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Combining laser spectroscopy in a Versatile Arc Discharge and Laser Ion Source, with Penning-trap mass spectrometry at the CERN-ISOLDE facility, this work reports on mean-square charge radii of neutron-rich mercury isotopes across the $N = 126$ shell closure, the electromagnetic moments of $^{207}$Hg and more precise mass values of $^{206-208}$Hg. The odd-even staggering (OES) of the mean square charge radii and the kink at $N = 126$ are analyzed within the framework of covariant density functional theory (CDFT), with comparisons between different functionals to investigate the dependence of the results on the underlying single-particle structure. The observed features are defined predominantly in the particle-hole channel in CDFT, since both are present in the calculations without pairing. However, the magnitude of the kink is still affected by the occupation of the $1i_{11/2}$ and $2g_{9/2}$ orbitals with a dependence on the relative energies as well as pairing., 20 pages, 12 figures, Phys. Rev. C in press

The in-gas-jet laser spectroscopy method relies on the production of uniform and low-temperature gas jets to fully resolve the atomic hyperfine structure and efficiently determine fundamental nuclear properties of short-lived isotopes from, e.g., the hardly accessible actinide and transactinide elements. In this article we present the studies devoted to designing, producing, and characterizing the flow properties of a convergent-divergent (de Laval) hypersonic nozzle with a superior performance for laser spectroscopy applications. A novel flow mapping technique, based on resonance ionization spectroscopy (RIS), has been employed to characterize the local flow properties of an argon gas jet formed by this nozzle, revealing a 61.5-mm long, highly collimated atomic jet at a uniform low temperature of 16.6(5) K [Mach 8.11(12)] that will enable laser spectroscopy experiments on heavy-exotic nuclei with an unprecedented spectral resolution and a high efficiency. These results have been compared with those obtained by planar laser induced fluorescence spectroscopy (PLIFS) studies and show a good agreement between the two techniques and a significant improvement in efficiency of the RIS mapping method with respect to PLIFS. The data are compared to state-of-the-art fluid-dynamics calculations that were carried out to obtain the nozzle contour and simulate its performance, as well as to explain the observation of a possible onset of argon nucleation.

© 2019 Elsevier B.V. Radio-frequency (RF) ion guides, also known as Linear Paul Traps, are powerful devices to efficiently transport ion beams from high to low pressure regions while keeping good ion optical properties. A set of ion guides comprising three different RF quadrupole (RFQ) structures has been designed using ion-trajectory simulations to improve the performance of the In-Gas Laser Ionizations and Spectroscopy (IGLIS) technique currently under development at KU Leuven. Results of the commissioning tests for the total transport efficiency and transient time through the ion guides as well as the longitudinal energy spread and transverse emittance are found to be in agreement with ion trajectory simulations considering a realistic ion-atom interaction potential. ispartof: Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms vol:463 pages:148-153 ispartof: location:SWITZERLAND, CERN, Geneva status: published

A new automated control system is developed for the In-Gas Laser Ionization and Spectroscopy (IGLIS) laboratory at KU Leuven. The IGLIS Control System is capable of stabilizing a narrowband single-mode tunable diode laser with a standard deviation of 1.14 MHz. Furthermore, the system controls and synchronizes all data acquisition for multiple techniques from resonant laser spectroscopy in gas cell or in gas jet to atomic Planar-Laser Induced Fluorescence (PLIF) spectroscopy of copper atoms seeded in a supersonic gas jet. The IGLIS Control System is validated by measuring the hyperfine splitting parameters of the ground state transition at 327 nm in 63Cu. A new automated control system is developed for the In-Gas Laser Ionization and Spectroscopy (IGLIS) laboratory at KU Leuven. The IGLIS Control System is capable of stabilizing a narrowband single-mode tunable diode laser with a standard deviation of 1.14 MHz. Furthermore, the system controls and synchronizes all data acquisition for multiple techniques from resonant laser spectroscopy in gas cell or in gas jet to atomic Planar-Laser Induced Fluorescence (PLIF) spectroscopy of copper atoms seeded in a supersonic gas jet. The IGLIS Control System is validated by measuring the hyperfine splitting parameters of the ground state transition at 327 nm in 63Cu. ispartof: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms vol:463 pages:297-301 ispartof: location:SWITZERLAND, CERN, Geneva status: published

