Discripancy for gamma spectra around 0 - 1 MeV


I am simulating a NaI detector in front of a 14.1 MeV monoenergetic point source kept in an aluminum pipe. This pipe is surrounded by an environment with lots of WATER, Concrete structure with HDPE and borated HDPE shielding. The actual experimental source is 14.1 MeV pulsating source. Upon comparing the spectrums I see a big discrepancy around 0-1 MeV especially around 0.511 MeV annihilation peak, everything else matches. I manually corrected the spectrum for I-128 Beta decay, which has reduced some discrepancy, still, a lot of scope for improvement is there.

I am using QGSP_BIC_HP physics list, with G4ThermalNeutrons.

Has anyone else seen this kind of issue with NaI sensors? If you know a solution to this please let me know.

Looking forward to some constructive advice.

Thank very much!


what is the source particle? What is Geant4 version?
Do natural background and electronic noise excluded from the data?

You may compare results for QGSP_BIC and QGSP_BIC_HP.
You may also try out QGSP_BIC_EMZ .