  A double-gap topology is used for filtering central-diffractive events from a
proton-proton minimum-bias data sample at a centre-of-mass energy
{$\sqrt{s}=7$\,TeV}. This topology is defined by particle activity in the ALICE
central barrel and absence of particle activity outside. The fraction of events
satisfying the double-gap requirement $R_{DG}$ is found to be
{$7.63\pm0.02(stat.)\pm0.87(syst.)\times 10^{-4}$}. The background of this
double-gap fraction is estimated by studying the contributions of
non-diffractive, single- and double-diffractive dissociation processes as
modelled by Monte Carlo event generators, and is found to be about 10%.
