[Show abstract][Hide abstract] ABSTRACT: The available data for E2 transition strengths in the region between
neutron-deficient Hf and Pt isotopes are far from complete. More and precise
data are needed to enhance the picture of structure evolution in this region
and to test state-of-the-art nuclear models. In a simple model, the maximum
collectivity is expected at the middle of the major shell. However, for actual
nuclei, this picture may no longer be the case, and one should use a more
realistic nuclear-structure model. We address this point by studying the
spectroscopy of Hf. We remeasure the 2^+_1 half-lives of 172,174,176Hf, for
which there is some disagreement in the literature. The main goal is to
measure, for the first time, the half-lives of higher-lying states of the
rotational band. The new results are compared to a theoretical calculation for
absolute transition strengths. The half-lives were measured using \gamma-\gamma
and conversion-electron-\gamma delayed coincidences with the fast timing
method. For the determination of half-lives in the picosecond region, the
generalized centroid difference method was applied. For the theoretical
calculation of the spectroscopic properties, the interacting boson model is
employed, whose Hamiltonian is determined based on microscopic energy-density
functional calculations. The measured 2^+_1 half-lives disagree with results
from earlier \gamma-\gamma fast timing measurements, but are in agreement with
data from Coulomb excitation experiments and other methods. Half-lives of the
4^+_1 and 6^+_1 states were measured, as well as a lower limit for the 8^+_1
states. We show the importance of the mass-dependence of effective boson charge
in the description of E2 transition rates in chains of nuclei. It encourages
further studies of the microscopic origin of this mass dependence. New data on
transition rates in nuclei from neighboring isotopic chains could support these
Physical Review C 04/2015; DOI:10.1103/PhysRevC.91.044301 · 3.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Six lifetimes have been determined in the 5/2+  band from vi13/2 parentage in 167Yb by means of Recoil distance Doppler-shift (RDDS) measurements carried out at the Cologne FN tandem. The deduced transition strengths and the level scheme are reasonably described by Particle plus triaxial rotor model (PTRM) calculations except for the behavior of the quadrupole collectivity in the two signatures of the 5/2+ band. In that band, the quadrupole collectivity of the favored signature is appreciably larger than this of the unfavored signature. The effect increases with increasing the spin. Naturally, the rigid PTRM cannot explain these features, but the structure of its wave functions suggests a possible solution. It is associated with the enhanced contribution of low-Ω orbitals from vi13/2 parentage in the favored signature compared to the unfavored one. This could selectively increase the deformation of the favored signature band members and give rise to a dynamic shape coexistence taking place between the two signatures which needs quantitative explanation by future theoretical work.
Journal of Physics Conference Series 09/2014; 533(1):012034. DOI:10.1088/1742-6596/533/1/012034
[Show abstract][Hide abstract] ABSTRACT: Gamow-Teller (GT) transitions in atomic nuclei are sensitive to both nuclear shell structure and effective residual interactions. The nuclear GT excitations were studied for the mass number A=42, 46, 50, and 54 "f-shell" nuclei in (He3, t) charge-exchange reactions. In the Ca42→Sc42 reaction, most of the GT strength is concentrated in the lowest excited state at 0.6 MeV, suggesting the existence of a low-energy GT phonon excitation. As A increases, a high-energy GT phonon excitation develops in the 6-11 MeV region. In the Fe54→Co54 reaction, the high-energy GT phonon excitation mainly carries the GT strength. The existence of these two GT phonon excitations are attributed to the 2 fermionic degrees of freedom in nuclei.