A study of the laser-ionized and mass-separated neutron-deficient isotopes Au 176 , 177 , 179 was performed using the Resonance Ionization Laser Ion Source and the Windmill detection setup at ISOLDE, CERN. New and improved data on complex fine-structure α decays of the three isotopes were deduced, providing insight into the low-lying levels in the daughter nuclei Ir 172 , 173 , 175 . New information on the properties of β -decay daughter products Pt 177 , 179 was also obtained. From the first in-source laser spectroscopy measurements of the hyperfine structure in the atomic 267.6-nm transition of Au 176 , the nuclear magnetic moments for both high- and low-spin α -decaying states were deduced. Together with the values determined from the additivity relations, they were used to propose the most probable spins and configurations for both states. The α -decay branching ratios were determined as b α ( Au ls 176 ) = 58 ( 5 ) % and b α ( Au hs 176 ) = 29 ( 5 ) % .

The mean-square charge radii of $^{207,208}$Hg ($Z=80, N=127,128$) have been studied for the first time and those of $^{202,203,206}$Hg ($N=122,123,126$) remeasured by the application of in-source resonance-ionization laser spectroscopy at ISOLDE (CERN). The characteristic \textit{kink} in the charge radii at the $N=126$ neutron shell closure has been revealed, providing the first information on its behavior below the $Z=82$ proton shell closure. A theoretical analysis has been performed within relativistic Hartree-Bogoliubov and non-relativistic Hartree-Fock-Bogoliubov approaches, considering both the new mercury results and existing lead data. Contrary to previous interpretations, it is demonstrated that both the kink at $N=126$ and the odd-even staggering (OES) in its vicinity can be described predominately at the mean-field level, and that pairing does not need to play a crucial role in their origin. A new OES mechanism is suggested, related to the staggering in the occupation of the different neutron orbitals in odd- and even-$A$ nuclei, facilitated by particle-vibration coupling for odd-$A$ nuclei.

A study of the ground states of the laser-ionized and mass-separated odd-odd isotopes 180,182Au was performed using the Resonance Ionization Laser Ion Source, Windmill detection setup and ISOLTRAP Multi-Reflection Time-of-Flight Mass Spectrometer at ISOLDE, CERN. A complex fine-structure α-decay pattern of 180Au was deduced, providing insight into the low-lying levels in the daughter nucleus 176Ir. An α-decay branching ratio of bα(180Au)=0.58(10)% and a half-life of T1/2=7.2(5) s have also been derived, allowing for the calculation of the reduced α-decay widths and determining the degree of hindrance of respective α-decay branches. From complementary first in-source laser spectroscopy measurements of the hyperfine structure in atomic transitions of 180,182Au, the nuclear magnetic moments of μ(180Au)=−0.83(9)μN and μ(182Au)=1.66(9)μN were extracted with an inclusion of a correction for the hyperfine anomaly. Based on the observed hyperfine structure patterns, and on the comparison of the measured and calculated μ values, a preferred ground-state spin and parity Iπ(180Augs)=(1+) is proposed, and the earlier assignment of Iπ(182Augs)=(2+) is confirmed. For 180Au, the most probable proton-neutron Nilsson configuration of π3/2−[532]⊗ν5/2−[512] suggests the same proton state as in the heavier deformed odd-odd nuclei 182,184Au.

