Question

What is the third peak in the spectrum of Sodium-22?

Answer 1

I collected this spectrum from a sodium-22 sample in an instructional laboratory. The sample was set in front of a germanium detector which sends a signal into a pulse height analyzer and plots a histogram according to the energies of detected particles. I know that the first peak at 511 keV is due to positronium annihilation and the second peak is due to a ~1.27 MeV gamma decay. But I can't find any information on what the third peak at ~1.78 MeV ought to be. Does anyone know what the source of this feature is? Note that is isn't very large but it is obviously significant and prominent.