15.04.2008
Investigation of Cu isotopes delivers new findings on core structure
In a recent issue of Physical Review Letters (Phys. Rev. Lett. 100, 112502, 2008) a collaboration including scientists of the Excellence Cluster Universe provides latest results on the nuclear structure of Copper (Cu) isotopes. These nuclides are an interesting target as they form the bridge between iron – the heaviest element built in the stellar fusion chain – and heavier elements that are mainly produced by the r-(rapid) process nucleosynthesis in exploding supernova shells.
The investigations by this cooperation focused on the properties of Cu nuclei. The findings shed some light on the transition from elements with a mass number of A=60 (nickel-iron group) to heavier elements with A=80 marking the beginning of the r-process. The experiments will help to improve the reliability of nuclear models used to understand astrophysical processes responsible for the building the bridge to the elements heavier than iron.
Nuclei with closed proton or neutron shells are stabilized – this is the case for nuclei with 2, 8, 20, 28, 50, 82 nucleons or 126 neutrons, also referred to as „magic numbers“. This means excitations of single particles to higher shells require larger energies and therefore low-lying collective states involving many of such single particle excitations do not occur. This is of importance when atomic nuclei are enriched with additional neutrons either by direct neutron capture or by the capture of electrons by protons, converting them into neutrons. These processes require unoccupied neutron states in the nuclei.
In this context scientists have debated the local existence of a magic neutron number N=40 along the isotopic chain of Nickel with a proton number Z=28 for a long time. In case of a strong shell closure, all allowed neutron states would be blocked because of the Pauli principle. Otherwise, excitations of neutrons to orbitals above N=40 can open vacancies below which then are available for additional neutrons.
To develop a better understanding of the nuclear structure of this region, the group led by Irina Stefanescu (Instituut voor Kern- en Stralingsfysica, Katholieke Universiteit Leuven, Belgium) studied neutron-rich Cu isotopes with Z=29. Using the ISOLDE- and the REX-ISOLDE facilities at CERN, the scientists bombarded a uranium carbide target with energetic protons. As a result relatively short-lived Cu isotopes with odd mass numbers 67, 69, 71 and 73 were produced. These were impinged on palladium and tin foils to excite the Cu nuclei via Coulomb interaction. Returning to their ground state the isotopes emit gamma rays which are detected by the MINIBALL gamma ray spectrometer that has been co-developed by the Maier-Leibnitz-Laboratorium (MLL). The spectroscopy of this radiation provides information on the structure, as the obtained results can be compared with predictions from different nuclear models, like the shell model.
The experiments have yielded interesting new insights into the structure of the isotopes studied. For the first time, the scientists managed to experimentally determine the electromagnetic transition probabilities beyond Cu-65. An important finding is that the low-energy level schemes of the odd-mass Cu isotopes at and beyond the neutron number N = 40 are governed by three different nuclear modes: proton single particle excitations, particle-core-coupled states and a collective mode that lies at a surprisingly low energetic level. The presence of collectivity at the low excitation energy is likely to play an interesting role in the evolution of the structure of nuclei near the doubly magic nuclei Ni-68 and Ni-78. In particular, a magic number N=40 effectively well-localised around Ni-68 and indications for a weakening of the Z=28 closure open new paths along which the capture processes relevant in astrophysics may proceed.
