Exotic electromagnetic transitions in neutron-rich carbon isotopes

Structure and electromagnetic properties of exotic neutron-rich carbon isotopes are studied by shell model calculations. A p-sd shell model Hamiltonian is modified by enhancing the effects of the tensor force in the p-sd cross shell matrix elements as well as with corrections in the T=1 monopole terms. A considerable suppression of the magnetic dipole (M1) transition in ¹⁷C from the 1/2₁⁺ state recently observed is found to be well explained by the modified Hamiltonian. The anomalous hindrance of the quadrupole (E2) transitions in ¹⁶C and ¹⁸C is also shown to be reproduced by our new Hamiltonian. (author)