A dc Hall-effect apparatus, based on conventional van der Pauw specimen geometry, has been developed for in situ measurements under van de Graaff electron irradiation (0.7-2.2 MeV). An associated cryostat permits ambient temperatures of 95-300 K. A key element is a flat permanent magnet of field strength 3.55 kG. Initial test measurements have been performed on wide-bandgap semiconductor materials, including an HVPE-grown AlGaN/GaN heterostructure field-effect transistor structure.