Sensitive high-resolution ion microprobe

The SHRIMP originated in 1973 with a proposal by Prof. Bill Compston,[2] 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.

[7] These results and the SHRIMP analytical method itself were initially questioned[8][9] but subsequent conventional analysis were partially confirmed.

Refined ion optic designs in the mid-1990s prompted development and construction of the SHRIMP-RG (Reverse Geometry) with improved mass resolution.

More recent uses include the determination of Ordovician sea surface temperature,[24] the timing of snowball Earth events[25] and development of stable isotope techniques.

[26][27] 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.

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.

[29] The primary beam is 45° incident to the plane of the sample surface with secondary ions extracted at 90° and accelerated at 10 kV.

Turbomolecular pumps evacuate the entire beam path of the SHRIMP to maximise transmission and reduce contamination.

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+.