Collective properties of low-lying octupole excitations in 20882Pb 126, 6020Ca 40 and 288O 20

Abstract

The octupole strengths of three nuclei: β-stable nucleus 208 82Pb 126, neutron skin nucleus 60 20Ca 40 and neutron drip line nucleus 28 8O 20 are studied by using the self-consistent Hartree–Fock calculation with the random phase approximation. The collective properties of low-lying excitations are analyzed by particle–vibration coupling. The results show that there is the coexistence of the collective excitations and the decoupled strong continuum strength near the threshold in the lowest isoscalar states in 60 20Ca 40 and 28 8O 20. For these three nuclei, both the low-lying isoscalar states and giant isoscalar resonance carry isovector strength. The ratio B(IV)/ B(IS) is checked and it is found that, for 208 82Pb 126, the ratio is equal to (( N− Z)/ A) 2 in good accuracy, while for 60 20Ca 40 and 28 8O 20, the ratios are much larger than (( N− Z)/ A) 2. The study shows that the enhancement of the ratio is due to the excess neutrons that have small binding energies in 60 20Ca 40 and 28 8O 20.