SHRIMP
Encyclopedia
The sensitive high resolution ion microprobe (SHRIMP) is a large-diameter, double-focusing secondary ion mass spectrometer
(SIMS) sector instrument
produced by Australian Scientific Instruments in Canberra, Australia. The SHRIMP microprobe
focuses a primary beam of ions on a sample sputtering
secondary ions
which are focussed, filtered and measured according to their energy and mass.
The SHRIMP is primarily used for geological and geochemical applications. It can rapidly measure the isotopic and elemental abundances in minerals at a micrometre-scale and is therefore well-suited for the analysis of complex minerals, as often found in metamorphic
terrains, some igneous rock
s, and for relatively rapid analysis of statistical valid sets of detrital minerals from sedimentary rocks. The most common application of the instrument is in uranium-thorium-lead
geochronology
, although the SHRIMP can be used to measure other isotopic
and elemental abundances.
that exceeded the sensitivity and resolution of ion probes available at the time in order to analyse individual mineral grains. Ion optic designer Steve Clement based the prototype instrument (now referred to as 'SHRIMP-I') on a design by Matsuda which minimised aberrations in transmitting ions through the various sectors. The instrument was built from 1975 and 1977 with testing and redesigning from 1978. The first successful geological applications occurred in 1980.
The first major scientific impact was the discovery of Hadean
(>4000 million year old) zircon grains at Mt. Narryer in Western Australia and then later at the nearby Jack Hills
. These results and the SHRIMP analytical method itself were initially questioned but subsequent conventional analysis confirmed the results. SHRIMP-I also pioneered ion microprobe studies of titanium
, hafnium
and sulfur
isotopic systems.
Growing interest from commercial companies and other academic research groups, notably Prof. John de Laeter
of Curtin University (Perth, Western Australia), led to the project in 1989 to build a commercial version of the instrument, the SHRIMP-II, in association with ANUTECH, the Australian National University's commercial arm. Refined ion optic designs in the mid-1990s prompted development and construction of the SHRIMP-RG (Reverse Geometry) with improved mass resolution. Further advances in design have also led to multiple ion collection systems, negative-ion stable isotope measurements and on-going work in developing a dedicated instrument for light stable isotopes.
Fifteen SHRIMP instruments have now been installed around the world and SHRIMP results have been reported in more than 2000 peer reviewed scientific papers. SHRIMP is an important tool for understanding early Earth history having analysed some of the oldest terrestrial material including the Acasta Gneiss
and further extending the age of zircons from the Jack Hills. Other significant milestones include the first U/Pb ages for lunar zircon and Martian apatite
dating. More recent uses include the determination of Ordovician
sea surface temperature
, the timing of snowball Earth
events and development of stable isotope techniques.
The SHRIMP instrument was also discussed in Bill Bryson's
popular science book A Short History of Nearly Everything
where he describes a visit to the founding SHRIMP Laboratory at the Australian National University to learn more about geochronology and the oldest rocks on Earth.
geochronology analytical mode, a beam of (O2)1- primary ions are produced from a high-purity oxygen gas discharge in the hollow Ni
cathode of a duoplasmatron
. The ions are extracted from the plasma and accelerated at 10 kV. The primary column uses Köhler illumination
to produce a uniform ion density across the target spot. The spot diameter can vary from ~5 µm to over 30 µm as required. Typical ion beam density on the sample is ~10 pA/µm2 and an analysis of 15–20 minutes creates an abalation pit of less than 1 µm.
. A mechanically-operated slit provides fine-tuning of the energy spectrum transmitted into the magnetic sector and an electrostatic quadrapole lens is used to reduce aberrations in transmitting the ions to the magnetic sector.
. Essentially, the path of a less massive ion will have a greater curvature through the magnetic field than the path of a more massive ion. Thus, altering the current in the electromagnet focuses a particular mass species at the detector.
is used for ion counting.
s evacuate the entire beam path of the SHRIMP to maximise transmission and reduce contamination. The sample chamber also employs a cryopump
to trap contaminants, especially water. Typical pressures inside the SHRIMP are between ~7 x 10−9 mbar in the detector and ~1 x 10−6 mbar in the primary column.
towards the target where it sputters “secondary” ions from the sample. These secondary ions are accelerated along the instrument where the various isotopes of uranium
, lead
and thorium
are measured successively, along with reference peaks for Zr2O+, ThO+ and UO+. Since the sputtering yield differs between ion species and relative sputtering yield increases or decreases with time depending on the ion species (due to increasing crater depth, charging effects and other factors), the measured relative isotopic abundances do not relate to the real relative isotopic abundances in the target. Corrections are determined by analysing unknowns and reference material (matrix-matched material of known isotopic composition), and determining an analytical-session specific calibration factor.