Hyperfine-structure constants for the 6s2S1/2 and 6p2P1/2 atomic states of the Iπ=11/2− gold isomers Aum177,191,193,195 have been measured at CERN-ISOLDE, using the in-source laser resonance-ionization spectroscopy technique. From the measured hyperfine constants the differences between hyperfine anomalies for these atomic states have been deduced. These differential hyperfine anomaly values have been used to determine the 6s-state hyperfine anomaly relative to the stable Au197 with advanced atomic calculations. Magnetic dipole moments for the gold isomers in question have been deduced, taking into account the corresponding relative hyperfine-anomaly values. It has been shown that the commonly used prescription for the extraction of the magnetic moment values for the gold isotopes should be reconsidered. The magnetic moments calculated by this prescription have been reevaluated by properly accounting for the hyperfine anomaly, which is as large as 10% for several gold isotopes.

© 2019 Elsevier B.V. Laser spectroscopy enables the determination of fundamental atomic and nuclear properties with high precision. In view of the low production rates of the heaviest elements, a high total efficiency is a crucial requirement for any experimental setup to be used in on-line experiments. The setup requires the use of gas stopping techniques to slow down the radionuclides of interest. In previous studies laser spectroscopy was performed inside a gas-filled stopping cell with a limited spectral resolution of a few GHz. Collisional broadening inside stopping cells ultimately limits the precision of laser spectroscopic studies and hampers in particular hyperfine spectroscopy. The spectral linewidth is reduced by an order of magnitude when the laser spectroscopy is performed in a well-collimated gas jet formed by the exit nozzle of a gas stopping cell. In addition, the exposure of the jet to high-repetition rate laser light which saturates the optical transitions allows maintaining a high total efficiency. Here, we present a new setup dedicated to laser spectroscopy of the heaviest elements with an improved resolution, which is presently under construction. This setup combines the efficient filament neutralization demonstrated for nobelium with the improved resolution of in-gas-jet spectroscopy. ispartof: Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms vol:463 pages:272-276 ispartof: location:SWITZERLAND, CERN, Geneva status: published

A comprehensive study of the isotope 178Au has been made at the CERN-ISOLDE facility, using resonance laser ionization. Two long-lived states in 178Au were identified—a low-spin ground state and a high-spin isomer—each of which were produced as pure beams. Using the ISOLTRAP precision Penning trap, the excitation energy of the isomeric state in 178Au was determined to be E∗=189(14)keV. The α-decay fine structure patterns of the two states were studied using the Windmill decay station, providing information on the low-lying states in the daughter nucleus 174Ir. Nuclear spin assignments of I(178Aug)=(2,3) and I(178Aum)=(7,8) are made based on the observed β-decay feeding and hyperfine structure intensity patterns. These spin assignments are used for fitting the hyperfine structures of the two states from which values for the magnetic dipole moments are extracted. The extracted moments are compared with calculations using additivity relations to establish the most probable configurations for 178Aug,m.

In-source resonance ionization spectroscopy of the neutron-rich actinium isotopes $^{225\ensuremath{-}229}\mathrm{Ac}$ has been performed at the ISAC facility in TRIUMF, probing a $^{2}D_{3/2}\ensuremath{\rightarrow}^{4}P_{5/2}^{\ensuremath{\circ}}$ atomic transition. New data on the magnetic dipole moments and changes in mean-square charge radii $\ensuremath{\delta}\ensuremath{\langle}{r}^{2}\ensuremath{\rangle}$ of $^{226,228,229}\mathrm{Ac}$ have been obtained. The comparison of the measured isotope shifts and magnetic dipole coupling constants $a(^{4}P_{5/2}^{\ensuremath{\circ}})$ of $^{225,227}\mathrm{Ac}$ with a high-resolution data set is used to identify systematic uncertainties on the deduced $\ensuremath{\delta}{\ensuremath{\langle}{r}^{2}\ensuremath{\rangle}}^{A,215}$ and magnetic dipole moment values. The charge radii odd-even staggering is evaluated for the odd-$N$ isotopes, showing that $^{226}\mathrm{Ac}$ has an inverted odd-even staggering that might be linked with a reflection-asymmetric shape. Comparison of the magnetic dipole moments of $^{225,227,229}\mathrm{Ac}$ with Nilsson-model estimates supports the assumption of octupole deformation in isotopes $^{225,227}\mathrm{Ac}$ and its gradual decrease toward isotope $^{229}\mathrm{Ac}$. The changes in mean-square charge radii are compared to self-consistent calculations employing multiple modern energy density functionals: SLy5s1, BSk31, and DD-MEB1. For SLy5s1 in particular, self-consistent time-reversal breaking calculations of odd-odd nuclei incorporating finite octupole deformation are reported for the first time. For these calculations, the overall best agreement is obtained when the octupole degree of freedom is taken into account.

