  The intrinsic lifetime of the upper level in the bound-bound 3d$^9$4s$^2$
$^2$D$_{3/2}$ $\rightarrow$ 3d$^9$4s$^2$ $^2$D$_{5/2}$ radiative transition in
Ni$^-$ was measured to be 15.1 $\pm$ 0.4 s. The experiment was performed at
cryogenic temperatures in one of the ion-beam storage rings of the DESIREE
(Double ElectroStatic Ion Ring Experiment) facility at Stockholm University.
The storage lifetime of the Ni$^-$ ion-beam was measured to be close to five
minutes at a ring temperature of 13 K.
