Lifetime measurements in 63^{63}Co and 65^{65}Co

Lifetimes of the $9/2^-_1$ and $3/2^-_1$ states in $^{63}$Co and the $9/2^-_1$ state in $^{65}$Co were measured using the recoil distance Doppler shift and the differential decay curve methods. The nuclei were populated by multi-nucleon transfer reactions in inverse kinematics. Gamma rays were measured with the EXOGAM Ge array and the recoiling fragments were fully identified using the large-acceptance VAMOS spectrometer. The E2 transition probabilities from the $3/2^-_1$ and $9/2^-_1$ states to the $7/2^-$ ground state could be extracted in $^{63}$Co as well as an upper limit for the $9/2^-_1\rightarrow7/2^-_1$ $B$(E2) value in $^{65}$Co. The experimental results were compared to large-scale shell-model calculations in the $pf$ and $pfg_{9/2}$ model spaces, allowing to draw conclusions on the single-particle or collective nature of the various states.Comment: 8 pages, 8 figures, 1 table, accepted for publication in Physical Review

Morphological and chemical characterization of tooth enamel exposed to alkaline agents

Objectives: In this study, morphological and chemical changes in teeth enamel exposed to alkaline agents, with or without surfactants, have been investigated. In addition, chemical effects of the organic surface layer, i.e. plaque and pellicle, were also investigated. Methods: The present study was conducted using several techniques: Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results: From XPS-measurements, it was found that exposure to alkaline solutions resulted in a massive removal of carbon from the tooth surface, and that the addition of surfactants increased the rate under present conditions. Based on the results from the FTIR-analysis, no substitution reactions between phosphate, carbonate and hydroxide ions in the enamel apatite could be detected. From a minor SEM-analysis, degradation and loss of substance of the enamel surface was found for the exposed samples. From XRD-analysis, no changes in crystallinity of the enamel apatite could be found between the samples. Conclusions: The findings in this study show that exposure to alkaline solutions results in a degradation of enamel surfaces very dissimilar from acidic erosion. No significant erosion or chemical substitution of the apatite crystals themselves could be discerned. However, significant loss of organic carbon at the enamel surface was found in all exposed samples. The degradation of the protective organic layer at the enamel surface may profoundly increase the risk for caries and dental erosion from acidic foods and beverages

The Effect of the Harmonic Content Generated by AC/DC Modular Multilevel Converters on HVDC Cable Systems

With the increasing penetration of renewable and decentralized energy sources into the power grid, an extended use of DC voltages is expected on both distribution and transmission\ua0levels. Generation of DC voltages by means of voltage source converters is associated with a wide spectrum of harmonic distortions at converter terminals, both on the ac and on the dc\ua0sides. This can lead to partial discharges in power cables, which deteriorate insulation material thus weakening its performance and reducing cable life-time. In the previously published paper, the effect of harmonic distortion on appearance of partial discharges in cable insulation was evaluated. Here, the study related to the PD behavior has been made focusing on the\ua0influence of multilevel converter parameters such as the number of submodules and the switching frequency chosen to generate different voltage stresses. In addition, effects of using a function generator for producing desired waveform instead of the dSPACE board as in the previous work, are highlighted. In particular, it is shown implementation of the former made it\ua0possible utilizing the Pry-Cam synchronization and thus the realization of the PD pattern

Lifetime measurements in neutron-rich Fe and Co isotopes

Lifetimes of yrast states in neutron-rich Fe and Co isotopes were measured using the differential Recoil Distance Doppler Shift (RDDS) and the differential decay curves methods. The nuclei of interest were populated in multi-nucleon transfer in inverse kinematics. The deduced B(E2) values are compared with large-scale shell-model calculations, leading to a better understanding of the mechanisms at the origin of the onset of collectivity in the region just below 68Ni

