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Question Energy of prompt photon from G4NeutronHPCaptureFS is higher than that in reality 

Keywords: neutron capture final state photon energy
Forum: Hadronic Processes
Date: 02 Jul, 2007
From: Jing Liu <jingliu@mppmu.mpg.de>

Hi,

I am using Geant4.8.3 to simulate the neutron interactions with germanium detectors. The energy of neutron from the source is smaller than 20 MeV. So I am using the high precise neutron process in my simulation.

In the experimental setup, the neutron source is put in a big paraffin collimator. From the energy spectrum recorded by the germanium detector, I observed a gamma line with energy of 2223.0 keV (the statistic error is very small), which is from the neutron capture process: n + H -> D + gamma. However, the same gamma line in the simulation has the energy of 2224.6 keV.

In order to understand the discrepancy, I looked into the codes of the class "G4NeutronHPCaptureFS", I found the following code:

thePhotons = theFinalStatePhotons.GetPhotons(eKinetic);

which, I think, tries to set the final state photon energy based on the neutron energy and the Geant4 neutron data sets file:

G4NDL3.10/Capture/FS/1_1_Hydrogen (This file contains the energy of the excitation level of D, that is, 2224.6 keV)

My guess is that the "GetPhotons" function reads in the energy of the excitation level of D, but somehow does not calculate the energy of the final state photon accordingly, which should be a little smaller. In order to check that, I added the following tracing codes right after the above codes:

 for(i=0; i<thePhotons->size(); i++)
   G4cout<<"Egtot = " << thePhotons->operator[](i)->GetTotalEnergy()<<G4endl;

It gave the value of 2224.6 keV. To further test it, I changed the values in the file 1_1_Hydrogen to 2223.0 keV, and the tracing codes gave me 2223.0 keV. Seems my guess is true.

Is there anyone having similar problems? Is this a real Geant4 bug or I misunderstand something?

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