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CSIRO-GEMOC Nuclear Microprobe CSIRO
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.

Proton Induced X-ray Emission

Proton Induced Gamma-ray Emission

Backscattering

Elastic Recoil Detection Analysis

Nuclear Reaction Analysis

Channelling Contrast Microscopy

Ionoluminescence



Return to NMP microanalysis

PIXE - Proton Induced X-ray Emission

The passage of an energetic (several MeV) proton past atoms in the sample can lead to the ionization of inner atomic-shell electrons. The subsequent filling of the resulting inner shell vacancies by atomic transitions from outer shells leads to the emission of characteristic X-rays (and/or Auger electrons) for each element. These help to uniquely identify the presence of the element in the sample. At the same time, the deceleration of the proton in the sample is slow and leads to low levels of continuum background. This ultimate limit on detection limits is about two orders of magnitude better than with electron beams.

The large mass of a proton relative to sample electrons means that the slowing down behaviour of MeV energy protons is smooth and predictable, and the ions experience very little scattering and deflection. These properties, together with the fact that the PIXE interaction is well characterized, has enabled the development of standardless, quantitative analysis methods based on the detection of these PIXE X-rays. At the CSIRO we use the GeoPIXE software package for PIXE analysis and quantitative imaging.

PIXE, with its high X-ray production yields and low continuum background, provides good detection limits for all elements heavier than Z~13 (Al), with the possible exception of the middle rare-earth elements (MREE). At the CSIRO, typical detection limits in silicate minerals are around 1-2 ppm for a 5 minute analysis time. Longer counting times yield detection limits down to 0.3 ppm, as shown in the spectrum below for an imaging application, and light matrices, such as diamond allow detection limits of less than 0.1 ppm to be achieved (Ryan et al., 2001).

PIXE Spectrum

PIXE spectrum from a kimberlitic melt inclusion in clinopyroxene showing representative concentrations and detection limits (99% confidence level). 25 elements were detected. Spectrum was extracted from a sub-region of the image area. Charge for sub-region is 47 µC.

PIXE is a simultaneous, multielement technique. This means that any element present above the detection limits will be seen, analyzed and imaged. Therefore, it is not necessary to preselect the elements of interest in a particular sample suite; any anomalous elements will be seen immediately.

Like most ion-beam techniques, PIXE is non-destructive. Therefore, the sample is not destroyed or consumed in the analysis. The sample is still available to be characterized by other methods. This also means that there is no fall-out or debris that can contaminate other parts of the sample.

Using the Dynamic Analysis matrix transform algorithm, PIXE data can be used to generate quantitative, true element images of trace element spatial variation in minerals.

MeV protons penetrate several tens of microns into minerals, and travel along straight trajectories. Together with the non-destructive nature of such beams, this permits the in situ analysis and imaging of intact fluid inclusions in minerals.



PIXE is the mainstay of many nuclear microprobe laboratories, and especially the CSIRO-GEMOC Nuclear Microprobe which is dedicated to geological applications. PIXE provides a smoothly varying sensitivity at ppm levels across most of the perioidic table making it ideal for geological work.

Examples of PIXE applied to geological problems at the CSIRO include:

  • Analysis and imaging of the major and trace element composition of ore-fluids trapped as Fluid Inclusions in minerals provides a unique tool for the study of ore-forming proesses.

  • In the study of the mineralogical residence and spatial distribution of precious metals and ore elements in sulfide ores and mineral assemblages, PIXE microanalysis and imaging yields essential information for ore processing, and also for basic studies of ore-forming processes.

  • Trace element signatures, observed in heavy indicator minerals (garnet, chromite and ilmenite) using the proton microprobe, provide information on the physical and chemical environment of these minerals at their source and the potential for the formation and preservation of coexisting diamond. At the CSIRO, these signatures have enabled the development of powerful Diamond Exploration tools.

  • The composition of mantle-derived garnet, transported to the upper crust in volcanic eruptions, provides information on P,T and rock chemistry at depth in the mantle, at the time of eruption. This is the basis for a CSIRO method for 4D Imaging of the Mantle structure, chemistry and processes, to depths exceeding 250 km.

  • Trace element signatures in resistate minerals (minerals that are preserved during weathering) are providing important clues for the development of new tools for copper-gold deposit exploration.

  • PIXE microanalysis and trace element imaging provide powerful tools for the study of geological processes preserved as spatial zoning in minerals.

  • PIXE trace element analysis provides a valuable tool for the experimental study of element partitioning. The sensitivity of PIXE permits more natural (minor or trace) doping levels to be used.

Further Reading:

For further information contact: Dr Jamie Laird or Dr Chris Ryan
Phone +61-3-8344 8375
Fax orders +61-8-6436 8586
CSIRO Earth Science and Resource Engineering
c/o School of Physics, University of Melbourne, VIC 3010, Australia
CSIRO Australia

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