Measurement of the energy and time resolution of a undoped CsI + MPPC array for the Mu2e experiment

This paper describes the measurements of energy and time response and resolution of a 3×3 array made of undoped CsI crystals 3×3×20 cm3 coupled to large area Hamamatsu Multi Pixel Photon Counters 12×12 mm2. The measurements have been performed using the electron beam of the Beam Test Facility in Frascati (Rome, Italy) in the energy range 80–120 MeV. The measured energy resolution, estimated with the FWHM, at 100 MeV is 16.4%. This resolution is dominated by the energy leakage due to the small dimensions of the prototype. The time is reconstructed by fitting the leading edge of the digitized signals and applying a digital constant fraction discrimination technique. A time resolution of about 110 ps at 100 MeV is achieved.