SHRIMP II

Description

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.