Observation of low- and high-energy gamow-teller phonon excitations in nuclei

Y. Fujita et al. ; 5 pags. ; 3 figs. PACS numbers: 24.30.Cz, 25.55.Kr, 27.40.+zGamow-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. © 2014 American Physical Society.This work was in part supported by MEXT, Japan (Grants No. 13002001, No. 15540274, and No. 18540270); MICINN, Spain (Grant No. FPA200806419-C02-01). Y. F. and B. R. acknowledge the support of the Japan-Spain collaboration program by JSPS and CSIC.Peer Reviewe

First g(2+) measurement on neutron-rich 72 Zn, and the high-velocity transient field technique for radioactive heavy-ion beams

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

Low-lying octupole isovector excitation in Nd-144

International audienceThe nature of low-lying 3− levels in Nd144 was investigated in the Nd143(n,γγ) cold neutron-capture reaction. The combination of the high neutron flux from the research reactor at the Institut Laue-Langevin and the high γ-ray detection efficiency of the EXILL setup allowed the recording of γγ coincidences. From the coincidence data precise branching ratios were extracted. Furthermore, the octagonal symmetry of the setup allowed angular-distribution measurements to determine multipole-mixing ratios. Additionally, in a second measurement the ultra-high resolution spectrometer GAMS6 was employed to conduct lifetime measurements using the gamma-ray induced Doppler-shift technique (GRID). The confirmed strong M1 component in the 33−→31− decay strongly supports the assignment of the 33− level at 2779keV as low-lying isovector octupole excitation. Microscopic calculations within the quasiparticle phonon model confirm an isovector component in the wave function of the 33− level, firmly establishing this fundamental mode of nuclear excitation in near-spherical nuclei

Low collectivity of the first 2⁺ states of ²¹²,²¹⁰Po

The lifetimes of the first 2⁺ excited states of ²¹²,²¹⁰Po were measured in two transfer reactions ²⁰⁸Pb(¹²C,⁸Be)²¹²Po and ²⁰⁸Pb(¹²C,¹⁰Be)²¹⁰Po by the Recoil Distance Doppler Shift (RDDS) method and by the Doppler Shift Attenuation method (DSAM), respectively. The derived absolute B(E2) values of 2.6(3) W.u. for ²¹²Po and 1.83(28) W.u. for ²¹⁰Po indicate low collectivity. It is shown that the properties of the yrast 2₁⁺, 4₁⁺, 6₁⁺ and 8₁⁺ states in both nuclei cannot be described consistently in the framework of nuclear shell models. It is also demonstrated in the case of ²¹⁰Po that Quasi-particle Phonon Model (QPM) calculations cannot overcome this problem thus indicating the existence of a peculiarity which is neglected in both theoretical approaches

High-spin structure in the transitional nucleus 131Xe:Competitive neutron and proton alignment in the vicinity of the N = 82 shell closure

International audienceThe transitional nucleus Xe131 is investigated after multinucleon transfer in the Xe136+Pb208 and Xe136+U238 reactions employing the high-resolution Advanced γ-Tracking Array (AGATA) coupled to the magnetic spectrometer PRISMA at the Laboratori Nazionali di Legnaro, Italy, and as an elusive reaction product in the fusion-evaporation reaction Sn124(B11,p3n)Xe131 employing the High-efficiency Observatory for γ-Ray Unique Spectroscopy (HORUS) γ-ray array coupled to a double-sided silicon strip detector at the University of Cologne, Germany. The level scheme of Xe131 is extended to 5 MeV. A pronounced backbending is observed at ℏω≈0.4MeV along the negative-parity one-quasiparticle νh11/2(α=−1/2) band. The results are compared to the high-spin systematics of the Z=54 isotopes and the N=77 isotones. Large-scale shell-model calculations employing the PQM130, SN100PN, GCN50:82, SN100-KTH, and a realistic effective interaction reproduce the experimental findings and provide guidance to elucidate the structure of the high-spin states. Further calculations in Xe129−132 provide insight into the changing nuclear structure along the Xe chain towards the N=82 shell closure. Proton occupancy in the π0h11/2 orbital is found to be decisive for the description of the observed backbending phenomenon

First g(2+) measurement on neutron-rich 72Zn, and the high-velocity transient field technique

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. ©2012 American Physical SocietyThis work was supported by the EU FP6 contract no. EURONS N◦RII3-CT-2004-506065, by the IAP Program, P6/23 BriX, Belgian Science Policy, by the Bulgarian Science Fund DID-02/16 and DFNR-02/5, by the Spanish Ministerio de Ciencia e Innovación under contract nos. FPA2007-66069 and FPA2009-13377-C02-02, the Spanish Consolider-Ingenio 2010 Programme CPAN (CSD2007-00042), and by the AustralianResearch Council Discovery Grant No. DP0773273.The work of E.F. has been supported in part by the ExtreMe Matter Institute EMMI.Peer Reviewe

BAND STRUCTURE IN 84Sr

Excited levels in 84Sr have been populated in the reactions 76Ge (12C, 4n) and 81Br (6Li,3n). Singles and coincidence gamma-ray spectra as well as angular distributions were measured. Lifetimes were measured by the recoil-distance method. The positive parity levels were sorted out into four bands ; the existence of at least two negative parity bands was established. The properties of 84Sr were compared to those of neighbouring nuclei

Plunger lifetimes and electromagnetic transition strengths in odd 167Yb

Nine lifetimes have been determined for the first time in the 5/2−[523] and 5/2+[642] 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+[642] 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 νi13/2 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