Scintillation yield from electronic and nuclear recoils in superfluid helium-4

Superfluid He-4 is a promising target material for direct detection of light (<1 GeV) dark matter. Signal channels for dark matter - nucleus interactions in superfluid helium include prompt photons, triplet excimers, rotons and phonons, but measurement of these signal strengths have yet to be performed for low energy nuclear recoils. A study of scintillation yield from electronic and nuclear recoils was carried out in superfluid He-4 at ~1.75 Kelvin, with deposited energy in the range of 10-1000 keV. Scintillation from a 16 cm³ volume of superfluid He-4 was read out by six PMTs immersed in the superfluid.Yields of both prompt and delayed scintillation components were measured. Elastic scattering of 2.8 MeV neutrons (generated by a deuterium-deuterium neutron generator) from superfluid He-4, with a liquid organic scintillator module used as far-side detector, was used to determine the scintillation signal yield for a variety of nuclear recoil energies. For comparison, Compton scattering of Cs-137 gamma-rays from the superfluid He-4, with NaI scintillators used as far-side detectors, was used to determine the scintillation signal yield of electronic recoils.