Further Evidence for Shape Coexistence in

Isomers close to doubly magic _{28}^{78}Ni_{50} provide essential information on the shell evolution and shape coexistence near the Z=28 and N=50 double shell closure. We report the excitation energy measurement of the 1/2^{+} isomer in _{30}^{79}Zn_{49} through independent high-precision mass measurements with the JYFLTRAP double Penning trap and with the ISOLTRAP multi-reflection time-of-flight mass spectrometer. We unambiguously place the 1/2^{+} isomer at 942(10) keV, slightly below the 5/2^{+} state at 983(3) keV. With the use of state-of-the-art shell-model diagonalizations, complemented with discrete nonorthogonal shell-model calculations which are used here for the first time to interpret shape coexistence, we find low-lying deformed intruder states, similar to other N=49 isotones. The 1/2^{+} isomer is interpreted as the bandhead of a low-lying deformed structure akin to a predicted low-lying deformed band in ^{80}Zn, and points to shape coexistence in ^{79,80}Zn similar to the one observed in ^{78}Ni. The results make a strong case for confirming the claim of shape coexistence in this key region of the nuclear chart.

Applying machine learning methods for the analysis of two-dimensional mass spectra.

In a measurement of isomeric yield-ratios in fission, the Phase-Imaging Ion-Cyclotron-Resonance technique, which projects the radial motions of ions in the Penning trap (JYFLTRAP) onto a position-sensitive micro-channel plate detector, has been applied. To obtain the yield ratio, that is the relative population of two states of an isomer pair, a novel analysis procedure has been developed to determine the number of detected ions in each state, as well as corrections for the detector efficiency and decay losses. In order to determine the population of the states in cases where their mass difference is too small to reach full separation, a Bayesian Gaussian Mixture model was implemented. The position-dependent efficiency of the micro-channel plate detector was calibrated by mapping it with 133Cs+ ions, and a Gaussian Process was trained with the position data to construct an efficiency function that could be used to correct the recorded distributions. The obtained numbers of counts of excited and ground-state ions were used to derive the isomeric yield ratio, taking into account decay losses as well as feeding from precursors.

High-precision measurements of the hyperfine structure of cobalt ions in the deep ultraviolet range.

High-precision hyperfine structure measurements were performed on stable, singly-charged [Formula: see text]Co ions at the IGISOL facility in Jyväskylä, Finland using the collinear laser spectroscopy technique. A newly installed light collection setup enabled the study of transitions in the 230 nm wavelength range from low-lying states below 6000 cm[Formula: see text]. We report a 100-fold improvement on the precision of the hyperfine A parameters, and furthermore present newly measured hyperfine B paramaters.

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.

Evidence of a sudden increase in the nuclear size of proton-rich silver-96.

Understanding the evolution of the nuclear charge radius is one of the long-standing challenges for nuclear theory. Recently, density functional theory calculations utilizing Fayans functionals have successfully reproduced the charge radii of a variety of exotic isotopes. However, difficulties in the isotope production have hindered testing these models in the immediate region of the nuclear chart below the heaviest self-conjugate doubly-magic nucleus 100Sn, where the near-equal number of protons (Z) and neutrons (N) lead to enhanced neutron-proton pairing. Here, we present an optical excursion into this region by crossing the N = 50 magic neutron number in the silver isotopic chain with the measurement of the charge radius of 96Ag (N = 49). The results provide a challenge for nuclear theory: calculations are unable to reproduce the pronounced discontinuity in the charge radii as one moves below N = 50. The technical advancements in this work open the N = Z region below 100Sn for further optical studies, which will lead to more comprehensive input for nuclear theory development.

