Evidence for shape coexistence in the N=Z nucleus 7236Kr36

Gamma rays associated with the decay of states in the N=Z nucleus ⁷²Kr have been identified following the ¹⁶O(⁵⁸Ni, 2n) ⁷²Kr reaction at a mean beam energy of 170 MeV. Identification was made using the Daresbury Recoil Separator. The first excited state was found to be at 709.1 ± 0.3 keV and to be populated with a cross section of 60 ± 25 υb. The pattern of gamma rays associated with ⁷²Kr indicates the co-existence of nuclear shapes.     

Microscopic treatment of shape coexistence in even-even nuclei and related extension of IBM

Intruder quasi-rotational bands in tin isotopes are treated on the bases of a microscopic theory developed previously and extended to include proton pp-RPA bosons and hh-RPA bosons in addition to neutron bosons. Nucleons in the core are shown to play a decisive role in the intruder band. It is pointed out that the proton hole and particle bosons and the neutron bosons conspire to deform the nucleus and lower the intruder states to the observed relatively low excitation energies. Some general features of shape coexistence in nuclei are discussed.     

Description of the properties of the low-lying energy states in 100Mo with IBM2

The properties of the low-lying energy states for the 100Mo isotope is investigated within the framework of the proton-neutron interacting model IBM2.By considering the relative energy of the d proton boson to be diferent from that of the neutron boson and taking into account the dipole interacting among like-boson LπLπand LνLν,the low-lying energy spectrum is reproduced well.Particularly,the relative position of the energies for 2+1,0+2,2+2 and 4+1states shifted correctly fit the experimental data.The electromagnetic properties,including the key observable B(E2)reduced transition branching ratios and the E2 reduced matrix elements of the experimental data,are well described.Our calculations show possible shape coexistence in the 100Mo nucleus.     

Nuclear shape transition in neutron-deficient gold isotopes

On-line resonance ionization mass spectrometry has been applied to determine the isotope shift and hyperfine structure of^185–189Au and tne I=11/2 isomer of¹⁸⁹Au in the 6s²S_1/2 6p²P_1/2=268 nm) transition. The Au atoms were obtained as daughters of mass-separated Hg isotopes produced at the ISOLDE facility at CERN, ionized by a three-step resonant photoionization process, detected and mass-selected by time of flight. A drastic change of the nuclear charge radius was observed between¹⁸⁷Au and¹⁸⁶Au, which is interpreted as an onset of strong deformation of ₂ 0.25 in¹⁸⁶Au and¹⁸⁵Au.This work has been supported by the Bundesministerium für Forschung und Technologie. One of us, J. Wood was supported in part by the U.S. Department of Energy, Contract DE-AS05-80 ER 10599.     

Description of the Structural Properties of Low-Lying States in ^102Ru with IBM2

We describe the properties of low-lying states of ^102Ru within the framework of the proton-neutron interacting model IBM2. The theoretical predictions of the ground state, quasi-γ and quasi-β bands, and the ratios of the B（E2） transition strengths are reproduced very well. The structural properties of ^102Ru are identified in the parameters space of the interacting boson model （IBM2）. The characteristic feature of the energy spectrum structure exhibits that ^102Ru is very close to the critical point of Uπv（5）-Oπv（6） transition and towards Uπv（5） symmetry. The key sensitive quantities of the B（E2） branching ratio clearly indicate that ^102Ru is a primary Oπv（6） symmetry, while with a somewhat Uπv（5） symmetry. It is possible that the shape coexistence persists in ^102Ru, whereas the evident fingerprint of the shape coexistence has not been observed.     

Description of the evolution of mixed-symmetry states in the N=78 isotonic chain with IBM2

The characteristics of the lowest mixed-symmetry states 2_ms⁺ and 1_ms⁺ for ¹³²Xe, ¹³⁴Ba and ¹³⁶Ce in the even-even N=78 isotones are investigated within the framework of the IBM2 model. The lowest mixed-symmetry state 2_ms⁺ levels for both a single isolated state in ¹³²Xe and ¹³⁶Ce and a fragmented state in ¹³⁴Ba are reproduced by the predictions. The agreement between the IBM2 calculation and the experimental values is good for the B(E2) and B(M1) transition probabilities both quantitatively and qualitatively. The predicted summed B(M1) strength follows the experimental data, remaining nearly constant as a function of proton number along the chain of the N=78 isotones.     

