Strange behaviour simulating a HPGe measurement of uranium contaminated surfaces

Dear Geant4 users,
I’m simulating the a measurement of 235U contaminated surfaces with an HPGe detector.
For this purpose, I have defined a source volume wich generates gammas at 185.7 keV ( 235 U gamma line) , after simulation I got the attached result.
The peak correspond to 235U, however I don’t understand where the particles detected at the region 500-700 keV comes from.
I’m using the physics list QGSP_BERT_HP recommended for low energy dosimetric applications.

Is there something I’m missing?


Thanks a lot for your help.

The spectrum from 0.5 to 0.7 MeV is due to Ge(n, n’) reactions. There are two prominent peaks at 0.596 MeV (74Ge(n,n’)) and 0.691 MeV (72Ge (n,n’)).

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