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.

enter image description here

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?


2 Answers 2


The 511-keV photons in the $^{22}Na$ spectrum are annihilation photons. They definitly have Compton interactions as seen in the diagram. The 1250-keV peak is a gamma in the daughter of the sodium positron decay. It also has a Compton edge and a backscatter region. ${22}Na$ gamma spectrum


Photons are photons.

If photons from one source experiences a certain kind of physics, then photons from other sources do too. So, short answer: yes.

And they can produce Compton edges as well.

  • 1
    $\begingroup$ As an example, take a look at a gamma spectrum from Na-22. $\endgroup$
    – Bill N
    Sep 14, 2015 at 17:07

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