NUCLEAR MICROPROBE RUTHERFORD BACKSCATTERING FOR 12C/13C-RATIO ANALYSIS IN TEM SAMPLES – A NOVEL NON-DESTRUCTIVE METHOD WITH HIGH LATERAL RESOLUTION

Recently it has been reported that the Ion Microprobe can be used for carbon isotope measurements of individual microfossils (House et al., 2000: Geology 28). This method, however, is limited by matrix effects and by being slightly destructive, which makes it essentially non-applicable to biological and transmission electron microscopy (TEM) samples.

By combining traditional Rutherford Backscattering analysis with the Nuclear Microprobe technique a non-destructive microanalytical method for carbon isotope analysis has been developed. In RBS, a-particles are used as beam particles and the energy of the elastically scattered particles are measured in the backward direction. At the energy used, 2.77 MeV, the effect from the kinematic spread is enough to separate a-particles scattered from the two different carbon isotopes. The energy 2.77 MeV is chosen because over a narrow energy resonans interval there is an enhancement in the cross section for ¹³C relative to ¹²C of about 40, due to deviations from the pure Rutherford cross-section.

The technique was developed at the Nuclear Microprobe at the Department of Nuclear Physics at Lund University, Sweden. At present, the beam can be focused to approximately 1 µm, but an optimized equipment should make analysis of sub-micrometer objects possible.

This non-destructive method has been developed for analysis of rare samples, and should be a valuable complement to the Ion Microprobe techniques in general and for terrestrial and extraterrestrial microbiological and micropaleontological research.