In-beam gamma-ray spectroscopy of heavy actinides at the JAEA Tandem accelerator

The existence of long-lived super heavy elements depends upon whether stability against fission can be sufficiently enhanced by shell effects. The largest effects are expected near the so-called Island of Stability (IoS), where the next spherical shell gaps are believed to occur. According to various models, the most likely locations for the next spherical shell closures are proton number Z=114, 120 or 126 and neutron number N=184. In the path to the IoS, small energy gaps also arise among the single-particle orbtals of deformed isotopes near (Z,N) = (100,152) and (108,162). These gaps, usually referred to as deformed shell gaps, significantly enhanced the stability of isotopes in their proximity, and extend our reach towards the IoS. Furthermore, some valence proton and neutron orbitals in neutron-rich actinides near N=152 correspond to substates, lowered by deformation, of orbits associated with the IoS. Extending our knowledge of the properties of actinides near these deformed shell gaps, and in particular a more precise mapping of the single-particle orbitals, can provide important benchmarks for theoretical models and permit more reliable extrapolations to the superheavy element region. Neutron-rich actinides, however, are still rather poorly known due to the difficulties connected with their production and identification. At the JAEA Tandem accelerator laboratory in Tokai, Japan, we developed a new setup that permits the in-beam gamma-ray spectroscopy of heavy actinides in experiments using rare radioactive actinide targets. With our setup, we investigated the structure of heavy actinides such as ²⁴⁸Cf (Z=98, N=150), ²⁴⁹Cf(Z=98, N=151), ²⁵⁴Es(Z=99, N=155) and ²⁵²Fm(Z=100,N=152). The nuclei of interest were either Coulomb excited (²⁴⁹Cf, ²⁵⁴Es), or produced using multi-nucleon transfer reactions of a ¹⁸O beam impinging onto a ²⁴⁹Cf actinide target ( ²⁴⁸Cf, ²⁵²Fm). A brief overview of some recent results will be presented.