Created attachment 853 [details] macro file for Hadr03 and output snippets for four hadronic models A post on the Geant4 forum ("Missing n,n+a for 14.1 MeV neutrons in Silicon", initial post 2024-01-27 in Physics Processes, Models and Cross Sections category), observed that Geant4 does not seem to include the 28Si(n,n alpha)24Mg reaction at 14.1 MeV for any of its physics models. That observation was based on 5x10^9 neutrons incident on a 25 um thick 28Si target. No 24Mg ion products were observed when rough calculations suggested that there should be a few hundred. This effect was confirmed using the Hadr03 program from the Geant4 11.2.0 distribution extended examples in which 10^8 14.1 MeV neutrons were incident on a 1000m thick 28Si target (see macro in first section of attachment). No 24Mg ions were observed and there was no indication of even a single 28Si(n,n alpha)24Mg reaction occurring. Hadr03 was compiled and run using Geant4-v11.2.0 multithreaded on Fedora 39, Cmake version 3.27.7, g++ version 13.2.1. It was not modified except to try different Hadron Inelastic Physics modules in PhysicsList.cc. All of the hadronic models were tried one at a time (commenting out all the others). This included FTFP_BERT_HP, QGSP_BIC_HP, QGSP_BIC_AllHP, QGSP_BIC, FTFP_BERT, QBBC, INCLXX, Shielding. Representative output snippets for some of the different physics modules are in the remainder of the attachment. The important point is that there are many events corresponding to a wide variety of energetically favourable reactions, but not a single one for 28Si(n,n alpha)24Mg. This behaviour is unexpected since, from the EXFOR reaction data base, the 28Si(n,n alpha)24Mg reaction has ~6mb cross section which is roughly ~1/200 of the total inelastic cross section (~1220mb) for reactions in that data base. For 10^8 incident 14.1 MeV neutrons in the Hadr03 example, millions of total reactions occur on 28Si (see output) so there should be tens of thousands of 24Mg ions produced. However, there are none.
