Lifetime measurement of low-lying excited states in neutron-rich B and C isotopes

Lifetime measurements of low-lying excited states in neutron-rich B and C isotopes are presented. Firstly we report on lifetime measurements of the first 2+ states in 16,18C by means of the recoil-shadow method applied to a radioactive-ion beam produced at RIKEN. The observed lifetimes show suppressed collectivity of around 1 W.u. for both the isotopes. Secondly, a lifetime measurement of bound excited states in 13B with N = 8, which include the low-lying E1 state at 3.68 MeV, is discussed. The measured mean lifetime of 30(10) fs corresponds to a small E1 strength of about 1.2x10-3 W.u., which makes a sharp contrast in strength to the large low-lying E1 strength observed in the neighboring isotone 12Be. Both results are discussed in terms of neutron-decoupling phenomena for low-lying E1 and E2 collective motion in neutron-rich nuclei.