Laser spectroscopy of cooled zirconium fission fragments

Impact of Nuclear Deformation and Pairing on the Charge Radii of Palladium Isotopes

The impact of nuclear deformation can been seen in the systematics of nuclear charge radii, with radii generally expanding with increasing deformation. In this Letter, we present a detailed analysis of the precise relationship between nuclear quadrupole deformation and the nuclear size. Our approach combines the first measurements of the changes in the mean-square charge radii of well-deformed palladium isotopes between A=98 and A=118 with nuclear density functional calculations using Fayans functionals, specifically Fy(std) and Fy(Δr,HFB), and the UNEDF2 functional. The changes in mean-square charge radii are extracted from collinear laser spectroscopy measurements on the 4d^{9}5s ^{3}D_{3}→4d^{9}5p ^{3}P_{2} atomic transition. The analysis of the Fayans functional calculations reveals a clear link between a good reproduction of the charge radii for the neutron-rich Pd isotopes and the overestimated odd-even staggering: Both aspects can be attributed to the strength of the pairing correlations in the particular functional which we employ.

Charge Radius of Neutron-Deficient ^{54}Ni and Symmetry Energy Constraints Using the Difference in Mirror Pair Charge Radii

The nuclear root-mean-square charge radius of ^{54}Ni was determined with collinear laser spectroscopy to be R(^{54}Ni)=3.737(3)  fm. In conjunction with the known radius of the mirror nucleus ^{54}Fe, the difference of the charge radii was extracted as ΔR_{ch}=0.049(4)  fm. Based on the correlation between ΔR_{ch} and the slope of the symmetry energy at nuclear saturation density (L), we deduced 21≤L≤88  MeV. The present result is consistent with the L from the binary neutron star merger GW170817, favoring a soft neutron matter EOS, and barely consistent with the PREX-2 result within 1σ error bands. Our result indicates the neutron-skin thickness of ^{48}Ca as 0.15-0.21 fm.

g Factor of the ^{99}Zr (7/2^{+}) Isomer: Monopole Evolution in the Shape-Coexisting Region

The gyromagnetic factor of the low-lying E=251.96(9)  keV isomeric state of the nucleus ^{99}Zr was measured using the time-dependent perturbed angular distribution technique. This level is assigned a spin and parity of J^{π}=7/2^{+}, with a half-life of T_{1/2}=336(5)  ns. The isomer was produced and spin aligned via the abrasion-fission of a ^{238}U primary beam at RIKEN RIBF. A magnetic moment |μ|=2.31(14)μ_{N} was deduced showing that this isomer is not single particle in nature. A comparison of the experimental values with interacting boson-fermion model IBFM-1 results shows that this state is strongly mixed with a main νd_{5/2} composition. Furthermore, it was found that monopole single-particle evolution changes significantly with the appearance of collective modes, likely due to type-II shell evolution.

Evidence for Coexisting Shapes through Lifetime Measurements in ^{98}Zr

The lifetimes of the first excited 2^{+}, 4^{+}, and 6^{+} states in ^{98}Zr were measured with the recoil-distance Doppler shift method in an experiment performed at GANIL. Excited states in ^{98}Zr were populated using the fission reaction between a 6.2  MeV/u ^{238}U beam and a ^{9}Be target. The γ rays were detected with the EXOGAM array in correlation with the fission fragments identified by mass and atomic number in the VAMOS++ spectrometer. Our result shows a very small B(E2;2_{1}^{+}→0_{1}^{+}) value in ^{98}Zr, thereby confirming the very sudden onset of collectivity at N=60. The experimental results are compared to large-scale Monte Carlo shell model and beyond-mean-field calculations. The present results indicate the coexistence of two additional deformed shapes in this nucleus along with the spherical ground state.

