In Section I a general formula for α, the coefficient of recombination of ions in gases, is developed. Finally, we calculate projected sensitivities to dark matter–nucleon elastic scattering, demonstrating that even very small (sub-kg) target masses can probe wide regions of as-yet untested dark matter parameter space. We simulate radioactive backgrounds from gamma rays and construct an overall background spectrum expectation also including neutrons and solar neutrinos. We describe signal production and signal sensing probabilities, and estimate the resulting electron recoil discrimination. The energy of adsorption amplifies the phonon/roton signal before calorimetric sensing, producing a gain mechanism that can reduce the technology’s recoil energy threshold below the calorimeter energy threshold. Kinetic excitations of the superfluid medium (rotons and phonons) are detected using quantum evaporation and subsequent atomic adsorption onto a calorimeter suspended in vacuum above the target helium. In this concept, both scintillation photons and triplet excimers are detected using calorimeters, including calorimeters immersed in the superfluid. A superfluid helium target has several advantageous properties, including a light nuclear mass for better kinematic matching with light dark matter particles, copious production of scintillation light, extreme intrinsic radiopurity, high impedance to external vibration noise, and a unique “quantum evaporation” signal channel enabling the detection of phononlike modes via liberation of He4 atoms into a vacuum. We name this concept “HeRALD,” helium roton apparatus for light dark matter. Most importantly, the analysis method presented in this work can be extended to other noble liquids to explore the dependencies for electrical breakdown in those media.Ī promising technology concept for sub-GeV dark matter detection is described, in which low-temperature microcalorimeters serve as the sensors and superfluid He4 serves as the target material. We show that the results from this analysis provide an explanation for the supposed electrode gap-size effect and also allow for a determination of the breakdown-field distribution for arbitrary shaped electrodes. The dependence of the breakdown probability on the field strength as extracted from the breakdown field distribution data is used to show that breakdown is a surface phenomenon closely correlated with Fowler–Nordheim field emission from asperities on the cathode. A data-based approach for determining the electrode-surface-area scaling of the breakdown field is presented. The distribution of the breakdown field is obtained for temperatures between 1.7 K and 4.0 K, pressures between the saturated vapor pressure and 626 Torr, and with electrodes of different surface polishes. We report results from a study on electrical breakdown in liquid helium using near-uniform-field stainless steel electrodes with a stressed area of ∼ 0.7 cm 2.
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