Investigation of shell structure in exotic nuclei at RIBF in RIKEN, Japan
Zusammenfassung der Projektergebnisse
The test of existing doubly-magic shell closures and the search for modifications of shell structure in nuclei far off stability are of central interest with respect to our fundamental understanding of the nuclear many-body system. At the same time shell structure in nuclei far away from stability has a dramatic influence on the pathways leading to the production of the chemical elements above iron in our universe. With the new RIBF facility at the RIKEN Nishina Center in Tokyo, Japan, exotic nuclei like 42Si,54Ca, 100Sn have come into reach at much higher intensities to allow for a detailed spectroscopy of their nuclear properties. Within the framework of this project the group has significantly contributed and worked on three work packages which are beam development and cross section measurements, detector development and experiment setup, as well as several experimental works and data analysis. The study of inelastic excitations in 42Si was one of the first experimental programs that was performed at the RIBF facility. For 42Si we tested theoretical calculations that predict the breakdown of the N=28 shell for Z=14 and the existence of an oblate deformed ground state in 42Si. Additionally, we studied multi nucleon removal reactions of 44S and 40S to populate 4+ states in 40Si and 38Si in order to obtain valuable information on the evolution of the quadrupole collectivity In neutron-rich Si isotopes provided by the systematic trend of the ratio R4/2. Using the CAITEN implantation and decay detector set-up, the half-lives of 37AI and 38AI have been measured to 11.5(4) ms and 9.0(7) ms, respectively. Also beta-n branching ratios could be deduced for the two respective decays. The level schemes for 37Si and 38Si were deduced employing coincidence spectroscopy following event-by-event identification of the implanted nuclei. Comparison to large scale nuclear shell model calculations indicate that the classical shell gap at magic neutron number N = 28 between the f7/2 and p3/2 orbitals gets reduced by 0.3MeV in this region leading to low-energy states with intruder configuration in 37Si. This work was finished in 2013 with the PhD thesis of Konrad Steiger. Despite the high priority allocated by the RIKEN PAC to our other proposed experiments, the reduced duty cycle of the RIBF facility, unforeseen technical problems in the development of medium mass beams like Krypton and Xenon and the impact of the severe earthquake in Japan in spring 2011 lead to a delay in the overall experimental campaign. A big fraction of the scientific goals described in the application could be reached and the effort of the group had a large impact on a so called EURICA campaign at the RIBF facility at RIKEN. Due to the great success of the RISING campaign at GSI Darmstadt especially in an experiment lead by the Munich group a new effort called EURICA (EUROBALL RIKEN Cluster Array) campaign was started. Here the applicant played an important role in the proposal, planning, the installation and commissioning at RIKEN. By connecting 12 Cluster detectors with silicon detectors developed in Munich and funded by the DFG Project in the so called SIMBA and WAS3ABI detectors and the BigRIPS fragment separator for experimental campaigns at the RIBF in 2012/13, the world’s most intense secondary beam rates from inflight separation and the high efficiency and granularity detectors provide unique opportunities for nuclear structure studies. A 124 Xe beam was developed at RIBF especially for our experiments RIBF09 and RIBF83. After a first test run of this new beam in 2011 it turned out that some improvements on the accelerator will significantly improve the intensity in the near future. Several stages of improvement allowed us to perform the experiment RIBF09 finally in 2013. With an average of 30pnA of 124Xe beam at 345AMeV energy we produced and identified 2500 100Sn nuclei and a large data sample of its neighbors. In addition 94Cd, 92Ag, 90Pd had been identified for the first time and a first evidence for the existence of 104Te, 98Sn, 96In has been found. The decay spectroscopy analysis of these Eurica data has just started but exceeds the time range of the project.
Projektbezogene Publikationen (Auswahl)
- Low-lying level structure of the neutron-rich nucleus 109Nb: A possible oblate-shape isomer, Phys. Lett. B 696 (2011) 186-190
H. Watanabe et al.
(Siehe online unter https://doi.org/10.1016/j.physletb.2010.12.028) - β-Decay Half-Lives of Very Neutron-Rich Kr to Tc Isotopes on the Boundary of the r-Process, Phys. Rev. Lett. 106, 052502 (2011)
S. Nishimura et al.
(Siehe online unter https://doi.org/10.1103/PhysRevLett.106.052502) - Well Developed Deformation in 42Si, Phys. Rev. Lett. 109, 182501 (2012)
S. Takeuchi et al.
(Siehe online unter https://doi.org/10.1103/PhysRevLett.109.182501) - Isomers in 128Pd and 126Pd: Evidence for a Robust Shell Closure at the Neutron Magic Number 82 In Exotic Palladium Isotopes, Phys. Rev. Lett. 111, 152501 (2013)
H. Watanabe et al.
(Siehe online unter https://doi.org/10.1103/PhysRevLett.111.152501) - Production cross section measurements of radioactive isotopes by BigRIPS separator at RIKEN Rl Beam Factoty, Nuc. Instr. and Meth. 8 317(2013)756-768
H. Suzuki et al.
(Siehe online unter https://doi.org/10.1016/j.nimb.2013.08.049) - Study of the superallowed β-decay of 100Sn, Acc. Prog. Rep. Riken 47 (2014)
D. Lubos et al.
