The SPICE/HeRALD collaboration is performing R&D to enable studies of sub-GeV
dark matter models using a variety of target materials. Here we report our
recent progress on instrumenting a superfluid 4He target mass with a
transition-edge sensor based calorimeter to detect both atomic signals (e.g.
scintillation) and 4He quasiparticle (phonon and roton) excitations. The
sensitivity of HeRALD to the critical "quantum evaporation" signal from 4He
quasiparticles requires us to block the superfluid film flow to the
calorimeter. We have developed a heat-free film-blocking method employing an
unoxidized Cs film, which we implemented in a prototype "HeRALD v0.1" detector
of ∼10~g target mass. This article reports initial studies of the atomic
and quasiparticle signal channels. A key result of this work is the measurement
of the quantum evaporation channel's gain of 0.15±0.012, which will
enable 4He-based dark matter experiments in the near term. With this gain
the HeRALD detector reported here has an energy threshold of 145~eV at 5 sigma,
which would be sensitive to dark matter masses down to 220~MeV/c2.Comment: 14 pages, 9 figure