Ionization Yield Measurement for SuperCDMS Germanium Detectors

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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.