The neutron-rich isotopes of cadmium up to the N ¼ 82 shell closure have been investigated by highresolution laser spectroscopy. Deep-uv excitation at 214.5 nm and radioactive-beam bunching provided the required experimental sensitivity. Long-lived isomers are observed in 127 Cd and 129 Cd for the first time. One essential feature of the spherical shell model is unambiguously confirmed by a linear increase of the 11=2 À quadrupole moments. Remarkably, this mechanism is found to act well beyond the h 11=2 shell. DOI: 10.1103/PhysRevLett.110.192501 PACS numbers: 21.10.Ky, 21.60.Cs, 31.15.aj, 32.10.Fn When first proposed, the nuclear shell model was largely justified on the basis of magnetic-dipole properties of nuclei [1]. The electric quadrupole moment could have provided an even more stringent test of the model, as it has a very characteristic linear behavior with respect to the number of valence nucleons [2,3]. However, the scarcity of experimental quadrupole moments at the time did not permit such studies. Nowadays, regardless of experimental challenges, the main difficulty is to predict which nuclei are likely to display this linear signature. The isotopes of cadmium, investigated here, proved to be the most revealing case so far. Furthermore, being in the neighborhood of the ''magic'' tin, cadmium is of general interest for at least two additional reasons. First, theory relies on nuclei near closed shells for predicting other, more complex systems. Second, our understanding of stellar nucleosynthesis strongly depends on the current knowledge of nuclear properties in the vicinity of the doubly magic tin isotopes [4]. Moreover, specific questions concerning the nuclear structure of the cadmium isotopes require critical evaluation, such as shell quenching [5,6], sphericity [7], deformation [8,9], or whether vibrational nuclei exist at all [10]. Some of these points will be addressed here quite transparently, while others require dedicated theoretical work to corroborate our conclusions. In this Letter we report advanced measurements by collinear laser spectroscopy on the very neutron-rich cadmium isotopes. Electromagnetic moments in these complex nuclei are found to behave in an extremely predictable manner. Yet, their description goes beyond conventional interpretation of the nuclear shell model.The measurements were carried out with the collinear laser spectroscopy setup at ISOLDE-CERN. High-energy protons impinging on a tungsten rod produced low-to medium-energy neutrons inducing fission in a uranium carbide target. Proton-rich spallation products, such as cesium, were largely suppressed in this manner. Further reduction of surface-ionized isobaric contamination was achieved by the use of a quartz transfer line [11], which allowed the more volatile cadmium to diffuse out of the target while impurities were retained sufficiently long to decay. Cadmium atoms were laser ionized, accelerated to an energy of 30 keV, and mass separated. The ion beam was injected into a gas-filled radio-frequency Paul trap [12]...

Differences in mean-square nuclear charge radii of ^{100-130}Cd are extracted from high-resolution collinear laser spectroscopy of the 5s ^{2}S_{1/2}→5p ^{2}P_{3/2} transition of the ion and from the 5s5p ^{3}P_{2}→5s6s ^{3}S_{1} transition in atomic Cd. The radii show a smooth parabolic behavior on top of a linear trend and a regular odd-even staggering across the almost complete sdgh shell. They serve as a first test for a recently established new Fayans functional and show a remarkably good agreement in the trend as well as in the total nuclear charge radius.

The magnetic moments and isotope shifts of the neutron-deficient francium isotopes [202][203][204][205] Fr were measured at ISOLDE-CERN with use of collinear resonance ionization spectroscopy. A production-todetection efficiency of 1% was measured for 202 Fr. The background from nonresonant and collisional ionization was maintained below one ion in 10 5 beam particles. There are surprisingly few nuclear observables with which theorists can elucidate the nuclear force and interacting many-fermion problem. The laser spectroscopy technique reported here has measured two of these (the magnetic moment and mean-square charge radius) via the hyperfine interaction and isotope shift. This is achieved without introducing assumptions associated with any particular approach, making these measurements suitable for testing modern nuclear models. A variety of laser spectroscopy techniques now exists for studying shortlived radioactive isotopes, which broadly focus on either high resolution (< 100 MHz linewidth) or high sensitivity (< 1 atom=s) [1,2].We report here the first measurements of 202;203;205 Fr, reaching 11 neutrons from the N ¼ 126 shell closure. This has been made possible by a new highly sensitive, highresolution technique of bunched collinear-beam resonance ionization spectroscopy (CRIS). The CRIS technique combines for the first time velocity bunching (provided by the collinear geometry [3,4]) and time bunching (to eliminate the duty loss of required pulsed laser systems). The high sensitivity is reached through a combination of the excellent overlap of laser and beam, and the high quantum efficiency of ion detectors. This new technique may be applied generally to all nuclides, but it is at the limits of nuclear stability that it will manifest its particular advantages.The CRIS method was first proposed more than 30 years ago [5], and its sub-Doppler resolution was demonstrated by Schultz et al., who measured the radioactive isotopes of ytterbium [6]. Since these isotopes were produced as a continuous beam, the duty cycle losses associated with pulsed lasers introduced a loss in efficiency by a factor of 30, which contributed to a low total experimental efficiency of 0.001%. With the installation of a gas-filled radio frequency quadrupole ion trap (ISCOOL) at ISOLDE, bunched ion beams that match the duty cycle of the pulsed lasers are now available [7]. This motivated the development of a dedicated experiment to exploit the CRIS technique applied to time-bunched beams. In the present case it allowed for the first time measurements of the neutrondeficient francium isotopes with half-lives as short as 300 ms and production rates below 100 atoms=s.In our work the francium isotopes were produced through spallation reactions induced by 1.4 GeV protons, incident on a high temperature uranium carbide target (2000 C) and surface ionized (ionization potential 4.07 eV [8]) with use of a tantalum ionizer tube. A schematic of the experiment is presented in Fig. 1. The beam was accelerated to 50 keV, mass separated, and sub...

This paper reports on the hyperfine-structure and radioactive-decay studies of the neutron-deficient francium isotopes [202][203][204][205][206] Fr performed with the Collinear Resonance Ionization Spectroscopy (CRIS) experiment at the ISOLDE facility, CERN. The high resolution innate to collinear laser spectroscopy is combined with the high efficiency of ion detection to provide a highly sensitive technique to probe the hyperfine structure of exotic isotopes. The technique of decay-assisted laser spectroscopy is presented, whereby the isomeric ion beam is deflected to a decay-spectroscopy station for alpha-decay tagging of the hyperfine components. Here, we present the first hyperfine-structure measurements of the neutron-deficient francium isotopes [202][203][204][205][206] Fr, in addition to the identification of the low-lying states of 202;204 Fr performed at the CRIS experiment.

New technical developments have led to a 2 orders of magnitude improvement of the resolution of the collinear resonance ionization spectroscopy (CRIS) experiment at ISOLDE, CERN, without sacrificing the high efficiency of the CRIS technique. Experimental linewidths of 20(1) MHz were obtained on radioactive beams of francium, allowing us for the first time to determine the electric quadrupole moment of the short lived [t_{1/2}=22.0(5) ms] ^{219}Fr Q_{s}=-1.21(2) eb, which would not have been possible without the advantages offered by the new method. This method relies on a continuous-wave laser and an external Pockels cell to produce narrow-band light pulses, required to reach the high resolution in two-step resonance ionization. Exotic nuclei produced at rates of a few hundred ions/s can now be studied with high resolution, allowing detailed studies of the anchor points for nuclear theories.