DUSEL R&D: Detection of impurities in cryogenic liquids with extreme sensitivity

近年来，基于液体惰性气体探测器技术的实验激增。稀有气体具有电离电势低、闪烁效率高、电子寿命长、成本低等优点，是一种极具吸引力的粒子探测器介质。这些探测器目前被用来解决各种各样的基本物理问题，包括无中微子双β衰变、中微子振荡、罕见的µ子衰变、暗物质搜索和太阳中微子。这些问题中的许多都是地下实验室的科学任务的核心，类似的探测器也被提出用于杜塞尔的最初一系列实验。到目前为止，大多数液体探测器的尺寸相对较小，目标质量从几公斤到几百公斤不等。然而，下一代实验正准备将这一界限远远超出当前的技术水平。要使实验在这种新体制下取得成功，就必须对低温液体进行提纯，远远超过目前的技术所能达到的水平。对于现有的TPC型探测器，电负性杂质的浓度必须小于约100ppt氧当量，以避免电荷信号在漂移通过探测器体积时的衰减。因此，下一组实验将需要杂质浓度从1到10 ppt，或自由电子寿命从数十到数百毫秒。该奖项将使该团队能够开发一种检测极低浓度杂质的新方法，有可能将电流灵敏度提高几个数量级。作为这项工作更广泛影响的一部分，这种杂质探测器将在许多可能由杜塞尔主持的基础物理实验中得到广泛应用。此外，这项工作还将对进一步促进研究生教育产生更广泛的影响。

Recent years have seen a proliferation of experiments based on the liquid noble gas detector technology. Noble gases are an attractive medium for particle detectors due to their low ionization potential, high scintillation efficiency, long electron lifetime and low cost. These detectors are currently employed to address a wide variety of fundamental physics questions, including neutrinoless double beta decay, neutrino oscillations, rare muon decays, dark matter searches and solar neutrinos. Many of these questions are central to the scientific mission of an underground lab and similar detectors have been proposed for the initial suite of experiments at DUSEL. Most liquid detectors to date are of relatively modest size, with target masses ranging from a few kilograms to a few hundred kilograms. However, next-generation experiments are poised to push the boundaries far beyond the current state of the art. For an experiment to succeed in this new regime, it will be necessary to purify the cryogenic liquids far beyond what current technologies have achieved. For existing TPC-style detectors, the concentration of electronegative impurities must be less than about 100 ppt oxygen equivalent to avoid attenuation of the charge signal as it drifts through the detector volume. Therefore, the next set of experiments will require impurity concentrations from one to ten ppt, or free electron lifetimes from tens to hundreds of milliseconds. This award will enable the group to develop a new method for detecting extremely low concentrations of impurities, with the potential to extend current sensitivities by several orders of magnitude. As part of the Broader Impacts of this work, this impurity detector would find wide application for many of the fundamental physics experiments which are likely to be hosted by DUSEL. In addition, this work will also have significantly broader impact by serving to further the education of graduate students.