tomography (PET). While electronics already can reach intrinsic timing capabilities well below 50 ps, and photosensors also provide accurate timing performances, scintillators lag behind. Herein, in order to achieve sufficient sensitivity, dense inorganic crystals such as LYSO or BGO with thicknesses of 10–30 mm are required. The response of such crystals is relatively slow, about 40 and 300 ns, respectively. Alternative to these crystals, cost-effective organic scintillators have very fast responses (2–3 ns rise times) but lack sufficient stopping power for medical applications. In this work, we investigate combining the favorable characteristics of high density but slow crystals such as BGO and LYSO, with low density but very fast plastics. Taking advantage of scintillation mechanics, where gamma rays deposit their energy within just few hundred µm from initial interaction point, we propose a new geometrical arrangement, where thin layers of crystal and plastic are placed in an alternating configuration. Simulation results show that for slab thicknesses of less than 200 µm, the majority of events are initiated in the crystal and shared between crystal and plastic. We have also analyzed light transport along the layers for a variety of different treatments to the scintillator walls for high aspect ratio detector designs. This topology allows one not only timing resolution improvement through production of a big amount of quasi-prompt photons on top of the standard scintillation pulse, but also precise determination of the 3D localization of the gamma interaction point without compromising the stopping power and, therefore, detection sensitivity of the system. Pulses have been analyzed through their quad-exponential model and a theoretical investigation of approaches to harness the superb timing of a variable population of quasi-prompt photons has been made.