Strongly enhanced low energy alpha-particle decay in heavy actinide nucleiand long-lived superdeformed and hyperdeformed isomeric states

Unidentified low energy and very enhanced a-particle groups have been obser ved in various actinide fractions produced via secondary reactions in a CER N W target which had been irradiated with 24-GeV protons. In particular, 5. 14, 5.27 and 5.53 MeV a-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-N o sources, respectively. The measured energies are a few MeV lower than the known ground state to ground state alpha -decays in the corresponding neut ron-deficient actinide nuclei. The half-lives are 10(4) to 10(7) shorter th an expected from energy versus lifetime relationship for such low-energy al pha -particles in this region of nuclei. The deduced evaporation residue cr oss sections are in the mb region, about 104 times higher than expected. No t only is it impossible to identify these alpha -decays with any known acti vity in the whole nuclear chart, but they also could not be due to hypothet ically unknown isomeric states in various conceivable neutron deficient nuc lei, nor due to unknown isomeric states in the rare-earth region. Based on the fact that in other experiments we have found isomeric states in the sec ond and third minima of the potential for other heavy ion reaction products , one can now understand in a quantitative way, both the unusual low energi es, the unusual enhanced lifetimes and the unusual large production cross s ections, in terms of production of similar isomeric states in appropriate a ctinide isotopes. Some consequences regarding the production of the long-li ved superheavy elements axe also discussed.