The Absorption of X-Rays

Abstract

THE atomic X-ray absorption coefficients of the elements have commonly been represented by simple formulæ of the type τ_a = kZ^xλ^y(k a factor involving fundamental atomic constants, Z the atomic number of the absorbing element, λ the wave-length of the X-rays, x and y exponents not very different from 4 and 3 respectively). These formulæ have been derived in a variety of ways (J. J. Thomson, A. H. Compton, L. de Broglie, H. A. Kramers), and the complete expression for the absorption coefficient of an element over all ranges or X-ray wave-lengths has been represented as the sum of a number of such terms—each term corresponding to the fluorescent excitation of a distinct series or sub-series (K,L₁.…), and dropping out for wave-lengths longer than that of the corresponding absorption edge. Experimental determinations of the absorption coefficients have shown fair agreement with theory, both with respect to the general run of the coefficients on each side of a discontinuity, and to the magnitudes of the discontinuities.