Energy inversion of thef 7/2 andh 9/2 neutrons in yrast states of154Yb

Abstract

The N=84 nucleus 154yb was investigated through the l~ reaction. We found the 16 + yrast level in this nucleus to be formed by the (~h2w2 xJfT/2hg/2) configuration, in contrast to the 9 " 2 2 + 9lower-Z lsotones where (Xhlv2~f 7/2)16 is the yrast state. Manybody shell model calculations with empirical two-body interactions support our conclusion. PACS: 21.60. Cs; 23.20. Lv;27.70.+q The lowering of the vh~/2 single particle state is a characteristic structure feature in the spherical nuclei above 14~Gd where the valence protons fill the h~/2 orbital. This is evident in the even-Z N=83 isotones where the 9h~/2 energy regularly decreases from 1.40MeV in ~47Gd down to 0.57MeV in lS3yb. For the yet unknown lSgOs nucleus one expects [1] that the h9/2 state has dropped below 9f7/2 and would form the nuclear ground state. A corresponding energy decrease is known to occur in the N=84 isotones for the (9f7/2 h9/2 ) 8 + yrast state, which here already in lS6Hf has dropped below the (9f27/2)6 + level, giving rise to A I=8 a-decay [2]. The analogous 4qp-configuration xh~l/29fT/2 h9/2 forms [3,4] the well established 18 + yrast state of ~5~ and ~S2Ers4. It decays to the ground state through a cascade of 9 stretched E2 transitions where the first one proceeds to the (xh]l/2vf27/2)16 + yrast level. In higher-Z isotones these 4qp-states are so far not clearly identified. We have now investigated the 154yb yrast states and can show that in this nucleus the two 4qp-configurations are inverted at I=16; the(xh~/29fT/2hs/2)16 + level is now yrast state, while the maximum aligned 16 + level of rch2n/2 ~f~/2 character lies above it and is not fed in the main yrast cascade.The ~5~yb yrast levels were populated through the ~~ ot2p) reaction at 4.7MeV/u beam energy. The experiments were carried out at the Dedicated to Prof. Dr. O. Schult on the occasion of his 60 t~ birthday 9 Visitor from Institut of Nuclear Physics, Cracow, Poland HMI Berlin with a 300ns-pulsed beam from the VICKSI accelerator, using the OSIRIS -(-detector array, combined with a large-volume four-sector neutron detector at 0 ~ for identification of unknown evaporation channels.The total of 5 • recorded coincidences gave the ls4Yb level scheme of Fig. 1. The four lowest transitions were known from 1S4Lu(9+) B-decay [5,6], and an earlier in beam study [7] had established the main yrast cascade up to ~5.TMeV, but with other transition orderings than we found. Our data firmly established a new yrast isomer at 4.479MeV. Its 18.6(15)ns half life was extracted from the t-(af data for the 622, 782, and 869keV transitions. The half life of the previously known lower-lying 8 + isomer was redeterrnined using the tT-( data of the three strongest transitions below and above it. The resulting 28(2)ns 8 + half life is about twice shorter than the value from the earlier measurement [7 ], where only one isomeric state was known.Assignment of level spins and parities from our data is limited as the presence of the neutron detector required all "(-detectors to be located near ...

γ-ray spectroscopy approaching the limits of existence of atomic nuclei: A study of the excited states ofPt168andPtHornillos1, O’Donnell2, Simpson3 et al. 2009Phys. Rev. C10110View full textAdd to dashboardCite

“…Long-lived 8 + states have been observed in other nuclei in the region. For example, in the case of 152 Er 84 , a half-life of 1.8 ns [22] has been reported for the 8 + state while in 154 Yb 84 T 1/2 (8 + ) = 28 ns [23]. In both cases, the excitation energy of the yrast I π = 8 + state is lowered, relative to its position in the heavier isotopic neighbor, while the energy of the yrast I π = 6 + state continues to increase with decreasing neutron number.…”

Multiparticle configurations of excited states in Lu155

“…Previous studies of isotones in this region have revealed that as the occupancy of the πh 11/2 orbital increases with increasing Z, states involving configurations with neutrons in the h 9/2 orbital are lowered in energy relative to those with neutrons in the f 7/2 orbital [1][2][3][4]. This observation has been attributed to an increasing attraction between h 11/2 protons and h 9/2 neutrons as the occu- * Present address: Department of Physics, University of Surrey, Guildford GU2 7XH, United Kingdom † Present address: Department of Physics, University of Guelph, Guelph, Ontario, N1G 2W1 Canada pancy of the h 11/2 proton orbital increases.…”

Spectroscopy ofYb155: Structure evolution in theN=85isotones

“…The calculations were performed in the model space ν(h 9/2 , f 7/2 , i 13/2 ) and two-body interactions were taken into account. The single particle energies and the two-body interactions were extracted from the neighboring nuclei 154 Yb [5] …”

“…For the eveneven Yb isotopes, the low-lying excitations exhibit a clear structural evolution with neutron number N . For instance, with N decreasing from 88 to 84, 158 Yb shows the behavior of soft rotation [1]; 156 Yb undergoes an evolution from a quasivibrational to quasirotational structure with increasing spin [2,3]; and 154 Yb is interpreted in terms of the quasiparticle coupling [4,5]. For the odd-A Yb isotopes in this transitional region, collective structures built on the f 7/2 , h 9/2 , and i 13/2 neutron orbitals have been observed in 157 Yb [6,7] and the coexistence of different nuclear shapes has been revealed.…”