Laser Spectroscopy Programme at the Jyväskylä IGISOL

This paper reviews the status of the laser spectroscopy programme being carried using the IGISOL mass separator in combination with an RFQ cooler-buncher. Measurements in the zirconium region are being extended to the yttrium isotopes. Two K = 8 isomers, in ¹⁷⁶Yb and ¹³⁰Ba, are found to have smaller mean square charge radii than their ground states, and the isotope shifts of stable osmium isotopes have been measured off-line by collinear laser spectroscopy.     

Odd-even staggering in nuclear charge radii of neutron-rich europium isotopes

Nuclear magnetic dipole and electric quadrupole moments and changes in mean square charge radii for the neutron-rich^155–159Eu isotopes have been measured using resonance ionization spectroscopy at the IRIS facility. It has been found that the isotopes withN>92, unlike the isotopes with 89≦N≦92, have an ordinary character of odd-even staggering in nuclear charge radii. This means that the octupole deformation attributed previously to the europium nuclei around¹⁵⁴Eu does not display itself in the charge radii of heavier europium isotopes.     

Hyperfine structure in the 6p5d configuration and isotope shifts in transitions between the 6s5d and the 6p5d configurations in Ba I

High-resolution laser fluorescence spectroscopy, using a single-mode dye laser acting on a collimated atomic beam, has been performed to determine the hyperfine-structure (hfs) constants in six states of the 6p 5d configuration of¹³⁵Ba and¹³⁷Ba. Isotope shifts (IS) for eleven transitions between the 6s 5d and the 6p 5d configurations have also been measured. From an analysis of the energy levels, intermediate angular wavefunctions have been deduced. The wavefunctions are used to evaluate experimental hyperfine parameters from the experimental hfs constants. The parameters are, for the magnetic-dipole interaction compared with theoretical values, and for the electricquadrupole interaction used to estimate the nuclear quadrupole moments for the odd isotopes. The IS in the measured transitions are analysed using a King-plot, with the first resonance line in Ba II as the reference. Specific mass and field shifts are evaluated for the measured transitions. The field shifts have been used to determine the change in mean-square radius between the natural abundant Ba-isotopes.     

COMPLIS experiments: COllaboration for spectroscopy Measurements using a Pulsed Laser Ion Source

Laser spectroscopy measurements have been carried out on very neutron-deficient isotopes of Au, Pt and Ir, produced as daughter elements from a Hg ISOLDE beam. For these transitional region nuclides, the hyperfine structure (HFS) and isotope shift (IS) were measured by Resonance Ionization Spectroscopy (RIS). Magnetic moments μ, spectroscopic quadrupole moments Q_s and changes of the nuclear mean square charge radius δ〈r_c ²〉along isotopic series have been extracted. For some results, a detailed comparison with theoretical predictions is presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hyperfine structure and isotope shift of the neutron-rich barium isotopes139–146Ba and148Ba

The hyperfme structure and isotope shift in the 6s ² S _1/2?6p ²P_3/2 line of Ba II (455.4 nm) have been measured by collinear fast-beam laser spectroscopy for the neutron-rich isotopes^139–146Ba and¹⁴⁸Ba. Nuclear moments and mean square charge radii of these isotopes have been recalculated. The isotope shift of the isotope¹⁴⁸Ba (T_1/2=0.64 s) could be studied for the first time, yieldingδ〈r²〉^138,148=1.245(3) fm².     

Optical isotope shifts of146Sm and151Sm

We have measured the optical isotope shifts of¹⁴⁶Sm and¹⁵¹Sm by laser resonance fluorescence. From these measurements the changes in the mean square nuclear radii are: Δ〈r²〉 (A=144 to 146)=0.266(10) fm², and Δ〈r²〉 (A=151 to 152)=0.262(10) fm². These results, together with those of the stable isotopes, show that the average nuclear expansion of samarium can be accounted for by the liquid drop model with deformations.     

Mean square charge radii of radium isotopes and octupole deformation in the 220–228Ra region

The first investigation of isotope shifts in both the atomic and ionic resonance lines of radium has been carried out using the technique of on-line collinear fast-beam laser spectroscopy at the ISOLDE facility at CERN. The measurements cover 19 isotopes in the mass range 208⩽A⩽232 with half-lives between 23 ms and 1600 y. The differences in the nuclear mean square charge radii δ〈r²〉 have been evaluated and - together with earlier published spins and moments from the hyperfine structure - related to nuclear deformation. In particular the inversion of the odd-even staggering effect for the isotopes ²²¹Ra, ²²³Ra and ²²⁵Ra can be interpreted by the presence of octupole instability and adds weight to the concept of near-stable octupole deformation in the odd-A isotopes which already explained their spins and magnetic moments.     

