Nuclear moments of indium isotopes reveal abrupt change at magic number 82.

In spite of the high-density and strongly correlated nature of the atomic nucleus, experimental and theoretical evidence suggests that around particular 'magic' numbers of nucleons, nuclear properties are governed by a single unpaired nucleon1,2. A microscopic understanding of the extent of this behaviour and its evolution in neutron-rich nuclei remains an open question in nuclear physics3-5. The indium isotopes are considered a textbook example of this phenomenon6, in which the constancy of their electromagnetic properties indicated that a single unpaired proton hole can provide the identity of a complex many-nucleon system6,7. Here we present precision laser spectroscopy measurements performed to investigate the validity of this simple single-particle picture. Observation of an abrupt change in the dipole moment at N = 82 indicates that, whereas the single-particle picture indeed dominates at neutron magic number N = 82 (refs. 2,8), it does not for previously studied isotopes. To investigate the microscopic origin of these observations, our work provides a combined effort with developments in two complementary nuclear many-body methods: ab initio valence-space in-medium similarity renormalization group and density functional theory (DFT). We find that the inclusion of time-symmetry-breaking mean fields is essential for a correct description of nuclear magnetic properties, which were previously poorly constrained. These experimental and theoretical findings are key to understanding how seemingly simple single-particle phenomena naturally emerge from complex interactions among protons and neutrons.

Spectroscopy of short-lived radioactive molecules.

Molecular spectroscopy offers opportunities for the exploration of the fundamental laws of nature and the search for new particle physics beyond the standard model1-4. Radioactive molecules-in which one or more of the atoms possesses a radioactive nucleus-can contain heavy and deformed nuclei, offering high sensitivity for investigating parity- and time-reversal-violation effects5,6. Radium monofluoride, RaF, is of particular interest because it is predicted to have an electronic structure appropriate for laser cooling6, thus paving the way for its use in high-precision spectroscopic studies. Furthermore, the effects of symmetry-violating nuclear moments are strongly enhanced5,7-9 in molecules containing octupole-deformed radium isotopes10,11. However, the study of RaF has been impeded by the lack of stable isotopes of radium. Here we present an experimental approach to studying short-lived radioactive molecules, which allows us to measure molecules with lifetimes of just tens of milliseconds. Energetically low-lying electronic states were measured for different isotopically pure RaF molecules using collinear resonance ionisation at the ISOLDE ion-beam facility at CERN. Our results provide evidence of the existence of a suitable laser-cooling scheme for these molecules and represent a key step towards high-precision studies in these systems. Our findings will enable further studies of short-lived radioactive molecules for fundamental physics research.

Deformation versus Sphericity in the Ground States of the Lightest Gold Isotopes.

The changes in mean-squared charge radii of neutron-deficient gold nuclei have been determined using the in-source, resonance-ionization laser spectroscopy technique, at the ISOLDE facility (CERN). From these new data, nuclear deformations are inferred, revealing a competition between deformed and spherical configurations. The isotopes ^{180,181,182}Au are observed to possess well-deformed ground states and, when moving to lighter masses, a sudden transition to near-spherical shapes is seen in the extremely neutron-deficient nuclides, ^{176,177,179}Au. A case of shape coexistence and shape staggering is identified in ^{178}Au which has a ground and isomeric state with different deformations. These new data reveal a pattern in ground-state deformation unique to the gold isotopes, whereby, when moving from the heavy to light masses, a plateau of well-deformed isotopes exists around the neutron midshell, flanked by near-spherical shapes in the heavier and lighter isotopes-a trend hitherto unseen elsewhere in the nuclear chart. The experimental charge radii are compared to those from Hartree-Fock-Bogoliubov calculations using the D1M Gogny interaction and configuration mixing between states of different deformation. The calculations are constrained by the known spins, parities, and magnetic moments of the ground states in gold nuclei and show a good agreement with the experimental results.

Muonic atom spectroscopy with microgram target material.

