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

暗物质和中微子的性质是现代物理学中最引人注目的问题之一。暗物质粒子可以通过观察它们与目标中的原子核散射时产生的核反冲来探测。当中微子与原子核相干散射时，也会产生核反冲。如果观测到这一过程，将有助于探测非标准中微子相互作用，寻找惰性中微子，并了解核心坍缩超新星的爆炸机制。随着核反冲能量的下降，两种散射率都急剧增加。探测器的能量阈值越低，可以检测到的事件越多。SURF地下实验室最近的活动刺激了南达科他州对州立大学博士项目扩张的支持。该奖项计划的活动为博士生提供了参与地下科学研究的机会。该活动也为在77 K下未掺杂的NaI/CsI在民用领域的实际应用铺平了道路。例如，未掺杂的NaI在77 K下的闪烁信号比室温下的NaI/CsI（Tl）快得多。它可用于提高PET的时间和位置分辨率，从而更精确地成像患者的相关组织。降低闪烁探测器能量阈值的直接方法是增加其光产率。该奖项的重点是优化77 K下未掺杂NaI/CsI的光读出。在室温下，碘化钠和掺有铊的碘化铯NaI/CsI（Tl）都是优异的闪烁体。PI计划将未掺杂的NaI/CsI直接耦合到光电倍增管（pmt）上，将其冷却到77 K以最大化光读出。比较了pmt的低温性能；研究了未掺杂的NaI/CsI晶体的尺寸和各种表面处理，以优化光收集效率。与现有的室温下使用NaI/CsI（Tl）的实验相比，目标是验证是否有可能通过将光产率提高两倍来将能量阈值降低两倍。未掺杂NaI/CsI在77 K下的闪烁光衰减时间与NaI/CsI（Tl）在室温下的闪烁光衰减时间有很大不同。它们对信号和背景辨别的影响也将被研究。

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.