In a gamma-ray spectrum, the peak due to pair production in the detector and escape, from the sensitive part of the detector, of two photons of 511 keV resulting from annihilation.
Source:
PAC, 1982, 54, 1533. 'Glossary of terms used in nuclear analytical chemistry (Provisional)' on page 1541 (https://doi.org/10.1351/pac198254081533)