Sensitive high resolution ion microprobe

History and scientific impact

The SHRIMP originated in 1973 with a proposal by Prof. Bill Compston, trying to build an ion microprobe at the Research School of Earth Sciences of the Australian National University that exceeded the sensitivity and resolution of ion probes available at the time in order to analyse individual mineral grains. Optic designer Steve Clement based the prototype instrument (now referred to as 'SHRIMP-I') on a design by Matsuda which minimised aberrations in transmitting ions through the various sectors. The instrument was built from 1975 and 1977 with testing and redesigning from 1978. The first successful geological applications occurred in 1980.

The first major scientific impact was the discovery of Hadean (>4000 million year old) zircon grains at Mt. Narryer in Western Australia and then later at the nearby Jack Hills. These results and the SHRIMP analytical method itself were initially questioned but subsequent conventional analysis were partially confirmed. SHRIMP-I also pioneered ion microprobe studies of titanium, hafnium and sulfur isotopic systems.

Growing interest from commercial companies and other academic research groups, notably Prof. John de Laeter of Curtin University (Perth, Western Australia), led to the project in 1989 to build a commercial version of the instrument, the SHRIMP-II, in association with ANUTECH, the Australian National University's commercial arm. Refined ion optic designs in the mid-1990s prompted development and construction of the SHRIMP-RG (Reverse Geometry) with improved mass resolution. Further advances in design have also led to multiple ion collection systems (already introduced in the market by a French company years before), negative-ion stable isotope measurements and on-going work in developing a dedicated instrument for light stable isotopes.

Fifteen SHRIMP instruments have now been installed around the world and SHRIMP results have been reported in more than 2000 peer reviewed scientific papers. SHRIMP is an important tool for understanding early Earth history having analysed some of the oldest terrestrial material including the Acasta Gneiss and further extending the age of zircons from the Jack Hills. Other significant milestones include the first U/Pb ages for lunar zircon and Martian apatite dating. More recent uses include the determination of Ordovician sea surface temperature, the timing of snowball Earth events and development of stable isotope techniques.

Primary column

In a typical U-Pb geochronology analytical mode, a beam of (O₂)¹⁻ primary ions are produced from a high-purity oxygen gas discharge in the hollow Ni cathode of a duoplasmatron. The ions are extracted from the plasma and accelerated at 10 kV. The primary column uses Köhler illumination to produce a uniform ion density across the target spot. The spot diameter can vary from ~5 µm to over 30 µm as required. Typical ion beam density on the sample is ~10 pA/µm² and an analysis of 15–20 minutes creates an ablation pit of less than 1 µm.

Sample chamber

The primary beam is 45° incident to the plane of the sample surface with secondary ions extracted at 90° and accelerated at 10 kV. Three quadrupole lenses focus the secondary ions onto a source slit and the design aims to maximise transmission of ions rather than preserving an ion image unlike other ion probe designs. A Schwarzschild objective lens provides reflected-light direct microscopic viewing of the sample during analysis.

Electrostatic analyzer

The secondary ions are filtered and focussed according to their kinetic energy by a 1272 mm radius 90° electrostatic sector. A mechanically-operated slit provides fine-tuning of the energy spectrum transmitted into the magnetic sector and an electrostatic quadrupole lens is used to reduce aberrations in transmitting the ions to the magnetic sector.

Magnetic sector

The electromagnet has a 1000 mm radius through 72.5° to focus the secondary ions according to their mass/charge ratio according to the principles of the Lorentz force. Essentially, the path of a less massive ion will have a greater curvature through the magnetic field than the path of a more massive ion. Thus, altering the current in the electromagnet focuses a particular mass species at the detector.

Detectors

The ions pass through a collector slit in the focal plane of the magnetic sector and the collector assembly can be moved along an axis to optimise the focus of a given isotopic species. In typical U-Pb zircon analysis, a single secondary electron multiplier is used for ion counting.

Vacuum system

Turbomolecular pumps evacuate the entire beam path of the SHRIMP to maximise transmission and reduce contamination. The sample chamber also employs a cryopump to trap contaminants, especially water. Typical pressures inside the SHRIMP are between ~7 x 10⁻⁹ mbar in the detector and ~1 x 10⁻⁶ mbar in the primary column.

Mass resolution and sensitivity

In normal operations, the SHRIMP achieves mass resolution of 5000 with sensitivity >20 counts/sec/ppm/nA for lead from zircon.

Isotope dating

For U-Th-Pb geochronology a beam of primary ions (O₂)¹⁻ are accelerated and collimated towards the target where it sputters “secondary” ions from the sample. These secondary ions are accelerated along the instrument where the various isotopes of uranium, lead and thorium are measured successively, along with reference peaks for Zr₂O⁺, ThO⁺ and UO⁺. Since the sputtering yield differs between ion species and relative sputtering yield increases or decreases with time depending on the ion species (due to increasing crater depth, charging effects and other factors), the measured relative isotopic abundances do not relate to the real relative isotopic abundances in the target. Corrections are determined by analysing unknowns and reference material (matrix-matched material of known isotopic composition), and determining an analytical-session specific calibration factor.