  The integral cross section of the $^{12}$C($n,p$)$^{12}$B reaction has been
determined for the first time in the neutron energy range from threshold to
several GeV at the n_TOF facility at CERN. The measurement relies on the
activation technique, with the $\beta$-decay of $^{12}$B measured over a period
of four half-lives within the same neutron bunch in which the reaction occurs.
The results indicate that model predictions, used in a variety of applications,
are mostly inadequate. The value of the integral cross section reported here
can be used as a benchmark for verifying or tuning model calculations.
