Ion microprobes have been
around in various forms for many years. It was not until the mid 70's
that the ion probe was viewed as having the potential to be the geologist's
ultimate weapon. The ion probe uses a focused beam of primary ions to
sputter away the sample surface. A small fraction of the sputtered material
is ionized and can then be accelerated into a mass spectrometer. A characteristic
of secondary ion mass spectrometry (SIMS) is a plethora of atomic and
molecular species which often cause isobaric interferences. The first
ion microprobes relied on low mass resolution mass spectrometers and
tried to strip away interferences by monitoring other peaks containing
the interfering elements. This method is fraught with difficulty because
it relies on the correct identification of all potential interferences.
The first SIMS instrument capable of high mass resolution was the Cameca
ims-3f. This instrument works as an ion microscope, that is, a direct
image of the spatial distribution of the isotopes in the target can
be obtained. High mass resolving power could only be achieved on this
instrument at the expense of beam transmission - entrance and exit slits
had to be very narrow thereby reducing the amount of beam transmission.
The first SHRIMP (SHRIMP I) was conceived by Professor William Compston of the Australian National University, Canberra, as a response to the lack of a commercially available ion microprobe capable of isotopic analyses of geological materials. Steve Clement was called on to produce a blueprint for the ion microprobe and he chose the ion optical design of Professor H Matsuda as the best for the purpose.
The beam transport theory indicated that the mass analyzer
needed to made as large as possible so as to maintain sensitivity at
the high mass resolving powers required to discriminate against molecular
isobaric interferences. The magnet turning radius was set at 1000 mm
with the other lens elements scaled accordingly from Matsuda (1974).
Construction began in 1977 and by 1981 the first isotopic analyses were
made. SHRIMP I was the first ion microprobe capable of accurate U-Pb
analyses on a microscale (ca. 30 microns). The fundamentals for the
U-Pb calibration were published by Compston et al. (1984) and the capability
for rapid analysis led to the discovery of Earth's oldest zircons (Froude
et al. 1983). SHRIMP was also turned to the study of S isotopic distributions
in ore minerals (Eldridge et al. 1987) and also to studies of extraterrestrial
minerals with large isotopic anomalies (Ireland et al. 1985).
SHRIMP II was the commercial prototype that was completed in 1990. It uses the same ion optical design as SHRIMP II but with a modified secondary extraction geometry that gives a three fold increase in the extracted secondary ion yield.
SHRIMP RG uses the same source chamber and primary column as SHRIMP II but it is a reverse geometry mass spectrometer capable of higher mass resolution for a given sensitivity.
Sensitive High Resolution Ion MicroProbe - Reverse Geometry
Our SHRIMP RG incorporates a completely different mass analyzer to that used on the SHRIMP II models. So, what is reverse geometry, or for that matter, forward geometry?
Double-focusing mass spectrometers consist of an electrostatic analyser (ESA) and a magnetic sector. The purpose of the ESA is to remove velocity dispersion from the mass filtered beam by producing an equal and opposite dispersion to that produced in the magnet. In other words, the magnet produces momentum dispersion and we only want mass dispersion. The double-focusing refers to the refocus of the ion beams of a single mass without any dispersion from the angular trajectory of the ion beams or the velocity of the ions.
When the ESA precedes the magnet, it is referred to as forward geometry. Thus, magnet before ESA is reverse geometry.
There are advantages and disadvantages for both geometries. In a reverse geometry mass spectrometer, mass separation occurs relatively early in the beam path and so only one mass is passed through to the collector. Hence, multiple collection is not an option on the SHRIMP RG. However, because the analyser is only transmitting a single mass ion beam, the abundance sensitivity (the degree to which scattered ions interfere with the peak of interest) is much reduced because any neighboring intense ion beam is rejected well out of sight of the collector.
Perhaps the biggest advantage of the SHRIMP RG is the larger magnet dispersion afforded by the reverse geometry. Given similar source and collector slits, SHRIMP RG should yield four times the mass resolution of the SHRIMP FG designs with the same sensitivity. The SHRIMP RG is based on a Matsuda (1990) design. Comparisons for various mass spectrometers of two figure of merit are given below.
C/X is the ratio of dispersion to magnification; the desired
attribute for the mass spectrometer is to have as much dispersion as
possible without magnifying the image. For example, any of the mass
spectrometers above could separate two peaks by 2 cm (dispersion) but
they will have very different mass resolutions. AF is the aberration
limted transmission factor. Basically this is the ultimate resolution
achievable with infinitely narrow (or at least 1 ion wide) slits. This
is not an achievable goal because the transmission would be totally
compromised. For the SHRIMP RG design, the dispersion is 3.0 x 10e6
microns and the magnification is 0.32. Hence, if the source slit is
100 microns, it will appear to be only 32 microns across at the collector.
The SHRIMP RG schematic shows the same primary column and source chamber as the SHRIMP II design, but a completely different mass analyzer. Upon exiting the source chamber, the secondary ion beam is spread horizontally by two quadrupole lenses (Q1, Q2) in the QQH chamber. The action of the quadrupoles is to compress in the vertical direction and diverge in the horizontal (as looking down the beam line). If only one quadrupole was used, the beam would diverge and all beam transmission would be lost; the pair of lenses is used such that a crossover is produced between them and the divergence is halted by the second of the quads (see the schematic of the vertical profile in the bottom of the figure). The beam is passed into the 46° magnet producing a momentum spectrum which can be imaged at a scannable slit in the beginning of the ESA. Immediately before the "energy" slit is a third quadrupole lens which is used in a similar position on the SHRIMP II design. After the ESA, is a fourth quadrupole which simply acts as a projector lens to magnify the image produced at the collector.
Steve Clement commenced construction-design for the SHRIMP RG in 1993
and fabrication of the ANU prototype began in 1995. A second SHRIMP
RG was built in parallel for the Stanford - USGS consortium by Australian
Scientific Instruments following the placement of an order in late 1994.
The SHRIMP RG was delivered on April 2, 1998 and installed
over the following ten days.