Vacuum x-ray diodes with μm-thick filters have been developed to measure specific x-ray line emissions from plasmas imploded by megampere driving currents. Detector–filter combinations are presented for argon L-shell radiation (0.25–0.46 keV), for the 0.92-keV neon He-α line, and for the 1.13-keV sodium He-α line. For example, a power of 50 GW is measured in the neon He-α line.