Photon detector with PbWO4 crystals and APD readout

Abstract

Quantum chromodynamics (QCD), the theory of strong interactions, with the statistical treatment predicts a phase transition from a confined state of quarks and gluons as normal nuclear matter into a deconfined state called quark gluon plasma (QGP). The universe on the current cosmology has evolved from an initial state of the extreme energy density to the present state through a rapid expansion and cooling. Speculated is the phase transition described by QCD at the early universe. The only one is the relativistic heavy-ion collisions in the current experimental technique for raising the phase transition in a laboratory. Some signatures of QGP with heavy-ion collisions should be searched and discovered in order to understand the thermal history of the universe. The most promising probe in heavy-ion collisions is expected to be photons because of their large mean free paths in the strongly interacting reaction regime. Photons can escape from the reaction region and carry pure information on it. The emission of photons in heavy-ion collisions is focused on. The present purpose is to measure photons in the energy range from several hundred MeV to several GeV with high accuracy. A larger amount of particles is, however, produced in relativistic heavy-ion collisions than that of elementary reactions such as electron-position and proton-antiproton collisions in a unit solid angle. Almost all of produced particles are not the probes but background photons originating from hadronic decay processes of neutral mesons. An electromagnetic calorimeter with high granularity is required in order to measure photons accurately in such a high particle multiplicity environment. A high-segmentation and high-resolution photon detector is under development aiming at high-quality photon measurements in relativistic heavy-ion collisions. A prototype detector with lead-tungstate (PbWO4) crystals and avalanche photodiode (APD) readout has been tested with high-energy beams. Evaluations of the performance evaluation are presented.