Particle detection at cryogenic temperatures with undoped CsI

Scintillators are widely used as particle detectors in particle physics. Scintillation at cryogenic temperatures can give rise to detectors with particle discrimination for rare-event searches such as dark matter detection. We present time-resolved scintillation studies of Cesium Iodide (CsI) under excitation of both alpha and gamma particles over a long acquisition window of 1 ms to fully capture the scintillation decay between room temperature and 4 K. This allows a measurement of the light yield independent of any shaping time of the pulse. We find the light yield of CsI to increase up to two orders of magnitude from that of room temperature at cryogenic temperatures, and the ratio of alpha to gamma excitation to vary significantly, exceeding 1 over a range of temperatures between 10 and 100 K. This property could be useful in separating alpha backgrounds from the low energy nuclear recoil signal region. We also find the time structure of the emitted light to follow similar exponential decay time constants between alpha and gamma excitation, with the temperature behavior consistent with a model of self-trapped exciton de-excitation. Based on these properties, undoped CsI is an interesting candidate for use in cryogenic particle detectors.