High-Precision Q-Value Measurement Confirms the Potential of

The ground-state-to-ground-state ß-decay Q value of ^{135}Cs(7/2^{+})→^{135}Ba(3/2^{+}) has been directly measured for the first time. The measurement was done utilizing both the phase-imaging ion-cyclotron resonance technique and the time-of-flight ion-cyclotron resonance technique at the JYFLTRAP Penning-trap setup and yielded a mass difference of 268.66(30) keV between ^{135}Cs(7/2^{+}) and ^{135}Ba(3/2^{+}). With this very small uncertainty, this measurement is a factor of 3 more precise than the currently adopted Q value in the Atomic Mass Evaluation 2016. The measurement confirms that the first-forbidden unique ß^{-}-decay transition ^{135}Cs(7/2^{+})→^{135}Ba(11/2^{-}) is a candidate for antineutrino mass measurements with an ultralow Q value of 0.44(31) keV. This Q value is almost an order of magnitude smaller than those of nuclides presently used in running or planned direct (anti)neutrino mass experiment.

Erratum: Precision Mass Measurements on Neutron-Rich Rare-Earth Isotopes at JYFLTRAP: Reduced Neutron Pairing and Implications for r-Process Calculations [Phys. Rev. Lett. 120, 262701 (2018)].

This corrects the article DOI: 10.1103/PhysRevLett.120.262701.

Precision Mass Measurements on Neutron-Rich Rare-Earth Isotopes at JYFLTRAP: Reduced Neutron Pairing and Implications for r-Process Calculations.

The rare-earth peak in the r-process abundance pattern depends sensitively on both the astrophysical conditions and subtle changes in nuclear structure in the region. This work takes an important step towards elucidating the nuclear structure and reducing the uncertainties in r-process calculations via precise atomic mass measurements at the JYFLTRAP double Penning trap. ^{158}Nd, ^{160}Pm, ^{162}Sm, and ^{164-166}Gd have been measured for the first time, and the precisions for ^{156}Nd, ^{158}Pm, ^{162,163}Eu, ^{163}Gd, and ^{164}Tb have been improved considerably. Nuclear structure has been probed via two-neutron separation energies S_{2n} and neutron pairing energy metrics D_{n}. The data do not support the existence of a subshell closure at N=100. Neutron pairing has been found to be weaker than predicted by theoretical mass models. The impact on the calculated r-process abundances has been studied. Substantial changes resulting in a smoother abundance distribution and a better agreement with the solar r-process abundances are observed.

Towards high-resolution laser ionization spectroscopy of the heaviest elements in supersonic gas jet expansion.

Resonant laser ionization and spectroscopy are widely used techniques at radioactive ion beam facilities to produce pure beams of exotic nuclei and measure the shape, size, spin and electromagnetic multipole moments of these nuclei. However, in such measurements it is difficult to combine a high efficiency with a high spectral resolution. Here we demonstrate the on-line application of atomic laser ionization spectroscopy in a supersonic gas jet, a technique suited for high-precision studies of the ground- and isomeric-state properties of nuclei located at the extremes of stability. The technique is characterized in a measurement on actinium isotopes around the N=126 neutron shell closure. A significant improvement in the spectral resolution by more than one order of magnitude is achieved in these experiments without loss in efficiency.

Single and Double Beta-Decay Q Values among the Triplet

The atomic mass relations among the mass triplet ^{96}Zr, ^{96}Nb, and ^{96}Mo have been determined by means of high-precision mass measurements using the JYFLTRAP mass spectrometer at the IGISOL facility of the University of Jyväskylä. We report Q values for the ^{96}Zr single and double ß decays to ^{96}Nb and ^{96}Mo, as well as the Q value for the ^{96}Nb single ß decay to ^{96}Mo, which are Q_{ß}(^{96}Zr)=163.96(13), Q_{ßß}(^{96}Zr)=3356.097(86), and Q_{ß}(^{96}Nb)=3192.05(16) keV. Of special importance is the ^{96}Zr single ß-decay Q value, which has never been determined directly. The single ß decay, whose main branch is fourfold unique forbidden, is an alternative decay path to the ^{96}Zr ßß decay, and its observation can provide one of the most direct tests of the neutrinoless ßß-decay nuclear-matrix-element calculations, as these can be simultaneously performed for both decay paths with no further assumptions. The theoretical single ß-decay rate has been re-evaluated using a shell-model approach, which indicates a ^{96}Zr single ß-decay lifetime within reach of an experimental verification. The uniqueness of the decay also makes such an experiment interesting for an investigation into the origin of the quenching of the axial-vector coupling constant g_{A}.

