[en] X-ray beams emerging from the new SLAC electron-positron storage ring (PEP) can impinge on the walls of tangential divertor channels. A fixed mask made of OFHC copper is installed in the channel to limit wall heating. The mask is cooled with water flowing axially at 30⁰C. Beam strikes on the mask cause highly localized heating in the channel structure. Analyses were completed to determine the temperatures and thermally-induced stresses due to this heating. The current design and operating conditions should result in the entrance to the fixed mask operating at a peak temperature of 105⁰C with a peak thermal stress at 26% of yield