What semiconductor material is used in Beta-voltaic cells?

Beta-voltaic batteries are batteries that harness the kinetic energy from fast � particles. These cells have extremely long lives and large energy density which makes them ideal for use in pace-makers, weather buoys, remote sensors and space probes, but detailed information on the functional components of the batteries aforementioned is difficult to find. Does anyone know what material is used to make the PN junction in � cells?