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The shape of exotic even-mass [182][183][184][185][186][187][188][189][190] Pb isotopes was probed by measurement of optical isotope shifts providing mean square charge radii (hr 2 i). The experiment was carried out at the ISOLDE (CERN) on-line mass separator, using in-source laser spectroscopy. Small deviations from the spherical droplet model are observed, but when compared to model calculations, those are explained by high sensitivity of hr 2 i to beyond mean-field correlations and small admixtures of intruder configurations in the ground state. The data support the predominantly spherical shape of the ground state of the proton-magic Z 82 lead isotopes near neutron midshell (N 104). The subtle interplay between individual and collective behavior of a finite number of strongly interacting fermions leads to aspects of mesoscopic systems that can only be studied in atomic nuclei [1]. For neutron-deficient nuclides around the closed proton shell at Z 82, this interplay leads to the appearance of states with different shapes at low excitation energy. These so-called shape coexisting states can be interpreted as particle-hole excitations across the closed proton shell gap [2] whereby the interaction of the valence proton particles and holes with the neutrons drives the nucleus into deformation. The phenomenon of shape-coexistence is subject to intensive experimental and theoretical studies [3,4]. Alpha-decay experiments have revealed a triplet of low-lying 0 states in the 186 Pb nucleus, which is located at neutron midshell between N 82 and 126 [1]. Excited bands built on top of the 0 states were observed in [182][183][184][185][186][187][188][189][190], and recent lifetime measurements confirmed the deformed character of the bands [11]. For 186 188 Pb, it was concluded that the ground state and the 2 1 state have a very different structure, the 0 ground state of predominantly spherical and the 2 1 state of predominantly prolate character. Monopole transition strengths between the 0 states were used to estimate the mixing between the normal and intruder configuration [12] and revealed limited configuration mixing in the 190;192;194 Pb ground-state wave function [13,14]. But as the excited 0 states become lower in energy when approaching N 104 ( 186 Pb), the mixing could increase substantially.Several theoretical models have been applied to describe the structure of the neutron-deficient lead isotopes with their coexisting and mixed spherical, prolate, and oblate states, such as phenomenological shape mixing calculations [13,14], symmetry guided shell model and interacting boson model truncations [15,16], and beyond mean-field approaches [4,17,18]. All models that provide a consistent picture of the available data suggest that the ground state of lead isotopes is dominated by spherical configurations, even when the prolate and oblate rotational bands come down very low in energy around N 104, and the barrier that separates the corresponding structures in the total energy surface is very small. But all models also ...

