Wall-Dominated Quantum-Evaporation Efficiency in Superfluid He-4 Detectors: A Multi-Bounce Monte Carlo Study

This technical report presents a multi-bounce ballistic Monte Carlo of phonons and rotons in a cylindrical superfluid He-4 cell, as used in light-dark-matter detectors read out by quantum evaporation (e.g. HeRALD, DELight). The kinematics of quantum evaporation and the roton–atom scattering probabilities at the free surface are taken from, or constructed to agree with, the published literature. The contribution of this work is cell-scale transport: each quasiparticle is followed through any number of wall and surface encounters. Main results (within the model):- Only about 2% of quasiparticles evaporate without touching a wall, so the walls control the signal.- With the measured diffuse wall reflectivity of about 30%, the evaporation efficiency is about 0.07, or 0.02 if phonon–atom coupling is as weak as experiments suggest. Low-loss diffuse walls raise it to about 0.72.- Specular walls trap quasiparticles because the parallel wave vector is conserved.- In a 3 cm cell, 75–97% of evaporation events arrive later than 100 µs.- Wall absorption explains 65% of the efficiency variance in a global sensitivity analysis. This is a model study; it has not been peer reviewed and contains no new experimental data. The simulation code is available as a separate record (see Related works).