The OPERA event generator and the data tuning of nuclear re-interactions

The OPERA event generator "NEGN" is based on an adapted version of the event generator developed for the NOMAD experiment [M. Veltri, presentation at the NUINT01 conference]. It includes all the features implemented in the NOMAD generator which were cross-checked with a large sample of neutrino interactions, finely reconstructed at the level of single particles. This sample allowed for a study of the hadronic system fragmentation and to tune the inclusion of nuclear effects, like the intra-nuclear cascades. Many of these effects are also relevant, for different reasons, in the OPERA simulation.