Shrimp History

History Matsuda Designs Geochronology

In the mid 1970s a revolution was taking place whereby chemical separation of rocks and minerals was being replaced by in situ analysis. Electron microprobes had been developed that allowed chemical compositions to be determined in minerals through the measurement of the intensity of chemical specific, characteristic X-rays. The electron probe performs well for major elements, but the background radiation restricted the measurement of trace elements. The logical extension was the adaptation of Secondary Ion Mass Spectrometry to allow measurement of low abundance elements. The cornerstone of SIMS is the sputtering of material with a primary ion beam to produce an ion beam of the target. This ion beam can be analysed in a mass spectrometer for ionic abundance. Elemental abundances can therefore be determined limited only by counting statistics. However, isotopic compositions ca also obtained and hence the radiogenic decay of elements such as U and Th cause changes in Pb isotope ratio that can be related to geological time.The first ion microprobes were able to resolve individual unit masses, but could not deal with the complex molecular interferences produced in the sputtering process. Professor William Compston saw the solution to this problem in the design of a large ion microprobe capable of high mass resolution while maintaining high sensitivity. And so SHRIMP was born. Steve Clement, an RSES graduate student who designed his own mass spectrometer for his PhD, was retained to design SHRIMP I based on ion optical parameters of Professor H. Matsuda of Osaka University.

Matsuda Designs

All SHRIMP mass spectrometers are based on the ion optic designs of Professor H. Matsuda of Osaka University. Professor Matsuda used his TRIO program to evaluate the mass spectrometric terms and reduce the aberrations on the final image at the collector. The original SHRIMP I design was published by Matsuda (1974) as part of a suite of solutions minimizing second order aberrations. The ion optical parameters were massaged into the final design by Steve Clement, working on optimising mass resolution and sensitivity through beam transport theory.

The same mass spectrometric parameters were used for SHRIMP II, but the SHRIMP RG relies on designs for minimization of third order aberrations (Matsuda, 1990).