Hello Jack,
Your attempted filtering by ICRES was unfortunately ineffective: information on residual nuclei from point-wise non-elastic neutron interactions is stored in the fheavy.inc common (this will be better highlighted in our manual and training material shortly).
The filtering by ICODE.eq.300 (low-energy neutron interaction) is alright, the one over JTRACK.eq.8 (particle originating the interaction was a neutron) should be redundant (but it doesn’t harm). The gamma energy accumulation looks alright.
In order to intercept only neutron capture events on Gd, I’d suggest filtering by ICHTAR.eq.64 (the target was some isotope of Gd) and then loop over the heavies stack (see fheavy.inc) from 1 to NPHEAV, make sure there is only one particle there (it should be the residual) – this should be the signature of a capture event. Then fish out A and Z via IBHEAV(KHEAVY(1)) and ICHEAV(KHEAVY(1)), make sure that they match IBTAR+1 and ICHTAR respectively (then you have the explicit channel in your hands), and do your analysis under this scenario.
Doing this I see events with total energy of gammas of 7.8 MeV, 8.5 MeV, etc as a result of capture on the different isotopes of Gd in its natural composition. One does not stumble upon events with 15 MeV accumulated gamma energy.
Take it for a spin and let us know if issues persist or if you need further hints.
Cheers,
Cesc