  Cosmic rays are ubiquitous in interstellar environments, and their
bombardment of dust-grain ice mantles is a possible driver for the formation of
complex, even prebiotic molecules. Yet, critical data that are essential for
accurate modeling of this phenomenon, such as the average radii of cosmic-ray
tracks in amorphous solid water (ASW) remain unconstrained. It is shown that
cosmic ray tracks in ASW can be approximated as a cylindrical volume with an
average radius that is mostly independent of the initial particle energy.
Interactions between energetic ions and both a low-density amorphous (LDA) and
high-density amorphous (HDA) ice target are simulated using the Geant4-DNA
Monte Carlo toolkit, which allows for tracking secondary electrons down to
subexcitation energies in the material. We find the peak track core radii,
$r_\mathrm{cyl}$, for LDA and HDA ices to be 9.9 nm and 8.4 nm, respectively -
somewhat less than double the value of 5 nm often assumed in astrochemical
models.