Muonic atom spectroscopy-the measurement of the x rays emitted during the formation process of a muonic atom-has a long standing history in probing the shape and size of nuclei. In fact, almost all stable elements have been subject to muonic atom spectroscopy measurements and the absolute charge radii extracted from these measurements typically offer the highest accuracy available. However, so far only targets of at least a few hundred milligram could be used as it required to stop a muon beam directly in the target to form the muonic atom. We have developed a new method relying on repeated transfer reactions taking place inside a 100 bar hydrogen gas cell with an admixture of 0.25% deuterium that allows us to drastically reduce the amount of target material needed while still offering an adequate efficiency. Detailed simulations of the transfer reactions match the measured data, suggesting good understanding of the processes taking place inside the gas mixture. As a proof of principle we demonstrate the method with a measurement of the 2p-1s muonic x rays from a 5  µ g gold target.

Isotope Shifts of Radium Monofluoride Molecules.

Isotope shifts of ^{223-226,228}Ra^{19}F were measured for different vibrational levels in the electronic transition A^{2}Π_{1/2}←X^{2}Σ^{+}. The observed isotope shifts demonstrate the particularly high sensitivity of radium monofluoride to nuclear size effects, offering a stringent test of models describing the electronic density within the radium nucleus. Ab initio quantum chemical calculations are in excellent agreement with experimental observations. These results highlight some of the unique opportunities that short-lived molecules could offer in nuclear structure and in fundamental symmetry studies.

Laser Spectroscopy of Neutron-Rich

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 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 nonrelativistic 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.

Large Shape Staggering in Neutron-Deficient Bi Isotopes.

The changes in the mean-square charge radius (relative to ^{209}Bi), magnetic dipole, and electric quadrupole moments of ^{187,188,189,191}Bi were measured using the in-source resonance-ionization spectroscopy technique at ISOLDE (CERN). A large staggering in radii was found in ^{187,188,189}Bi^{g}, manifested by a sharp radius increase for the ground state of ^{188}Bi relative to the neighboring ^{187,189}Bi^{g}. A large isomer shift was also observed for ^{188}Bi^{m}. Both effects happen at the same neutron number, N=105, where the shape staggering and a similar isomer shift were observed in the mercury isotopes. Experimental results are reproduced by mean-field calculations where the ground or isomeric states were identified by the blocked quasiparticle configuration compatible with the observed spin, parity, and magnetic moment.

First Glimpse of the N=82 Shell Closure below Z=50 from Masses of Neutron-Rich Cadmium Isotopes and Isomers.

We probe the N=82 nuclear shell closure by mass measurements of neutron-rich cadmium isotopes with the ISOLTRAP spectrometer at ISOLDE-CERN. The new mass of ^{132}Cd offers the first value of the N=82, two-neutron shell gap below Z=50 and confirms the phenomenon of mutually enhanced magicity at ^{132}Sn. Using the recently implemented phase-imaging ion-cyclotron-resonance method, the ordering of the low-lying isomers in ^{129}Cd and their energies are determined. The new experimental findings are used to test large-scale shell-model, mean-field, and beyond-mean-field calculations, as well as the ab initio valence-space in-medium similarity renormalization group.

Studies of pear-shaped nuclei using accelerated radioactive beams.

There is strong circumstantial evidence that certain heavy, unstable atomic nuclei are 'octupole deformed', that is, distorted into a pear shape. This contrasts with the more prevalent rugby-ball shape of nuclei with reflection-symmetric, quadrupole deformations. The elusive octupole deformed nuclei are of importance for nuclear structure theory, and also in searches for physics beyond the standard model; any measurable electric-dipole moment (a signature of the latter) is expected to be amplified in such nuclei. Here we determine electric octupole transition strengths (a direct measure of octupole correlations) for short-lived isotopes of radon and radium. Coulomb excitation experiments were performed using accelerated beams of heavy, radioactive ions. Our data on (220)Rn and (224)Ra show clear evidence for stronger octupole deformation in the latter. The results enable discrimination between differing theoretical approaches to octupole correlations, and help to constrain suitable candidates for experimental studies of atomic electric-dipole moments that might reveal extensions to the standard model.

