Microbeam

With the microbeam, the particle beam is squeezed down to about 1 to 4 microns in size. Analysis of the sample is then done using conventional RBS or PIXE techniques.

Normal analysis of materials in this laboratory uses a beam of particles about 1 or 2 mm in diameter. The results are therefore averaged over a similar sized portion of the user’s sample.

With the microbeam, the particle beam is squeezed down to about 1 to 4 microns in size. Analysis of the sample is then done using conventional RBS or PIXE techniques.

Analysis with very small beam size would not be useful if the experimenter could not tell where the beam is hitting on the sample. The microbeam system has several features to assist in placing the beam. The first is a closed-circuit TV system using a moderate power microscope lens. This allows a view on target of about 3 mm diameter. The second is a scanning mode similar to a scanning electron microscope. The beam is scanned in a TV-like raster over about 1.2 mm of the sample. The electrons knocked loose by the beam are captured and displayed on a computer screen, to give a picture similar to a TV microscope view. When the user locates the desired site on the sample, this picture is used to place the beam precisely on the feature of interest. Data can then be taken on that spot for as long as necessary.

The system can also take data while scanning the beam over the sample. The result is a picture or "map" of the location of chemical elements within the scan. While this scan mode can be a very powerful method of location, it is less desirable for final data. The RBS or PIXE spectrum taken continuously at one single point can give good results within 5 to 10 minutes. When the same amount of data are spread out over 65,536 different locations, the information in any one point is very small. Maps often take one to four hours for useful results, while a good spectrum at one selected point rarely takes more than 15 minutes.