Strongly Enhanced Low Energy α-Particle Decay in Heavy Actinide Nuclei and Long-Lived Superdeformed and Hyperdeformed Isomeric States

Relatively low energy and very enhanced α-particle groups have been observed in various actinide fractions produced via secondary reactions in a CERN W target which had been irradiated with 24-GeV protons. In particular, 5.14, 5.27 and 5.53 MeV α-particle groups with corresponding half-lives of 3.8±1.0 y, 625±84 d and 26±7 d, have been seen in Bk, Es and Lr-No sources, respectively. The measured energies are a few MeV lower than the known ground state to ground state α-decays in the corresponding neutrondeficient actinide nuclei. The half-lives are 104 to 107 shorter than expected from the systematics of α-particle decay in this region of nuclei. The deduced evaporation residue cross sections are in the mb region, about 104 times higher than expected. A consistent interpretation of the data is given in terms of production of long-lived isomeric states in the second and third wells of the potential-energy surfaces of the parent nuclei, which decay to the corresponding wells in the daughters. The possibility that the isomeric states in the third minimum are actually the true or very near the true ground states of the nuclei, and consequences regarding the production of the long-lived superheavy elements, are discussed. PACS numbers: 23.60.+e, 21.10.Tg, 25.60.Pj, 27.90.+b