Respond to Nuclear Recoils in Pure CsI Detectors at temperatures below 77K

The COHERENT collaboration achieved the first detection of Spallation Neutron Source (SNS) neutrinos through coherent elastic neutrino-nucleus scattering (CEvNS) using a 14 kg CsI(Na) detector. However, the sensitivity of this detector was compromised by background Cherenkov radiation generated in the quartz window of the photomultiplier tube (PMT). To address this, a transition from PMTs to silicon photomultiplier (SiPM) arrays is proposed, as SiPMs do not have a quartz window and are thus immune to this source of background. SiPMs, however, exhibit high dark count rates at room temperature and require cooling, either to 77 K using liquid nitrogen or to 40 K with a pulse-tube refrigerator for optimal performance. For such cryogenic operations (around 40K), undoped crystals are preferred due to their superior light yield—approximately double that of doped crystals at 300 K. The combination of SiPMs with undoped CsI crystals is expected to enhance the number of detectable CEvNS events by at least an order of magnitude compared to the original CsI(Na) detector under similar exposure. One of the challenges in this study is understanding the signature of neutrino-induced low-energy nuclear recoils in CsI. Key aspects of the detector technology, including the cryogenic operation of SiPMs and undoped CsI crystals, have been validated at 77 K, and the quenching factor (QF) has been measured at the Triangle Universities Nuclear Laboratory (TUNL) at this temperature. We have extended the measurements to investigate the detector's response to nuclear recoils at temperatures lower than 77 K. I will report the results of these measurements.