Secondary ion mass spectrometry
Secondary ion mass spectrometry is a technique used in materials science and surface science to analyze the composition of solid surfaces and thin films by sputtering the surface of the specimen with a focused primary ion beam and collecting and analyzing ejected secondary ions...
(SIMS) sector instrument
Sector instrument
A sector instrument is a general term for a class of mass spectrometer that uses a static electric or magnetic sector or some combination of the two as a mass analyzer. A popular combination of these sectors has been the BEB...
produced by Australian Scientific Instruments in Canberra, Australia. The SHRIMP microprobe
Microprobe
A microprobe is an instrument that applies a stable and well-focused beam of charged particles to a sample.-Types:When the primary beam consists of accelerated electrons, the probe is termed an electron microprobe, when the primary beam consists of accelerated ions, the term Ion Microprobe is used...
focuses a primary beam of ions on a sample sputtering
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
secondary ions
Secondary ionization
The process of secondary ionization consists of removing a second electron from an atom or molecule when another electron has been removed before ....
which are focussed, filtered and measured according to their energy and mass.
The SHRIMP is primarily used for geological and geochemical applications. It can rapidly measure the isotopic and elemental abundances in minerals at a micrometre-scale and is therefore well-suited for the analysis of complex minerals, as often found in metamorphic
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
terrains, some igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s, and for relatively rapid analysis of statistical valid sets of detrital minerals from sedimentary rocks. The most common application of the instrument is in uranium-thorium-lead
Uranium-lead dating
Uranium-lead is one of the oldest and most refined of the radiometric dating schemes, with a routine age range of about 1 million years to over 4.5 billion years, and with routine precisions in the 0.1-1 percent range...
geochronology
Geochronology
Geochronology is the science of determining the age of rocks, fossils, and sediments, within a certain degree of uncertainty inherent to the method used. A variety of dating methods are used by geologists to achieve this, and schemes of classification and terminology have been proposed...
, although the SHRIMP can be used to measure other isotopic
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
and elemental abundances.
History and scientific impact
The SHRIMP originated in 1973 with a proposal by Prof. Bill Compston to build an ion microprobe at the Research School of Earth Sciences of the Australian National UniversityAustralian National University
The Australian National University is a teaching and research university located in the Australian capital, Canberra.As of 2009, the ANU employs 3,945 administrative staff who teach approximately 10,000 undergraduates, and 7,500 postgraduate students...
that exceeded the sensitivity and resolution of ion probes available at the time in order to analyse individual mineral grains. Ion optic designer Steve Clement based the prototype instrument (now referred to as 'SHRIMP-I') on a design by Matsuda which minimised aberrations in transmitting ions through the various sectors. The instrument was built from 1975 and 1977 with testing and redesigning from 1978. The first successful geological applications occurred in 1980.
The first major scientific impact was the discovery of Hadean
Hadean
The Hadean is the geologic eon before the Archean. It started with the formation of the Earth about 4.7 Ga and ended roughly 3.8 Ga, though the latter date varies according to different sources. The name "Hadean" derives from Hades, Greek for "Underworld", referring to the "hellish"...
(>4000 million year old) zircon grains at Mt. Narryer in Western Australia and then later at the nearby Jack Hills
Jack Hills
The Jack Hills are a range of hills in Mid West Western Australia. They are best known as the source of the oldest material of terrestrial origin found to date: zircons that formed around 4.4 billion years ago...
. These results and the SHRIMP analytical method itself were initially questioned but subsequent conventional analysis confirmed the results. SHRIMP-I also pioneered ion microprobe studies of titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
, hafnium
Hafnium
Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...
and sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
isotopic systems.
