In‐Flight Observation of Positron Annihilation by ILDAS

Abstract

We report a 511‐keV photon flux enhancement that was observed inside a thundercloud and is a result of positron annihilation. The observation was made with the In‐flight Lightning Damage Assessment System (ILDAS) on board of an A340 test aircraft. The aircraft was intentionally flying through a thunderstorm at 12‐km altitude over Northern Australia in January 2016. Two gamma ray detectors showed a significant count rate increase synchronously with fast electromagnetic field variations registered by an on‐board antenna. A sequence of 10 gamma ray enhancements was detected, each lasted for about 1 s. Their spectrum mainly consists of 511‐keV photons and their Compton component. The local electric activity during the emission was identified as a series of static discharges of the aircraft. A full‐scale Geant4 model of the aircraft was created to estimate the emission area. Monte Carlo simulation indicated that the positrons annihilated in direct vicinity or in the aircraft body.