Backscattered peaks. Close inspection of a gamma-ray spectrum may reveal a wide peak with energy <250keV which does not correspond to a known photon from the source. Such a feature, termed a Backscatter peak, is due to gamma-rays which first interact by Compton scattering with the shielding.

What is the back scatter peak?

Backscattering peak is one of the main features of the pulse height spectrum from a gamma ray detector. This arises mainly from materials outside like source baking, photomultiplier tube housing, shielding etc.

What is photo peak?

The photopeak refers to the region of the pulse height spectrum caused by the complete photoelectric absorption of gamma rays by the scintillator crystal of a gamma camera.