Examining the impact of alpha-decay energies on the odd-even staggering in half-lives: alpha-decay spectroscopy of 207-209Ac

The alpha decays of 207-209Ac, located near the proton drip-line, were reinvestigated by means of alpha-decay spectroscopy. They were produced in the fusion reaction 36Ar +176Hf and separated in flight using the gas-filled recoil separator SHANS. A new alpha-decay line at 7483(15) keV was observed and assigned as the decay from the ground state of 208Ac to the excited (2+) or (4+) state in 204Fr. In addition, the previously known alpha-decay properties of 207-209Ac were measured with improved precision. The half-lives obtained, together with the existing decay data of other N 126 actinium isotopes, were compared with the values calculated with the Wentzel-Kramers-Brillouin approximation and without considering the alpha-preformation factors. It is shown that the calculated half-lives exhibit a distinct odd-even staggering (OES) following the same trend as the experimental data, whereas the amplitudes of the exhibited OES are 30-60 % of the real ones. This implies that apart from the alpha-preformation factors, the alpha-decay energies also contribute significantly to the OES of alpha-decay half-lives.