Secondary Ion Mass Spectrometry
Schematic overview of a generalised SIMS instrument:
- primary ions (usually Cs+ or O2-) generated in the ion source are accelerated and focused onto target area on the sample (mineral) surface
- primary ions ablate (or “sputter”) secondary ions from the sample surface, resulting in a complex spectrum of secondary ions emitted (even for simple targets) so;
- secondary ions are accelerated, focused and energy-filtered using an ESA and then
- mass- (or momentum-) filtered by a magnetic sector field analyzer (as in TIMS), then
- ion species of interest are sequentially directed into and counted in a detector (usually either a electron multiplier or Faraday Cup).
The sputtering process is usually continuous, with the primary ion beam on all the time during the analysis and the secondary ions to be analyzed selected by changing the magnetic field to direct the secondary ions sequentially into the detector.
There are some variations on this basic design but most SIMS instruments in geoscientific use have these components. The combination of a magnetic and an electrostatic sector produces a double focusing instrument.
There are two SIMS instrument types currently in routine use for geochronology: the French-made Cameca and the Sensitive High-Resolution Ion MicroProbe (or SHRIMP) developed at the Australian National University.
Detailed information about routine laboratory running costs, sample and preparation a SIMS laboratory containing a CAMECA SIMS instrument (MS Word document, 7 Mb) is downloadable from here. Additional details about routine operation and maintenance of a Cameca SIMS instrument is also available; please Contact Us.