Quenching factor measurement of the undoped CsI at cryogenic temperature

We report on a measurement of the quenching factor of undated CsI at 77K, inspired by the COHERENT collaboration's experience measuring CEvNS using CsI[Na]. A serious background limiting its sensitivity was the Cherenkov radiation emitted from the PMT quartz window by charged particles. A switch from PMTs to SiPM arrays can be used to eliminate this background since SiPM arrays do not have a quartz window. In order to reduce the high dark count rate of SiPMs at room temperature, they need to be cooled to cryogenic temperatures. The cryogenic operation calls for the switch from doped CsI crystals to undoped ones, since the latter at 40 Kelvin have about twice as high light yield as the former at 300 Kelvin. Before we start using the combination of CsI and SiPM we need to study the properties of csi at low temperature. The first step we have done is the Quenching Factor measurement of undoped CsI at 77 K. The Am241 source is used to measure the electron recoil energy. The neutron source form Triangle Universities Nuclear Laboratory (TUNL) is used to do the nuclear recoil energy measurement. The nuclear recoil energy range is 3.9-59.7keV. We will present the experiment and analysis details in the presentation.