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Investigating the relative importance of low-energy (<20 eV) electrons in astrochemistry via Geant4 Monte Carlo Simulations

Presentation #135.05 in the session Molecular Cloud Chemistry.

Published onJun 29, 2022
Investigating the relative importance of low-energy (<20 eV) electrons in astrochemistry via Geant4 Monte Carlo Simulations

The energetic processing of ice mantles inside dark, dense molecular clouds via photochemistry and radiation chemistry is thought to be the dominant mechanism for the extraterrestrial synthesis of prebiotic molecules. The interactions of extraterrestrial ices with cosmic ray particles with energies up to 1020 eV produce ionization, triggering a cascade of secondary low-energy (<20 eV) electrons and, consequently, radiation-induced chemical reactions. Here, we explore differences in the fluxes of both the low-energy secondary electrons that are the agents of radiation chemistry, as well as of low-energy photons, which are the instigators of photochemistry. Order-of-magnitude calculations suggest that the effects of low-energy secondary electrons are at least as significant as those of photons in the interstellar synthesis of prebiotic molecules, particularly at electron energies corresponding to resonances associated with dissociative electron attachment. However, this Fermi-type calculation of the low-energy electron flux employs several simplifying assumptions, including a single, average value for the energy required to produce electrons. To refine that work, we use Geant4-DNA, an open-source Monte Carlo simulation toolkit capable of modeling particle interactions with matter, to examine the energy deposited by ionizing radiation in a far more physically realistic way. In addition to presenting our models for the cosmic ray spectrum in the cloud and the geometry of the dust grain, we present a comparison of models for the distribution of secondary electron energies. We find that Geant4-DNA models for proton interaction processes show increased secondary electron production below 10 eV relative to de facto standard simulation results in radiation chemistry (Pimblott et al., 2007).

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