Experimental and Theoretical Considerations for Proton Irradiation Processing of Early Solar System Solids

The radiation environment of the early system was explored to understand how enhanced Solar Energetic Particles (SEP) could process early solar materials in a nebular environment. A series of proton irradiations using particle accelerators were performed on a suite of materials representative of primitive chondritic meteorite constituents: this included mineral fragments of olivine, feldspar, pyrite, and a weathered ordinary chondrite; sintered mixtures of olivine/gabbro, olivine/anorthosite, and olivine/corundum; and synthetic materials including forsterite, fused silica, and single crystalline Si wafers. Proton (p) irradiation conditions included energies between 50 keV-3.8 MeV, fluxes between 1012-1014 p/cm2-s, and fluences between 1014-1017 p/cm2. The irradiated materials were examined using a variety of insitu and post-irradiation characterization techniques; insitu techniques included video monitoring, radiation detection using a Geiger-Mueller tube, and mass spectrometry using a Residual Gas Analyzer. Post-experimental examination included optical and electron microscopy and radiation count rate experiments.The experimental results can be broadly classified into physical, chemical, and nuclear processing. Physical processing refers to mechanical fracture, melting, and radiation damage; chemical processing includes the formation of organic compounds, volatilization of certain elements and compounds, and the reduction of iron oxides to metal droplets; while nuclear processing include the formation of radioactive isotopes below energies typically investigated in spallation studies. The results provide several new mechanisms on how enhanced SEP events can process constituent chondrite components. Enhanced SEP processing could have a role in comminuting larger material to sub-mm sized fragments, melting fragments of chondrule precursors and chondrules, developing the igneous textures associated with some refractory inclusions (Calcium-Aluminum Inclusions, CAIs), annealing radiation damage, forming or modifying organic compounds found in the matrix of meteorites, volatilizing labile elements such as sodium and sulfur, reducing iron oxides to develop metals outside of the expected condensation sequence, and transmuting silicon and other isotopes from normal solar abundances below traditionally examined energies. Collectively the results suggest that enhanced SEP interactions with early solar material may open up a new field of study in planetary sciences