Energy deposition higher than incoming neutron kinetic energy

Hello everyone,

In my application, I simulate the propagation of neutrons through an ICRU sphere with the purpose of measuring the energy deposition -> absorbed dose -> equivalent dose. I use Geant4 version 10.6 and a PhysicsConstructor objects configuration taken from example Hadronic 06 with thermal scattering On.

I am having doubts regarding the energy deposition of the neutrons and their secondaries, specifically with thermal neutrons (around 0.025 eV). I can tell by looking at the tracking verbosity that when some thermal neutrons hit the ICRU sphere, let’s say for example one of 0.024 eV, it gets captured and a gamma photon (2.2 MeV) and a deuteron (1.31 keV) are produced. As you can see part of this in the following picture:

The gamma photon doesn’t deposit any energy itself, but leaves a track of electrons which deposit up to ca. 2 MeV, while the deuteron deposits all of its energy into the ICRU sphere. Ending up with ca. 2.1 MeV energy deposited coming from a 0.024 eV neutron. I just want to be sure if this is actually correct, as I thought the energy should be conserved.

I am still a beginner in both Geant4 and physics, so any help clearing this out would be greatly appreciated.

Thank you very much in advance!

This is what happens in real life: the neutron is captured by an atom which changes its isotopic number and initially is a an excited state, which decays to ground state emiting a gamma. The nucleus is not simulated in Geant4 as it needs no tracking.

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Thanks for your explanation! So, what I’m seeing in my simulation is correct? I can imagine that the 2.22 MeV of the gamma being produced come from the binding energy of the deuteron, and then through Compton scattering it deposits some of this energy into the ICRU sphere.