The construction and development of SHRIMP I: An historical outline

In 1973 Bill Compston advocated the building of an ion microprobe at the Research School of Earth Sciences (RSES) at the Australian National University (ANU). The commercial ion probes available at this time were too small to have sufficient sensitivity for trace element analysis and too low in mass resolution to avoid molecular interferences. The project commenced in 1974 with the appointment of a former ANU PhD student Steve Clement who had expertise in beam transport theory. To achieve high sensitivity and high mass resolution, beam transport theory indicated that a much larger magnet than in any commercially available mass spectrometer would be required. Clement chose an ion optical design, by Professor Matsuda of Osaka University in 1974, which had the required combination of high mass resolution and high transmission. Clement's job was to produce the detailed scientific designs and machine drawings for the new instrument as well as testing the completed instrument. Clement coined the term SHRIMP-Sensitive High Resolution Ion MicroProbe. By the end of 1977 nearly all the components had been manufactured and the big electromagnet had been successfully tested. In the following year the secondary mass analyzer was assembled and tested using a thermal ionization source and showed great promise with flat-topped peaks at 5000 resolution and 50% transmission with 50 V energy spread. At this stage the machine had far exceeded the specifications for the available commercial ion probes. Continued development during 1981 to the point where the original design specifications were fully realized was time consuming since learning how to use the entirely novel instrument was no simple task; no one else had an instrument like SHRIMP. The application of the instrument to zircon U–Pb geochronology established the necessary operating conditions for measuring Pb isotopic compositions and the elemental ratios Pb/U and U/Zr from 20 μm diameter spots on single zircon grains. Application of this in the early 1980s started a revolution in Precambrian geology by the ability to produce rapidly accurate and precise age determinations on structurally complex zircon samples.