I128 neutron capture gamma cascade issue

I have a simulation where I have a large volume Sodium Iodide (NaI) detector composed of different NaI segments with a borehole in the middle where a neutron source is placed. I am trying to simulate interactions of the neutrons with the NaI detector at various neutron energies and so far I am finding the results to be reasonable except with regards to neutron capture on 127I. The total energy of the capture reaction is consistent with the Q-value and the energy of the captured neutron, but the gammas that are produced are not consistent with the known levels in 128I. I have been getting 2 gamma rays for each capture, one high in energy of varying magnitude and one of either 135keV or 153keV. This is not correct given that the I128 photonevaporation data file has many more gammas than this, as expected.

This is especially puzzling because when a neutron captures on Na23 to produce Na24, the Q-value is conserved and the resulting gamma ray cascades are consistent with the data files. I have played around with the environment variables and found that changing these values does not affect the 128I situation:

====== ParticleHP Physics Parameters ========

UseOnlyPhotoEvaporation ? 1
SkipMissingIsotopes ? 1
NeglectDoppler ? 0
DoNotAdjustFinalState ? 1
ProduceFissionFragments ? 0
UseWendtFissionModel ? 0
UseNRESP71Model ? 0

I have also changed the G4DeexPrecoParameters and that also doesn’t seem to have an effect:

G4DeexPrecoParameters* param = G4NuclearLevelData::GetInstance()->GetParameters();

Hopefully this is enough information, any help would be greatly appreciated.

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You may have a look at the Capture file in the G4NDL database that defines the gammas produced at Capture. I have written a guide to this format that soon will be public, write to me privately pedro.arce@ciemat.es. You can also find in the guide the C++ code that manages Capture, in case you find it useful.

Did you manage to figure out where the problem lies? I have issues with simulating I-127. Which version of Geant4 are you using?