Charge Radii of Neutron Deficient ^{52,53}Fe Produced by Projectile Fragmentation

Bunched-beam collinear laser spectroscopy is performed on neutron deficient ^{52,53}Fe prepared through in-flight separation followed by a gas stopping. This novel scheme is a major step to reach nuclides far from the stability line in laser spectroscopy. Differential mean-square charge radii δ⟨r^{2}⟩ of ^{52,53}Fe are determined relative to stable ^{56}Fe as δ⟨r^{2}⟩^{56,52}=-0.034(13)  fm^{2} and δ⟨r^{2}⟩^{56,53}=-0.218(13)  fm^{2}, respectively, from the isotope shift of atomic hyperfine structures. The multiconfiguration Dirac-Fock method is used to calculate atomic factors to deduce δ⟨r^{2}⟩. The values of δ⟨r^{2}⟩ exhibit a minimum at the N=28 neutron shell closure. The nuclear density functional theory with Fayans and Skyrme energy density functionals is used to interpret the data. The trend of δ⟨r^{2}⟩ along the Fe isotopic chain results from an interplay between single-particle shell structure, pairing, and polarization effects and provides important data for understanding the intricate trend in the δ⟨r^{2}⟩ of closed-shell Ca isotopes.

Simple Nuclear Structure in (111-129)Cd from Atomic Isomer Shifts

Isomer shifts have been determined in ^{111-129}Cd by high-resolution laser spectroscopy at CERN-ISOLDE. The corresponding mean square charge-radii changes, from the 1/2^{+} and the 3/2^{+} ground states to the 11/2^{-} isomers, have been found to follow a distinct parabolic dependence as a function of the atomic mass number. Since the isomers have been previously associated with simplicity due to the linear mass dependence of their quadrupole moments, the regularity of the isomer shifts suggests a higher order of symmetry affecting the ground states in addition. A comprehensive description assuming nuclear deformation is found to accurately reproduce the radii differences in conjunction with the known quadrupole moments. This intuitive interpretation is supported by covariant density functional theory.

Critical-point boundary for the nuclear quantum phase transition near A=100 from mass measurements of (96,97)Kr

Mass measurements of (96,97)Kr using the ISOLTRAP Penning-trap spectrometer at CERN-ISOLDE are reported, extending the mass surface beyond N=60 for Z=36. These new results show behavior in sharp contrast to the heavier neighbors where a sudden and intense deformation is present. We interpret this as the establishment of a nuclear quantum phase transition critical-point boundary. The new masses confirm findings from nuclear mean-square charge-radius measurements up to N=60 but are at variance with conclusions from recent gamma-ray spectroscopy.

Nuclear spins and moments of Ga isotopes reveal sudden structural changes between N=40 and N=50

Collinear laser spectroscopy was performed on Ga (Z=31) isotopes at ISOLDE, CERN. A gas-filled linear Paul trap (ISCOOL) was used to extend measurements towards very neutron-rich isotopes (N=36-50). A ground state (g.s.) spin I=1/2 is measured for 73Ga, being near degenerate with a 3/2{-} isomer (75  eV≲E{ex}≲1  keV). The 79Ga g.s., with I=3/2, is dominated by protons in the πf{5/2} orbital and in 81Ga the 5/2{-} level becomes the g.s. The data are compared to shell-model calculations in the f{5/2}pg{9/2} model space, calling for further theoretical developments and new experiments.

Laser spectroscopy of niobium fission fragments: first use of optical pumping in an ion beam cooler buncher

A new method of optical pumping in an ion beam cooler buncher has been developed to selectively enhance ionic metastable state populations. The technique permits the study of elements previously inaccessible to laser spectroscopy and has been applied here to the study of Nb. Model independent mean-square charge radii and nuclear moments have been studied for ;{90,90 m,91,91 m,92,93,99,101,103}Nb to cover the region of the N=50 shell closure and N approximately 60 sudden onset of deformation. The increase in mean-square charge radius is observed to be less than that for Y, with a substantial degree of beta softness observed before and after N=60.

First precision mass measurements of refractory fission fragments

Atomic masses of 95-100Sr, 98-105Zr, and [corrected] 102-110Mo and have been measured with a precision of 10 keV employing a Penning trap setup at the IGISOL facility. Masses of 104,105Zr and 109,110Mo are measured for the first time. Our improved results indicate significant deviations from the previously published values deduced from beta end point measurements. The most neutron-rich studied isotopes are found to be significantly less bound (1 MeV) compared to the 2003 atomic mass evaluation. A strong correlation between nuclear deformation and the binding energy is observed in the two-neutron separation energy in all studied isotope chains.