  ComPair is a prototype gamma-ray telescope for the development of key
technologies for next-generation gamma-ray detectors consisting of four
subsystems: a 10-layer double-sided silicon strip detector tracker, a cadmium
zinc telluride calorimeter, a cesium iodide calorimeter, and a plastic
anti-coincidence detector (ACD). The ACD acts as an active shield to veto
charged particle events and consists of 5 plastic scintillating panels. ComPair
was launched as a balloon payload from Ft. Sumner, New Mexico and completed a
6-hour flight on August 27, 2023. Here we detail the design adn calibration of
the ComPair ACD, and report on the ACD's veto efficiency and other performance
metrics during the ComPair fight.
