轻稀土核的B（E2）异常和形状变化

The light rare-earth nuclei with mass number A≈130 lie in the transitional region, they are soft and triaxial deformation occurs easily. Shape change from spherical to deformed when proton numbers move away from Z=50 or neutron numbers from the N=82 closed shells, and there is a strong competition between collective movement and particle excitation at high spin. Lifetime measurements of high spin states for the even Ce isotopes in this mass region show that the reduced transition probabilities B(E2) decrease with increasing spin, and exhibit a minimum in the backbanding region. Theoretical calculations can describe well features of the backbend for these nuclei, and successfully predicted that a minimum of B(E2) values appears in the vicinity of backbending. However, anomalously high B(E2) values for the 12+ state were reported in 128Ce、130Ce、132Ce. This effect is inapprehensible, any theoretical calculation can not reproduce these extremely high B(E2) values. Later, a completely contrary result exhibiting smaller B(E2) strength in 128Ce、132Ce was presented. It is necessary to repeat the lifetime measurements in 130Ce in order to undoubtedly clarify the anomalous behaviour of B(E2) for the Ce isotopes. It is also interesting to study shape change of odd-A nuclei because the unpaired nucleon is a sensitive porbe of properties of the core, a wealth of information on nuclear structure can be obtained. Out of this motive, we have determined lifetimes of high spin states in light rare-earth nuclei 129,130,131Ce and 131Pr through the fussion-evaporation reaction induced by heavy ion using Doppler Shift attenuation method in γ-γ coincidence measurement with multidetectors. The results indicate that collectivity of 129Ce is reduced with respect to the even-even core 128Ce. The transition quadrupole moments of the positive- and negative- parity bands exhibit a considerably similar behavior with increasing angular momentum. It would appear that a pair of h11/2 quasiprotons possibly have a dominant influence on property of the core. The previously reported anomalously high B(E2) values around backband in 130Ce was not observed, whereas a minimum was seen in the present work. A triaxial deformation with γ=7°could be inferred. Shape change caused by band crossing was found in the lifetime measurements of high spin yrast states in 131Pr and 131Ce. A comparison with the neighbouring even-even nucleus 130Ce indicates that influence of the valence neutron on shape change of core is greater than that of the valence proton. Besides, level lifetimes of 90Mo have been measured through the reaction 59Co(35Cl, 2p2n)90Mo. Enhanced B(M1) in the positive-parity decay sequence and apppreciable staggering of the large B(E2) valuces in the negative-parity states were observed. Transitions between positive- and negative- parity states exhibit enhanced B(E1) values, looking like existence of octupole correlation. As regards this result further study is needed because 90Mo is not such a nucleus possessing octupole correlation predicted by theory.

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