Binding Energy of

The masses of the neutron-rich copper isotopes ^{75-79}Cu are determined using the precision mass spectrometer ISOLTRAP at the CERN-ISOLDE facility. The trend from the new data differs significantly from that of previous results, offering a first accurate view of the mass surface adjacent to the Z=28, N=50 nuclide ^{78}Ni and supporting a doubly magic character. The new masses compare very well with large-scale shell-model calculations that predict shape coexistence in a doubly magic ^{78}Ni and a new island of inversion for Z<28. A coherent picture of this important exotic region begins to emerge where excitations across Z=28 and N=50 form a delicate equilibrium with a spherical mean field.

Precision Mass Measurements of

Masses adjacent to the classical waiting-point nuclide ^{130}Cd have been measured by using the Penning-trap spectrometer ISOLTRAP at ISOLDE/CERN. We find a significant deviation of over 400 keV from earlier values evaluated by using nuclear beta-decay data. The new measurements show the reduction of the N=82 shell gap below the doubly magic ^{132}Sn. The nucleosynthesis associated with the ejected wind from type-II supernovae as well as from compact object binary mergers is studied, by using state-of-the-art hydrodynamic simulations. We find a consistent and direct impact of the newly measured masses on the calculated abundances in the A=128-132 region and a reduction of the uncertainties from the precision mass input data.

Use of a Continuous Wave Laser and Pockels Cell for Sensitive High-Resolution Collinear Resonance Ionization Spectroscopy.

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.

Shape coexistence in the neutron-deficient even-even (182-188)Hg isotopes studied via coulomb excitation.

Coulomb-excitation experiments to study electromagnetic properties of radioactive even-even Hg isotopes were performed with 2.85 MeV/nucleon mercury beams from REX-ISOLDE. Magnitudes and relative signs of the reduced E2 matrix elements that couple the ground state and low-lying excited states in Hg182-188 were extracted. Information on the deformation of the ground and the first excited 0+ states was deduced using the quadrupole sum rules approach. Results show that the ground state is slightly deformed and of oblate nature, while a larger deformation for the excited 0+ state was noted in Hg182,184. The results are compared to beyond mean field and interacting-boson based models and interpreted within a two-state mixing model. Partial agreement with the model calculations was obtained. The presence of two different structures in the light even-mass mercury isotopes that coexist at low excitation energy is firmly established.

Half-life determination of 215At and 221Ra with high-purity radioactive ion beams.

At CERN-ISOLDE, high-purity radioactive ion beams of 219Fr and 221RaF were investigated with α-decay spectroscopy at the CRIS and ASET experiments in the course of three different experimental campaigns. The half-life of 215At, α-decay daughter of 219Fr, is measured to be 36.3(3)[9]µs, and that of 221Ra was determined to be 26.2(1)[6]s, both of which are well in line with the trends in this region of the nuclear landscape but at odds with some of the reported literature.

Production study of Fr, Ra and Ac radioactive ion beams at ISOLDE, CERN.

The presented paper discusses the production of radioactive ion beams of francium, radium, and actinium from thick uranium carbide (UC x ) targets at ISOLDE, CERN. This study focuses on the release curves and extractable yields of francium, radium and actinium isotopes. The ion source temperature was varied in order to study the relative contributions of surface and laser ionization to the production of the actinium ion beams. The experimental results are presented in the form of release parameters. Representative extractable yields per µ C are presented for 222 - 231 Ac, several Ra and Fr isotopes in the mass ranges 214 ≤ A ≤ 233 and 205 ≤ A ≤ 231 respectively. The release efficiency for several isotopes of each of the studied elements was calculated by comparing their yields to the estimated in-target production rates modeled by CERN-FLUKA. The maximal extraction efficiency of actinium was calculated to be 2.1(6)% for a combination of surface ionization using a Ta ion source and resonant laser ionization using the two-step 438.58 nm, and 424.69 nm scheme.

Collinear resonance ionization spectroscopy of neutron-deficient francium isotopes.

