Ionization Yield Measurement for SuperCDMS Germanium Detectors

我的研究计划的目标是提高SuperCDMS SNOLAB实验的科学范围。本研究的目的是在一个便携式绝热退磁制冷机（ADR）中，利用一个克级探测器测量中子束中锗的电离产额。 SuperCDMS合作在SNOLAB的低温硅和锗探测器中寻找暗物质粒子。该技术的低能量阈值将使其对轻暗物质粒子（约0.1- 10 GeV/c2）具有世界领先的灵敏度。为了解释这些数据，一个关键的信息是锗中核反冲的电离产率，它表征了每单位能量沉积产生的自由电荷载流子的数量。锗在低于1 keV的电离产额还没有很好地理解。Collar等人（PRD 103，122003（2021））和SuperCDMS Soudan实验（手稿待定）的光中子校准的最新数据彼此不一致，同时偏离了理论预测。了解锗的电离产额将消除由未知产额值引起的大的系统不确定性，并且将提供SuperCDMS SNOLAB数据的及时高质量数据解释。在这个为期一年的计划中提出的科学方法是通过散射实验测量核反冲电离产额。实验装置包括一个单能中子束入射到锗探测器上，然后在已知的散射角的二次探测器阵列。2019年，我在杜克大学的三角大学核实验室（TUNL）用ADR中的克级硅探测器进行了电离产额测量，中子束为58 keV。在拟议的实验中，我的研究小组（包括一名博士后研究员，两名研究生和两名本科生）将领导锗的测量，使用我以前使用过的必要ADR基础设施，并将从西北大学租借到我在UofT的实验室。利用SuperCDMS合作设计和制造克级锗探测器的持续努力，我的团队将于2022年春季在我在UofT的实验室的ADR中调试探测器，并根据光束质量在TUNL光束或替代光束设施（如蒙特利尔大学的串联设施）进行电离产额测量。这项行动计划在2022年夏天进行，随后进行数据分析。拟议的研究将显著影响SuperCDMS SNOLAB的研究工作，通过提供在反冲能量低于1 keV时锗电离产率的精确测量，这将解决现有数据和理论之间的差异。这项研究将改善对SuperCDMS SNOLAB数据的解释，并将影响全球正在进行的努力。

The goal of my research program is to enhance the science reach of the SuperCDMS SNOLAB experiment. The objective of the proposed research is to measure the ionization yield of germanium with a gram-scale detector in a portable Adiabatic Demagnetization Refrigerator (ADR) in a neutron beam. The SuperCDMS collaboration searches for dark matter particles in cryogenic silicon and germanium detectors at SNOLAB. This technology's low energy threshold will give it world--leading sensitivity to light dark matter particles (between ~0.1--10 GeV/c2). To interpret the data, one critical piece of information is the ionization yield of nuclear recoils in germanium, which characterizes the amount of free charge carriers created per unit energy deposition. The ionization yield in germanium below 1 keV is not well understood. The recent data from Collar et al. (PRD 103, 122003 (2021) ) and from the photoneutron calibration by the SuperCDMS Soudan experiment (manuscript pending) are not consistent with each other, while at the same time deviating from the theoretical predictions. Understanding the ionization yield of germanium would eliminate the large systematic uncertainty caused by the unknown yield value, and would provide prompt high quality data interpretation of the SuperCDMS SNOLAB data. The scientific approach proposed in this one-year program is to measure the nuclear recoil ionization yield through a scattering experiment. The experimental setup consists of a mono-energetic neutron beam incident onto the germanium detectors then an array of secondary detectors at known scattering angles. In 2019, I led an ionization yield measurement with a gram-scale silicon detector in an ADR, with a 58 keV neutron beam, at the Triangle Universities Nuclear Laboratory (TUNL) at Duke University. In the proposed experiments, my research group (including one postdoctoral fellow, two graduate and two undergraduate students), will lead the measurements with germanium, using the necessary ADR infrastructure that I used previously and will be on loan to my laboratory at UofT from Northwestern University. Leveraging the ongoing effort in the SuperCDMS collaboration of designing and fabricating a gram-scale germanium detector, my team will commission the detector in the ADR in my laboratory at UofT in spring 2022, and perform an ionization yield measurement at the TUNL beam, or at an alternative beam facility like the tandem facility at Université de Montréal, based on the beam quality. This operation is planned to happen in the summer of 2022, with data analysis to follow. The proposed research will significantly impact the research efforts of the SuperCDMS SNOLAB by providing precise measurements of the ionization yields of germanium at recoil energies below 1 keV, that will resolve the discrepancies among the existing data and theory. This research will improve the interpretation of the data from the SuperCDMS SNOLAB and will impact ongoing efforts globally.