The response of lead-tungstate scintillators (PWO) to photons with energies in the range 13 MeV-64 MeV

The response of a matrix of 25 lead tungstate (PWO) scintillator detectors, operated at −25°C, to photons in the range 13MeV–64MeV has been measured at the tagged-photon facility at MAX-lab, Lund. The tapered PWO crystals, each with a length of 200mm and a cross-section of 24.4×24.4mm2 in the front end, read out by 19mm photomultiplier tubes, were arranged in a 5×5 matrix. The response was measured for photons directed towards the centre of the central crystal as well as for photons directed towards the corner of the central crystal, where four crystals meet. The obtained energy resolution surpasses what has been published so far and is close to the limit given by Poisson statistics and escaped energy. For photons directed towards the centre(corner) of the central crystal the relative energy resolution, defined as (FWHM/2.35)/Eγ, decreases from 7.3%(11.0%) at Eγ=13MeV to 3.3%(3.6%) at Eγ=64MeV. The reconstructed point of impact of a photon in this energy range is determined with an uncertainty (one standard deviation) of 7.3±0.1mm.