The magnetic moments and isotope shifts of the neutron-deficient francium isotopes (202-205)Fr were measured at ISOLDE-CERN with use of collinear resonance ionization spectroscopy. A production-to-detection 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. Through a comparison of the measured charge radii with predictions from the spherical droplet model, it is concluded that the ground-state wave function remains spherical down to (205)Fr, with a departure observed in (203)Fr (N=116).

Determination of the Terbium-152 half-life from mass-separated samples from CERN-ISOLDE and assessment of the radionuclide purity.

Terbium-152 is one of four terbium radioisotopes that together form a potential theranostic toolbox for the personalised treatment of tumours. As 152 Tb decay by positron emission it can be utilised for diagnostics by positron emission tomography. For use in radiopharmaceuticals and for activity measurements by an activity calibrator a high radionuclide purity of the material and an accurate and precise knowledge of the half-life is required. Mass-separation and radiochemical purification provide a production route of high purity 152Tb. In the current work, two mass-separated samples from the CERN-ISOLDE facility have been assayed at the National Physical Laboratory to investigate the radionuclide purity. These samples have been used to perform four measurements of the half-life by three independent techniques: high-purity germanium gamma-ray spectrometry, ionisation chamber measurements and liquid scintillation counting. From the four measurement campaigns a half-life of 17.8784(95) h has been determined. The reported half-life shows a significant difference to the currently evaluated half-life (ζ-score = 3.77), with a relative difference of 2.2 % and an order of magnitude improvement in the precision. This work also shows that under controlled conditions the combination of mass-separation and radiochemical separation can provide high-purity 152Tb.

Precise determination of the unperturbed 8B neutrino spectrum.

A measurement of the final state distribution of the (8)B ß decay, obtained by implanting a (8)B beam in a double-sided silicon strip detector, is reported here. The present spectrum is consistent with a recent independent precise measurement performed by our collaboration at the IGISOL facility, Jyväskylä [O. S. Kirsebom et al., Phys. Rev. C 83, 065802 (2011)]. It shows discrepancies with previously measured spectra, leading to differences in the derived neutrino spectrum. Thanks to a low detection threshold, the neutrino spectrum is for the first time directly extracted from the measured final state distribution, thus avoiding the uncertainties related to the extrapolation of R-matrix fits. Combined with the IGISOL data, this leads to an improvement of the overall errors and the extension of the neutrino spectrum at high energy. The new unperturbed neutrino spectrum represents a benchmark for future measurements of the solar neutrino flux as a function of energy.

Half-life determination of 155Tb from mass-separated samples produced at CERN-MEDICIS.

Terbium-155 has been identified for its potential for single-photon emission computed tomography (SPECT) in nuclear medicine. For activity measurements, an accurate and precise half-life of this radionuclide is required. However, the currently evaluated half-life of 5.32(6) d with a relative standard uncertainty of 1.1% determines the precision possible. Limited literature for the half-life measurements of this radionuclide is available and all reported investigations are prior to 1970. Further measurements are therefore needed to confirm the accuracy and improve the precision of the half-life for its use in the clinical setting. Two samples produced and mass separated at the CERN-MEDICIS facility have been measured at the National Physical Laboratory by two independent techniques: liquid scintillation counting and high-purity germanium gamma-ray spectrometry. A half-life of 5.2346(36) d has been determined from the weighted mean of the half-lives determined by the two techniques. The half-life reported in this work has shown a relative difference of 1.6% to the currently evaluated half-life and has vastly improved the precision.

Early onset of ground state deformation in neutron deficient polonium isotopes.

In-source resonant ionization laser spectroscopy of the even-A polonium isotopes (192-210,216,218)Po has been performed using the 6p(3)7s (5)S(2) to 6p(3)7p (5)P(2) (λ=843.38 nm) transition in the polonium atom (Po-I) at the CERN ISOLDE facility. The comparison of the measured isotope shifts in (200-210)Po with a previous data set allows us to test for the first time recent large-scale atomic calculations that are essential to extract the changes in the mean-square charge radius of the atomic nucleus. When going to lighter masses, a surprisingly large and early departure from sphericity is observed, which is only partly reproduced by beyond mean field calculations.