Isotope Fractionation of Solar Wind Implanted into the Genesis Concentrator Target Determined by Neon in the Gold Cross and Implantation

Neon abundance and isotopic composition measurements are now done for all 4 arms of the Genesis concentrator gold cross (consisting of gold on stainless steel; AuSS). The data prove that the entire concentrator target was radially homogeneously irradiated (Fig. 1). An implantation experiment showed, however, that AuSS, characterized by a rough surface, experienced a significantly larger backscatter loss of Ne than a shiny Au target and as predicted by SRIM [1] (Fig. 2). Backscatter loss in AuSS was also largely independent from the angle of incidence. These observations will rule out a successful determination of isotopic fractionation of oxygen and nitrogen in the real concentrator targets through analyses of Ne implanted into AuSS. We conclude that the mass fractionation caused by the Genesis concentrator must be determined by Ne measurements on one of the concentrator targets directly. Preliminary experiments on SiC are underway. Introduction: The concentrator onboard the Genesis spacecraft increased the fluence of solar wind (SW) ions on a special target to allow for high precision analyses of isotopic composition of oxygen and nitrogen [2, 3]. The SW is a proxy of the solar nebula composition and therefore important to understand origin and evolution of the different O isotopic reservoirs observed in different solar system objects. The concentrator was an electrostatic mirror. The concentration process caused isotope fractionation as function of the radial distance from the center of the target on the order of up to 3.8% per amu as measured with Ne isotopes along the gold cross used to mount the single concentrator targets onto the base plate [4]. However, measured data disagreed with simulations of the concentrator performance. Here we present, first, Ne data from all 4 arms of the gold cross testing the radial homogeneity of the irradiation throughout the entire concentrator target. Second, an artificial implantation experiment was carried out to study backscatter loss and isotope fractionation of Ne implanted into AuSS to elucidate the observed discrepancy between measured and simulated data. Experimental: In the gold cross arms Ne concentration and isotope composition were analyzed on single spots (≤ 100μm) by UV laser ablation along all 4 arms as shown for the first 2 arms in [4]. The analysis of small spot sizes was permitted by a very sensitive mass spectrometer equipped with a molecular drag pump that almost quantitatively conveys the gas into the ion source [5]. For the implantation experiment Ne (1E+14/cm at 72keV) and Ne (1E+13/cm at 74keV) were implanted into flight spares of AuSS, AuoS (Au on sapphire, a target with shiny surface) and DOS (diamondlike carbon). Targets were irradiated at 3 different angles of incidence: 0°, 45°, and 55° during one experiment (Fig.2c), in order to approximate implantation conditions of the concentrator in space (main angle of incidence 50°-55°). Gas was extracted by a UV laser (213nm) from areas of 350 x 350μm and measured in a noble gas mass spectrometer. The amount of expected backscatter loss was determined by SRIM for Ne and Ne for each material and angle of incidence. Besides the comparison of measured abundances in rough and smooth Au, this experiment allows also to test SRIM predictions of backscatter losses. Results and Discussion: a) Genesis concentrator gold cross. Fig. 1 shows the Ne concentrations and the Ne/Ne ratio, given as permil deviation from unfractionated SW [6], for all arms of the gold cross. Both quantities agree in all arms along the entire radius within their 95% confidence limits. Ne concentration monotonically increases from edge to center by a factor of 10. Concentration factors range from 5 (edge) to 50 (center), determined relative to Ne implanted into the bulk solar wind AuoS collector (6.8E+11atoms/cm). The Ne/Ne ratio at the edge (14.05) is light compared to the unfractionated SW value of 13.77 and monotonically decreases towards the center of the target to 13.04. At 22.4 mm radius the unfractionated solar wind Ne isotopic composition is encountered. The agreement between all 4 arms suggests that the entire concentrator target was radially symmetrically irradiated, which excludes any major misalignment of the concentrator assemblage during operation. This is an important finding for the O [7] and eventual N data measured in the concentrator targets, since restricted target areas do not allow extensive tests and analyses are not feasible using other elements than Ne. Fig. 1 also shows the simulated Ne and Ne/Ne curves. As discussed in [4] one possible reason for the disagreement between simulated and 1485.pdf 40th Lunar and Planetary Science Conference (2009)