Two-Particle Separation Energies in the Superdeformed Well

The location of nuclear closed shells, as evidenced through discontinuities in binding energy and one-and two-particle separation energy systematics, remains one of the simplest tests of global nuclear models. How shell gaps evolve, whether with increasing mass, increasing neutron: proton ratio or increasing deformation, is still uncertain, and it has recently been suggested that one must go beyond a static meanfield picture to include the effects of dynamic fluctuations in the nuclear shape even in the ground state. The identification of key properties which may distinguish between competing approaches is thus vital. Comparison of the binding energies of superdeformed nuclei in the A approximate to 190 region shows that two-proton separation energies are higher in the superdeformed state than in the normal state, despite the probably lower Coulomb barrier and lower total binding energy. Possible reasons for this difference are discussed. This somewhat counterintuitive result provides a critical test for global nuclear models

Gamma decay of the possible 1- two-phonon state in 140Ce excited via inelastic scattering of 17O

The \u3b3 decay from the low-lying dipole states of 140Ce excited via inelastic scattering of 17O at bombarding energy of 340 MeV was measured using the high resolution AGATA-Demonstrator array in coincidence with scattered ions detected in two segmented \u394E-E silicon detectors of the TRACE array. Particular attention is here given to the decay of the first 1- state at 3643 keV which is considered to be of two-phonon character. The gamma-gamma coincidence method was applied to select desired decay branch. No direct decay from this state was observed to 2+ and 3- phonon states which would be the proof of the pure harmonic coupling. The comparison between experimentally obtained differential cross sections and analysis with distorted wave Born approximation (DWBA) allowed to conclude that the first 1- state has a different nature than higher-lying pygmy dipole states. This was possible using the form factor obtained by folding a microscopically calculated transition density

Lifetime measurments of excited states in neutron-rich fission fragments

Lifetimes of short-lived excited states in a wide range of neutron-rich fission fragments were measured using the recoil distance Doppler shift (RDDS) technique, which was applied to fusion-fission reactions in inverse kinematics for the first time. The fission fragments were identified event-by-event in mass, charge, and atomic number using the VAMOS magnetic spectrometer at GANIL. Gamma rays originating from the fission fragments were measured with the EXOGAM array of Ge Clover detectors around the target position. Using a degrader, the change in the Doppler shift of the gamma ray allows the application of the RDDS method. Details of the experimental technique will be discussed and the status for the ongoing analysis for odd-mass yttrium isotopes will be presented

Evolution of nuclear shapes in odd-mass yttrium and niobium isotopes from lifetime measurements following fission reactions

Lifetimes of excited states in Y99,Y101,Nb101,Nb103, and Nb105 were measured in an experiment using the recoil distance Doppler shift method at GANIL (Grand Accélérateur National d'Ions Lourds). The neutron-rich nuclei were produced in fission reactions between a U238 beam and a Be9 target. Prompt γ rays were measured with the EXOGAM array and correlated with fission fragments that were identified in mass and atomic number with the VAMOS++ spectrometer. The measured lifetimes, together with branching ratios, provide B(M1) and B(E2) values for the strongly coupled rotational bands built on the [422]5/2+ ground state in the Y and Nb nuclei with neutron number N≥60. The comparison of the experimental results with triaxial particle-rotor calculations provides information about the evolution of the nuclear shape in this mass region

Nature of low-lying electric dipole resonance excitations in Ge 74

Isospin properties of dipole excitations in Ge-74 are investigated using the (alpha,alpha'gamma) reaction and compared to (gamma,gamma') data. The results indicate that the dipole excitations in the energy region of 6 to 9MeV adhere to the scenario of the recently found splitting of the region of dipole excitations into two separated parts: one at low energy, being populated by both isoscalar and isovector probes, and the other at high energy, excited only by the electromagnetic probe. Relativistic quasiparticle time blocking approximation (RQTBA) calculations show a reduction in the isoscalar E1 strength with an increase in excitation energy, which is consistent with the measurement