Low Mass Francium and Emanation Isotopes of High Alpha-Stability

Abstract

Isotopes of francium with 126 or fewer neutrons have been looked for in bombardments of ${\mathrm{Th}}^{232}$ with 350-Mev protons from the 184-inch cyclotron. ${\mathrm{Fr}}^{212}$, with an apparent half-life of 19.3 minutes for branching decay by alpha-emission (44 percent) to ${\mathrm{At}}^{208}$ and by orbital electron-capture (56 percent) to ${\mathrm{Em}}^{212}$, has been found. ${\mathrm{Em}}^{212}$ is shown to be a 23-minute alpha-emitter. ${\mathrm{At}}^{208}$ decays primarily (99.5 percent) by orbital electron-capture to ${\mathrm{Po}}^{208}$, but shows 0.5 percent alpha-branching. The francium and emanation isotopes have alpha-half-lives completely out of line with the simple predictions based on the previously known isotopes of these elements. Their high alpha-stability is believed to be due to a closed shell of 126 neutrons in analogy to the behavior of elements 83-85. The non-existence of long-lived francium in nature is discussed in the terms of this and other recent work on francium isotopes.