Cosmic ray tracks in astrophysical ices: Modeling with the Geant4-DNA Monte Carlo Toolkit

TL;DR

This study models cosmic ray tracks in astrophysical ices using the Geant4-DNA Monte Carlo toolkit, providing new estimates for track radii that are crucial for understanding molecule formation in space.

Contribution

It introduces a novel simulation approach to determine cosmic ray track radii in amorphous ices, refining previous assumptions used in astrochemical models.

Findings

Peak track core radii are 9.9 nm for LDA ice and 8.4 nm for HDA ice.

Track radii are mostly independent of initial particle energy.

Results suggest smaller radii than previously assumed in models.

Abstract

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.