Transmission resonance spectroscopy in the third minimum of232Pa

Abstract

The fission probability of ${}^{232}$Pa was measured as a function of the excitation energy in order to search for hyperdeformed (HD) transmission resonances using the ($d,pf$) transfer reaction on a radioactive ${}^{231}$Pa target. The experiment was performed at the Tandem accelerator of the Maier-Leibnitz Laboratory (MLL) at Garching using the ${}^{231}$Pa($d,pf$) reaction at a bombarding energy of ${E}_{d}=12$ MeV and with an energy resolution of $\ensuremath{\Delta}E=5.5$ keV. Two groups of transmission resonances have been observed at excitation energies of ${E}^{*}=5.7$ and 5.9 MeV. The fine structure of the resonance group at ${E}^{*}=5.7$ MeV could be interpreted as overlapping rotational bands with a rotational parameter characteristic to a HD nuclear shape (${\ensuremath{\hbar}}^{2}/2\ensuremath{\Theta}=2.10\ifmmode\pm\else\textpm\fi{}0.15$ keV). The fission barrier parameters of ${}^{232}$Pa have been determined by fitting talys 1.2 nuclear reaction code calculations to the overall structure of the fission probability. From the average level spacing of the $J=4$ states, the excitation energy of the ground state of the third minimum has been deduced to be ${E}_{\text{III}}={5.05}_{\ensuremath{-}0.10}^{+0.40}$ MeV.