Scintillation yield from electronic and nuclear recoils in superfluid $^4$He

TL;DR

This study measures and compares scintillation yields from electronic and nuclear recoils in superfluid helium-4, providing data crucial for developing helium-based dark matter detectors and exploring signal discrimination techniques.

Contribution

It provides the first detailed measurements of scintillation yields from both electronic and nuclear recoils in superfluid helium-4 at cryogenic temperatures.

Findings

Measured scintillation yield for electronic recoils: 1.25 phe/keVee.

Compared yields to a semiempirical model based on helium interactions.

Studied delayed scintillation components for recoil type and energy dependence.

Abstract

Superfluid $^4$He is a promising target material for direct detection of light ($<$ 1 GeV) dark matter. Possible signal channels available for readout in this medium include prompt photons, triplet excimers, and roton and phonon quasiparticles. The relative yield of these signals has implications for the sensitivity and discrimination power of a superfluid $^4$He dark matter detector. Using a 16~cm$^3$ volume of 1.75~K superfluid $^4$He read out by six immersed photomultiplier tubes, we measured the scintillation from electronic recoils ranging between 36.3 and 185 keV$_\mathrm{ee}$, yielding a mean signal size of $1.25^{+0.03}_{-0.03}$~phe/keV$_\mathrm{ee}$, and nuclear recoils from 53.2 to 1090 keV$_\mathrm{nr}$. We compare the results of our relative scintillation yield measurements to an existing semiempirical model based on helium-helium and electron-helium interaction cross sections. We also study the behavior of delayed scintillation components as a function of recoil type and energy, a further avenue for signal discrimination in superfluid $^4$He.