A new high-precision (p,t) study of the 158 Gd nucleus was carried out with the Q3D spectrometer at the University of Munich. The result is the observation for the first time of a deformed nucleus with 13 excited 0 ϩ states below an excitation energy of approximately 3.1 MeV. Seven of these 0 ϩ states are observed for the first time and an additional three are new confirmations of previous tentative assignments. This abundance of 0 ϩ states provides significant new information on these poorly understood excitations. 158 Gd can now be viewed as a unique laboratory for further investigations on the nature of 0 ϩ excitations in nuclei.The nature of low lying K ϭ0 ϩ bands in deformed nuclei remains a mystery. Traditionally the first excited K ϭ0 ϩ bands along with the K ϭ2 ϩ bands were labeled as single-phonon ''␤'' and ''␥'' vibrational excitations. The K ϭ2 ϩ excitations are well understood theoretically and shown to vary smoothly in collectivity across a given isotopic chain ͓1-3͔. The nature of K ϭ0 ϩ excitations however still remains enigmatic and therefore the focus of intense discussions as well as a flurry of activity from both theoretical and experimental aspects. Data on K ϭ0 ϩ bands have traditionally been relatively sparse. However, recent improvements in technology have remedied the situation by enabling spectroscopy, reaction, and lifetime measurements of a large number of K ϭ0 ϩ bands that were previously inaccessible in nuclei.The results are puzzling at best. First, in many deformed nuclei of the rare-earth region, there are several excited K ϭ0 ϩ bands below the pairing gap. Second, there are variations in collectivity amongst the K ϭ0 ϩ bands in the same nucleus, as well as enormous variations in collectivity of the first excited K ϭ0 ϩ bands in very narrow isotopic regions.Numerous attempts have been made to address the nature of low-lying K ϭ0 ϩ bands via various nuclear models. The IBM and the newly developed critical point symmetries ͓4 -6͔ or partial dyamical symmetries ͓7,8͔ are amongst the newest approaches as well as the more traditional QPNM ͓9͔ ͑quasi-particle-phonon nuclear model͒.The microscopic calculations of Soloviev et al. within the QPNM ͓9͔ yield a Hamiltonian of phonons, quasiparticles ͑qp͒, and phonon-qp interactions. Calculations have been done for several rare-earth deformed nuclei and in each case, the result is a spectrum that typically includes five excited K ϭ0 ϩ bands below 2.3 MeV. The exact nature of these K ϭ0 ϩ bands then depends on the number of phonons and qp pairs included.A review of existing data and a discussion of several possible interpretations is given in Ref. ͓10͔. Suffice it to say that K ϭ0 ϩ bands are one of the fundamental excitations in nuclear spectra and their nature is not yet fully understood. The numerous recent publications addressing this subject point to the immense current interest on this topic. In order to carry out a meaningful discussion or a comprehensive theoretical effort to understand the nature of these K ϭ0 ϩ bands, it is first nece...

In recent experiments at the velocity filter Separator for Heavy Ion reaction Products (SHIP) (GSI, Darmstadt), an extended and improved set of α-decay data for more than 20 of the most neutron-deficient isotopes in the region from lead to thorium was obtained. The combined analysis of this newly available α-decay data, of which the (186)Po decay is reported here, allowed us for the first time to clearly show that crossing the Z = 82 shell to higher proton numbers strongly accelerates the α decay. From the experimental data, the α-particle formation probabilities are deduced following the Universal Decay Law approach. The formation probabilities are discussed in the framework of the pairing force acting among the protons and the neutrons forming the α particle. A striking resemblance between the phenomenological pairing gap deduced from experimental binding energies and the formation probabilities is noted. These findings support the conjecture that both the N = 126 and Z = 82 shell closures strongly influence the α-formation probability.

Excited low-spin states of 92 Zr have been studied with the (n,n γ ) reaction. Comprehensive data on the electromagnetic decay of states with excitation energies up to about 3.8 MeV in particular, lifetimes, γ -ray branching ratios, multipole mixing ratios, and absolute transition strengths have been obtained. The detailed spectroscopic information about the low-spin level scheme enables us to address the predominant proton-neutron symmetry for low-spin states of 92 Zr. These data are compared to those of corresponding states in the N = 52 isotone 94 Mo and to a shell model calculation using 88 Sr as an inert core. However, neither a purely collective picture nor the restricted shell model calculation yields a fully satisfactory description of the observed structures. [1][2][3][4]. In neighboring 96 Ru 52 , the 2 + 1,ms state was found [5,6], and candidates for two-phonon MS states were assigned from E2/M1 mixing ratios, branching ratios, and lifetime limits [6].In vibrational nuclei, signatures of MS states, accessible through γ -ray spectroscopy at rather low excitation energies, are strong M1 transitions to symmetric states with the same phonon number with matrix elements of about | J f sym M1 J i ms | ≈ 1µ N , and weakly collective E2 transitions to symmetric states, since the latter transitions stem from the annihilation of a MS phonon Q ms . In contrast, we expect collective E2 transitions with transition strengths of several Weisskopf units between states with the same proton-neutron symmetry, e.g., from the MS two-phonon states to the 2 + 1,ms state from the annihilation of a symmetric phonon Q s . 0556-2813/2005/71(5)/054304(15)/$23.00 054304-1