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 *** Congratulations to Dr. Bin Gu and Prof. Jorge Kohanoff of the ESC2RAD team for their paper on the stopping of protons in water ! This opens several interesting possibilities to calculate, via ab-initio methods, the stopping of protons in biomolecules ***


ESC2RAD is a Research and Innovation H2020 project financed by the European Commission. It aims at studying Space radiation effects on both functional materials (like solar cells) and biological matter in contexts that are relevant for current and future Space and planetary missions and, in general, to improve the basic understanding of radiation-matter interaction, especially in those energy ranges where there is room to ameliorate current Monte Carlo models. While the Space radiation effects community rely mostly on Monte Carlo modelling, appropriate for high energy impacting particles, the ESC2RAD consortium will focus on that energy range where typical assumptions made by such approaches break down, and first principles (parameters-free) approaches from the quantum chemistry/electronic structure community (Time Dependent Density Functional Theory, coupled with ab-initio Molecular Dynamics) allow to gain further insights. These energy ranges, between a few MeV down to ~keV for solar cells, and in the case  of biological matter even down to eV, are important to study respectively the damage to solar cells via atomic displacements induced by trapped radiation or shock accelerated Solar Energetic Particles (SEPs) and to study the effects of both protons hitting a biological target (proxy for an astronaut) after having been slowed down by the spacecraft walls or a regolith shielding on Mars and of low energy secondary electrons generated in the water surrouding the biological molecules.

Monte Carlo particle transport codes, like Geant4 (a "condensed history" Monte Carlo particle transport code) for solid state targets and Geant4-DNA (a track structure code which is the low energy extension of Geant4) for biological matter are very efficient and successfull in many cases. Nonetheless, the estimation of atomic displacements in target solid state materials is based on approximations like the binary collision approximations, a completely amorphous target, estimation of number of defects often based on a simplied (Kinchin Pease) model, and complete separation between ionic and electronic degrees of freedom, whicle the coupling of these may even change fundamental quantieis in the widely used Non Ionising Energy Loss (NIEL) model used for solar cells, for example imacting the value of the threshold displacement energy. For what concerns the impact of radiation on biological matter, track structure codes likeGeant4-DNA actually follow the track structure in water, and then superposing a coarse grained (geometrical) model of the biological molecules, they estimate its damage via the density of ionization events, which compared to a certain threshold energy gives an ideaa of the strand breaks. Our aim is to improve the modeling of radiation effects in both these fields:

1. For the most active layer in triple junctions solar cells (GaInP/GaAs/Ge), used for present missions like the ExoMars Trace Gas Orbiter, for the Juno spacecraft around Jupiter, and planned to be used in future missions like JUICE and ExoMars2022, we will investigate a) channeling effects (ions travelling in between crystal planes of the ordered structure); b) the dynamical nature of the threshold energy for displacement, its possible dependence on crystalline orientation and on the synergy with electronic excitations; c) the evolution of collisional casacades in a regime in which nuclear stopping is also influenced by electronic stopping.

2. New radiation-resistant materials like the hybrid perovskites, which constitute a potential disruptive technology for Space as well for Earth, will also be investigated.

3. For biological targets, to devise a strategy to study the effects of both medium-low energy protons and low energy secondary electrons on water and (small, for the moment) biological molecules. This study will go beyond the optical data model (based on linear response theory) currently used for the dielectric fuction and the cross sections in Geant4-DNA. 

4. A final roadmap for those studies that, in a chemical physics/condensed matter perspective, are needed in the field of radiation damage on biological targets will also be provided.



Contact: BIRA-IASB, Av. Circulaire 3, Brussels, Belgium.