In a γ-ray spectrum, the peak due to pair production in the detector and escape, from the sensitive part of the detector, of one of the 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)