HeRALD stands for Helium Roton Apparatus for Light Dark matter. The experiment consists of a superfluid helium target where DM particles would undergo nuclear recoil interactions. Depending on the amount of energy transferred, which depends on the DM particle mass, He scintillation light and/or phonon and roton excitations will be produced. The phonons and rotons can cause quantum evaporation of individual He atoms when they reach the liquid surface. TES based sensors suspended above the He liquid surface are used to detect both the scintillation light and the evaporated He atoms. The detection of quantum evaporation is a very powerful technique, as it takes 1 meV of energy to evaporate one atom, but when that atom lands on the sensor surface it releases an energy of order 10 meV as it “sticks”.
A critical technology needed for a quantum evaporation system is sperfluid He film stopping, in order to have a dry sensor in a chamber containing superfluid He. The dry sensor is required for the detection of evaporated He atoms to work. HeRALD has demonstrated film stopping based on Cs film coating of the supports holding the sensor. The figure on the left shows the quantum evaporation signal (from arXiv:2307.11877). The detector setup (top) and detail of the sensor wafer within (bottom) are shown in the photos.
