Abstract
Under conditions routinely used for electron microprobe analyses (15 kV, sample current of 0.015 μA, beam diameter of 5–50 μm) FKα X-ray intensities of Durango fluorapatite sections with the surface perpendicular to the c axis increase up to 100% during the first 60 s of exposure to the beam. After longer periods of exposure to an electron beam the intensity falls to values below the initial intensity. This effect is strongly anisotropic. Sections parallel to the c axis show a similar behavior but on a time scale approximately 20 times longer. There appears to be no relaxation or decay of the effect when the same spot is reanalyzed after periods of up to several weeks. Under similar microprobe operating conditions, topaz shows no change in FKα intensity, and fluorite shows a decline in intensity with increasing duration of beam exposure.
The intensity variation and its anisotropy can be explained by diffusion of F to the surface driven by the electrical field produced by primary beam electrons implanted at a depth below the analyzed region. The increase in intensity is believed to be due to the diffusion of F ions to sites near the surface where their X-rays are subject to much less absorption than in the apatite matrix. The anisotropy of the effect is explained by the known, structurally controlled anisotropy of diffusion in apatite. Cl appears to behave in a similar manner except that the initial enhancement of X-ray intensity is less pronounced, since Cl X-rays are not so strongly absorbed in the apatite matrix.
Accurate analysis of apatite will require a series of analyses on the same spot extrapolated to a value at the initiation of beam exposure. Because of permanent long-term changes in X-ray intensity with cumulative beam exposure, apatite should not be used as a primary standard. Samples exposed to an electron beam for cathodoluminescence studies should not be analyzed without removing the altered surface layer (~5 μm).
