This is an accepted version of a paper published in Nature. This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination.Citation for the published paper: Hinke, C., Boehmer, M., Boutachkov, P., Faestermann, T., Geissel, H. et al. (2012) "Superallowed Gamow-Teller decay of the doubly magic nucleus 100 Sn" Nature, 486 (7403): [341][342][343][344][345] Access to the published version may require subscription.

The decay of excited states in the waiting-point nucleus 130 Cd 82 has been observed for the first time. An 8 two-quasiparticle isomer has been populated both in the fragmentation of a 136 Xe beam as well as in projectile fission of 238 U, making 130 Cd the most neutron-rich N 82 isotone for which information about excited states is available. The results, interpreted using state-of-the-art nuclear shell-model calculations, show no evidence of an N 82 shell quenching at Z 48. They allow us to follow nuclear isomerism throughout a full major neutron shell from 98 Cd 50 to 130 Cd 82 and reveal, in comparison with 76 Ni 48 one major proton shell below, an apparently abnormal scaling of nuclear two-body interactions. DOI: 10.1103/PhysRevLett.99.132501 PACS numbers: 21.60.Cs, 23.20.Lv, 26.30.+k, 27.60.+j The pioneering work of Goeppert-Mayer [1] and Haxel, Jensen, and Suess [2] in realizing that the experimental evidence for nuclear magic numbers could be explained by assuming a strong spin-orbit interaction constituted a major milestone in our understanding of the internal structure of the atomic nucleus. However, it has been recognized for more than 20 years that the single-particle ordering which underlies the shell structure (and with it the magic numbers) may change for nuclei approaching the neutron dripline. It has been argued that the neutron excess causes the central potential to become diffuse, leading to a modification of the single-particle spectrum of neutron-dripline nuclei [3,4]. In addition, a strong interaction between the energetically bound orbitals and the continuum also affects the level ordering. The consequence of these modifications can be a shell quenching; i.e., the shell gaps at magic neutron numbers are less pronounced in very neutronrich nuclei than in nuclei closer to stability. At the extreme, these gaps may even disappear. Alternatively, the tensor part of the nuclear force has been shown to cause shell reordering for very asymmetric proton and neutron numbers [5,6].The N 82 isotones below the doubly magic nucleus 132 Sn are crucial for stellar nucleosynthesis due to the close relation between the N 82 shell closure and the A 130 peak of the solar r-process abundance distribution. Based on the mass models available at that time, it was shown in the 1990s that the assumption of a quenching of the N 82 neutron shell closure leads to a considerable improvement in the global abundance fit in r-process calculations [7,8], in particular, a filling of the troughs around A 120 and 140. On the other hand, recently, alternative descriptions of the phenomenon have been given without invoking shell quenching at all [9,10]. Unfortunately, the very PRL 99,

No abstract

Excited states in 96 Ag were populated in fragmentation of an 850-MeV/u 124 Xe beam on a 4-g/cm 2 Be target. Three new high-spin isomers were identified and the structure of the populated states was investigated. The level scheme of 96 Ag was established, and a spin parity of (13 − ), (15 + ), and (19 + ) was assigned to the new isomeric states. Shell-model calculations were performed in various model spaces, including πν(p 1/2 , g 9/2 , f 5/2 , p 3/2 ) and the large-scale shell-model space πν(gds), to account for the observed parity changing M2 and E3 transitions from the (13 − ) isomer and the E2 and E4 transitions from the (19 + ) core-excited isomer, respectively. The calculated level schemes and reduced transition strengths are found to be in very good agreement with the experiment.

The four proton-hole nucleus, 204 Pt, was populated in the fragmentation of an E/A = 1 GeV 208 Pb beam. The yrast structure of 204 Pt has been observed up to angular momentum I = 10 by detecting delayed γ-ray transitions originating from metastable states. These long-lived excited states have been identified to have spin-parities of I π = (10 + ), (7 − ) and (5 − ) and half-lives of T 1/2 = 146(14) ns, 55(3) µs and 5.5(7) µs, respectively. The structure of the magic N = 126 204 Pt nucleus is discussed and understood in terms of the spherical shell model. The data suggests a revision of the two-body interaction for N = 126, Z < 82, which determines the evolution of nuclear structure towards the r-process waiting point nuclei.PACS numbers: 29.30. Kv, 23.20.Lv The evolution of the properties of atomic nuclei with respect to neutron and proton numbers is a key question of nuclear physics. The study of unstable, neutron-rich nuclei represents one of the foremost pursuits of modern nuclear physics. Over the coming decade new radioactive ion beam facilities are being built with the main objectives being to probe neutron-rich nuclei. Within recent years surprising phenomena have been observed in neutron-rich nuclei such as neutron skins, halos and dramatic changes in the ordering and spacing of energy levels [1].While the stability of the N = 82 shell gap is an active topic of research [2,3], an open question is whether or not there is a quenching of the N = 126 shell gap as protons are removed from doubly magic 208 Pb. The proton dripline has been experimentally reached up to heavy elements [4], our present knowledge of the neutron dripline is limited to light species. The part of the nuclear chart with the least information on neutron-rich nuclei is the 76 Os to 82 Pb region, with experimental knowledge on only a few isotopes. This mass region is however an ideal testing ground of nuclear theories. With the removal of just a few protons and neutrons the landscape evolves from spherical to elongated prolate through disk shaped oblate and triaxial forms [5]. Consequently the information gained on neutron-rich, N = 126 nuclei is essential for the understanding of nuclear structure in heavy nuclei. From a longer-term perspective, experi-

Heavy neutron-rich nuclei were populated via the fragmentation of a E/A = 1 GeV 208 82 Pb beam. Secondary fragments were separated and identified and subsequently implanted in a passive stopper. By the detection of delayed γ rays, isomeric decays associated with these nuclei have been identified. A total of 49 isomers were detected, with the majority of them observed for the first time. The newly discovered isomers are in 204,205 80 Hg, 201,202,204,205 79 Au, 197,203,204 78 Pt, 195,[199][200][201][202][203] 193,[197][198][199] 196 75 Re, 190,191 74 W, and 189 73 Ta. Possible level schemes are constructed and the structure of the nuclei discussed. To aid the interpretation, shell-model as well as BCS calculations were performed.

We have studied the f decay of the Tz = -1 , f y2 shell nuclei 54Ni, 50Fe, 46Cr, and 42Ti produced in fragmentation reactions. The proton separation energies in the daughter T. = 0 nuclei are relatively large («4-5 MeV) so studies of the y rays are essential. The experiments were performed at GSI as part of the Stopped-beam campaign with the RISING setup consisting of 15 Euroball Cluster Ge detectors. From the newly obtained high precision /1-decay half-lives, excitation energies, and f branching ratios, we were able to extract Fermi and Gamow-Teller transition strengths in these f) decays. With these improved results it was possible to compare in detail the Gamow-Teller (GT) transition strengths observed in beta decay including a sensitivity limit with the strengths of the Tz = +1 to Tz = 0 transitions derived from high resolution (3He,f) reactions on the mirror target nuclei at RCNP, Osaka. The accumulated B(GT) strength obtained from both experiments looks very similar although the charge exchange reaction provides information on a broader energy range. Using the "merged analysis" one can obtain a full picture of the £(GT) over the full Qp range. Looking at the individual transitions some differences are observed, especially for the weak transitions. Their possible origins are discussed.