Undoped NaI/CsI Directly Coupled to PMTs at 77 K for Rare-Event Searches

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Dark matter and neutrino properties are among the most compelling problems to be studied in modern physics. Dark matter particles may be detected through the observation of nuclear recoils produced when they scatter with nuclei in a target. Nuclear recoils can also be created when neutrinos coherently scatter with nuclei. This process, if ever observed, would help in probing non-standard neutrino interactions, searching for sterile neutrinos, and understanding the explosion mechanism of core-collapse supernovae. Both scattering rates increase dramatically as the energies of nuclear recoils go down. The lower the energy threshold of a detector, the more events can be detected. Recent activity in the SURF underground laboratory stimulated support from the South Dakota state to the expansion of PhD programs at state universities. The activity planned with this award provides an opportunity for a PhD student to take part in research related to underground science. The activity may also pave the way for practical applications of undoped NaI/CsI at 77 K in civil uses. For example, un-doped NaI at 77 K has a much faster scintillation signal than NaI/CsI(Tl) at room temperature. It can be used to improve the time and position resolution of PET, resulting in more precise imaging of the concerned tissue of a patient.A straight forward way to lower the energy threshold of a scintillation detector is to increase its light yield. This award is focused on optimizing the light readout from un-doped NaI/CsI at 77 K. Both sodium iodide and cesium iodide doped with thallium, NaI/CsI(Tl), are excellent scintillators at room temperature. The PI plans to couple un-doped NaI/CsI directly to the photomultiplier tubes (PMTs), cool them down to 77 K to maximize the light readout. Low temperature properties of the PMTs will be compared; dimensions and various surface treatments of un-doped NaI/CsI crystals will be examined to optimize the light collection efficiency. The goal is to verify whether it is possible to lower the energy threshold by a factor of two by increasing the light yield by a factor of two, compared to existing experiments utilizing NaI/CsI(Tl) at room temperature. The decay times of scintillation light from un-doped NaI/CsI at 77 K are quite different from those of NaI/CsI(Tl) at room temperature. Their effect on signal and background discrimination will also be examined.