Resistive Anode Encoder (RAE)

The Cameca ims-series ion probes are designed to operate in two distinct modes: as an ion microprobe and as an ion microscope. In this second mode the secondary ion optics preserve the geometry of the sample surface throughout the mass spectrometer. This permits the relative variations in trace element concentrations within the sample to be imaged. Using a micro-channel plate amplifier in combination with a 'resistive anode' it is possible to generate a digitized image. The spatial resolution of this technique is between 1 to 2 µm and can detect variations at concentration levels reaching into the 10s of ppm. Here we give as an example an image of the hafnium distribution in a zircon imaged as the isotope 178Hf. The grain which is embedded in epoxy is 60 µm wide; total data collection time was only two minutes. An overgrowth rich in Hf is clearly visible as are irregular patches within the grain interiors reflecting post crystallization alteration. A series of diagonal cracks, rich in hafnium, are also visible. The installation of the RAE provides a valuable tool to petrogenitic studies.





The RAE consist of an additional piece of vacuum housing placed between the end of the secondary flight tube and the 'standard' channel plate and phosphorus screen. The red handle mechanically swings the RAE into our out of the beam line. The following two views show exterior and interior views of the detector.






As part of a study into the deformation history of the Western Alps we acquired a number of images for assessing sample homogeneity and to determine if sample deformation history is reflected by a redistribution in the major elements in structurally oriented mica grains. The following examples demonstrate the type and quality of data which this approach can generate. The element for each individual frame is shown at the bottom of that frame.