SHRIMP-II - The Sensitive High Resolution Ion Micro Probe II is a high precision Secondary Ion Mass Spectrometer (SIMS).
Ion microprobes make in situ isotopic and chemical 'surface' analysis of solid targets by bombarding the sample with an ion beam with a diameter of several microns typically employing Kohler focussing. The high mass resolution of SHRIMP is achieved by the use of double-focussing mass spectrometer (simultaneous energy and mass refocussing) with a very large turning radius of Magnet and Electrostatic Analyser.
The operating principle of SHRIMP is quite simple. A high-energy (10kV) beam of ions (O-) is focussed onto a small area (<30 µm diameter) on the surface of the target. The ion bombardment erodes (sputters) atoms and molecules from the target, some of which are themselves ionised. These secondary ions are gathered using electrostatic lenses and transfered to a mass spectrometer, by which they are separated according to their relative masses. A limiting factor in the accuracy of all ion probe analyses is the mass spectrometer's ability to distinguish between (resolve) secondary ions of atoms and molecules that are extremely similar in mass (isobars).
In the secondary ion spectra of complex compounds, isobars with fractional mass differences of <3 x 10-4 are common. The high mass resolution of SHRIMP is achieved by the use of double-focussing mass spectrometer (simultaneous energy and mass refocussing) with a very large turning radius (magnet radius 1 m, electrostatic analyser radius 1.27 m) The resulting instrument has a beam line over 7 m long and weighs more than 12 tonnes.
The SHRIMP II ion microprobe is the highest precision instrument in the broad category of Secondary Ion Mass Spectrometers (SIMS). The SHRIMP-II existing in our Center is the first mass-spectrometer of SHRIMPs with a multi-collector instrument. So it has a better precision and productivity of measurements.
SHRIMP has many applications. The general is the uranium-lead zircon dating. The other applications are:
uranium-lead zircon dating on monazites
locale analyze of stable isotopes (O, S, N) in minerals
locale elemental rare-earth analyze in minerals
Scientific tasks resolving with SHRIMP-II:
Investigation of nucleo-synthesis in stars
Definition of geochronological margins of Palaeozoic
Dating of the Earth's crust
Detection of amount of trace elements in inclusions in Diamonds
Investigation of isotopic rations of Ti in meteorites
Science of materials
Physics of solid state
Ion microprobe analysis of zircon is now a routine technique. Combined with optical and electron imaging techniques, especially cathodoluminescence, it allows the assessment of age heterogeneity or difference between age of core and overgross.
Examples of Ahrens-Wetherill concordia plots for various types of zircons:
Example of plot for local elemental rare-earth analysis in zircons:
Example of Tera-Wasserburg plot (observed, uncorrected for common lead) for young zircons:
Requirements for the sample preparation for SRHIMP-II
Samples for ion microprobe SHRIMP-II are embedded in mounts (polished blocks). You can send us samples (zircons, other minerals, or glass, ceramics, semiconductors and etc.) and we will prepare them for the analisis. Otrhervise you can prepare specimens by yourselves. The mount must satisfy the following requirements:
Standard mount has a cylindrical shape 25.4 mm (1 inch) in diameter and 5-6 mm as thick. It is shown in the figure bellow.
As a rule, sample is manufactured from the epoxy resin: practice shows that the resins Epofix and Specifix are optimal, since they do not contain the admixtures, which may distort a result. Grains of substance to be analyzed (minerals, glass, ceramics, semiconductors and etc.) are mounted in resin and then they are ground and polished in order to expose their internal parts. Grain size must not be less than 2 - 2,5 diameters of primary beam (most frequently used size of the spot is 15 microns). So grain size must be larger than 30 microns. Grains of the analyzed substance must be located not nearer than 4 mm from the edge of mount. The surface, which contains the analyzed material, must be perfectly flat and smooth; scratches and cavities disrupt the process of the emission of secondary ions. Final polishing is carried out by diamond abrasive 1 or 1/4 micron.
The arrangement of analyzed grains is documented in reflected light on several scales: in the small (magnification x20-x40) for convenience of navigation over a mount and the large (magnification x200-x300) for the selection of a certain analytical spot. Transmitted light observation and cathode-luminescence are also necessary.
The frontal and rear planes of mount must be parallel. After cleaning of the surface of mount it is coated by 150 angstroms layer of gold (99,999%).