Pure CsI electromagnetic calorimeter design for the Super Tau-Charm Facility

The Super Tau-Charm Facility (STCF) is a high-luminosity electron-positron collider proposed in China. It will operate in the energy region of 2-7 GeV with a peak luminosity of at least 0.5 x 1035 cm-2 s-1, providing an excellent platform for studying the tau-charm physics. The STCF detector includes an electromagnetic calorimeter (EMC), which plays a crucial role in the accurate measurement of photon energy. To address the challenge of achieving superior photon energy resolution in the high-rate environment caused by the high luminosity, a fast-response EMC based on pure CsI crystals is proposed for the STCF experiment. In this paper, we present the conceptual design of a pure CsI EMC for STCF. The light yield of a pure CsI unit designed for the STCF EMC was measured using cosmic rays. Without considering the mechanical structures of the EMC and the sub-detectors in front of the EMC, the performance of the pure CsI based EMC was studied through Geant4 simulations incorporating the measured light yield, and the EMC design was optimized based on the results. The simulation was carried out under the condition of 100 p.e./MeV light yield and the high background environment anticipated in the STCF experiment. By employing a waveform fitting method, an energy resolution of better than 2.5% for 1 GeV photons was achieved, which satisfies the major performance requirement for the STCF EMC. However, validation studies should be conducted, including the contribution of different types of experimental backgrounds and the accuracy of the background counting rate.