Created attachment 636 [details]
Strange decays of C12, N12, C13, N13 in v10.4

My apologies for a long description, but I have to explain the whole story. In 2008-2009 I tested multifragmentation models of G4: G4StatMF, G4FermiBreakUp along with G4ExcitationHandler. At that time I proposed several fixes because in G4 v9.1 there were apparent deviations of the results of G4StatMF and G4FermiBreakUp from those of famous Fortran prototypes written by Alexander Botvina. I have to stress that these two implementations were based on exactly same physics. In v9.2 after all the corrections implemented we managed to obtain same results from Fortran and C++ versions of G4StatMF and G4FermiBreakUp. Generally, the higher excitation energy, the wider list of open decay channels, and, hence, the larger fragment multiplicity.

However, in v10.4 we found a very strange results of G4FermiBreakUpVI, please see the enclosed figure. Decay modes of C-12, N-12, C-13, N-13 do not depend on their excitation energy. C-12 always decays exactly into three fragments, while C-12 and N-13 - into a nucleon and a residue. Even more strange results found for N-12. Until the excitation of 6 MeV/nucleon is reached, this nucleus remain intact, and only above this threshold decays into p + C-11 become possible. 

From a more verbose output it becomes clear that the list of possible decay channels may contain, for example, 8 entries but they are all represented by the p + C-11 configuration.

We also tried v10.3, and even v9.6. The results of these two versions are identical, somehow more realistic, demonstrating some evolution with excitation energy,  but still very different from 9.2 and the Fortran prototype of the Fermi break-up model. We believe that the reliability of this model is crucial for accurate modelling of carbon-ion therapy, where a proper description of beam fragmentation is important.