[en] Requirements fro continuous operation of photodiodes in the presence of nuclear radiation has led to the development of a novel GaA1As/GaAs photodiode. Device structure and semi conductor properties of this design were optimized to balance the need for photodetection at λ /equals/ 820 nm against unwanted excess photocurrents induced by ionizing radiation. To achieve radiation hardness the photodiode employs a double heterojunction device structure. The spectral response of this detector, 1 keV to 10 MeV, may offer unique opportunities for use in high-temperature plasma diagnostics compared to typical bare Si:PIN x-ray photodiode characteristics. Application of this GaA1As/GaAs detector in a fiber optic link will be reviewed. Also, use of this detector in simple atomic-absorption-edge filtered x-ray detector channels is presented. 12 refs., 5 figs