What is the heaviest neutron-proton combination that holds together as an entity long enough for us to investigate its properties? This question frames one of the frontiers of nuclear-structure research, and tests the limits of our current experimental technologies as well as our theoretical understanding of nuclear matter.
Experiments that focus on synthesizing the heaviest elements (Z=112-118), e.g. [Oga12], use reactions with pico barn cross sections, where the primary data is from ground-state alpha decays and with little or no information on possible excited states*. A complementary approach is to explore and expand the limits of nuclear spectroscopy to higher angular momenta where excitations and de-excitations of the heaviest nuclei can be studied. These detailed studies of higher spins in the heaviest nuclei are currently limited at Z~104 [Gre12], where the accessible nuclei are well deformed in their ground states. As such, they provide critical information on both collective correlations of the core, as parameterized by the shapes and moments of inertia, as well as single-particle phenomena that manifest themselves through deformation-aligned K-isomers or rotation-aligned quasiparticles. These two degrees of freedom and the rich interplay between them can be quantified through detailed measurements of level structures and transition matrix elements, which ultimately provide the most stringent tests for theoretical models that aim to understand the superheavy frontier.
The spectroscopy of these shell-stabilized nuclei that resist fission competition up to high angular momenta utilize both fusion-evaporation as well as inelastic and transfer reactions. While fusion reactions push inexorably towards the highest proton orbitals, inelastic and transfer reactions with heavy beams and radioactive targets enable studies of deformed neutron orbitals around N=154, relevant for superheavy nuclei. These measurements currently stretch the capabilities of the most advanced multi-detector arrays in the world. The unprecedented granularity, resolving power, and detection efficiency of GRETA will be essential for measurements aimed at understanding the limits of nuclear existence.
* Recently, gamma-ray decays have been reported from excited states in element Z = 109 and Z = 107, produced in the 243Am(48Ca, xn) 115 reaction [Rud14, Gat15].
[Gat15] J. M. Gates et al., to be published.