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。本研究的目的是利用便携式绝热退磁制冷机（ADR）中的克级探测器在中子束中测量锗的电离产率。超级cdms合作项目在SNOLAB的低温硅和锗探测器中寻找暗物质粒子。这项技术的低能量阈值将使其对轻暗物质粒子（在~0.1- 10 GeV/c2之间）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核反冲的电离产率，它表征了每单位能量沉积产生的自由载流子的数量。锗在1 keV以下的电离产率尚不清楚。Collar et al. （PRD 103, 122003(2021)）的最新数据与SuperCDMS Soudan实验（待定稿）的光子中子校准数据相互不一致，同时偏离理论预测。了解锗的电离产率将消除因产率值未知而引起的较大的系统不确定性，并将为SuperCDMS SNOLAB数据提供及时的高质量数据解释。在这个为期一年的项目中提出的科学方法是通过散射实验来测量核反冲电离产率。实验装置由单能量中子束入射到锗探测器上，然后在已知散射角的二次探测器阵列上进行。2019年，我在杜克大学三角大学核实验室（TUNL）领导了一项电离产率测量，在ADR中使用克级硅探测器，具有58 keV中子束。在提议的实验中，我的研究小组（包括一名博士后，两名研究生和两名本科生）将使用我以前使用过的必要的ADR基础设施，并将从西北大学租借到我在UofT的实验室，领导锗的测量。利用SuperCDMS合作中正在进行的设计和制造克级锗探测器的努力，我的团队将于2022年春季在UofT实验室的ADR中委托探测器，并根据光束质量在TUNL光束或在其他光束设施（如universitede montracal的串联设施）上进行电离产率测量。该操作计划在2022年夏天进行，随后进行数据分析。该研究将对SuperCDMS SNOLAB的研究工作产生重大影响，提供精确测量反冲能量低于1 keV时锗的电离产率，从而解决现有数据与理论之间的差异。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工作。

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.