New Insights into Frontier of Nuclear Structure of Neutronrich Nuclei by Means of Prompt Fission γ Ray Measurements at Gammasphere(Ⅰ: Section 1 & Section 2)

Key words: 
• neutronrich nuclei /
• high spin /
• yrast /
• fission /
• ²⁵²Cf /
• γ-γ-γ and γ-γ(θ) coincidences /
• Gammasphere /
• chiral symmetry breaking /
• triaxiality /
• tilted axis cranking (TAC) calculations /
• soft chiral vibration /
• N=83 isotonic chain /
• doublymagic ¹³²Sn /
• shell model calculation /
• magnetic rotation /
• similarities of spectroscopy /

Abstract: New insights have been gained into the frontiers of nuclear structure of neutronrich nuclei by means of γ-γ-γ and γ-γ(θ) coincidences of prompt γ rays emitted in the spontaneous fission of ²⁵²Cf at Gammasphere. Over 5.7×10¹¹ tripleand higher fold coincidence events and the lesscompressed cube data provide excellent conditions for searches and studies over a wide unknown range with more neutron excess. Highspin yrast and near yrast level schemes of neutronrich nuclei in regions of physics interest have been identified for the first time, or extensively extended and expanded compared to previous preliminary measurements.
Chiral symmetry breaking was recently identified in eveneven neutronrich^{110, 112}Ru and ¹⁰⁸Mo isotopes. The former have the largest lowering of ground state energy when axial symmetry is broken, and near maximum triaxiality was deduced in the isotopes. By exhibiting all the fingerprints for chiral doubling, especially the best energy degeneracy, the doublet bands observed in these Ru and Mo isotopes are the best examples of chiral properties reported in this region. The evolution of chirality from γsoft 108Ru to triaxial ^{110, 112}Ru is proposed. Tilted axis cranking (TAC) calculations extended by random phase approximation (RPA) calculations can explain the features of the doublet bands in terms of a soft chiral vibration for these eveneven nuclei. The chirality in these eveneven nuclei cannot be reduced to the simple geometrical picture as in oddodd nuclei. Instead, in these eveneven nuclei the tendency to chirality comes about from the interplay of all the neutrons in the open shell.
Systematic studies of the N=83 isotonic chain in the vicinity of the doublymagic 132Sn have yielded a wealth of spectroscopic information in this attractive region. The new data of N=83 isotones 135Te (Z=52), 136I (Z=53), ¹³⁷Xe (Z=54), ¹³⁸Cs (Z=55) and ¹³⁹Ba (Z=56), especially the observation of the longsought level scheme of ¹³⁸Cs, and shell model calculations, indicate the key role played by the coupling of the excitations of the few g7/2 valence protons outside the Z=50 major shell closure and the f7/2 valence neutrons outside the N=82 major shell closure. Resemblance of spectroscopy and counterparts were observed between the ¹³²Sn region and ²⁰⁸Pb region. Tilted rotation(magnetic rotation) was observed in ¹³⁵Te, which is the first observation of magne tic rotation in the vicinity of ¹³²Sn.