Growing interest from commercial companies and other academic research groups, notably Prof. John de Laeter
John Robert de Laeter
John Robert de Laeter, AO, FTSE, FAIP was an Australian scientist with a distinguished career across several fields in Nuclear physics, Cosmochemistry, Geochronology, Isotope geochemistry...
of Curtin University (Perth, Western Australia), led to the project in 1989 to build a commercial version of the instrument, the SHRIMP-II, in association with ANUTECH, the Australian National University's commercial arm. Refined ion optic designs in the mid-1990s prompted development and construction of the SHRIMP-RG (Reverse Geometry) with improved mass resolution. Further advances in design have also led to multiple ion collection systems, negative-ion stable isotope measurements and on-going work in developing a dedicated instrument for light stable isotopes.
Fifteen SHRIMP instruments have now been installed around the world and SHRIMP results have been reported in more than 2000 peer reviewed scientific papers. SHRIMP is an important tool for understanding early Earth history having analysed some of the oldest terrestrial material including the Acasta Gneiss
Acasta Gneiss
The Acasta Gneiss is a rock outcrop of Hadean tonalite gneiss in the Slave craton in Northwest Territories, Canada. Located on an island about 300 kilometres north of Yellowknife, the Acasta River rock deposit, believed to be 4.031 to 3.58 billion years old, is the oldest known intact crustal...
and further extending the age of zircons from the Jack Hills. Other significant milestones include the first U/Pb ages for lunar zircon and Martian apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
dating. More recent uses include the determination of Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
sea surface temperature
Sea surface temperature
Sea surface temperature is the water temperature close to the oceans surface. The exact meaning of surface varies according to the measurement method used, but it is between and below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a...
, the timing of snowball Earth
Snowball Earth
The Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma . Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical...
events and development of stable isotope techniques.
The SHRIMP instrument was also discussed in Bill Bryson's
Bill Bryson
William McGuire "Bill" Bryson, OBE, is a best-selling American author of humorous books on travel, as well as books on the English language and on science. Born an American, he was a resident of Britain for most of his adult life before moving back to the US in 1995...
popular science book A Short History of Nearly Everything
A Short History of Nearly Everything
A Short History of Nearly Everything is a popular science book by American author Bill Bryson that explains some areas of science, using a style of language which aims to be more accessible to the general public than many other books dedicated to the subject...
where he describes a visit to the founding SHRIMP Laboratory at the Australian National University to learn more about geochronology and the oldest rocks on Earth.
Design and operation
Primary column
In a typical U-PbUranium-lead dating
Uranium-lead is one of the oldest and most refined of the radiometric dating schemes, with a routine age range of about 1 million years to over 4.5 billion years, and with routine precisions in the 0.1-1 percent range...
geochronology analytical mode, a beam of (O2)1- primary ions are produced from a high-purity oxygen gas discharge in the hollow Ni
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
cathode of a duoplasmatron
Duoplasmatron
Duoplasmatron, an invention of Manfred von Ardenne, is a type of ion beam source. The duoplasmatron operates as follows: a cathode filament emits electrons into a vacuum chamber. A gas such as argon is introduced in very small quantities into the chamber, where it becomes charged or ionized through...
. The ions are extracted from the plasma and accelerated at 10 kV. The primary column uses Köhler illumination
Köhler illumination
Köhler illumination is a method of specimen illumination used for transmitted and reflected light optical microscopy. Köhler illumination acts to generate an extremely even illumination of the sample and ensures that an image of the illumination source is not visible in the resulting image...
to produce a uniform ion density across the target spot. The spot diameter can vary from ~5 µm to over 30 µm as required. Typical ion beam density on the sample is ~10 pA/µm2 and an analysis of 15–20 minutes creates an abalation pit of less than 1 µm.
Sample chamber
The primary beam is 45° incident to the plane of the sample surface with secondary ions extracted at 90° and accelerated at 10 kV. Three quadrapole lenses focus the secondary ions onto a source slit and the design aims to maximise transmission of ions rather than preserving an ion image unlike other ion probe designs. A Schwarzchild objective lens provides reflected-light direct microscopic viewing of the sample during analysis.Electrostatic analyzer
The secondary ions are filtered and focussed according to their kinetic energy by a 1272 mm radius 90° electrostatic sectorElectrostatic analyzer
An electrostatic analyzer or ESA is an instrument used in ion optics that employs an electric field to allow the passage of only those ions or electrons that have a given specific energy. It usually also focuses these particles into a smaller area...
