Odd peak in energy deposition spectrum

This may be a physics question in general, but I am seeing it using GEANT4 and cannot explain it rapidly.

I am looking at gamma ray energy deposition in a scintillator, to understand how different geometries could help improve gamma to neutron discrimination. At this point, I am only concerned with the energy deposition, not the scintillation light or light collection.

I have used both 500 keV and 1 MeV gamma rays, and am looking at the energy deposition spectrum.

What I see is that below the full 500 keV or 1MeV full energy deposition peak, but above the Compton edge, is a peak that is 34.743 keV less than the full energy peak. I cannot explain this with a double compton scatter. It seems to me like an escape peak, but the energy is very large, and the only element in my scintilaltor with an energy close is Cerium, but it only makes up 4% by mass, and the peak is very prominent.

Any guesses?

2scints_1mev.pdf (50.1 KB)

2scints_0.5mev.pdf (30.3 KB)

I imagine it the Kalpha1 X-ray escape peak due to cerium.

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May I ask what is the geometry and material composition of the scintillator?


@John_McFee I took out the cerium and the peak disappeared, it is indeed cerium.

@atolosad This is modeled off of 6Li glass scintillator. The geometry is a flat plate. The dip in the spectrum is from 90 degree compton scatters. Because the gamma rays are coming normal to the scintillator and in the middle, 90 degrees has a high probability of re-absorption

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