1. Positron

2. Pair Production  Photons above twice the electron rest mass energy can create a electron positron pair. Minimum = 0.012 ÅMinimum = 0.012 Å  The nucleus is involved for momentum conservation. Probability increases with ZProbability increases with Z  This is a dominant effect at high energy. ee  ee Z

3. Total Photon Cross Section  Photon cross sections are the sum of all effects. Photoelectric , Compton  incoh, pair Photoelectric , Compton  incoh, pair  J. H. Hubbell (1980) Carbon Lead