Super Heavy Nuclei probe the extremes of nuclear structure in terms of number of nucleons that can form a bound system. Their existence and decay properties are among the fundamental problems in nuclear physics, being strongly related to the nuclear shell structure: the heaviest nuclei with Z > 100 would be Coulomb unstable if a large shell correction would not provide additional binding energy, rising the fission barrier up to 8 MeV. The understanding of this problem is related to the general question whether theoretical models can reliably predict the nuclear shell structure.
At present, different groups are testing models based on energy density functional (EDF), which predict how single particle shells evolve along the nuclear landscape. In Milano, work is done to introduce in EDF tensor and particle-vibration coupling components. The success of these models in stable and unstable known nuclei will allow to obtain prediction of shells, level density and spectroscopic factors in unknown regions.
The properties of shell-stabilized nuclei are very little known and their survival with angular momentum and excitation energy is an open question. Such information can be obtained by in-beam gamma spectroscopy experiments with very cold heavy ions fusion reactions (i.e. with large negative Q-value), providing a test to the robustness of shell corrections and fission barriers against rotation and temperature T (T<1 MeV). The role of the dipole excitations in this formation process should also be theoretically investigated. These experiments are very demanding: they require the combined use of an efficient gamma-array coupled to a detection system which identifies, out of a huge fission background, very weak fusion products, with cross sections around few microb. The Milano group will propose such experiments to the LNL Laboratory of INFN. They require the combined use of i) the first phase of the new generation Ge array AGATA, ii) an array of large volume BaF2 and LaBr3 scintillator detectors (belonging to Milano) to improve the gamma efficiency, iii) a start detector array of plastic scintillators for the selection of the nuclei of interest by time of flight techniques. The latter represents a "key" detector for this particular research and important modification will be needed to enhance its selectivity to cross sections below 0.1 mb. In particular, a closer target distance geometry will be used, and a system of multichannel plates will be added to improve the trigger conditions (estimated cost 3 kE); scaler modules are also requested for monitoring of the dead time due to fission.
The setup will be first tested by looking, as a function of temperature, at heavy nuclei with cross-sections around 1 mb, such as Rn, Ra and Th isotopes (around A=230), which are known to exhibit stable octupole deformation up to high spins. In the future nuclei populated with lower cross-sections (around 1 microb or less) will be investigated.