ELECTRON YIELD DETECTOR ACCESSORY
 Fig. 7 Electron yield detector accessory. This detector is mounted on a lid to fit either the 3-grid or 5-grid ion chamber detector sample box. It may be used separately or simultaneously with the fluorescent detector by rotating 45° to the x-ray beam. It consists of the detector plus a 45 V battery pack to apply -45 V to the front window and sample position of the detector. The collector grid at +45 V is either framed Ni mesh or aluminized Mylar (both furnished) located at the center of the detector. The incident x-ray beam passes through the front window, through the collector grid and impinges directly upon the sample. The Auger electrons emitted from the sample surface are stopped in He flowing slowly through the detector. The He is ionized by the Auger electrons emitted by the absorbing element. The secondary electrons are collected by the detector. Since ~ 30 eV is required to create an ion pair in He, there is a gain factor equal to Auger energy/30 or about 100-200 for 3d transition metal elements. The detector should be connected directly to a current amplifier (Keithley) operated at ~ 109 gain. This simple detector is very useful for measuring either high concentration samples or the near surface of samples. High concentration samples may be measured in the bulk (metal pieces, chunks, etc.) or as powders. Powdered samples should be ground as fine as possible then smeared with a spatula onto sticky tape (furnished) which is pasted over the sample opening to make the detector gas tight. Very little sample is required--if you can see it, you can measure it. Perhaps the greatest advantage of e-yield data for concentrated samples is that the technique is insensitive to particle size or sample thickness. This means that edge resonances and EXAFS are always at their full amplitude. A further advantage is that the data are nearly free of diffraction glitches from single crystal samples. Since the signal arises from ejected electrons, the diffracted x-ray photons create very little ionization in the He gas flowing through the detector. Hydrogen gas may also be used. It is a better dielectric and will give a little better signal to noise. There is good sensitivity for surfaces. It is possible to get data from a mono-layer on an inert low Z matrix, e. g. silicon or glass. For surfaces where the element of interest is also a part of the bulk, the penetration depth depends upon the energy of the emitted Auger electron, typically a few hundred Å. The technique is also useful for gaseous samples that can be swept into the detector in the He carrier gas. Ideal concentrations are ~ 1000 ppm; generally, if you can smell it you can. [ Basic 3-grid ion chamber detector ] [ Home ] [ Top of Page ] |