Shape-coexistence enhanced by multi-quasiparticle excitations in A~190 mass region 石跃 北京大学 导师：许甫荣教授 2010.7.25

Outline Introduction The model Calculation and discussion Conclusion

Shape-coexistence in Pb isotopes with N~104 introduction Shape-coexistence in Pb isotopes with N~104 prolate oblate prolate oblate G. D. Dracoulis et al. Phys. Rev. C 67(2003)051301(R) J. Pakarinen et al. Phys. Rev. C 72(2005)011304(R) 186Pb 188Pb

Realistic calculation show soft and shallow PES introduction Realistic calculation show soft and shallow PES R. Bengtsson, W. Nazarewicz, ZPA 334 (1989) 269

Shape mixing is especially pronounced for light Po isotopes introduction It is the mixing that complicates the shape idetification of low spin band structures in light Po isotopes. 190Po 190Po F. R. May et al., PLB 68 (1976) 113 Energy difference between Neighbouring Countors is 100 keV!!

Two or three level mixing analysis introduction Two or three level mixing analysis

introduction 188Pb M. Bender et al. Phys. Rev. C 69(2004)064303 V. Hellemans et al. Phys. Rev. C 77(2008)064324

introduction Two mechanisms to stabilize different subminima in potential-energy surface Collective rotation Broken-pair excitation The minima with larger deformations are progressively favoured with increasing angular momentum (large MOI). possible K isomerism!!

Possibilities of K isomerism introduction Possibilities of K isomerism The formation of high-K states can enhance the stability of different subminima through increasing the potential barriers between them. The filling of strongly deformed-driving orbitals of the unpaired nucleons can polarize the soft nucleus to a larger deformation. The above two ingredients, combined with a low Ex of high-K state, provide good conditions for the formation of K isomers. Decays from isomeric states give valuable structure information. Isomerism is a key feature in giving access to observables required for shape measurements. motivation

Oblate shape motivation Oblate shape is very rare. introduction Oblate shape Oblate shape is very rare. The rareness of oblate deformation compared to prolate shape may be related to the detailed form of mean-field. The study of oblate deformed K-isomers give insights into oblate single-particle levels. motivation

3.Adiabatic blocking: PLB435(1998)257. model 1.Single particle energy: Woods-Saxon potential with universal parameter. 2.Paring interaction: Lipken-Nogami treatment of paring with its strength determined by average pairing gap method. 3.Adiabatic blocking: PLB435(1998)257.

Systematic calculations has been performed at both oblate and prolate deformations a) For oblate deformation Pb, Po isotopes with N=104~114. b) On prolate side N=102,104 isotones with Z=74~82.

calculation

calculation

calculation prolate

Shape transition!! Pb,Po isotopes discussion M. Ionescu-Bujor, et al. PLB 650 (2007) 141

discussion While almost all the ground states of the calculated nuclei have relatively small deformations except for light polonium isotopes with N≦112, nearly all the calculated high-K states have large axially deformed shapes (see Tables I and II), indicating significant shape polarizations of the broken pairs of nucleons. Further, we found that the orbital blockings can stabilize the minimum of the configuration-constrained PES for these nuclei.

Support from experiment in neighbouring odd A nuclei. discussion Support from experiment in neighbouring odd A nuclei. N. Fotiades et al. PRC 56(1997)723; A. N. Andreyev et al. PRL 89 (1999)1819;

summary summary Systematic configuration-constrained PES calculations have been performed to study the shape-coexistence of two-qp isomeric states in neutron-deficient Hg, Pb, and Po isotopes. Calculations are in good agreement with available experimental data. An abundance of high-K states are predicted to exist at both prolate and oblate deformations. The high-K states are relatively rigid and well-deformed compared to the respective g.s.s, hence providing favourable conditions for the formation of high-K isomers.

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