First operation of undoped CsI directly coupled with SiPM at 77 Kelvin*

The light yield of a combination of an undoped CsI crystal plus two SiPMs at about 77 Kelvin was measured to be 38.88 ±2.96 photo-electrons (PE) per keV electron-equivalent (keVee) using the 26.3 keV γ-ray peaks from an ²⁴¹Am radioactive source. The feasibility of using an undoped CsI crystal coupled with two SiPMs at 77 K at a lower energy region was the first attempt in the world. The high light yield together with some other technical advantages makes it a great neutrino and dark matter detector at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory (ORNL).