High resolution in probe isotopic analysis by in depth profiling: the example of solar H, Li, B, C and N implanted in lunar soils

Details High resolution in probe isotopic analysis by in depth profiling: the example of solar H, Li, B, C and N implanted in lunar soils High resolution in probe isotopic analysis by in depth profiling: the example of solar H, Li, B, C and N implanted in lunar soils

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....9316c&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #9316

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Ion microprobe analysis offers the rather unique opportunity to measure selectively isotopic compositions with a nanometric spatial resolution. This can be obtained by doing isotopic analysis by in depth profiling. In such type of analysis the sample which must have a rather flat surface is sputtered by a primary ion beam of variable size (typically micrometer size) which can be rastered over wide areas (up to several hundreds of micrometers). Because collisions caused by the sputtering into the sample between the high energy (10-15kV) primary ions and the atoms from the sample typically concern a few atomic layers, the secondary ions are emitted from a narrow depth. The in depth resolution in such analysis is dictated by the sputtering rate of the sample which depends on (i) the nature of the sample, (ii) the primary beam intensity and (iii) the size of the surface sputtered. The sputtering rate also controls, for a given concentration of the element of interest, the secondary ion intensity and thus limits from the counting statistics the precision which can be reached on isotopic measurements. Thus the analytical conditions which must be chosen for a given type of sample and analysis are always a compromise which depends on the size of flat surfaces on the sample, the concentration of the element to be analysed and the thickness of the layers containing this element. This technique has been used with the Nancy Cameca ims 1270 ion microprobe to try to detect and to measure the isotopic composition of solar wind (SW) light elements (H, Li, B, C, N and O) implanted at the surface of mineral grains from lunar soils [1-3]. Because of their energies SW-derived particles are typically implanted in lunar grains within the first 150 nanometers, while they (especially H) may be remobilized by diffusion at greater depths. For example, for a sputtering rate of 4.2 x 10-3 nm/(nA x sec) a counting rate of 5-50 SW-^6Li atoms / sec was obtained on lunar soils. This experimental approach allowed to identify successfully SW H, Li, C and N in lunar grains and to put narrow limits on their isotopic compositions. [1] Chaussidon M. and Robert F. (1999) Nature 402, 270-273. [2] Hashizume K. et al. (2000) Science 290, 1142-1145. [3] Hashizume K. et al. (2002) Lunar and Planetary Science XXXIII, abstract #1465.

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