Dilepton and photon decay of giant resonances built on excited states of 56Ni

Nuclear states in 56Ni, at an initial excitation energy E*=70 MeV, were populated via the isospin T=0 reaction 16O+40Ca. The dilepton- (e+e-) pairs and photon-decay yields were measured simultaneously. Statistical decay calculations describe well both measurements on an absolute scale. The isospin mixing parameter extracted from these measurements (α2=0.065) is consistent with other studies of the isospin symmetry in highly excited compound nuclei. The 1σ upper limit for the observed monopole strength is 4.3 nb, which is 3 times higher than the one sum rule value calculated with the statistical model for the decay of the compound nucleus. copyright 1997 The American Physical Society