Neutron-deficient Hg177-185 isotopes were studied using in-source laser resonance-ionization spectroscopy at the CERN-ISOLDE radioactive ion-beam facility in an experiment combining different detection methods tailored to the studied isotopes. These include either α-decay tagging or multireflection time-of-flight gating for isotope identification. The endpoint of the odd-even nuclear shape staggering in mercury was observed directly by measuring for the first time the isotope shifts and hyperfine structures of Hg177-180. Changes in the mean-square charge radii for all mentioned isotopes, magnetic dipole, and electric quadrupole moments of the odd-A isotopes and arguments in favor of I=7/2 spin assignment for Hg177,179 were deduced. Experimental results are compared with density functional theory (DFT) and Monte Carlo shell model (MCSM) calculations. DFT calculations using Skyrme parametrizations predict a jump in the charge radius around the neutron N=104 midshell, with an odd-even staggering pattern related to the coexistence of nearly degenerate oblate and prolate minima. This near-degeneracy is highly sensitive to many aspects of the effective interaction, a fact that renders perfect agreement with experiments out of reach for current functionals. Despite this inherent difficulty, the SLy5s1 and a modified UNEDF1SO parametrization predict a qualitatively correct staggering that is off by two neutron numbers. MCSM calculations of states with the experimental spins and parities show good agreement for both electromagnetic moments and the observed charge radii. A clear mechanism for the origin of shape staggering within this context is identified: A substantial change in occupancy of the proton πh9/2 and neutron νi13/2 orbitals., SCOPUS: ar.j, info:eu-repo/semantics/published

A new approach to observe the radiative decay of the $^{229}$Th nuclear isomer, and to determine its energy and radiative lifetime, is presented. Situated at a uniquely low excitation energy, this nuclear state might be a key ingredient for the development of a nuclear clock, a nuclear laser and the search for time variations of the fundamental constants. The isomer's $\gamma$ decay towards the ground state will be studied with a high-resolution VUV spectrometer after its production by the $\beta$ decay of $^{229}$Ac. The novel production method presents a number of advantages asserting its competitive nature with respect to the commonly used $^{233}$U $\alpha$-decay recoil source. In this paper, a feasibility analysis of this new concept, and an experimental investigation of its key ingredients, using a pure $^{229}$Ac ion beam produced at the ISOLDE radioactive beam facility, is reported., Comment: 11 pages, 5 figures, 2 tables

