The SHRIMP is a high precision mass spectrometer designed for in situ measurements of isotopes. (An added bonus is all the puns and jokes you can make about the name.)
At RSES we have 3 (yes, three!) SHRIMPs. These are SHRIMP II (it came after SHRIMP I, which has recently been decommissioned), SHRIMP RG (reverse geometry) and SHRIMP SI, which is dedicated to analysing light stable isotopes. SHRIMP is used for many diverse purposes: to date minerals by U-Th-Pb dating; analyse stable isotopes (eg carbon, sulphur and oxygen) in biogenic and inorganic materials; to look at isotope anomalies in meteorites and lunar soil. The SHRIMP is especially good for geological materials because:
- the high mass resolution means that we can distinguish between the mass peak of interest and nearby interferences
- the small spot size of each analysis means that different zones of the same mineral grain can be analysed in situ. This is especially important for zoned minerals.
What goes into the SHRIMP?
Unfortunately, one cannot just put a rock into the SHRIMP and analyse it. After a rock has been crushed and minerals separated (using density and magnetic properties of each mineral), minerals are hand picked and mounted onto a 25 mm epoxy mount along with standard minerals of known isotopic composition. The mount has to be polished and coated with conductive material (eg gold or aluminium) to prevent charge build up.
How does SHRIMP work?
A focussed primary beam of oxygen or caesium ions are focussed to a 10-30 micron (0.01-0.03 mm) spot on the sample. This sputters material from the surface of the sample, some of which is ionised. These secondary ions are then extracted and sent off down the mass spectrometer.
The mass spec bit of the SHRIMP makes up the bulk of the machine. And SHRIMP is big. The secondary ion beam (that is, the ions sputtered from the sample) travel through an electrostatic analyser (ESA, just like in the SSAMS, but bigger). The ESA is two parallel, curved plates held at a particular electronic potential. Ions of different energy will therefore travel different paths.
After the ESA, the secondary beam is focussed through the magnetic analyser, which separates ions by momentum. (except in SHRIMP RG, which is reverse geometry so the magnet is before the ESA). According to the “right hand rule” an ion travelling through a magnetic field will experience a force at right angles to the magnetic field and the ion’s velocity. The upshot of all this physics is that the ions move in a circular motion, and the radius of the circle (in a constant magnetic field) is a function of the velocity and mass of the ion.
Finally, after travelling some metres and being separated by velocity and momentum, the ions are focussed into the ion counter.
For more information on SHRIMP, including a history of the design and development, check out shrimp.anu.edu.au