Steps Toward Accurate Large Area Analyzes of Genesis Solar Wind Samples: Evaluation of Surface Cleaning Methods Using Total Reflection X-ray Fluorescence Spectrometry

Total reflection X-ray fluorescence spectrometry (TXRF) was used to analyze residual surface contamination on Genesis solar wind samples and to evaluate different cleaning methods. The Genesis mission collected solar wind during a period of 854 days by embedding the charged particles into collectors made of various ultra clean materials such as silicon, sapphire and silicon-on-sapphire. The sample return capsule unexpectedly crashed on return to Earth fracturing the collectors and exposing them to the desert soil of the landing side. The ubiquitous contaminants are separated from the atoms of solar wind by only 5-15 nm, presenting significant challenges for solar wind analysis as well as the development of cleaning techniques. Currently, an ultrapure water and ozone UV radiation treatment is routinely applied to the collectors by the curatorial team at NASA’s Johnson Space Center. Additional cleaning steps involving various forms of acid treatment and/or carbon dioxide snow treatment are being evaluated as well. To gauge the suitability of the cleaning method, two samples were analyzed following cleaning by lab-based TXRF. The analysis comprised of an overview and a crude manual mapping of the samples by orienting them with respect to the incident X-ray beam in such way that different regions were covered. The results showed that cleaning with concentrated hydrochloric acid and a combination of hydrochloric acid and hydrofluoric acid decreased persistent inorganic contaminants substantially on one sample. Application of carbon dioxide snow for surface cleaning tested on the other sample appears to be effective in removing one persistent Genesis contaminant, namely germanium. Unfortunately, the TXRF analysis results of the second sample were impacted by relatively high background contamination. This was mostly due to the relatively small sample size and that the solar wind collector was already mounted with silver glue for resonance ion mass spectrometry (RIMS) on an aluminum stub. Further studies are planned to eliminate this problem. In an effort to identify the location of very persistent contaminants, selected samples were also subjected to environmental scanning electron microscopy. The results showed excellent agreement with TXRF analysis