Below is a diagram of part of a structure which I find in a gamma spectrum taken from a hall in which a neutron source is undergoing spallation.
The structure which I'm interested in can be seen around 172-174KeV. I can't find suitable candidate isotopes which produce this structure. However, it could be explained as being a backscatter peak from 511KeV photons. This would make sense, and perhaps explain the strange shape (?). My question is whether annihilation photons themselves can go on to produce a backscatter peak? If not, why not?