I ran my previously described simulation with /tracking/verbose 2 on 155Gd (note that I am using physics list QGSP_BERT_HP in Geant4 10.7.2 with G4NDL4.6). I then examined the individual neutron capture occurrences along the tracks. For all occurrences, the capture gamma rays (i.e., for neutron capture on a a single nucleus) summed to less than or equal to the Q-value for the (n,gamma) reaction. Below is a snippet of the output for one capture occurrence on 155Gd(n,gamma)156Gd.
G4WT0 > 2 2.4 -17.7 -0.00061 0 0 0.000628 77.1 Target nCapture
G4WT1 > Step# X(mm) Y(mm) Z(mm) KinE(MeV) dE(MeV) StepLeng TrackLeng NextVolume ProcName
G4WT0 > :----- List of 2ndaries - #SpawnInStep= 14(Rest= 0,Along= 0,Post=14), #SpawnTotal= 14 ---------------
G4WT1 > 0 62.5 -69.6 -0.00857 0.297 0 0 0 Target initStep
G4WT0 > : 2.4 -17.7 -0.00061 2.76 gamma
G4WT0 > : 2.4 -17.7 -0.00061 0.753 gamma
G4WT0 > : 2.4 -17.7 -0.00061 0.7 gamma
G4WT0 > : 2.4 -17.7 -0.00061 0.653 gamma
G4WT0 > : 2.4 -17.7 -0.00061 0.614 gamma
G4WT0 > : 2.4 -17.7 -0.00061 0.533 e-
G4WT1 > 1 62.9 -70.9 -0.274 0 0.000111 1.48 1.48 Target phot
G4WT0 > : 2.4 -17.7 -0.00061 0.551 gamma
G4WT1 > :----- List of 2ndaries - #SpawnInStep= 8(Rest= 0,Along= 0,Post= 8), #SpawnTotal= 8 ---------------
G4WT0 > : 2.4 -17.7 -0.00061 0.508 gamma
G4WT1 > : 62.9 -70.9 -0.274 0.246 e-
G4WT0 > : 2.4 -17.7 -0.00061 0.401 e-
G4WT1 > : 62.9 -70.9 -0.274 0.0424 gamma
G4WT0 > : 2.4 -17.7 -0.00061 0.38 gamma
G4WT1 > : 62.9 -70.9 -0.274 0.000393 e-
G4WT0 > : 2.4 -17.7 -0.00061 0.297 gamma
G4WT1 > : 62.9 -70.9 -0.274 0.00709 gamma
G4WT0 > : 2.4 -17.7 -0.00061 0.199 gamma
G4WT1 > : 62.9 -70.9 -0.274 8.54e-05 e-
G4WT0 > : 2.4 -17.7 -0.00061 0.0387 e-
G4WT1 > : 62.9 -70.9 -0.274 0.000129 e-
G4WT0 > : 2.4 -17.7 -0.00061 8.04e-06 Gd156
G4WT1 > : 62.9 -70.9 -0.274 0.000123 e-
G4WT0 > :----------------------------------------------------------------- EndOf2ndaries Info ---------------
G4WT1 > : 62.9 -70.9 -0.274 1.88e-05 e-
G4WT0 >
G4WT1 > :----------------------------------------------------------------- EndOf2ndaries Info ---------------
Attached is a screenshot of a spreadsheet I made for a run with 50 events (each event is the emission of a thermal neutron by the source) using a 155Gd target. 16 neutron captures occurred and the gamma rays emitted for each capture are shown. You can see that for each capture, the gamma ray energies sum to less than or equal to (within a small error) the Q-value. The orange boxes correspond to gammas that are strong lines in the NNDC data base for thermal capture (see link in previous post). I did not check in depth the other gamma ray energies because there are 1009 of them listed. The gamma ray energies in @CHAO’s above post do not match any in my simulations or those in the NNDC data base.
I also ran the simulation in verbose level 2 on 14N, 58Ni, 60Ni and G4_Ni, which have much simpler level structures. All the captures for all of the nuclei yielded capture gamma rays whose energies summed to less than or equal to the Q-value for each individual neutron capture gamma ray reaction. For 14N and 60Ni, the gamma rays all corresponded to gamma energies in the NNDC data base. However, for 58Ni they did not. I will investigate the gamma ray energies from capture on 58Ni and 155Gd further.