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Interactions of MeV ions with matter permit microanalysis and imaging of sample constituents using excited X-rays
(PIXE - Proton Induced X-ray Emission), gamma-rays from nuclear reactions
(PIGE - Proton Induced Gamma-ray Emission), outgoing nuclear reaction particles
(NRA - Nuclear Reaction Analysis), elastically scattered ions
(BS - Backscattering), (ERDA - Elastic Recoil Detection Analysis),
or visible and infrared emissions from the sample
(IL - Ionoluminescence). The ion beam can also be channelled down crystal axes and planes. Many of these techniques can be combined with channelling to study the lattice location of species (CCM - Channelling Contrast Microscopy). MeV protons are very penetrating while producing very little sample damage. These qualities permit the in situ, non-destructive analysis and imaging of buried structures such as solid and fluid inclusions in minerals.
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NRA - Nuclear Reaction AnalysisNRA is a general category of technique involving nuclear reaction between a target nucleus and beam particle. When the energy of the incident particle approaches or exceeds the Coulomb barrier, the potential barrier caused by charge repulsion, nuclear reactions can occur. Reactions generally result in the emission of a reaction product particle or a gamma-ray (see PIGE).The common reactions employed for microanalysis are (d,a), (d,p), (p,a), for particle detection and (p,p'gamma) and (p,a gamma) for gamm-ray detection. NRA with reaction product particle detection is particularly useful for C, N and O analysis. The notation used for nuclear reactions, (x,yz), denotes the incident beam particle "x" and the reaction products "y" and "z", where "p" is a proton, "d" is a deuteron (2H), "a" is an alpha particle (4He) and "g" or "gamma" denotes a gamma-ray. "p'" denotes an inelastically scattered proton; it leaves the target nucleus in an excited state.
A weakness of PIXE is the general inaccessibility of light elements (Z<13). While complimentary tools, such as the EMP help to fill this gap for major elements, providing routine major element data down to at least Na, the lightest elements (e.g. H, Li, B) require nuclear reaction or recoil methods. The Laboratoire Pierre Sue, Saclay, and the Bruyeres le Chatel nuclear microprobe facility, Paris, have developed nuclear reaction methods for the measurement of 1,2H, 6,7Li, 10,11B, 12C, 14N, 16O, 19F, 23Na, 24,25Mg, 27Al, 28Si, 32S and 35Cl in minerals (Courel et al., 1991; see review by Ryan and Griffin, 1993, and references therein). The isotopes 7Li, 19F, 23Na, 24,25Mg, 27Al, 28Si and 35Cl are determined using (p,p'gamma) reactions (PIGE) at proton energies from less than 2 MeV up to 3.5 MeV, suitable for simultaneous PIXE analysis of heavier elements. A similar approach is taken by the Queen's University group (MacArthur and Ma, 1991). H is determined by elastic recoil detection (ERDA) of protons induced by a 3 MeV 4He microbeam (Mosbah et al., 1990; Tirira et al., 1991). The remaining isotopes are determined using (d,a), (d,p), (p,a) and (p,a gamma) reactions. The corresponding elemental detection limits for all elements between H and F are 10-50 ppm. For the elements Na and heavier the detection limits grow from ~200 to ~2000 ppm. Further Reading:
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For further information contact: Dr.
Chris Ryan via email: (Chris.Ryan@csiro.au)
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