low energy neutron capture

Hello All,

I'm trying to understand the multi-group treatment for low-energy neutron.
As a test, I send a neutron beam in the last thermal energy group in water
and see the capture on H. From my understanding, at this energy all the
neutrons should end up in H(n, g). I modified mgdraw.f to track down the
neutrons. However, out of 100 primary neutrons (all equal weight), only
several generated gamma at 2.2MeV with same weight. Most of them stop being
transported (n, ) in the middle of the water "detector" while they are
still in a thermal energy group and have weight 1. Why is this? Is this
because photons are produced using multi-group treatment as well? Even so,
statistically I should still get ~100 weight photons out of 100 primary
neutrons? Could anyone help me understand the results? I attached the
relevant files.

Thank you very much!

Shirley