The Absorption of X-rays by Iron, Cobalt, Nickel and Copper

X-ray absorption coefficients of Fe, Co, Ni and Cu, for wave-lengths below the K absorption limit, have been measured by the use of a large spectrometer with a selected calcite crystal giving a beam of exceptional purity. The chief difficulty was to secure uniform films of the pure metals. It is found that the sequence of values of the atomic coefficients is that of the atomic numbers, not that of atomic weights, and that the variation with atomic numbers is as the fourth power, as previously suggested, the relation ${\mu }_a=22.4\mathrm{}\times {10}^{-27\mathrm{}}{N}^4{\lambda }^3+{\sigma }_a$, holding within experimental error for all the ten elements for which the results have been obtained, from $N=6\left(\mathrm{C}\right)$ to $N=82\left(\mathrm{Pb}\right)$, except possibly iron for which the experimental value is 3 per cent too low. The values for carbon, lead and tungsten are preliminary and are taken from data not yet published.