Lifetime of the21S0state of heliumlikeNi26+

We report the observation of the two-photon decay of the $2^1$ $S_0$ level in heliumlike ${\mathrm{Ni}}^{26+}$ and a measurement of the lifetime of this state. We find a lifetime of 150(16) ps, in agreement with the theoretical calculation of G. W. F. Drake [Phys. Rev. A 34, 2871 (1986)] which yields 154.3(5) ps.