The ''island of inversion'' nucleus 32 Mg has been studied by a (t, p) two neutron transfer reaction in inverse kinematics at REX-ISOLDE. The shape coexistent excited 0 þ state in 32 Mg has been identified by the characteristic angular distribution of the protons of the ÁL ¼ 0 transfer. The excitation energy of 1058 keV is much lower than predicted by any theoretical model. The low-ray intensity observed for the decay of this 0 þ state indicates a lifetime of more than 10 ns. Deduced spectroscopic amplitudes are compared with occupation numbers from shell-model calculations. The evolution of shell structure in exotic nuclei as a function of the proton (Z) and neutron (N) number is currently at the center of many theoretical and experimental investigations [1,2]. It has been realized that the interaction of the last valence protons and neutrons, in particular, the monopole component of the residual interaction between those nucleons, can lead to significant shifts in the single-particle energies, leading to the disappearance of classic shell closures and the appearance of new shell gaps [3]. A prominent example is the collapse of the N ¼ 20 shell gap in the neutron-rich oxygen isotopes where instead a new magic shell gap appears for 24 O at N ¼ 16 [4,5]. Recent work showed that the disappearance of the N ¼ 20 shell can be attributed to the monopole effect of the tensor force [3,6,7]. The reduced strength of the attractive interaction between the proton d 5=2 and the neutron d 3=2 orbitals causes the d 3=2 orbital to rise in energy and come closer to the f 7=2 orbital. In regions without pronounced shell closures correlations between the valence nucleons may become as large as the spacing of the single-particle energies. This can thus lead to particle-hole excitations to higher-lying single-particle states enabling deformed configurations to be lowered in energy. This may result in low-lying collective excitations, the coexistence of different shapes at low energies or even the deformation of the ground state for nuclei with the conventional magic number N ¼ 20. Such an effect occurs in the ''island of inversion'', one of most studied regions of exotic nuclei in the nuclear chart. In this region of neutron-rich nuclei around the magic number N ¼ 20 strongly deformed ground states in Ne, Na, and Mg isotopes have been observed [8-11]. Because of the reduction of the N ¼ 20 shell gap, quadrupole correlations can enable low-lying deformed 2p-2h intruder states from the fp shell to compete with spherical normal neutron 0p-0h states of the sd shell. In this situation the promotion of a neutron pair across the N ¼ 20 gap can result in deformed intruder ground states. Consequentially, the competition of two configurations can lead to the coexistence of spherical and deformed 0 þ states in the neutron-rich 30;32 Mg nuclei [12]. Coulomb excitation experiments have shown that 30 Mg has a rather small BðE2Þ value for the 0 þ gs ! 2 þ 1 transition [13,14] placing this nucleus outside the island of inversion. The excited deform...

Article:Bree, N., Wrzosek-Lipska, K., Petts, A. et al. (67 more authors) (2014) Shape coexistence in the neutron-deficient even-even 182-188Hg isotopes studied via Coulomb excitation.

No abstract

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,

3The general phenomenon of shell structure in atomic nuclei has been understood since the pioneering work of Goeppert-Mayer, Haxel, Jensen and Suess [1].They realized that the experimental evidence for nuclear magic numbers could be explained by introducing a strong spin-orbit interaction in the nuclear shell model potential.However, our detailed knowledge of nuclear forces and the mechanisms governing the structure of nuclei, in particular far from stability, is still incomplete. In nuclei with equal neutron and proton numbers (N = Z), the unique nature of the atomic nucleus as an object composed of two distinct types of fermions can be expressed as enhanced correlations arising between neutrons and protons occupying orbitals with the same quantum numbers. Such correlations have been predicted to favor a new type of nuclear superfluidity; isoscalar neutron-proton pairing [2][3][4][5][6], in addition to normal isovector pairing (see Fig. 1). Despite many experimental efforts these predictions have not been confirmed. Here, we report on the first observation of excited states in N = Z = 46 nucleus 92 Pd. Gamma rays emitted following the 58 Ni( 36 Ar,2n) 92 Pd fusionevaporation reaction were identified using a combination of state-of-the-art highresolution -ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutronproton coupling scheme, different from the previous prediction [2][3][4][5][6]. We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling [7,8]) in the ground and low-lying excited states of the heaviest N = Z nuclei. The strong isoscalar neutron-proton correlations in these N = Z nuclei are predicted to have a considerable impact on their level structures, and to influence the dynamics of the stellar rapid proton capture nucleosynthesis process.For all known nuclei, including those residing along the N = Z line up to around mass 80, a detailed analysis of their properties such as binding energies [9] and the spectroscopy of the excited states [10] strongly suggests that normal isovector (T = 1) pairing is dominant at low excitation energies. On the other hand, there are long standing predictions for a change in the heavier N = Z nuclei from a nuclear superfluid dominated by isovector pairing to a structure where isoscalar (T = 0) neutron-proton (np) pairing has a major influence as the mass number increases towards the exotic doubly magic nucleus 100 Sn [2-6], the heaviest N = Z nucleus to be bound. N = Z nuclei with mass number > 90 can only be produced in the laboratory with very low The two-neutron (2n) evaporation reaction channel following formation of the 94 Pd compound nucleus, leading to 92 Pd, was very weakly populated with a relative yield of less than 10 −5 of the total fusion cross section. Gamma rays from decays of excited states in 92 Pd were identified by comparing γ-ray spectra in coincidence with two emitted neutrons and no charged particles with γ-ray spectra in coincidence with oth...