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~104introduction

186Pb

prolate oblate prolate oblate

188PbG. D. Dracoulis et al. Phys. Rev. C 67(2003)051301(R)J. Pakarinen et al. Phys. Rev. C 72(2005)011304(R)

Realistic calculation show soft and shallow PES

R. Bengtsson, W. Nazarewicz, ZPA 334 (1989) 269

introduction

190Po

Energy difference between Neighbouring Countors is 100 keV!!

Shape mixing is especially pronounced for light Po isotopes

introduction

It is the mixing that complicates the shape idetificationof low spin band structures in light Po isotopes.

F. R. May et al., PLB 68 (1976) 113

190Po

Two or three level mixing analysisintroduction

188Pb

M. Bender et al. Phys. Rev. C 69(2004)064303V. 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

introduction

The minima with larger deformations are progressively favoured with increasing angular momentum (large MOI).

possible K isomerism!!

introduction

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.

Possibilities of K isomerism

motivation

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.

introduction

motivation

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.

model

a) For oblate deformation Pb, Po isotopes with N=104~114.b) On prolate sideN=102,104 isotones with Z=74~82.

Systematic calculations has been performed at both oblate and prolate deformations

calculation

calculation

calculation

calculation

prolate

discussion

M. Ionescu-Bujor, et al. PLB 650 (2007) 141

Pb,Po isotopes

Shape transition!!

• 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.

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;

discussion

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.

summary

谢谢