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.

Neutron-rich isotopes around lead, beyond N=126, have been studied exploiting the fragmentation of an uranium primary beam at the FRS-RISING setup at GSI. For the first time β-decay half-lives of 219 Bi and 211,212,213 Tl isotopes have been derived. The half-lives have been extracted using a numerical simulation developed for experiments in high-background conditions. Comparison with state of the art models used in r-process calculations is given, showing a systematic underestimation of the experimental values, at variance from close-lying nuclei.

The neutron-rich lead isotopes, up to 216 Pb, have been studied for the first time, exploiting the fragmentation of a primary uranium beam at the FRS-RISING setup at GSI. The observed isomeric states exhibit electromagnetic transition strengths which deviate from state-of-the-art shell-model calculations. It is shown that their complete description demands the introduction of effective three-body interactions and two-body transition operators in the conventional neutron valence space beyond 208 Pb. The shell model is nowadays able to provide a comprehensive view of the atomic nucleus [1]. It is a many-body theoretical framework, successful in explaining various features of the structure of nuclei, based on the definition of a restricted valence space where a suitable Hamiltonian can be diagonalized. This effective interaction originates from realistic two-body nuclear forces based on phenomenological nucleon-nucleon potentials, renormalized to be adapted to the truncated model space. Although the renormalization process can be treated in a rigorous mathematical way, the appearance of effective terms is often neglected in calculations, as a common but incorrect practice. The presence and relevance of these effective forces is well known also in other fields of physics, as for example in condensed matter studies [2]. Indeed, effective three-body terms appear already at the lower perturbation order [3]: PRL 109, 162502 (2012) P H Y S I C A L

This Letter reports on a systematic study of β-decay half-lives of neutron-rich nuclei around doubly magic 208 Pb. The lifetimes of the 126-neutron shell isotone 204 Pt and the neighboring [200][201][202] Ir, 203 Pt, 204 Au are presented together with other 19 half-lives measured during the "stopped beam" campaign of the rare isotope investigations at GSI collaboration. The results constrain the main nuclear theories used in calculations of r-process nucleosynthesis. Predictions based on a statistical macroscopic description of the first-forbidden β strength reveal significant deviations for most of the nuclei with N < 126. In contrast, theories including a fully microscopic treatment of allowed and first-forbidden transitions reproduce more satisfactorily the trend in the measured half-lives for the nuclei in this region, where the r-process pathway passes through during β decay back to stability. DOI: 10.1103/PhysRevLett.113.022702 PACS numbers: 25.70.Mn, 23.40.-s, 26.30.Hj, 27.80.+w In very hot, neutron-rich stellar environments, the r process of nucleosynthesis is ignited in a series of rapid neutron captures on seed nuclei of the Fe group, thus creating very exotic neutron-rich nuclei that β decay back to stability around the neutron shell closures with N ¼ 50, 82, and 126. In these "waiting-point" regions, matter is accumulated at masses A ∼ 80, 130, and 195, thus creating the so-called first, second, and third r-abundance peaks. These basic features of the r process were established more than half a century ago [1]. However, how the heavy nuclei from Ni to U are synthesized is one of the major unanswered questions of modern physics because of the large uncertainties in the path, time scale, and astrophysical conditions for the rapid neutron capture process to develop [2]. Observational constraints such as the elemental abundances in metal-poor stars or in solar system material help to determine astronomical sites where it might occur [3,4]. Concurrently, β-decay properties of very exotic nuclei near the path, such as β half-lives, are critical in determining the observed abundances [5]. Since many of the r-process progenitors cannot be accessed with present radioactive ion beam facilities, estimates of r-process nucleosynthesis generally rely upon predictions of stateof-the-art nuclear models, based on the properties of nuclei far from stability [6][7][8][9][10][11]. But at extreme values of isospin, theoretical predictions may be biased by microscopic structural effects that modify the shape of the β-strength function, such as nuclear shell quenching or deformation [12,13]. Until now, such theories have only been tested with information on β decay around the first two waiting points