Energy deposition by electrons in superfluid helium and design of a detector for solar neutrinos

We report recent progress in the development of HERON, a detector of low-energy solar neutrinos that will use as a target material 10 tons of liquid helium. The HERON detector is intended to detect p–p neutrinos from the sun in real time with a an energy threshold of approximately 50 keV. The recoil energy of electrons resulting from the elastic scattering of neutrinos is measured with calorimetric wafers above the free surface of the liquid at 40 mK. Both the EUV scintillation and the helium atoms produced by quantum evaporation, which result from phonos and rotons hitting the free surface, are detected by the wafers. Background rejection is achieved statistically by a determination of event location with respect to the walls of the helium containment. The wafers will be used to form a coded aperture array. If the wafers have an energy threshold such that they can detect single 16 eV photons, a neutrino event having a recoil electron with an energy greater than 50 keV could be located to within 10 cm or better anywhere in the 10 tons of helium.