Anomalous line shapes in the internal conversion electron spectrum from 227Th

Abstract

The conversion electron spectra of the 29.9 and 31.6 keV transitions in 223Ra which are excited by the α decay of 227Th have been studied using the Chalk River [Formula: see text] spectrometer. The shapes of the L and M subshell conversion lines of the 29.9 keV transition differ significantly from those of the corresponding lines of the 31.6 keV transition. This observation confirms the difference in line shapes first reported by Valadares, Walen, and Briancon. The conversion electron spectra which are coincident with α particles which penetrated normally through the source backing have also been measured. The coincidence arrangement selects the component of the conversion electron spectrum contributed by atoms recoiling freely into the spectrometer vacuum. The conversion lines of the 31.6 keV transition appear as single doppler-shifted components which are displaced toward higher momentum relative to the main peaks of the singles spectrum. The conversion lines of the 29.9 keV transition have a more complex structure. The doppler-shifted peak is less pronounced for these lines and 68 ± 10% of the intensity in these coincidence spectra appear in broad momentum distributions lying some hundreds of electron volts lower in energy than the doppler-shifted peak. These observations provide support for the explanation which Valadares, Walen, and Briancon have advanced to account for the anomalous appearance of the conversion lines of the 29.9 keV transition in their singles spectra. From the coincidence spectra measured in this work together with results from the charge state studies of Perrin and de Wieclawick, we infer that the LII, LIII, and MIII binding energies in 223Ra atoms of mean charge + 12 are 239 ± 32, 216 ± 25, and 212 ± 38 eV higher than the corresponding binding energies in 223Ra atoms of mean charge + 1.