Response of cooled PWO scintillators readout with avalanche photodiodes to low-energy gamma-rays

Identification of π 0 and η mesons by detecting both γ -rays from their decay is a prerequisite for suppressing undesired background in studies of photon transitions between the states of charmonium in the physics program of PANDA. To achieve this goal the detection threshold of the PANDA electromagnetic calorimeter (EMC) should be as low as possible. An experimental setup intended for measurements of the response of cooled PWO scintillators in the energy range 4.4–20 MeV has been designed and constructed. The setup uses γ -rays emitted in reactions induced by protons with light nuclei. Events with full-energy expended in PWO are selected by enclosing the studied 20 × 20 × 200 mm PWO-II scintillator in a cylinder of EJ-200 plastic scintillator, whose two halves are read out independently. A comparison of the relative Gaussian dispersions, σ / E , obtained by us for the three energies in the above energy range, indicates that a smooth dependence established at MAMI, for a matrix of 3 × 3 PWO-II scintillators between 40.9 and 674.5 MeV, also gives a valid extrapolation into the range of these very low energies.