Low energy dipole strength in light drip line nuclei

Abstract

We have studied electric dipole $(E1)$ strength distribution in neutron and proton drip line nuclei ${}_{ 4}^{12}{\mathrm{Be}}_{8},$ ${}_{ 4}^{14}{\mathrm{Be}}_{10},$ and ${}_{ 8}^{13}{\mathrm{O}}_{5}$ by large scale shell model calculations with the effect of loosely-bound single particle wave functions. Large $E1$ strength is found in the low excitation energy region below ${E}_{x}=3 \mathrm{MeV}$ in both ${}_{ 4}^{12}{\mathrm{Be}}_{8}$ and ${}_{ 8}^{13}{\mathrm{O}}_{5},$ while strong $E1$ strength is found only in high excitation energy region above ${E}_{x}=7 \mathrm{MeV}$ in ${}_{ 4}^{14}{\mathrm{Be}}_{10}.$ The effect of extended wave functions and the coherence in the transition amplitudes enhance significantly the $E1$ strength in the low energy region. A manifestation of melting of the shell magicity at $N=8$ and $Z=8$ is pointed out in the systematics of $E1$ transitions in Be and O isotopes.