It's embarassing to have to ask this, but I have never seen this issue discussed. Is there an upper limit on the energy of the gamma rays from the gamma ray bursts? Namely 1.022 MeV? Because any gamma ray of that energy or greater can and will produce pairs of electrons and positrons by supplying its energy to the quantum vacuum. In accelerator experiments a high energy photon cant get more than a few centimeters without doing this.