Characterization of a 5 x 5 LYSO Matrix Calorimeter Prototype

In this paper, we present measurements of the energy and the time resolution of the Lutetium-Yttrium Oxyorthosilicate (LYSO) calorimeter prototype for the Mu2e experiment. The prototype calorimeter matrix consisted of 25, 3 × 3 × 130 mm^3, LYSO crystals readout by 10 × 10 mm^2 Hamamatsu APDs. The prototype has been tested both with the tagged photon beam at MAMI, Mainz, Germany, and with the e^- beam of BTF, Frascati. The energy of particles used ranges between 61.3 and 187.1 MeV (80-130 MeV) for photon (electron) beams. While the MAMI data shows a negligible beam energy spread, we observe a 3%-5% contribution related to the beam in BTF sample. After correcting this effect, we obtain a global energy dependence of the energy resolution described by the formula σE/E_dep = a/√(E_dep/GeV) ⊕ b, with a = (0.59 ± 0.12)% and b = (3.63 ± 0.21)%. The simulation is in good agreement with data if we add an additional contribution due to the intrinsic LYSO fluctuation of 2.6%. We present also the measurements of the time resolution. The measurements have been performed using the e^- beam BTF in the energy range from 100 to 400 MeV. A resolution of (161 ± 3) ps at 100 MeV has been achieved. The energy dependence of the resolution has been fitted with following parametrization: σ_t(E) = a/ √E/GeV ⊕ b, where the stochastic term and the constant term are determined to be a = (50 ± 1) ps and b = (40 ± 3) ps, respectively.