Detailed information about the configuration of electrons around the actinium nucleus has been obtained in a study of emission spectra of actinium recently completed by W. F. Meggers, Chief of the National Bureau of Standards spectroscopy laboratory. The results also establish that the elements following actinium in the periodic table form a series of "rare earths" analogous to the series that follows lanthanum. (1) The spectrograms were made by M. Fred and F. S. Tomkins of the Argonne National Laboratory, and the actinium for the study was produced at the Argonne laboratory by transmuting radium.
, "Emission Spectra of Actinium," Appl. Spectrosc. 11, 89-91 (1957)