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Enabling Smart Computations to study space RADiation effects

Periodic Reporting for period 1 - ESC2RAD (Enabling Smart Computations to study space RADiation effects)

Reporting period: 2018-05-01 to 2019-04-30

The study of the impact of Space radiation, both Galactic Cosmic Rays (GCRs) and Solar Energetic Particles (SEPs), causing biological damage in astronauts and degradation of functional materials of the spacecraft, is of fundamental importance. Ionization and excitation of the atoms of the very biological molecule (direct channel) or of the water surrounding the molecule (indirect channel) are the main physical phenomena induced by radiation in a biological target. Understanding the effects which extend over a physical, chemical, biological step, occurring at different length and time scales is necessary to develop mitigation strategies against radiation.

In order to be able to boost Space and planetary exploration, another important aspect is to master the necessary infrastructure, like photovoltaic panels, in which degradation occurs because of a damage of the structure of the different layers in the solar cell.

Robust Monte Carlo particle transport approaches allow to model the early damage as stochastic, mostly uncorrelated collision processes. The target is treated in a relatively simple way, as a gas-like mixture of the constituent atoms. This approach is very successful for the high energy regime of the impacting particle, but looses accuracy when such particle has slowed down, as the electronic and molecular structure of the target then come into play. In such regime, ab-initio approaches (i.e. without any free parameter) are needed.

This main objectives of the project are to study the biological risks/degradation of materials in Space missions (interplanetary travel to Mars, a stay on Mars) via particle transport models, for interesting scenarios, and compare them to available measurements of radiation and doses, to validate the accuracy of key quantities at low energy regimes via ab-initio approaches, and to investigate new strategies to propose new protection solutions for biological damage and solutions aimed at minimizing the degradation of solar cells.

Some other societal challenges of our epoch linked to these topics are: the development of complex cancer therapies, making long-lasting photovoltaic panels, that could eventually collect energy from Space to be then redirected to Earth.
During this first period we have conducted research on:

1) Human Exploration of Mars. First human explorers on Mars will likely be involved in contextual surveys, search of candidate samples for astrobiology driven analysis, and supervised drilling operations at sites with high biosignature preservation potential. Oxia Planum is the landing site for the next ESA mission ExoMars 2020 and Mawrth Vallis region has been previously considered by ESA and NASA (Fig. 1). The radiation environment, induced by GCRs in quiet conditions and as induced by both SEPs and GCRs during the 28th October 2003 event was studied via Monte Carlo particle transport. Our results suggest that a 1-year mission on Mars in quiet times would not overpass the limits established for the so-called stochastic effects. A 30-day stay during which an event such as the one of the 28th October 2003 would have not left big safety margins for the so-called deterministic effects (those surely occurring beyond a certain threshold dose).

2) Analysis of new shielding materials. In order to allow humans to go safely to Mars and come back, a first efficient protection will be needed during the course of the interplanetary travel. A first investigation of the shielding efficiency (preliminarily on the sole physical step) of possible new melanin and ectoine-based polymers was performed. Since these molecules systems are found in radioresistance microorganisms, it could be possible to grow such hosts during the interplanetary travel itself or eventually on Mars, and use them to build emergency spacesuits or pharmacological solutions, reducing payload at launch. In Fig. 2, the behavior of selected materials in protecting a water slab for a two-year interplanetary mission under worst case space weather scenarios is shown. While there is essentially no dose reduction for GCRs, there is some improvement in shielding from SEPs, with respect to the traditional Al shielding, which encourages further investigations for either emergency space suits, new layered structures for emergency modules, or pharmacological radioprotectors at the chemical/biological step.

3) The details of the impact of a particle on water. In Monte Carlo approaches there is no distinction of the molecular structure and electronic properties between the candidate molecules studied, while such properties are actually relevant for understanding how such systems could eventually protect DNA at the chemical and biological steps, if injested. A real time Time Dependent Density Functional Theory (RT-TDDFT) study combined with Ehrenfest dynamics was performed on the electronic stopping Se (linked to ionization/excitation processes) in water. A new strategy that allows to compute the deposited energy using a parametrization of the stopping power that depends on the chemical identity, structural parameters, and detailed charge distribution of the sample has been proposed. Fig. 3 shows the configuration of the target liquid water and the Se is compared with SRIM results (an empirical model based on observation data) (fig4,5). The new statistical method will be applied to samples containing solvated DNA components and will be used to develop a new DNA-damage descriptor.

4) The impact of radiation on solar cells. In this context, we have focused on fundamental studies on proton impact on triple-junction solar cells, made of gallium indium phosphide, gallium arsenide, and germanium (Ge) semiconductor materials. Despite the fact that much of the damage in solar cells is caused by atomic displacements, there are still much uncovered details in the electronic stopping power. We have studied, using RT-TDDFT and the Ehrenfest dynamics, the channeling of the impacting particles (which actually minimize the perturbation induced in the target) through crystallographic planes of the Ge subjuction. Fig.6 7, 8 show some results. The stopping power for the channel [011] (Fig. 8) show a strong dependence of the stopping power on the trajectory. Future work will focus on the synergy with the nuclear stopping power, and on higher impact energies.
The studies performed show some progress beyond the state of the art: there are at present no estimation on the radiation environment and doses at the landing site for ExoMars 2020, melanized and ectoine-based shielding solutions have been started to be investigated only very recently, and here they have been compared and studied for potential efficiency in different radiation environments. The new sampling strategy proposed for water opens the path for further applications of such ab-initio calculations at larger length scales, and the importance of channeling effects in solar cells is only now starting to be modeled by ab-initio approached and understood in detail.

The advancements achieved so far are promising for nanodosimetry and for encouraging more detailed investigations on pharmacological solutions and layered low Z/high Z structures for emergency modules.At the socio-economic level, the visibility aquired through different activities and the attraction of broad public, will still extend the scope of the project to new horizons.