Structure of C 16 and the B(E2) problem

The B(E2) value for the decay of the 21+ state of C16 to ground has been the subject of much discussion. The existing measurements extend over a large range, almost an order of magnitude, from 0.63 to 4.15e2fm4. Analyses assuming a simple model of two neutrons coupled to a C14 core give reasonable agreement but require the inclusion of a large effective charge for the neutrons. To assess this situation, a large-scale (2+4)ℏω shell-model calculation of C16 has been made from which the wave functions have been used to obtain the B(E2) value. As a check, comparison is made with available data on the spectrum of C16 and intermediate-energy elastic proton scattering. It is found that with the new model a much smaller effective charge is needed, if at all, to explain the accepted B(E2) value.