. A mechanically-operated slit provides fine-tuning of the energy spectrum transmitted into the magnetic sector and an electrostatic quadrapole lens is used to reduce aberrations in transmitting the ions to the magnetic sector.
Magnetic sector
The electromagnet has a 1000 mm radius through 72.5° to focus the secondary ions according to their mass/charge ratio according to the principles of the Lorentz forceLorentz force
In physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...
. Essentially, the path of a less massive ion will have a greater curvature through the magnetic field than the path of a more massive ion. Thus, altering the current in the electromagnet focuses a particular mass species at the detector.
Detectors
The ions pass through a collector slit in the focal plane of the magnetic sector and the collector assembly can be moved along an axis to optimise the focus of a given isotopic species. In typical U-Pb zircon analysis, a single secondary electron multiplierElectron multiplier
An electron multiplier is a vacuum-tube structure that multiplies incident charges. In a process called secondary emission, a single electron can, when bombarded on secondary emissive material, induce emission of roughly 1 to 3 electrons...
is used for ion counting.
Vacuum system
Turbomolecular pumpTurbomolecular pump
A turbomolecular pump is a type of vacuum pump, superficially similar to a turbopump, used to obtain and maintain high vacuum. These pumps work on the principle that gas molecules can be given momentum in a desired direction by repeated collision with a moving solid surface...
s evacuate the entire beam path of the SHRIMP to maximise transmission and reduce contamination. The sample chamber also employs a cryopump
Cryopump
A cryopump is a vacuum pump that traps gases and vapours by condensing them on a cold surface. They are only effective on some gases, depending on the freezing and boiling points of the gas relative to the cryopump's temperature...
to trap contaminants, especially water. Typical pressures inside the SHRIMP are between ~7 x 10−9 mbar in the detector and ~1 x 10−6 mbar in the primary column.
Mass resolution and sensitivity
In normal operations, the SHRIMP achieves mass resolution of 5000 with sensitivity >20 counts/sec/ppm/nA for lead from zircon.Isotope dating
For U-Th-Pb geochronology a beam of primary ions (O2)1- are accelerated and collimatedCollimated light
Collimated light is light whose rays are parallel, and therefore will spread slowly as it propagates. The word is related to "collinear" and implies light that does not disperse with distance , or that will disperse minimally...
towards the target where it sputters “secondary” ions from the sample. These secondary ions are accelerated along the instrument where the various isotopes of uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
, lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
and thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
are measured successively, along with reference peaks for Zr2O+, ThO+ and UO+. Since the sputtering yield differs between ion species and relative sputtering yield increases or decreases with time depending on the ion species (due to increasing crater depth, charging effects and other factors), the measured relative isotopic abundances do not relate to the real relative isotopic abundances in the target. Corrections are determined by analysing unknowns and reference material (matrix-matched material of known isotopic composition), and determining an analytical-session specific calibration factor.
SHRIMP instruments around the world
| Instrument Number | Institution | Location | SHRIMP model | Year of commissioning |
| 1 | Australian National University | Canberra | I | 1980 |
| 2 | Australian National University | Canberra | II | 1992 |
| 3 | Curtin University of Technology | Perth | II | 1993 |
| 4 | Geological Survey of Canada | Ottawa | II | 1995 |
| 5 | Hiroshima University | Hiroshima | II | 1996 |
| 6 | Australian National University | Canberra | RG | 1998 |
| 7 | USGS & Stanford University | Stanford | RG | 1998 |
| 8 | National Institute of Polar Research | Tokyo | II | 1999 |
| 9 | Chinese Academy of Geological Sciences | Beijing | II | 2001 |
| 10 | All Russian Geological Research Institute | St. Petersburg | II | 2003 |
| 11 | Curtin University of Technology | Perth | II | 2003 |
| 12 | University of São Paulo | São Paulo | II | tba |
| 13 | Chinese Academy of Geological Sciences | Beijing | IIe | tba |
| 14 | Geoscience Australia | Canberra | IIe | 2008 |
| 15 | Korea Basic Science Institute | Ochang | IIe | tba |