  CryoCsI, the proposed prototype, is a cryogenic undoped CsI scintillating
detector, which has a much lower energy threshold potentially down to 0.5
keV$_{nr}$ compared to the doped CsI. This enhanced sensitivity of CryoCsI
allows for the observation of more Coherent elastic neutrino-nucleus scattering
events. Precise measurements of CEvNS can not only validate the predictions of
the Standard Model but also explore new physics. In conjunction with other
COHERENT detectors, CryoCsI has the potential to achieve world-leading
sensitivities in a broad range of physics topics within and beyond the SM. The
sensitivities of CryoCsI to hidden-sector dark matter, non-standard neutrino
interactions, and neutron radius are explored.
  This thesis delves into the construction of CryoCsI and efforts to enhance
its light yield from 20 to $50 \pm 2$ photoelectrons per keV
electron-equivalent (keV$_{ee}$). It will address challenges with cryogenic
SiPMs, including inferior energy resolution, optical cross-talk, and potential
limitations on detecting rare events. Understanding the light yield of
scintillating detectors for nuclear recoils is crucial, as explored through
alpha-particle and neutron quenching factor measurements. A QF of approximately
15\% was measured using a neutron beam at the Triangle Universities Nuclear
Lab. Proposed solutions to challenges like the overshoot effect observed in
PMTs will be discussed. Additionally, the thesis will explore design
considerations for minimizing background noise and optimizing the CsI crystal's
shape through optical simulations.
