VARIATION OF F-X-RAY AND CL-X-RAY INTENSITY DUE TO ANISOTROPIC DIFFUSION IN APATITE DURING ELECTRON-MICROPROBE ANALYSIS

Under conditions routinely used for electron microprobe analyses (15 k V, sample current of 0.015 muA, beam diameter of 5-50 mum) FKalpha X-r ay intensities of Durango fluorapatite sections with the surface perpe ndicular to the c axis increase up to 100% during the first 60 s of ex posure to the beam. After longer periods of exposure to an electron be am the intensity falls to values below the initial intensity. This eff ect is strongly anisotropic. Sections parallel to the c axis show a si milar behavior but on a time scale approximately 20 times longer. Ther e appears to be no relaxation or decay of the effect when the same spo t is reanalyzed after periods of up to several weeks. Under similar mi croprobe operating conditions, topaz shows no change in FKalpha intens ity, and fluorite shows a decline in intensity with increasing duratio n of beam exposure. The intensity variation and its anisotropy can be explained by diffusion of F to the surface driven by the electrical fi eld produced by primary beam electrons implanted at a depth below the analyzed region. The increase in intensity is believed to be due to th e 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 aniso tropy 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 apati te matrix. Accurate analysis of apatite will require a series of analy ses on the same spot extrapolated to a value at the initiation of beam exposure. Because of permanent long-term changes in X-ray intensity w ith 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 la yer (approximately 5 mum).