Extraterrestrial materials examined by mean of nuclear microprobe
H. Khodjaa,b, T. Smitha,b, C. Engrandc, G. Herzogd, C. Raepsaeta,b
Nuclear Instruments and Methods in Physics Research B 306 (2013) 245–248
Comet fragments, micrometeorites, and Interplanetary Dust Particles (IDPs) are small objects (<1 mm) of high scientific interest in cosmochemistry. More particularly, the determination of light element concen- trations, such as C and N, in cometary samples is of interest since it gives information on the regions where such materials formed. Analyses of such objects should be performed so as to extract as much information as possible while preserving sample integrity. For this purpose, we need instruments and methods that provide both microanalysis and detailed imaging. In these respects, the nuclear microprobe offers many potential advantages: (i) the spatial resolution, 1 um is well-matched to the typical object dimensions, (ii) with some reservations, it is non-destructive when carefully conducted, (iii) it is quantitative, and especially sensitive for light elements.
Early attempts for IBA applications to extraterrestrial materials were reported by Kugel and Herzog and paved the way to a potential application of PIXE to such samples. Following this work, the technique sensitivity coupled sometimes with the microbeam spatial resolution became progressively attractive for this community. It was however only at the end of the nineties that papers reporting NRA applications were published. At Saclay, we have conducted careful analyses of glass inclusions trapped in meteorites by combining PIXE and NRA, as it helps at understanding formation conditions. The general methodology was developed at that period, and was progressively refined with time.
One of the currently hottest topics in astronomy is astrobiology, which includes study of the hypothesis of extraterrestrial fertilization of Earth by meteoritic bombardment. For this, the estimation of the extraterrestrial sources of organic molecules is needed. A decade ago, we used NRA to map the light elements (C, N) in various micrometeorites and interplanetary dust particles (IDPs). Where possible, the results were compared with those obtained with other (destructive) techniques, and were found in good agreement.
More recently, we were joined a worldwide collective effort to characterize samples collected during the NASA mission Stardust. It was the first time since the Apollo missions that extraterrestrial materials were brought to Earth from a distant object. In this study, our goal was to obtain the light element inventory from fragments of the comet Wild-2, and more particularly the C/N ratio. These fragments were collected when the Stardust probe crossed the comet tail in 2004 and collected ejectiles. Objects were caught in a silica aerogel with the aim of minimizing the rate of deceleration and preserving as far as possible the grain integrity. A panel of elemental mappings of such samples is given in Figure 1.
Figure 2. Comparison of oxygen and carbon NRA experimental data (dots) from micrometeorite DC06-0515 and SIMNRA without (plain line) and with (dotted line) roughness. Carbon ROI integral vary with 6% between the 2 simulations.
A special attention was given to potential pitfalls associated with NRA microanalyses. Objects are generally analyzed with minimal preparation, thus avoiding a polishing sequence. Consequently sample topology may play a role in the spectrum shape. This effect is actually sensitive with semi-thick samples for which both the reaction cross-sections and the sample thickness vary. This variation should affect the spectral line shapes, particularly on the low-energy side. SIMNRA evaluations show that it requires however large thickness dispersion (FWHM/mean value >> 100 %) to have a significant impact on the spectrum counts, as it can be seen on Figure 2.
Contact CEA : Hicham Khodja.