Determination of the emission probability of the principal gamma ray of 135Xe

► This article describes the principles and processes involved in the determination of the 135Xe γ-ray emission probability. ► The 135Xe γ-ray emission probability was determined directly from measurements of absolute activity and γ-ray intensities. ► The absolute activity of 135Xe was measured using length-compensated internal gas proportional counters. ► The γ-ray intensity was measured using an HPGe detector. ► The principal gamma ray emission probability of 135Xe was experimentally determined to be 0.896±0.016 (k=1).