[Show abstract][Hide abstract] ABSTRACT: A γγ angular correlation experiment investigating the nucleus 193Au is presented. In this work the level scheme of 193Au is extended by new level information on spins, multipolarities and newly observed states. The new results are compared with theoretical predictions from a general Interacting Boson Fermion Model (IBFM) calculation for the positive-parity states. The experimental data is in good agreement with an IBFM calculation using all proton orbitals between the shell closures at Z = 50 and Z = 126. As a dominant contribution of the d3/2d3/2 orbital to the wave function of the lowest excited states is observed, a truncated model of the IBFM using a Bose-Fermi symmetry is applied to the describe 193Au. Using the parameters of a fit performed for 193Au, the level scheme of 192Pt, the supersymmetric partner of 193Au, is predicted but shows a too small boson seniority splitting. We obtained a common fit by including states observed in 192Pt. With the new parameters a supersymmetric description of both nuclei is established.
Nuclear Physics A 02/2014; 922:200. DOI:10.1016/j.nuclphysa.2013.12.004 · 2.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cross sections for the 168Yb(alpha,gamma)172Hf and 168Yb(alpha,n)171$Hf
reactions were measured by means of the activation method using alpha particles
with energies between 12.9 MeV and 15.1 MeV. The spectroscopy of the gamma rays
emitted by the reaction products was performed using three different HPGe
detector types, namely clover-type high-purity germanium detectors, a
low-energy photon spectrometer detector, and a coaxial high-purity germanium
detector. The results were compared to Hauser-Feshbach statistical model
calculations. Within certain assumptions, astrophysical conclusions could be
drawn concerning the production of the p nucleus 168Yb. The data in this work
can serve as a contribution to the current very fragmentary experimental data
base for charged-particle induced reactions. In addition, the absolute
intensity for nine gamma-ray transitions following the electron capture decay
of 171Hf could be derived.
Nuclear Physics A 10/2013; 916:149. DOI:10.1016/j.nuclphysa.2013.08.002 · 2.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Lifetimes of first 4(+) and 6(+) states have been measured in neutron-rich isotopes of Zr, Mo, Ru and Pd using the recoil distance Doppler shift method at GANIL. The nuclei were produced through a fusion-fission reaction in inverse kinematics. The fission fragments were fully identified in the large-acceptance VAMOS spectrometer and gamma -rays were detected in coincidence with the EXOGAM germanium array. Lifetimes of excited states in the range of 1-100 ps were measured with the Cologne plunger. Preliminary lifetime results are presented as well as a discussion on the evolution of the collectivity in this region.
The European Physical Journal Conferences 05/2013; 62:01002. DOI:10.1051/epjconf/20136201002
[Show abstract][Hide abstract] ABSTRACT: A Mo100(α,2n)Ru102 fusion evaporation experiment, performed at the HORUS cube spectrometer at the Cologne FN-Tandem accelerator, is presented and discussed. Via the analysis of γγ coincidences, it was possible to extend the level scheme with 40 new levels and 81 decays. Several new spins and multipole mixing ratios were determined using the γγ angular correlation technique. In addition, a DSAM analysis was performed and 14 new effective lifetimes were determined in the femtosecond range. Thus, several effective transition strengths were determined. Investigating the phononic structure, candidates for the quadrupole–octupole phonon multiplet 21+⊗31− were found. The data is compared to calculations within the Interacting Boson Model (IBM-1) in the U(5) limit, the O(6) limit as well as a transitional class U(5)–O(6). The low spin level energies show characteristics of multiphonon excitations and point towards 102Ru resembling an anharmonic vibrational nucleus. The B(E2)B(E2) transition strengths do not exactly match this picture, and are described in a better way using the U(5)–O(6) calculation.
Nuclear Physics A 04/2013; 903:18-39. DOI:10.1016/j.nuclphysa.2012.11.013 · 2.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nine lifetimes have been determined for the first time in the 5/2−5/2− and 5/2+5/2+ bands of 167Yb by means of Recoil distance Doppler-shift measurements carried out at the Cologne FN tandem. For the data analysis, a dedicated version of the Differential decay curve method was used. The newly deduced transition strengths and the level scheme are reasonably described by the Particle plus triaxial rotor model (PTRM) calculations except for the behavior of the quadrupole collectivity in the two signatures of the 5/2+5/2+ band. The stretched B(E2)ʼs in the favored signature are appreciably larger than those in the unfavored signature. The effect increases with spin. In the PTRM calculations, the wave functions of the favored signature levels contain larger contribution of low-Ω orbitals from νi13/2νi13/2 parentage than those of the unfavored levels. This could selectively increase the deformation of the favored signature band members. If so, rotation and Coriolis interaction would give rise to a dynamic shape coexistence taking place between the two signatures.
