The lifetime of the 2 ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_1$ state in He-like niobium (${\mathrm{Nb}}^{39+}$) has been precisely determined with a beam-foil time-of-flight technique. The isotope ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ (I=9/2) was chosen in order to minimize cascade effects through hyperfine quenching of the primary feeding state. Further precision was obtained by removing blends with a Doppler-tuned transmission filter. Other cascade contributions were analyzed and found to be negligible. The result is 45.45(16)×${10}^{\mathrm{-}12}$ sec and respresents increased sensitivity to corrections of O(Zα${)}^2$ in the Zα-series expansion for the lifetime by a factor of 5.

• Received 14 September 1993

DOI:https://doi.org/10.1103/PhysRevA.49.3553