Polarization of Resonance Radiation and Hyperfine Structure: the Cadmium Resonance Lines

The cadmium resonance lines $\lambda \lambda$ 2288 and 3261A excited by unpolarized radiation in a magnetic field parallel to the exciting light beam are found to be 76.3 percent and 86-87 percent polarized respectively. From spectroscopic evidence Schüler has assigned nuclear moments of 0 and $\frac{3}{2}$ to the even and odd isotopes of Cd. Separations in 2288 are small compared to Doppler breadth, while in 3261 the strong component of the $\frac{3}{2}$ isotope pattern merges with the 0 isotope line. If we suppose that the introduction of spin does not change the net transition probabilities between the gross levels concerned these polarization values both yield 2.53 for the ratio of abundance of even and odd isotopes. A recent calculation of C. G. Mitchell leads to other results, but is based on the apparently incompatible assumptions that intensities in the source are proportional to the isotope abundance but in absorption are determined by assigning equal weights to Zeeman levels making absorption by the spin isotope twice that by the non-spin isotope for equal abundance and uniform spectral distribution, conflicting with Kirchhoff's law.