MOSSBAUER ABSORBER THICKNESSES FOR ACCURATE SITE POPULATIONS IN FE-BEARING MINERALS

We define the ideal absorber thickness, t(ideal), in the usual way, as the absorber thickness that gives the largest signal to noise ratio in a given time, and show by measurements that it is reliably calculated in real situations by the formulae of Long et al. (1983). We identify problem areas where it is essential to use the correct t(ideal). We describe thickness effects (i.e., unavoidable spectral distortions arising from nonzero absorber thicknesses) for uniform and nonpolarizing absorbers and define the thin absorber thickness, t(thin), as the largest thickness for which thickness effects are negligible. We present a graphical method whereby t(thin) can be evaluated for real mineral absorbers having spectra composed of intrinsically broad lines. The limits of our methods are carefully outlined. The necessary background concerning thickness effects in multisite materials has not been developed in the literature and is therefore given in detail. Most often t(thin) much less than t(ideal), however, with Fe-poor end-members (or with minerals containing Fe as a trace) one can have t(thin) greater-than-or-equal-to t(ideal) at t(ideal) values corresponding to quite doable experiments. We recommend that, whenever accurate quantitative results are required, the first concern be to obtain the best possible measured spectrum by using t = t(ideal). One can then reliably obtain the intrinsic absorber cross section (thereby eliminating all thickness effects) by deconvoluting the measured data using methods such as those recently developed by Rancourt (1989).