MCNP neutron energy deposition in water

[oksuz_]

Hi,

I did an MCNP simulation to see the neutron energy deposition in water. I used 14-MeV neutrons, and big enough water body to make sure all the neutrons stay in the water and give all their energies to water. I used F6 tally. However, I got energy deposition of around 10.3 MeV, not 14 MEV. I suspected that maybe when a proton captures a neutron, MCNP does not score neutron's energy. And I included energy deposition of Deuteron and got ~20 KeV, Which is so small. I wanted to try the same thing for Oxygen, but MCNP does not track O individually ( as far as I know). Can anyone give me some suggestion? Maybe I am missing something.

 

[Astronuc]

Does the tracking of the 14.1 MeV neutron have a cutoff energy below which it is not counted?

If the neutron is absorbed, there is both recoil of the resulting nuclide, as well as radiative capture (gamma ray). When a proton captures a neutron, the exited deuteron emits a 2.2235 MeV gamma, which is slightly less than the gamma energy required to separate the neutron from the proton (photo-dissociation). This gamma would deposit energy by Compton scattering or photo-absorption. Similarly, oxygen would emit a gamma ray when it absorbs a neutron. With respect to H₂O, the cross-section is weighted according to the number of atoms in the molecule. Neutron scattering off water involves the protons, the oxygen to a lesser extent, and the whole molecule, at thermal energies.