An inductively heated hot cavity catcher laser ion source.

An inductively heated hot cavity catcher has been constructed for the production of low-energy ion beams of exotic, neutron-deficient Ag isotopes. A proof-of-principle experiment has been realized by implanting primary (107)Ag(21+) ions from a heavy-ion cyclotron into a graphite catcher. A variable-thickness nickel foil was used to degrade the energy of the primary beam in order to mimic the implantation depth expected from the heavy-ion fusion-evaporation recoils of N = Z (94)Ag. Following implantation, the silver atoms diffused out of the graphite and effused into the catcher cavity and transfer tube, where they were resonantly laser ionized using a three-step excitation and ionization scheme. Following mass separation, the ions were identified by scanning the frequency of the first resonant excitation step while recording the ion count rate. Ion release time profiles were measured for different implantation depths and cavity temperatures with the mean delay time varying from 10 to 600 ms. In addition, the diffusion coefficients for silver in graphite were measured for temperatures of 1470 K, 1630 K, and 1720 K, from which an activation energy of 3.2 ± 0.3 eV could be determined.

Production of pure samples of 131mXe and 135Xe.

Pure samples of (131m)Xe, (133m)Xe, (133)Xe and (135)Xe facilitate the calibration and testing of noble gas sampler stations and related laboratory instrumentation. We have earlier reported a Penning trap-based production method for pure (133m)Xe and (133)Xe samples. Here we complete the work by reporting the successful production of pure (131m)Xe and (135)Xe samples using the same technique. In addition, we present data on xenon release from graphite.

Precision mass measurements beyond 132Sn: anomalous behavior of odd-even staggering of binding energies.

Atomic masses of the neutron-rich isotopes (121-128)Cd, (129,131)In, (130-135)Sn, (131-136)Sb, and (132-140)Te have been measured with high precision (10 ppb) using the Penning-trap mass spectrometer JYFLTRAP. Among these, the masses of four r-process nuclei (135)Sn, (136)Sb, and (139,140)Te were measured for the first time. An empirical neutron pairing gap expressed as the odd-even staggering of isotopic masses shows a strong quenching across N = 82 for Sn, with a Z dependence that is unexplainable by the current theoretical models.

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.

Ultra-high resolution mass separator--application to detection of nuclear weapons tests.

A Penning trap-based purification process having a resolution of about 1 ppm is reported. In this context, we present for the first time a production method for the most complicated and crucially important nuclear weapons test signature, (133m)Xe. These pure xenon samples are required by the Comprehensive Nuclear-Test-Ban Treaty Organization to standardize and calibrate the worldwide network of xenon detectors.

Reactor decay heat in 239Pu: solving the γ discrepancy in the 4-3000-s cooling period.

The ß feeding probability of (102,104,105,106,107)Tc, 105Mo, and 101Nb nuclei, which are important contributors to the decay heat in nuclear reactors, has been measured using the total absorption technique. We have coupled for the first time a total absorption spectrometer to a Penning trap in order to obtain sources of very high isobaric purity. Our results solve a significant part of a long-standing discrepancy in the γ component of the decay heat for 239Pu in the 4-3000 s range.

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.

Quenching of the SnSbTe cycle in the rp process.

The nuclides 104-108Sn, 106-110Sb, 108,109Te, and 111I at the expected endpoint of the astrophysical rp process have been produced in 58Ni+natNi fusion-evaporation reactions at IGISOL and their mass values were precisely measured with the JYFLTRAP Penning trap mass spectrometer. For 106Sb, 108Sb, and 110Sb these are the first direct experimental mass results obtained. The related one-proton separation energies have been derived and the value for 106Sb, Sp=424(8) keV, shows that the branching into the closed SnSbTe cycle in the astrophysical rp process is weaker than expected.

QEC values of the superallowed beta emitters 34Cl and 38Km.

The superallowed beta-decay Q(EC) values of (34)Cl and (38)K(m) have been measured with an online Penning trap to be 5491.662(47) keV and 6044.223(41) keV, respectively. The new values are more precise than the previous high-precision reaction-based values but are consistent with them and establish that there are no significant systematic differences between the two types of measurements.