SHRIMP instruments

SHRIMP SI

SHRIMP SI (stable isotope) is an instrument dedicated to low mass range stable isotope analysis: O, S and C.  It is exceptional for analysing three O isotopes and four S isotopes at low concentrations.

SHRIMP SI is designed to facilitate stable isotope measurements alone. It includes some elements of both SHRIMP I and SHRIMP II. The features from SHRIMP I include the sample-stage motors being mounted externally to the vacuum chamber, and the intermediate extraction lens to produce a crossover in the secondary extraction. The mass analyser is the same design as SHRIMP II.

The multiple-collector for SHRIMP SI is designed for low mass. This means that unit mass spacing is further apart and allows the use of standard ETP multipliers in the multiple collector, with fully shielded containers for each multiplier. In measuring stable isotopes, there is little variation in the collector set up and so the design has been simplified with fewer control systems in the chamber.

SHRIMP SI includes a fully redesigned primary column. The beam transport is based around Kohler illumination for spot analysis, but also has a critical illumination capability for submicron illumination. The source chamber is designed for compartmentalized space and differential pumping between sub-chambers.

SHRIMP SI is funded through an ARC LIEF grant and includes partner organizations University of Melbourne, University of Queensland, University of Tasmania (CODES), University of Wollongong, Curtin University, University of Western Australia (through the John deLaeter Center for Mass Spectrometry), Geoscience Australia, CSIRO, and Australian Scientific Instruments.

SHRIMP II

SHRIMP II is used for both stable isotope work (negative secondary ions and a Cs+ primary beam) and also for trace element and U-Pb analyses (positive secondary ions and an oxygen primary beam).

Following on from the success of SHRIMP I, SHRIMP II was the commercial prototype. SHRIMP II incorporated a number of changes from SHRIMP I in terms of the design and configuration.

Kohler illumination was configured as the standard mode of operation and with a stronger demagnification in the condensor lens (7 in SHRIMP II vs 4 in SHRIMP I) allowed a higher throughput of ions and therefore higher beam densities and primary beam strengths (factor of three).

The slit-einzel triplet system in the source chamber was replaced with a quadrupole triplet, which has lower aberrations allowing more useful beam through the source slit.

All power supplies and many apertures/slits were under computer control allowing the user to largely configure the instrument from the terminal.

SHRIMP II was designed with a single collector but with the intention of incorporating a multiple collector (for Pb and stable isotope analysis). The primary column was also reconfigured to allow a Cs metal gun and duoplasmatron to be mounted in a Y configuration with electrostatic switching between beams. Following several modifications, the multiple collector is now in operation. The Y column was dropped following the change to the Kimball Physics Cs zeolite gun, which was mounted on a flange to match the duoplasmatron mounting flange. As such there was no advantage in having both mounted at the same time.

SHRIMP RG

SHRIMP RG provides exceptional high mass resolution for trace element analysis and U-Pb isotopes.

SHRIMP RG (reverse geometry) uses a different ion optical design to the other SHRIMPs. Matsuda (1990) produced a set of mass spectrometer designs to minimize third-order aberrations through the use of reverse-geometry double-focussing mass spectrometers. In reverse geometry, the magnet precedes the electrostatic analyser in the ion optical pathway. In the SHRIMP RG design, the ion optics is further complicated by the use of four quadrupole lenses. Q1 and Q2 are housed in a chamber immediately before the magnet. This QQH chamber also includes a hexapole lens to allow shaping of the beam particularly as a response to imperfect shaping of the magnetic field. The Q3 lens operates in a similar fashion to the quadrupole lens in the forward geometry design of SHRIMP I and allows matching between the magnetic and electrostatic sectors. The Q4 lens acts mainly as a projection lens to throw the image to an appropriate point for the collector.

The original Matsuda designs were modified so that the 'momentum' crossover after the magnet occurred before the ESA. This allows “energy” filtering on SHRIMP RG. The design of the SHRIMP RG yields approximately four times the mass resolution for the same slit sizes compared to SHRIMP II.

SHRIMP RG was completed in 1997 however it was apparent that the instrument was not performing as well as it should. The voltages on the electrostatic quadrupole lenses were very different to the theoretical voltages and good peak shapes could only be obtained by severely limiting the beam divergence. After lengthy examination of the input parameters it was found that the RG electrostatic lenses were clamping the beam too hard and were not consistent with the expectation of Matsuda’s design. Following the re-design of the lenses, the voltages for the quadupole lenses were close to theoretical and high mass resolution could be readily obtained.