Origin of high energy gamma ray emission by high Z targets irradiated by a petawatt laser*

Recent experiments using the Texas petawatt laser in Austin texas irradiating high Z targets (Au, Pt, Re etc) observed excess gamma ray emission > 8 MeV, much higher than predicted emission by hot electron bremsstrahlung. in this paper we examine the potential origin of this high energy gamma ray emission and explore concepts to manipulate and optimize its characteristics for potential applications.