Abstract. This paper describes the measurements of energy and time response and resolution of a 3 × 3 array made of undoped CsI crystals coupled to large area Hamamatsu Multi Pixel Photon Counters. 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.

To study an alternative to BaF 2 , as the crystal choice for the Mu2e calorimeter, thirteen pure CsI crystals from Opto Materials and ISMA producers have been characterized by determining their light yield (LY) and longitudinal response uniformity (LRU), when read with a UV extended PMT. The crystals show a LY of ∼ 100 p.e./MeV (∼ 150 p.e./MeV) when wrapped with Tyvek and coupled to the PMT without (with) optical grease. The LRU is well represented by a linear slope that is on average δ ∼ -0.6 % /cm. The timing performances of the Opto Materials crystal, read with a UV extended MPPC, have been evaluated with minimum ionizing particles. A timing resolution of ∼ 330 ps (∼ 440 ps) is achieved when connecting the photosensor to the MPPC with (without) optical grease. The crystal radiation hardness to a ionization dose has also been studied for one pure CsI crystal from SICCAS. After exposing it to a dose of 900 Gy, a decrease of 33% in the LY is observed while the LRU remains unchanged.

Abstract. This paper describes the measurements of energy and time response and resolution of a 3 × 3 array made of undoped CsI crystals coupled to large area Hamamatsu Multi Pixel Photon Counters. 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.