Isotope shift of182Hg and an update of nuclear moments and charge radii in the isotope range181Hg-206Hg

The technique of collinear fast-beam laser spectroscopy has been used to measure the isotope shifts of the even-even isotopes of Hg (Z=80) in the mass range 182≤A≤198 at the on-line mass separator ISOLDE at CERN. The atomic transition studied (6s 6p ³ P ₂- 6s7s ³ S ₁,λ=546.1 nm) starts from a metastable state, which is populated in a quasi resonant charge transfer process. The resulting changes in nuclear mean square charge radii show clearly that¹⁸²Hg follows the trend of the heavier, even, weakly oblate isotopes. Correspondingly the huge odd-even shape staggering in the light Hg isotopes continues and the nuclear shape staggering and shape coexistence persists down to the last isotope investigated,¹⁸¹Hg. An update of isotope shift and hyperfine structure data for^181–206Hg is given, with a revised evaluation of the differences in nuclear mean square charge radii and of spectroscopic quadrupole moments.     

On the charge distribution of calcium nuclei

The mean square charge radii and the quadrupole moments of Ca nuclei are discussed in the light of theoretical predictions. The very peculiar dependence of the charge radii on the mass number between double magic⁴⁰Ca and double magic⁴⁸Ca can be ascribed to changes of the nuclear deformation, whereas the volume of the nuclear charge remains constant for all the Ca isotopes. Furthermore, correlations between nuclear charge radii and binding energies are discussed.     

Laserspectroscopic studies of collective properties of neutron deficient Ba Nuclei

Isotope shifts and hyperfine structure of the BaI resonance-line (λ=553.6 nm) have been measured by dye laser induced resonance fluorescence on an atomic beam for^{135m, 129g, 129m, 126}Ba thus extending previous high resolution measurements of neutron deficient Ba nuclides (N<82). The experimental results, now available for 16Ba isotopes and isomers withA=140?126, are used to deduce differences of rms charge radii, magnetic dipole and electric quadrupole moments. While the groundstates display a pronounced odd-even staggering the h 11/2^? isomers^135mBa and^133mBa show a decreased staggering. Conspicuously the isomer shift of theg 7/2⁺ isomer^129m Ba proves to be negative. The nuclear structure information is discussed in the context of gammaspectroscopic studies of transitional nuclei with 50<N,Z<82 and on the basis of a quasi-particle-plus-triaxial rotor model. The isotope shift discrepancy observed is fairly well described by the droplet model.     

New neutron-deficient isotopes of lanthanum and cerium

The short-lived neutron-deficient isotopes of lanthanum and cerium were produced in the ³²S+^{96, 98}Ru reactions and separated according to their masses. Seven isotopes ^123–125La and ^124–127Ce have been first observed and their half-lives and low-energy γ-ray data are reported. In addition, low-energy γ-ray data are given for the ^122–125Ba and ¹²⁷La decays. The β-decay probabilities for Ce, La and Ba isotopes are discussed in terms of the average β-strength function.     

Messung der magnetischen Kerndipolmomente an freien43Ca-,87Sr-,135Ba-,137Ba-,171Yb- und173Yb-Atomen mit optischem Pumpen

As previously reported^3,4 the optical pumping method was succesfully used to determine the nuclear magnetic moments of¹³⁵Ba,¹³⁷Ba,¹⁷¹Yb and¹⁷³Yb. This method has now been extended to⁴³Ca and⁸⁷Sr. The nuclear moments of the Ba- and Yb-isotopes have been remeasured by means of an improved experimental technique. Especially the influence of the light shift has been investigated and the nuclear moments have been corrected by eliminating this shift. The magnetic field has been calibrated by means of optical pumping of²³Na. The nuclear magnetic moments obtained from the rf-transitions between the Zeeman-levels of the ground state¹ S _o are: μ(⁴³Ca)=?1.3172 (6)μ _N ; μ⁸⁷Sr)=?1.0924 (7) μ _N ; μ(¹³⁵Ba)=0.83656(2)μ _N ; μ(¹³⁷Ba)=0.93582(2)μ _N ; μ(¹⁷¹Yb)=0.491889 (8)μ _N ; μ(¹⁷³Yb)=?0.67744 (3) μ _N (diamagnetic correction was applied). The ratios of the nuclear moments of the elements with two odd isotopes are: μ(¹³⁷Ba)/(¹³⁵Ba)=1.11865 (3); μ(¹⁷³Yb)/μ(¹⁷¹Yb)=?1.37723 (7). In comparison with the results of nuclear induction the chemical shifts have been calculated to be: δ(Ca)≦± 5 · 10^?4 δ(Sr)=?6(7) ·10^?4 δ(Ba)=?7.8(4) ·10^?4 δ(Yb)=?23(10) · 10^?4.     

