Collective Properties of Low-lying Octupole Excitations in $^{208}_{82}Pb_{126}$, $^{60}_{20}Ca_{40}$ and $^{28}_{8}O_{20}$

摘要

The octupole strengths of $\beta$-stable nucleus $^{208}_{82}Pb_{126}$, aneutron skin nucleus $^{60}_{20}Ca_{40}$ and a neutron drip line nucleus$^{28}_{8}O_{20}$ are studied by using the self-consistent Hartree-Fockcalculation plus the random phase approximation (RPA) with Skyrme interaction.The collective properties of low-lying excitations are analyzed by usingparticle-vibration coupling. The results show that the lowest isoscalar statesabove threshold in $^{60}_{20}Ca_{40}$ and $^{28}_{8}O_{20}$ are thesuperpositions of collective excitations and unperturbed transitions from boundstate to nonresonance states. For these three nuclei, both the low-lyingisoscalar states and giant isoscalar resonance carry isovector strength. Theratio B(IV)/B(IS) is checked. It is found that, for $^{208}_{82}Pb_{126}$, theratios are equal to $(\frac{N-Z}{A})^2$ in good accuracy, while for$^{60}_{20}Ca_{40}$ and $^{28}_{8}O_{20}$, the ratios are much larger than$(\frac{N-Z}{A})^2$. This results from the excess neutrons with small bindingenergies in $^{60}_{20}Ca_{40}$ and $^{28}_{8}O_{20}$.