Neutron-rich nuclei in the vicinity of the N=126 closed shell
Z. Podolyak 1*, for the RISING collaboration
1 University of Surrey
A series of experiments devoted to the study of the neutron-rich N~126 region have been performed at GSI, Darmstadt, within the Rare Isotopes Investigations at GSI (RISING) project. Exotic nuclei were synthesised using relativistic projectile fragmentation of 208Pb and 238U beams. Following ion-by-ion particle identification, time-correlated gamma decays from individually identified nuclear species have been measured, allowing the clean identification of isomeric decays (using a passive stopper), and beta and conversion electron decays (using the RISING active Si stopper).
Highlights from this experimental campaign include: (i) the first identification of excited states in even-even neutron-rich, proton-hole nuclei beyond the N=126 closed shell (208Hg128, and also 209Tl128); (ii) the first identification of excited states in the N=126 isotones  203Ir, 204Pt and 205Au, using both isomeric gamma-ray decay and conversion-electron spectroscopy; and (iii) new experimental information on the prolate-oblate shape transition in the W-Os region .
State of the art shell model calculations have been performed in order to obtain a deeper understanding of the microscopic structure of the low-lying excitations in these N~126 nuclei. In order to get an acceptable correspondence for both excitation energies and transition strengths on these newly studied N=126 isotones with Z<82 (i.e., proton holes in 208Pb), small modifications in both the standard two-body matrix elements  are required. The possible consequences of these modifications on the structure of the more exotic N=126 nuclei as approaching the r-process waiting points will also be discussed.
Details of the experimental results, comparisons with shell model descriptions and future possible experimental directions for these studies will be presented.
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