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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)
Y.X.Luo, J.H.Hamilton, J.O.Rasmussen, A.V.Ramayya, C.Goodin, A.V.Daniel, N.J.Stone, S.J.Zhu, J.K.Hwang, S.H.Liu, C.J.Beyer, Ke Li, H.L.Crowell, D.Almehed, S.Frauendorf, A.Covello0, V.Dimitrov, Jing-ye Zhang, X.L.Che, Z.Jang, D.Fong, A.Gelberg, I.Stefanescu, A.Gargano0, E.F.Jones, P.M.Gore, I.Y.Lee, G.M.Ter-Akopian, Yu.Ts.Oganessian, M.A.Stoyer, R.Donangelo, W.C.Ma, J.D.Cole, J.Kormicki, X.Q.Zhang, S.C.Wu, J.Gilat, T.N.Ginter, S.J.Asztalos0
2010, 27(3): 229-251. doi: 10.11804/NuclPhysRev.27.03.229  Published:2010-09-20
Keywords: 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,
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rich110, 112Ru 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, 112Ru 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.
New Insights into Frontier of Nuclear Structure of Neutronrich Nuclei by Means of Prompt Fission γ Ray Measurements at Gammasphere(II: Section 3 & Section 4)
Y.X.Luo, J.H.Hamilton, J.O.Rasmussen, A.V.Ramayya, C.Goodin, A.V.Daniel, N.J.Stone, S.J.Zhu, J.K.Hwang, S.H.Liu, C.J.Beyer, Ke Li, H.L.Crowell, D.Almehed, S.Frauendorf, A.Covello0, V.Dimitrov, Jing-ye Zhang, X.L.Che, Z.Jang, D.Fong, A.Gelberg, I.Stefanescu, A.Gargano0, E.F.Jones, P.M.Gore, I.Y.Lee, G.M.Ter-Akopian, Yu.Ts.Oganessian, M.A.Stoyer, R.Donangelo, W.C.Ma, J.D.Cole, J.Kormicki, X.Q.Zhang, S.C.Wu, J.Gilat, T.N.Ginter, S.J.Asztalos0
2010, 27(4): 363-389. doi: 10.11804/NuclPhysRev.27.04.363  Published:2010-12-20
Keywords: shape transition, axially symmetric shape, triaxial deformation, onset of deformation, bandcrossing, odd-Z, octupole excitations, B(E1)/B(E2) values, electric dipole moment D0, sharp drop of dipole moments, octupole vibrations, Cs, La isotopes
By means of γ-γ-γ and γ-γ(θ) measurements of prompt fission γ rays from 252Cf using Gammasphere, new insights have been gained into the A~100 (Z~40, N>56) neutron rich region, which has long been of interest for shape transitions, shape coexistence and onset of deformations. The systematic investigations of odd-Z isotopes have revealed a shape transition from axially symmetric shapes with very large quadrupole deformations in Z=39 (Y) to near maximum triaxial shapes with moderate quadrupole deformations in Z=43 (Tc), 44 (Ru), 45 (Rh) isotopes. The Nb (Z=41) isotopes are found to be transitional with regard to this shape transition. The bandcrossings observed in the evenN Tc and Rh isotopes are attributed to alignment of a pair of h11/2neutrons. The longexpected sudden shape transition in Nb isotopic chain is now observed. The longsought highspin level scheme and deformed states in 100Nb (N=59) were identified. The ground state of 100Nb was found to be strongly deformed, in disagreement with existing theory. The shape coexistence was confirmed in the nucleus, and the sudden shape transition in the Nb isotopic chain was seen to occur between N=58 (spherical) to N=59 (strongly deformed), in contrast to the sudden shape transition from N=59 (spherical) toN=60 (strongly deformed) in Sr (Z=38), Y (Z=39) and Zr (Z=40) isotopic chains. The observation of strongly deformed ground state in 100Nb and its very large and irregular moments of inertia make 100Nb an especially interesting case for further theoretical studies.
Recent detailed studies of octupole excitations in odd-Z Cs (Z=55) and La (Z=57) isotopes have discovered a sharp drop in electric dipole moments D0 in the Cs isotopic chain 141, 143, 144Cs, and, most likely, the previously reported moderate drop in D0 in 145, 147La is extended sharply down in 148La. The sharp drops of D0 in Cs and La are most likely analogous to those reported in 146Ba and 224Ra, which were accounted for by the reflectionasymmetric mean field shell correction theory. The very large D0 in both 141Cs and 142Xe and the simplexdependent D0 deduced in 141Cs but not in 143Cs need further theoretical studies. Based on rotational frequency ratios, ω-(I)/ω+(I), octupole vibrations are proposed in Cs isotopes, like the cases in Xe isotopes, while octupole deformations are suggested in La isotopes.