For incident photon energies E larger than two times the rest mass of the electron (1.022 MeV), pair production can occur. The resulting positron annihilates with one of the surrounding electrons, typically producing two photons with 511 keV. In a real detector (i.e. a detector of finite size) it is possible that after the annihilation:
* Both photons deposit their energy in the detector. This results in a 511 keV peak.
* One of the two photons escapes the detector and only one of the photons deposits its energy in the detector, resulting in a peak with  E − 511 keV, the single escape peak.
* Both photons escape the detector, resulting in a peak with  E − 2 × 511 keV, the double escape peak.
The above Am-Be-source spectrum shows an example of single and double escape peak in a real measurement.