Energy Loss Due to Defect Formation from $^{206}$Pb Recoils in SuperCDMS
  Germanium Detectors
  The Super Cryogenic Dark Matter Search experiment (SuperCDMS) at the Soudan
Underground Laboratory studied energy loss associated with Frenkel defect
formation in germanium crystals at mK temperatures using in situ $^{210}$Pb
sources. We examine the spectrum of $^{206}$Pb nuclear recoils near its
expected 103 keV endpoint energy and determine an energy loss of
$\left(6.08\pm0.18\right)$ %, which we attribute to defect formation. From this
result and using TRIM simulations, we extract the first experimentally
determined average displacement threshold energy of
$\left(19.7^{+0.6}_{-0.5}\right)$ eV for germanium. This has implications for
the analysis thresholds of future germanium-based dark matter searches.
