Experiments have been conducted on the JANUS laser system to measure fuel density in imploded laser-fusion targets filled with deuterium, tritium, and neon. An x-ray crystal spectrometer was used to record the line emission of the 1s-2p line of the Ne+9 ion. Simultaneous measurement of the linewidth and the neutron yield indicated that symmetrically irradiated exploding pusher targets reached fuel densities of approximately 1 g/cm3.