Nuclear Physics A 04/2013; 903:1–17. DOI:10.1016/j.nuclphysa.2013.02.010 · 2.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Miniball germanium detector array has been operational at the REX (Radioactive ion beam EXperiment) post accelerator at the Isotope Separator On-Line facility ISOLDE at CERN since 2001. During the last decade, a series of successful Coulomb excitation and transfer reaction studies have been performed with this array, utilizing the unique and high-quality radioactive ion beams which are available at ISOLDE. In this article, an overview is given of the technical details of the full Miniball setup, including a description of the
-ray and particle detectors, beam monitoring devices and methods to deal with beam contamination. The specific timing properties of the REX-ISOLDE facility are highlighted to indicate the sensitivity that can be achieved with the full Miniball setup. The article is finalized with a summary of some physics highlights at REX-ISOLDE and the utilization of the Miniball germanium detectors at other facilities.
European Physical Journal A 03/2013; 49(3):1-32. DOI:10.1140/epja/i2013-13040-9 · 2.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report on the study of excited states in Kr-92,Kr-94,Kr-96 populated via projectile Coulomb excitation at safe energies. The radioactive ion beams at energies of 2.85 MeV/u were delivered by the REX-ISOLDE, facility at CERN and impinged on self-supporting (194,196) pt targets. The emitted y gamma-rays were detected by the Miniball detector-array. A detailed description of the experimental techniques used for extracting diagonal and transitional matrix elements and of the theoretical framework is given. The present experiment reveals the moderate evolution of the collective structure in the considered neutron-rich Kr isotopic chain, whicb is supported by the interacting boson model combined with the self-consistent mean-field method using a microscopic Gogny energy-density functional. The theory also suggests possible shape coexistence in the exotic nucleus Kr-96.
Nuclear Physics A 02/2013; 899:1. DOI:10.1016/j.nuclphysa.2013.01.013 · 2.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A complex beam detector consisting of a high-resolution beam profile monitor (BPM) and a time of flight (TOF) spectrometer with tracking capabilities was designed especially for the special needs of the Cologne center for accelerator mass spectrometry (CologneAMS). The beam detector assembly is designed to match the beam specifications of the 6 MV Tandetron AMS setup and its data acquisition system. It will have a reconfigurable structure, either as a fast TOF subsystem with a ca. 10 cm2 equivalent active area, or as a more complex BPM–TOF detector with beam tracking capabilities and a larger active area (16 cm2). The purpose of this detector is to suppress background during spectrometry of heavy ions (U, Cm, Pu, Am etc.) and to suppress isobaric interferences such as 36S in 36Cl spectra.
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 01/2013; 294:410–415. DOI:10.1016/j.nimb.2012.04.026 · 1.12 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recently, a new island of X(5) nuclei has been suggested around A=180 exemplified by some Osmium isotopes. To investigate the limits of its region, a Recoil-distance Doppler shift lifetime measurement has been performed for 182Pt. For the data analysis, the Differential decay curve method has been applied in a newly developed version convenient for low recoil velocities and a non-negligible fraction of nuclei stopped already in the target. The level energies and the newly deduced transition quadrupole moments in the yrast band reveal the persistence of X(5) features in the investigated nucleus, but other spectroscopic data and IBM and GCM calculations indicate shape coexistence and a position of 182Pt close but not at the critical point of the shape-transition.
