The X-HPD is a modern implementation of the hybrid concept first employed by the Philips SMART and the QUASAR tubes of the Lake Baikal experiment, aiming at improved performance and maximum simplicity. The glass envelope is essentially spherical and a spatial scintillator crystal is mounted in its center. Photoelectrons from the cathode are accelerated by a potential difference of 20–30 kV and deposit their kinetic energy in the scintillator (or Phosphor). The generated scintillation light is detected by a small and low cost photodetector, e.g. a conventional PMT. The spherical symmetry leads to a uniform collection efficiency and very small intrinsic time spread over the full viewing angle of 120° (3π solid angle).We report about the design, fabrication and test results of a first X-HPD prototype of 208mm diameter with a conical LYSO crystal anode. Monte Carlo studies with Geant4 led to a qualitative understanding of the light transport in the anode assembly.
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