The atomic nucleus is a quantum mechanical ensemble of nucleons (protons, neutrons). Its properties are determined by 'fundamental' interactions, as nucleon - nucleon interaction, Coulomb interaction, spin - orbit interaction etc. In this case understanding the structure of nuclei is essential for understanding the 'fundamental' interactions. Superheavy nuclei (SHE) represent ensembles of 'extremely' large numbers of protons and neutrons and are thus ideal test cases for 'fundamental' interactions under extreme conditions.
At SHIP about a decade ago an experimental program had been started to investigate nuclear structure of isotopes in the element range Z ≥ 100 by means of α - γ - decay spectroscopy.
Of special interest is the region of strongly deformed nuclei around Z = 102, N = 152. These isotopes have a special stability due to large energy gaps between certain proton and neutron energy levels. The stability is reflected by isomeric states in many of these isotopes. Nuclei in this region can still be produced in a sufficient amount which enables an accurate study of the energies and ordering of the proton and neutron levels. From these results valuable insights can be obtained on shell gaps in the region of predicted spherical SHE which is expected around Z = 114, 120 or 126 and N = 184 and cannot yet be investigated directly.
An overview of isotopes that have been investigated at SHIP within the last decade is given in fig. 1.
Present techniques allow for detailed nuclear structure investigations up to isotopes with production cross sections down to 10 pb in reasonable experiment times, which means up to isotopes 270,271Ds (Z = 110). Technical and experimental improvements, which are in progress or considered in next future will pin down the cross section limit to approximately 1 pb. Then, nuclear structure investigations of superheavy element up to Z = 118 will be in reach.
Figure 1: Excerpt of the charts of nuclei showing the isotopes in the range from fermium (Z = 100) to darmstadtium (Z = 110) of which the decay properties have been studied at SHIP. (Image source: GSI Helmholtzzentrum für Schwerionenforschung)
Systematic study of low lying Nilsson levels in odd mass einsteinium isotopes by α-γ-coincidence studies of the corresponding mendelevium mother nuclei.
Figure 2: α-γ decay patterns of odd-mass mendelevium isotopes; the decays are characterized by an unhindered α-transition in coincidence of two γ-transitions of E1 character; based on theoretically predicted low lying Nilsson levels, from the decay characteristics spin and parities of the ground - states of the mendelevium isotopes and the levels populated in the einsteinium daughter isotopes can be assigned. (Image source: GSI Helmholtzzentrum für Schwerionenforschung)
Discovery of 270mDs, the heaviest K - isomer
Figure 3: Time distribution of α-decays assigned to the decay of 270Ds; it clearly consists of two components arising from the decay of the ground-state 270gDs and the isomeric state 270mDs. The life-time of the K isomer is longer than that of the ground-state, proving the high stability of high K quasi-particle states. (Image source: GSI Helmholtzzentrum für Schwerionenforschung)
Identification of a high lying isomer in 251No; first K isomer in an odd mass transfermium isotope
Figure 4: a) γ-spectrum observed in coincidence with α decays of 255Rf; b) γ - spectrum observed in delayed coincidence with 251No nuclei, produced in bombardments of 206Pb with 48Ca; the γ - lines are interpreted as due to the decay of an isomeric state in 251No having a half-life of about 2 μs and an excitation energy E* > 1.7 MeV. The low energy γ rays (142.4, 203.1 keV) prove that the 9/2- level is populated by the decay of the isomer rather than the 7/2+ ground-state, indicates a high spin value of the isomeric state. (Image source: GSI Helmholtzzentrum für Schwerionenforschung)
α - γ - decay study of 262Bh, the heaviest nucleus for which X-ray and γ lines are measured in coincidence with α - particles
Figure 5: Two dimensional plot of prompt α - γ - coincidences observed for the decay of 262Bh (Z = 107); besides the γ-transitions of 38.9, 102.4 and 156.5 keV two lines at E = 130.1 keV and 136.9 keV are visible that fit to the expected Kα2,α1 - X - energies of element Dubnium ( Z = 105); 262Bh is thus not only the heaviest nucleus for γ - lines have been measured in coincidence with α - particles, but also the heaviest one, whose atomic number has been established by measuring characteristic X - rays. (Image source: GSI Helmholtzzentrum für Schwerionenforschung)