In rare cases, the removal of a single proton (Z) or neutron (N) from an atomic nucleus leads to a dramatic shape change. These instances are crucial for understanding the components of the nuclear interactions that drive deformation. The mercury isotopes (Z = 80) are a striking example1,2: their close neighbours, the lead isotopes (Z = 82), are spherical and steadily shrink with decreasing N. The even-mass (A = N + Z) mercury isotopes follow this trend. The odd-mass mercury isotopes 181,183,185Hg, however, exhibit noticeably larger charge radii. Due to the experimental difficulties of probing extremely neutron-deficient systems, and the computational complexity of modelling such heavy nuclides, the microscopic origin of this unique shape staggering has remained unclear. Here, by applying resonance ionization spectroscopy, mass spectrometry and nuclear spectroscopy as far as 177Hg, we determine 181Hg as the shape-staggering endpoint. By combining our experimental measurements with Monte Carlo shell model calculations, we conclude that this phenomenon results from the interplay between monopole and quadrupole interactions driving a quantum phase transition, for which we identify the participating orbitals. Although shape staggering in the mercury isotopes is a unique and localized feature in the nuclear chart, it nicely illustrates the concurrence of single-particle and collective degrees of freedom at play in atomic nuclei. In rare cases, the removal of a single proton (Z) or neutron (N) from an atomic nucleus leads to a dramatic shape change. These instances are crucial for understanding the components of the nuclear interactions that drive deformation. The mercury isotopes (Z = 80) are a striking example1,2: their close neighbours, the lead isotopes (Z = 82), are spherical and steadily shrink with decreasing N. The even-mass (A = N + Z) mercury isotopes follow this trend. The odd-mass mercury isotopes 181,183,185Hg, however, exhibit noticeably larger charge radii. Due to the experimental difficulties of probing extremely neutron-deficient systems, and the computational complexity of modelling such heavy nuclides, the microscopic origin of this unique shape staggering has remained unclear. Here, by applying resonance ionization spectroscopy, mass spectrometry and nuclear spectroscopy as far as 177Hg, we determine 181Hg as the shape-staggering endpoint. By combining our experimental measurements with Monte Carlo shell model calculations, we conclude that this phenomenon results from the interplay between monopole and quadrupole interactions driving a quantum phase transition, for which we identify the participating orbitals. Although shape staggering in the mercury isotopes is a unique and localized feature in the nuclear chart, it nicely illustrates the concurrence of single-particle and collective degrees of freedom at play in atomic nuclei. ispartof: Nature Physics vol:14 issue:12 status: published

© 2018 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the »https://creativecommons.org/licenses/by/4.0/» Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. The method of laser spectroscopy in supersonic gas jets was proposed for high-resolution and high-efficiency in-gas laser ionization and spectroscopy studies of short-lived nuclei. The flow properties of such supersonic gas jets have been characterized under off-line conditions. Planar laser-induced fluorescence spectroscopy of seeded copper atoms has been applied to nonintrusively measure velocity, temperature, and relative density profiles of gas jets formed by different de Laval nozzles. For validation, planar laser-induced fluorescence spectroscopy was applied on supersonic free jets with well-known flow parameters. The performance of the in-gas-jet laser spectroscopy method is determined by the achievable spectral resolution, which requires an optimization and a precise manufacturing of the nozzle inner contour as well as a pressure matching of the background medium at the nozzle exit. Our studies now enable a thorough understanding and quantification of these requirements and a determination of the final performance of the in-gas-jet method. Additionally, a comparison between the experimental results and the numerical calculations was performed for the temperature, velocity, and Mach number profiles of underexpanded and quasiuniform jets formed by a de Laval nozzle. ispartof: Physical Review X vol:8 issue:4 pages:041008-041008 status: published

Hyperfine-structure parameters and isotope shift of the 9/2$^−$ isomeric state in $^{187}$Au relative to $^{197}$Au for the 267.6-nm atomic transition have been measured for the first time using the in-source resonance-ionization spectroscopy technique. The magnetic dipole moment and change in the mean-square charge radius for this 9/2$^−$ isomer have been deduced. The observed large isomer shift relative to the 1/2$^+$ ground state in $^{187}$Au confirms the occurrence of the shape coexistence in $^{187}$Au proposed earlier from the analysis of the nuclear spectroscopic data and particle plus triaxial rotor calculations. The analysis of the magnetic moment supports the previously proposed 9/2$^−$, 1/2$^−$[541] assignment at moderate prolate deformation for $^{187}$Au$^m$.