Electric dipole transitions in neutron-rich nuclei

The structure Of neutron-rich nuclei in several isotopes is investigated by shell model calculations. We study the electric dipole (E1) transitions in C isotopes focusing on the interplay between the low-energy Pigmy strength and the giant dipole resonance (GDR). Reasonable agreement is obtained with available experimental data for the photoreaction cross sections in C-12, C-13, and C-14 with the inclusion of the quenching effects. A low-energy peak in the dipole strength in C-15 is associated with a single-particle motion of the 1s(1/2) valence neutron relative to the C-14 core. The calculated transition strength below the GDR in C isotopes heavier than C-15 is found to exhaust about 50-80% of the cluster sum rule value and 12-16% of the classical Thomas-Reiche-Kuhn sum rule value. Next, we point out that the quadrupole and magnetic moments in the odd C isotopes strongly depend on configuration, which will be useful to determine the spin parities and the deformations of the ground states of these nuclei. The electric quadrupole (E2) transitions in even C isotopes are also studied. The isotopic dependence of the E2 transition strength is found to be reasonably well explained, although the calculated strength largely overestimates the unexpectedly small strength observed in C-16. The El strength in N-18 and N-19 as well as in Ne isotopes is also investigated. (C) 2004 MAIK "Nauka/Interperiodica".