I have a neutron source in magnesium target. I use source_newgen.f.
The problem is that at source energy 20MeV exactly, the run gives Error code: 136.
The log file shows an error within the source_newgen.f file.
This error does not occur at any other source energy. The error also does not occur if I change the magnesium to other materials.
thank you for the bug report. The problem is identified and fixed.
The corrected FLUKA version will be available in the next days.
The problem appeared because you are using as beam exactly 20MeV neutrons, generating an out of boundary exception for one isotope.
In the mean time waiting for the new FLUKA, if you want to avoid the crash you can make the neutron beam energy slightly different than 20MeV e.g. 19.999MeV and the simulation should run