Design study of a "scintronic" crystal targeting tens of picoseconds time resolution for gamma ray imaging: the ClearMind detector

We describe the concept of a new gamma ray "scintronic" detector targeting a time resolution of the order of 25 ps FWHM, with millimetric volume reconstruction and high detection efficiency. Its design consists of a monolithic large PbWO₄ scintillating crystal with an efficient photocathode directly deposited on it. With an index of refraction higher for the photocathode than for the crystal, this design negates the total reflection effect of optical photons at the crystal/photo-detector optical interface, and thus largely improves optical coupling between the crystal and the photodetector. This allows to detect efficiently the Cherenkov light produced by 511 keV photoelectric conversions in PbWO₄, and to optimize the detector time resolution. Furthermore, the low-yield, fast scintillation light produced additionally by PbWO₄ increases the detected photon statistics by a factor 10, thus fostering accurate (3 dimensional) localization of the gamma ray interaction within the crystal and providing a fair measurement of the deposited energy. This paper lists the technological challenges that have to be overcome in order to build this "scintronic" detector. We show that all the key technologies have now been demonstrated and present results of a preliminary Monte Carlo simulation, which include an innovative event reconstruction algorithm to support the claimed performances of the detector.