Problem 2269

Summary: Angular distribution of residual nucleus in (n,2n) reaction
Product: Geant4 Reporter: h.araujo
Component: processes/hadronic/models/neutron_hpAssignee: dennis.herbert.wright
Status: RESOLVED INVALID    
Severity: normal CC: emilio.mendoza, Koichi.Murakami
Priority: P4    
Version: 10.5   
Hardware: PC   
OS: Linux   

Description h.araujo 2020-08-24 15:39:46 CEST 
I am validating 14 MeV neutron interactions in some isotopes against evaluated data, and I'm coming across a potential problem with the angular distribution of the residual nucleus in F-19(n,2n)F-18 reactions. The energy spectrum of the F-18 residual can be found in the MF#6 file of ENDF/B-VIII, and this agrees generally with what G4 is producing. However, the file suggests the nucleus should be isotropic (in the COM) at this neutron energy, which would boost to a cosine angular distribution peaking in the neutron direction (0 deg) in the lab. However, G4 is producing a cos distribution peaking at 90 degrees. Either I'm misinterpreting the ENDF6 file or the final nucleus is not being simulated correctly. The other processes, in particular (n,n) and (n,n') are correct.
Comment 1 dennis.herbert.wright 2020-08-26 15:44:35 CEST 
I will refer this to Emilio, our ENDF expert.
Comment 2 Emilio 2020-09-08 16:32:27 CEST 
In ENDF/B-VII.1 and ENDF/B-VIII.0 the F-18 nucleus is produced isotropically in the LAB system, not in the CMS.

Taking this into account, Geant4 produces the F-18 recoil nuclei with the correct angular distribution, i.e. with the distribution provided by the data library.

Emilio