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We have performed large-scale shell-model calculations of the half-lives and neutron-branching probabilities of the r-process waiting-point nuclei at the magic neutron numbers N = 50, 82, and 126. The calculations include contributions from allowed Gamow-Teller and first-forbidden transitions. We find good agreement with the measured half-lives for the N = 50 nuclei with charge numbers Z = 28-32 and for the N = 82 nuclei 129 Ag and 130 Cd. The contribution of forbidden transitions reduce the half-lives of the N = 126 waiting-point nuclei significantly, while they have only a small effect on the half-lives of the N = 50 and 82 r-process nuclei.

Isomeric low-lying states were identified and investigated in the 75 Cu nucleus. Two states at 61.8(5)-and 128.3(7)-keV excitation energies with half-lives of 370(40)-and 170(15)-ns were assigned as 75m1 Cu and 75m2 Cu, respectively. The measured half-lives combined with the recent spin assignment of the ground state allow one to deduce tentatively spin and parity of the two isomers and the dominant multipolarities of the isomeric transitions with respect to the systematics of the Cu isotopes. Shell-model calculations using an up-to-date effective interaction reproduce the evolution of the 1/2 − , 3/2 − , and 5/2 − states for the neutron-rich odd-mass Cu isotopes when filling the νg 9/2 . The results indicate a significant change in the nuclear structure in this region, where a single-particle 5/2 − state coexists with more and more collective 3/2 − and 1/2 − levels at low excitation energies.

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.

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A low-energy enhancement of radiative strength functions was deduced from recent experiments in several mass regions of nuclei, which is believed to impact considerably the calculated neutron capture rates. In this Letter we investigate the behavior of the low-energy γ-ray strength of the ^{44}Sc isotope, for the first time taking into account both electric and magnetic dipole contributions obtained coherently in the same theoretical approach. The calculations are performed using the large-scale shell-model framework in a full 1ℏω  sd-pf-gds model space. Our results corroborate previous theoretical findings for the low-energy enhancement of the M1 strength but show quite different behavior for the E1 strength.

We present a quantitative study of the role played by different components characterizing the nucleon-nucleon interaction in the evolution of the nuclear shell structure. It is based on the spintensor decomposition of an effective two-body shell-model interaction and the subsequent study of effective single-particle energy variations in a series of isotopes or isotones. The technique allows to separate unambiguously contributions of the central, vector and tensor components of the realistic effective interaction. We show that while the global variation of the single-particle energies is due to the central component of the effective interaction, the characteristic behavior of spin-orbit partners, noticed recently, is mainly due to its tensor part. Based on the analysis of a well-fitted realistic interaction in sdpf shell model space, we analyze in detail the role played by the different terms in the formation and/or disappearance of N = 16, N = 20 and N = 28 shell gaps in neutron-rich nuclei. The shell structure is a common feature of finite quantum systems. Amongst them, atomic nuclei represent unique objects characterized by the appearance of a specific shell structure. In particular, the magic numbers which correspond to the shell closures, will change depending on the N/Z ratio, i.e. when we move from nuclei in the vicinity of the β-stability line towards the particle driplines. This has attracted a lot of attention nowadays because an increasing number of nuclei far from stability have become accessible experimentally (e.g., [1] and references therein). The hope to reach even more exotic nuclei demands for an improved modelization, i.e. in the context of nuclear astrophysics. Since the underlying shell structure determines nuclear properties in a major way, changes of nuclear shell closures and the mechanisms responsible for that should be much better understood.Recently, the role of different components of the nucleon-nucleon (NN) interaction in the evolution of the shell structure has been actively discussed. Based on the analysis of the origin of a shell closure at N = 16, Otsuka et al [2] have suggested that a central spin-isospinexchange term, f (r)( σ · σ)( τ · τ ) of the NN interaction plays a decisive role in the shell formation.However, from a systematic analysis of heavier nuclei, another conjecture has been put forward, namely, the dominant role played by the tensor force [3]. The evidence is based on the comparison of the position of experimental one-particle or one-hole states in nuclei adjacent to semi-magic configurations with the so-called effective single-particle energies (ESPE's). Within the shell-model framework, the latter ESPE's are defined [4] as one-nucleon separation energies for an occupied orbital (or extra binding gained by addition of a nucleon to an unoccupied orbital) evaluated from a Hamiltonian containing nucleon single-particle energies (the bare single-particle energies with respect to a closed-shell core) plus the monopole part of the two-body residual interaction [5, 6], ...

Recent progress in experimental techniques allows us to study very exotic systems like neutron-rich nuclei in the vicinity of 78 Ni. The spectroscopy of this region can nowadays be studied theoretically in the large scale shell model calculations. In this work, we perform a shell model study of odd copper nuclei with N = 40-50, in a large valence space with the 48 Ca core, using a realistic interaction derived from the CD-Bonn potential. We present the crucial importance of the proton core excitations for the description of spectra and magnetic moments, which are for the first time correctly reproduced in theoretical calculations. Shell evolution from 68 Ni to 78 Ni is discussed in detail. A weakening of the Z = 28 gap when approaching the N = 50 shell closure, suggested by the experimental evidence, is confirmed in the calculations.The region of nuclei toward the 78 Ni is nowadays one of the most extensively studied in both experiment and theory. The doubly closed 78 Ni, with an unusual proton to neutron ratio, lies between two regions: of the light nuclei, where experimental evidence for changing magic numbers far from stability is well established [1], and of heavy ones, where no quenching of the known spin-orbit shell closures has been so far observed. The weakening of the Z = 28 gap when approaching the N = 50 shell closure has been suggested, e.gfor example, by Otsuka in Ref.[2] due to the proton-neutron part of the nuclear force, which has an opposite action on protons in the f 7/2 and f 5/2 orbitals while filling the neutron g 9/2 shell. Previous shell model (SM) calculations [3,4] have also revealed the possibility of a weakening of this shell closure.A related subject of a great experimental interest is the so-called monopole migration in the copper isotopes and the competition of single-particle and collective modes at low excitation energies [5][6][7]. A decade ago a sudden drop of the 5/2 − level in 73,71 Cu was observed [8], giving a hint that this state may become the ground state of 75 Cu. Such a scenario may be expected concerning a strong attractive monopole interaction between protons and neutrons occupying the f 5/2 and g 9/2 orbitals, respectively. The recent measurements of magnetic moments in the copper chain [7] established experimentally the inversion of 5/2 − and 3/2 − levels in 75 Cu. In the same work, the authors pointed out that, however in the available shell model calculations this inversion is present between 73 Cu and 79 Cu [9-11], it is not followed by a rapid lowering of the first excited 1/2 − level observed in experiment. Therefore, some important physics mechanism is either omitted or underestimated in the recently developed shell model interactions.In this work we show that the Z = 28 proton gap is eroded in neutron-rich nuclei, thus the physics part which is missing in all previous shell model calculations is not related to the interaction itself, but rather to the lack of proton core excitations excluded in valence spaces based on the 56 Ni core used up to now in SM calcul...