  The PICO experiment uses bubble chambers filled with superheated C$_3$F$_8$
for spin-dependent WIMP dark matter searches. One of the main sources of
background in these detectors is alpha particles from decays of environmental
$^{222}\mathrm{Rn}$, which nucleate bubbles that are visually indistinguishable
from WIMP candidate events. Alpha-induced bubbles can be discriminated
acoustically, because the signal from alpha events is consistently larger in
magnitude than that from nuclear recoil/WIMP-like events. By studying the
dynamics of bubbles nucleated by these two types of ionizing radiation from the
first stages of their growth, we present a physical model for the acoustic
discrimination for the first time. The distribution of acoustic energies that
we generate for a simulated sample of bubble nucleations by alpha particles and
nuclear recoils is compared directly to the experimental data.
