Created attachment 343 [details] Test program to reproduce the issue While simulating the passage of muons through an uranium target my simulation sometimes gets stuck. Investigating a bit deeper I found that the simulation actually runs, but that the event never finishes since many slow neutrons are generated inside the uranium, and they in turn induce some fission reactions which generate other neutrons and so on. The process looks similar to a "nuclear fission reactor". Typically, the reactor is ignited by a stopping muon or by a gamma. I see this behaviour very frequently compared to what one can reasonably expect: when running 10 instances of my simulation program, each one simulating 10^5 muons, typically one or two of them get stuck because of this mechanism I have been able to put together a simple simulation to show off the problem. I attach a tarball containing the code and the CMakeLists to reproduce the issue. I tried this program on my 64 bit ArchLinux system with gcc 4.9.2. With Geant4 10.00.p02 the simulation shows the "nuclear reactor" behaviour, while with Geant4 9.6.p03 the muon is stopped without producing any nuclear cascade. To better analyze what's going on during the simulation I applied this patch: https://www.hightail.com/download/UlRUa3NVNkc3bUEwTWNUQw to G4EventManager.cc to dump the secondaries. Here are the logs for the two runs with 9.6.p03 and 10.00.p02 (I stopped the latter with Ctrl-C after a few seconds): https://www.hightail.com/download/UlRUa3NVNkc3bURxYk1UQw https://www.hightail.com/download/UlRUa3NVNkc3bUROUjhUQw I don't know which behaviour is the correct one, and for what I know it could be due to changes in the code, changes in the models or changes in the datasets (for each Geant4 version I used the datasets which are automatically installed when configuring the build with -DGEANT4_INSTALL_DATA=ON). Yet the two are really different so something must be wrong somewhere.
The same problem is discussed on hypernews: http://hypernews.slac.stanford.edu/HyperNews/geant4/get/hadronprocess/1457.html
I have summarized our validation tests for muon-stopping with the BERT cascade in Hypernews (http://hypernews.slac.stanford.edu/HyperNews/geant4/get/hadronprocess/1457/1/1/1/1.html). We believe that the cascade model is performing correctly, reproducing the expected secondary neutron spectrum. It is possible that the rate of muon-induced fission is "too high," but without experimental data we don't have a simple way to valdidate that, or to adjust the model properly.
