First Autoscan automated fission track counting systems installed at University of Geneva, Switzerland and Trinity College, Dublin, Ireland
In a world-first, two new Autoscan automated fission track counting systems were installed during November 2007: one in the Department of Mineralogy at the University of Geneva in Switzerland, and one in the Geology Department at Trinity College at the University of Dublin.
Richard Spikings, who is in charge of the laboratory in Geneva, will be known to most fission trackers as the Editor of OnTrack for 1999/2000, when he was working at ETH Zurich. In March 2001, Richard left ETH to work at the University of Potsdam in Germany. In March 2002, Richard moved to Geneva to develop a thermochronology laboratory at the University of Geneva.
David Chew, who is in charge of the laboratory in Trinity College Dublin, was a post-doctoral researcher in Geneva from 2003-5. He returned to Dublin in late 2005, where he is presently a lecturer. His research interests are thermochronology and geochronology.
The two systems just installed are based on the state-of-the-art Carl Zeiss AxioImager Z1m microscope, using the Autoscan AS3000i/f stage system in conjunction with Sony linear sensors for improved positioning accuracy, and an Olympus/SIS CView 3 high-resolution digital colour camera. The systems are controlled by our well-established Trakscan software, which now also incorporates FastTracks, our new automated track counting module.
The automated counting process involves a special sample preparation technique, followed by a proprietary image capture sequence which was developed during a 3-year collaborative research project between Autoscan and the University of Melbourne. FastTracks consists of two software modules:
- The special image capture module, which operates within Trakscan and creates a 3-dimensional image "stack" from images captured in both transmitted and reflected light. These images are archived for later retrieval.
- FastTracks, an independent automatic track counting software module which enables the user to process the image stack offline, without further reference to the microscope setup (thus freeing the microscope for further image capture work).
FastTracks incorporates the automated track counting facility as well as a review function which allows the operator to review (and if necessary, modify) the results achieved by the program. An advanced graphical user interface makes this process a pleasure for the operator.The automated counting system has so far been applied successfully to fission track dating of apatite and muscovite, but should also work well for other minerals, such as zircon or monazite.
The new software can be used either in the traditional way in association with the EDM method of fission track dating (which uses a grain mount and an associated mica mount to enable measurement of uranium concentration in the grains), or with a new technique which uses an LA-ICPMS to ablate a tiny portion of each grain to be analysed. This is an alternative method for quantifying the uranium content, and obviates the need for an external detector and the associated irradiation phase. The disadvantage of the LA-ICPMS is that the area to be analysed is physically destroyed in the process, but the great advantage of the process is that the images captured prior to ablation allow re-analysis or examination at any future time. In fact, the process is much faster and simpler than the irradiation method.
At the extreme magnifications (1000x) used in the FTD technique, minor vibrations can become a major headache. Such vibrations are externally generated (personnel in the proximity walking about, cars, trams, trains or buses nearby, air-conditioning equipment etc.) and are usually transmitted to the microscope through the building structure. In new buildings especially, it is not uncommon for the load to be borne only by the external walls, while internal walls are mainly partitions. This means that any internal stressors can cause considerable cantilevering distortions of the floors. Such vibrations can cause optical distortion of the captured image, leading to incorrect counting results.
In some buildings, therefore, an anti-vibration table of some sort is almost mandatory in order to eliminate the external vibrations. The designs can range from a simple heavy slab (eg granite) elastically suspended on squash-balls or wheelbarrow inner tubes, to more elaborate home-made designs, right up to the relatively expensive commercial solutions, some of which even incorporate "dynamic vibration cancellation". This operates in a similar manner to the now commonly available noise-cancelling headphones, by detecting external vibrations and applying an out-of-phase signal to the platform.
The anti-vibration table that Richard is using in Geneva is a Vibraplane 9100, manufactured by Kinetic Systems, USA (please see www.kineticsystems.com/page180.html). The table-top sits on a vibration-dampening cushion of compressed air, and hence an air-compressor (preferably a "silent type", such as that manufactured by Jun-Air) is also required. Isolation efficiencies: >90% at 10Hz, >65% at 5Hz. The company has agents in many countries.
David Chew has installed a much simpler anti-vibration setup at Trinity College, but it also seems very effective : the walls of the hallowed building which houses his laboratory (and which was once a museum) are extremely solid. This has made it possible to attach a shelf directly to the wall, thus avoiding the influence of floor vibrations altogether. The shelf consists of a solid granite plate, on top of which there is a second granite plate. The top plate is elastically isolated from the bottom plate by the use of a number of squash balls (presumably David had to sacrifice his favourite sport in the interests of science).
There are a number of other possibilities of providing anti-vibration bases. Plans for some of these and links to the commercial ones are available on our website (www.autoscan.com) :
- In the Autoscan testing laboratory, we simply use a single (but large) granite slab which is supported on inner tubes from the wheels of a small trolley. This is quite effective, and isolates our microscope from the train line which is about 500 metres away, and which was previously a major distraction.
- Dr. Ulrich Glasmacher (at the University of Heidelberg) has assembled a highly effective anti-vibration setup. (Please see "Hardware/Anti-vibration tables" for plans)
- A company called Halcyonics manufactures anti-vibration tables which sense external vibrations and apply dynamic anti-phase signals which cancel the incoming vibrations as they occur.
These are very exciting times for Autoscan. Our history has been one of continuous product development in various disciplines. With the cementing of an even closer relationship than ever before with the FTD group at the University of Melbourne, we are now in a position to assist fission trackers in their onerous technique and hopefully make it more enjoyable, but most importantly more accurate and productive, than ever.
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This article has also been published in OnTrack Forum for Nov 2007 (Vol 15, Issue 29). I would like to thank Matthias Raab and Konstanze Stuebner for their kind assistance.
The photographs show the experimental setup in the two laboratories.
Geneva system with anti-vibration table
Geneva laboratory system
Dublin system, Trinity College
Sample preparation equipment, Dublin installation