Shape-coexistence enhanced by multi-quasiparticle excitations in A~190 mass region

1. 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

2. Outline Introduction The model Calculation and discussion Conclusion

3. Shape-coexistence in Pb isotopes with N~104 introduction 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

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

5. 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!!

6. Two or three level mixing analysis introduction

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

8. 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!!

9. 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 Kisomers. • Decays from isomeric states give valuable structure information. • Isomerism is a key feature in giving access to observables required for shape measurements. motivation

10. Oblate shape introduction • 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

11. 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.

12. calculation 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.

14. calculation

15. calculation

16. calculation prolate

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

18. 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.

19. 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;

20. 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.

21. 谢谢