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 ...

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Isotope shifts and hyperfine splitting parameters have been measured for the neutron-deficient odd-mass lead isotopes [183][184][185][186][187][188][189] Pb. The measurement was performed at the ISOLDE (CERN) online mass separator using the in-source resonance ionization spectroscopy technique. The nuclear root mean square charge radii r 2 and the electromagnetic moments μ and QS have been deduced. They follow the smooth trend of the heavier isotopes and indicate the absence of deformation.