Xe偶－偶同位素低激发态性质的IBM2描述

通过考虑同类核子相干对Ｓρ、Ｄρ间的四极相互作用，在ＩＢＭＺ中对Ｘｅ偶一偶同位素１１６Ｘｅ—１３２Ｘｅ的低激发态能谱和Ｅ２跃过几率进行了理论分析．计算结果有效地改善了ＩＢＭ中这些核的准γ带能谱的Ｓｔａａｓｅｒｉｎｇ现象，较精确地描述了实验观察到的低激发态的性质．     

New shape isomer in the self-conjugate nucleus 72Kr

A new isomeric 0(+) state was identified as the first excited state in the self-conjugate (N=Z) nucleus 72Kr. By combining for the first time conversion-electron and gamma-ray spectroscopy with the production of metastable states in high-energy fragmentation, the electric-monopole decay of the new isomer to the ground state was established. The new 0(+) state is understood as the band head of the known prolate rotational structure, which strongly supports the interpretation that 72Kr is one of the rare nuclei having an oblate-deformed ground state. This observation gives in fact the first evidence for a shape isomer in a N=Z nucleus.     

Unveiling the origin of shape coexistence in lead isotopes

Shape coexistence in the nuclei (182-192)Pb is analyzed with the Hartree-Fock-Bogoliubov approach and the Gogny force. Good agreement with the experimental energies is found for the coexisting spherical, oblate, and prolate states. Contrary to the established interpretation, it is found that the low-lying prolate and oblate 0+ states are predominantly characterized by neutron correlations whereas the protons behave, in general, rather as spectators than playing an active role.     

Description of the Structure of 88-96Sr in the IBM

The neutron-rich even-even Sr isotopes are studied in the interacting boson model. The semi-closed nucleus ⁸⁸Sr is studied in the framework of the IBMI, and ^90-96Sr are studied in the neutron-proton interacting boson model. The spectrum and E2, M1 transition properties are calculated and compared with experiment.     

Variational approach to shape coexistence in Se

Results are presented on shape coexistence and neutron–proton correlations at low and high spins in the N=Z nucleus ⁶⁸Se obtained within the complex version of the Excited Vampir variational approach. Effects of possible contaminations due to the center of momentum motion on the lowest two positive-parity bands are discussed.     

Evidence for shape coexistence in186Pt

High-spin states in¹⁸⁶Pt have been populated by the¹⁸⁸Os (α, 6n) reaction and were investigated with the OSIRIS spectrometer. A shape coexistence at high spins was established in the nucleus¹⁸⁶Pt, which lies on the border between light prolate and heavy oblate Pt nuclei. Two bands corresponding to predominantly prolate shapes and one band of predominantly oblate shape have been observed. For prolate shapes a (π h _9/2)² alignment and for oblate shapesa (vi _13/2)² alignment has been found.     

Lifetime measurements in Pt and the shape coexistence picture

Lifetimes for levels in the yrast band of ¹⁸⁴Pt have been measured up to spin 16⁺ using the recoil distance technique. The B(E2) values exhibit a marked increase in going from spin 2 to 10, consistent with a proposal that two bands of different deformations are mixing at low spin. This provides further support for shape coexistence occuring at low excitation energies in this region.     

An IBM2 Description of the E(5) Symmetry in 134Ba and 108pd

With the low-lying energy levels, E2 transition branching ratios and absolute transition rates of the 134Ba and 108Pd, are investigated in the neutron-proton interacting boson model (IBM2) which includes the quadrupole quadrupole interaction between like bosons and the E(5) symmetry, it shows that the IBM2 can describe the nuclei at critical point of a phase transition well.     

An IBM2 Description of the E(5) Symmetry in 134Ba and 108Pd

With the low-lying energy levels, E2 transition branching ratios and absolute transition rates of the ¹³⁴Ba and ¹⁰⁸Pd, are investigated in the neutron-proton interacting boson model (IBM2) which includes the quadrupole-quadrupole interaction between like bosons and the E(5) symmetry, it shows that the IBM2 can describe the nuclei at critical point of a phase transition well.