Study of Exotic Nuclei

A high-sensitive fluorescence cell has been developed with an aim to perform laser spectroscopy of exotic nuclei. This fluorescence cell has been tested off-line for stable isotope ¹³³Cs. Also, an investigation of the nuclear root mean square (r.m.s.) charge and neutron radii, and of the binding energies of the cesium long isotopic chain has been carried out in the relativistic mean field (RMF) and relativistic Hartree–Bogoliubov (RHB) formalisms. The RMF/RHB calculations are compared with the experimental data and are found to be in good agreement.     

Nuclear charge radii of molybdenum fission fragments

Radioisotopes of molybdenum have been studied using laser spectroscopy techniques at the IGISOL facility, University of Jyväskylä. Differences in nuclear charge radii have been determined for neutron deficient isotopes ^90,91Mo and neutron rich isotopes ^{102–106,108}Mo (and all stable isotopes). A smooth transition in the mean square charge radii is observed as the neutron number increases with no sudden shape change observed in the region around N=60$N=60$. As N   increases, the nuclear deformation appears to go beyond a maximum and a fall off at N=66$N=66$ is observed. The magnetic moments of the odd isotopes ^91,103,105Mo are also determined.     

High-precision masses of 29-33Mg and the N = 20 shell “closure”

High-precision mass measurements have been performed on the exotic magnesium isotopes ^29-33Mg using the MISTRAL radiofrequency spectrometer, especially suited for very short-lived nuclides. This method, combined with the powerful tool of resonant laser ionization at ISOLDE, has provided a significant reduction of uncertainty for the masses of the most exotic Mg isotopes: a relative error of 7×10^-7 was achieved for the weakly produced ³³Mg that has a half-life of only 90ms. Moreover, the mass of ³³Mg is found to change by over 250keV. Verifying and minimizing binding energy uncertainties in this region of the nuclear chart is important for understanding the lack of binding energy that is normally associated with magic numbers.     

Laser spectroscopy measurements of neutron-rich tellurium isotopes by COMPLIS

Laser spectroscopy based on resonant ionization of laser-desorbed atoms has been used to study the neutron-rich tellurium isotopes with the COMPLIS facility at ISOLDE-CERN. Isotope shifts and hyperfine structures of several neutron-rich Te isotopes: ^120?136Te and ^{123 m?133 m}Te have been measured. From the hyperfine structure we have extracted magnetic and quadrupole moments. Changes in the mean square charge radii have been deduced and their comparison with the known data for the other elements near Z?=?50 is presented. The experimental δ?<?r ²> values are compared with those obtained from relativistic mean field calculations.     

Isotope shifts in the atomic spectrum of xenon and nuclear deformation effects

Optical isotope shifts of four lines in the atomic spectrum of xenon have been measured using enriched samples of all stable xenon isotopes. The spectrograms were recorded with the aid of a pressure-scanned Fabry-Pérot interferometer and analysed by digital data techniques. The measured isotope shifts are shown to be self-consistent by means of a King plot. An estimate of the specific mass effect is given and the changesδ 〈r ²〉 of the mean square radius of the nuclear charge distribution are extracted from the measured shifts. These changesδ 〈r ²〉 are discussed in terms of the nuclear deformation parameterβ ². The results for the deformation of the stable even xenon isotopes are shown to be in good agreement with the systematic of deformation found for the neighbouring elements from Coulomb excitation experiments.     

IBFM for Ba isotopes and chaoticity

Fluctuation properties have been analysed for the energy levels predicted by IBFM calculations in the Ba isotopes¹²¹Ba to¹³¹Ba. The results indicate, in general, a situation which is close to the chaotic limit. For the lighter isotopes studied (121 and 123), a phase transition is obtained in the low-spin, positive parity states, from a situation close to regularity at low excitation energies, towards chaoticity at higher excitations.     

New neutron-deficient isotopes of barium and rare-earth elements

This paper deals with an investigation of the short-lived neutron-deficient isotopes of barium and rare-earth elements. By using the BEMS-2 isotope separator with a heavy-ion beam, we succeeded in producing 19 new isotopes with mass numbers ranging from 117 to 138. Five of these (¹¹⁷Ba, ^{129, 131}Nd and^{133, 135}Sm) turned out to be delayed proton emitters. Theβ-decay probabilities for the new isotopes have been analyzed in terms of the β-strength function. An analysis of the proton spectrum shape has been performed using the statistical model for delayed proton     

High-accuracy mass determination of unstable cesium and barium isotopes

Direct mass measurements of short-lived Cs and Ba isotopes have been performed with the tandem Penning trap mass spectrometer ISOLTRAP installed at the on-line isotope separator ISOLDE at CERN. Typically, a mass resolving power of 600 000 and an accuracy of δm ≈ 13 keV have been obtained. The masses of ^123,124,126Ba and ^122mCs were measured for the first time. A least-squares adjustment has been performed and the experimental masses are compared with theoretical ones, particularly in the frame of a macroscopic-microscopic model.