Journal of Physics Conference Series 05/2012; 366(1). DOI:10.1088/1742-6596/366/1/012036
[Show abstract][Hide abstract] ABSTRACT: The high-velocity transient-field (HVTF) technique was used to measure the g factor of the 2+ state of 72Zn produced as a radioactive beam. The transient-field strength was probed at high velocity in ferromagnetic iron and gadolinium hosts using 76Ge beams. The potential of the HVTF method is demonstrated and the difficulties that need to be overcome for a reliable use of the TF technique with high-Z, high-velocity radioactive beams are revealed. The polarization of K-shell vacancies at high velocity, which shows more than an order of magnitude difference between Z=20 and Z=30 is discussed. The g-factor measurement hints at the theoretically predicted transition in the structure of the Zn isotopes near N=40.
Physical Review C 03/2012; 85(3). DOI:10.1103/PhysRevC.85.034334 · 3.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using the reaction 170Yb(16O, 4n) at a beam energy of 87 MeV and the Recoil Distance Doppler-Shift Method, five lifetimes in the yrast band of 182Pt have been determined for the first time. For the data analysis, a dedicated version of the Differential Decay Curve Method has been employed. Calculations within the Interacting Boson Model and the General Collective Model indicate shape coexistence in the investigated nucleus.
Nuclear Physics A 03/2012; 877:19–34. DOI:10.1016/j.nuclphysa.2012.01.001 · 2.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Gamow-Teller transition strengths, B(GT), in pf-shell nuclei are of interest in nuclear physics as well as in nuclear astrophysics. A high-resolution (He-3, t) charge-exchange (CE) reaction was performed on the T-z = +1 nucleus Fe-54 at 0 degrees and at an intermediate incident energy of 140 MeV/nucleon for the study of precise GT transition strengths to the final T-z = 0 nucleus Co-54. By applying dispersion matching techniques for a high-quality He-3 beam at RCNP, an energy resolution of 21 keV and an angular resolution of 5 mr were realized. The bumplike structure of the GT resonance observed in low-resolution CE reactions at around the excitation energy (E-x) of 10MeV was resolved in individual L = 0, GT states. Excitation strengths were obtained for these GT states. If the R-2 value that is defined by the ratio between GT and Fermi unit cross sections is known, the B(GT) values can be determined from the excitation strengths. For the derivation of the R-2 value, the "merged analysis" combining the GT strength distribution from the Fe-54(He-3, t)Co-54 study and the half-life from a Ni-54 beta decay was used, where T = 1 isospin symmetry for A = 54 isobars was assumed. The GT strengths were compared with a shell-model calculation using the GXPF1 interaction. The final GT states can have the isospin values T = 0, 1, and 2. The isospin T of each GT state observed in the 8.3 <= E-x <= 12.0 MeV region of the Fe-54(He-3, t)Co-54 spectrum was identified by comparing the excitation strength with that of corresponding M1 state observed in a Fe-54(p, p')Fe-54 experiment. The B(GT) values of the states identified to have T = 2, in particular, are of importance for the calculation of the electron capture rates at the core-collapse stage of presupernovae. The B(GT) strengths were further compared with B(M1) strengths measured in the Fe-54(e, e')Fe-54 reaction. In the M1 excitation using an electromagnetic probe, isoscalar (IS) and isovector (IV) orbital type operators are active in addition to the IV spin type operator that mediates the GT transitions. The contributions of the IS and IV orbital terms were studied by calculating the ratio of the strengths of analogous M1 and GT transitions.
Physical Review C 02/2012; 85(2-2). DOI:10.1103/PhysRevC.85.024308 · 3.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The neutron-rich nuclei 94,96Kr were studied via projectile Coulomb excitation at the REX-ISOLDE facility at CERN. Level energies of the first excited 2(+) states and their absolute E2 transition strengths to the ground state are determined and discussed in the context of the E(2(1)(+)) and B(E2;2(1)(+)→0(1)(+)) systematics of the krypton chain. Contrary to previously published results no sudden onset of deformation is observed. This experimental result is supported by a new proton-neutron interacting boson model calculation based on the constrained Hartree-Fock-Bogoliubov approach using the microscopic Gogny-D1M energy density functional.