Molecular dynamics simulations of bubble nucleation in dark matter detectors

TL;DR

This study uses large-scale molecular dynamics simulations to test and refine the classical heat spike model for bubble nucleation in superheated liquids, relevant for dark matter detection, revealing faster heat diffusion and multiple bubble formation along alpha tracks.

Contribution

First direct molecular dynamics simulations of heat-spike-induced bubble formation in superheated liquids, providing insights into nanoscale processes and refining classical nucleation theory.

Findings

Energy per length for bubble nucleation matches theory

Allowed spike length and total energy are about twice as large as predicted

Multiple bubbles form along alpha particle tracks, explaining acoustic signals

Abstract

Bubble chambers and droplet detectors used in dosimetry and dark matter particle search experiments use a superheated metastable liquid in which nuclear recoils trigger bubble nucleation. This process is described by the classical heat spike model of F. Seitz [Phys. Fluids (1958-1988) 1, 2 (1958)], which uses classical nucleation theory to estimate the amount and the localization of the deposited energy required for bubble formation. Here we report on direct molecular dynamics simulations of heat-spike-induced bubble formation. They allow us to test the nanoscale process described in the classical heat spike model. 40 simulations were performed, each containing about 20 million atoms, which interact by a truncated force-shifted Lennard-Jones potential. We find that the energy per length unit needed for bubble nucleation agrees quite well with theoretical predictions, but the allowed spike length and the required total energy are about twice as large as predicted. This could be explained by the rapid energy diffusion measured in the simulation: contrary to the assumption in the classical model, we observe significantly faster heat diffusion than the bubble formation time scale. Finally we examine {\alpha}-particle tracks, which are much longer than those of neutrons and potential dark matter particles. Empirically, {\alpha} events were recently found to result in louder acoustic signals than neutron events. This distinction is crucial for the background rejection in dark matter searches. We show that a large number of individual bubbles can form along an {\alpha} track, which explains the observed larger acoustic amplitudes.