  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.
