NMP Ion Beam Techniques

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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.

IL - Ionoluminescence

Ionoluminescence is the production of light (UV, visible or IR) induced by the ion beam. The beam-solid interaction populates states in the band-gap introduced by activator impurity ions or structural defects. De-excitation of these states, or transitions across the band-gap (intrinsic IL), produces photons in the infra-red or visible, if the band-gap is sufficiently large. Other ions act as quenchers, robbing strength from the activator centres, and inhibiting luminescence (Marshall, 1988).

One attraction of IL is to provide the nuclear microprobe with a rapid imaging capability that reveals subtle growth zoning reflecting trace activator and quencher distributions, similar to those seen in Cathodoluminescence (CL). Furthermore, since the level schemes of impurity atoms are perturbed by their crystal field environments, IL spectra are sensitive to the sites of impurity ions in the host lattice.

The development of IL integrated into a scanning proton microprobe for geological applications is continuing in the Lund nuclear microprobe laboratory, Sweden. Their system permits the simultaneous detection of PIXE X-rays and IL photons using either a panchromatic detector or a spectrometer (160 - 900 nm), permitting IL structure to be imaged alongside the PIXE major and trace element distributions (Homman, 1994; Yang et al., 1993). A broadband spectrograph for simultaneous multi-wavelength detection is also planned.

They can also photograph IL images directly through their microscope system (Malmqvist, 1995). This provides a very rapid means of imaging the structure revealed in IL at visible-wavelength optical resolution. So while more detailed spectroscopic information is lost (just have RGB signals), the resultant images have excellent resolution and are collected quickly.

Work by Homman (1994) examined the sensitivity of IL to valence and crystal field in order to determine the oxidation state of Fe in minerals. In plagioclase crystals from environments of varying oxygen fugacity, they measured the intensity of a peak at 710-750 nm (Fig. 6), which had been previously associated with Fe³⁺ in the Al³⁺ site (Marshall, 1988), and found an encouraging correlation of this intensity with expected Fe³⁺/Fe^total (Homman, 1994).