Observation of hyperfine splitting on an x-ray laser transition

We report the first observation of hyperfine splitting on an x-ray laser transition. Hyperfine splitting can have a major impact on laser gain and help explain anomalous behavior and poor performance of x-ray lasers with odd Z. In our experiments we measure the line shape of the J=0{→}1, 3p{→}3s transition in Ne-like Nb and Zr and observe a 28 m{} splitting between the two largest hyperfine components in the Nb (Z=41) line at 145.9 {}, in good agreement with theory. As predicted, no splitting is observed in Zr (Z=40).