Single and double escape peaks

In a real detector, if the incident gamma energy is above 1022 keV, pair production events result in the production of two 511 keV annihilation gamma-rays. 

If only one of these gamma-rays escapes while the other is completely absorbed in the detector, 511 keV will be lost from the detector. This results in a separate peak in the spectrum representing Eg-511 keV, called the single escape peak.

If both annihilation gamma-rays escape this gives rise to the double escape peak at Eg-1022 keV.