Enhancement of neutron quadrupole motion beyond N=8

For Be and C isotopes with N>8, the shell-model study with 2h omega basis states shows the enhancement of neutron quadrupole matrix elements. The neutron-rich nuclei such as Be-14 and C-16 have a few neutrons in the 1s(1/2) and 0d(5/2) orbits, and the neutrons in these orbits give the large neutron quadrupole motion because these orbits have almost the same single-particle energies. A large ratio of proton and neutron quadrupole matrix elements in these nuclei indicates decoupled motions of proton and neutron, which are suggested in the measurements of the B(E2) values.