Energy deposited by neutrons in plastic scintillator

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Hello,

I am simulating neutrons in a plastic scintillator material composed by 50% Carbon and 50% Hydrogen. The incoming neutron spectrum is close to a decreasing exponential between 1MeV and 10MeV. (see figure specNeut_input.pdf)

I am having some problems to understand the spectrum of the energy deposited in my detector.

- Since the cross section for neutron scattering in H is about twice as large as in C for incoming energies between 1MeV and 3MeV, I would expect that there are about 2 times more interactions with protons than with C12 atoms in my scintillator (for this energy range). What I observe is a factor 4.

- The energy deposited when the neutron interacts with a C12 atom (via HadronElastic, followed by a ionIonization) is usually less than 20% that of the incoming photon, what I think is right. But there is a gap in the deposited energy between 200 and 300 keV where no events appear (see the red line in figure specNeut_components.pdf). I would expect the spectrum of the deposited energy to be a continuous line, exponentially decreasing as the incoming spectrum is.

- Concerning the deposited energy when the neutron interacts with a proton (via HadronElastic followed by hIonization), the energy transfered to the proton is in most of the cases all the energy from the neutron (see the blue line in figure specNeut_components.pdf). I would expect it to be a uniform distribution between 0 and Eini (see Krane 1988, "Introductory Nuclear Physics").

I do not quite understand what would explain this spectral shape. I am not including yet the Birks' effect in the simulation. I am only obtaining the energy deposited by each particle using fStep -> GetTotalEnergyDeposit() in my SteppingVerbose.

I would be very thankful if somebody could give me some hint.

Best regards, Estela.

   Attachment:
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2009/09/29/03.15-94667-specNeut_input.pdf
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2009/09/29/03.15-63313-specNeut_components.pdf