Scintillation and quantum evaporation generated by single monoenergetic electrons stopped in superfluid helium

An electron stopped in superfluid helium generates phonons and rotons in the liquid as well as uv photons via scintillation. We report recent measurements with single 364 keV electrons. A sapphire wafer with a superconducting transition-edge sensor is mounted above the liquid and can measure energy and timing information of individual events. We observe both uv photons and the quantum evaporation of helium atoms resulting from phonons and rotons generated by the ionizing particle in the liquid. The production of photons and rotons is strikingly different for an electron and for an alpha particle. The origin of the differences is associated with the different density of excitations along the tracks of an alpha particle and an electron.