The half-life of the ${}^{249}\mathrm{Bk}$ ground state was determined by means of $\gamma$-ray spectroscopy, following for 728 days the growth of its $\alpha$-decaying daughter nuclide ${}^{249}\mathrm{Cf}$. Using a chemically purified source containing ${}^{249}\mathrm{Bk}$ and ${}^{137}\mathrm{Cs}$ nuclides, $\gamma$-ray singles measurements were carried out using a 25% coaxial Ge detector. The areas of the strongest 333.37- and 388.17-keV $\gamma$-ray peaks, produced in the $\alpha$ decay of ${}^{249}\mathrm{Cf}$, and the 661.66-keV peak, produced in the ${\beta }^{-}$ decay of ${}^{137}\mathrm{Cs}$, were determined. The measured activity of the latter was used to account for geometrical and dead-time corrections to the efficiency of the spectrometer, thus minimizing the systematic uncertainties associated with long-time, $\gamma$-ray counting measurements. Using the growth with time of the ratio of the 388.17- and 661.66-keV $\gamma$-ray peaks, a value of $T_{1/2}=327.2\pm 0.3$ d (the uncertainty quoted is one standard deviation, $1\sigma$) for the half-life of the ${}^{249}\mathrm{Bk}$ ground state was determined. The ${\beta }^{-}$-decay end-point energy of ${}^{249}\mathrm{Bk}$ was measured with a passivated implanted planar silicon detector to be $123.6\pm 0.4$ keV.

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