  A measurement of jet activity in the rapidity interval bounded by a dijet
system is presented. Events are vetoed if a jet with transverse momentum
greater than 20 GeV is found between the two boundary jets. The fraction of
dijet events that survive the jet veto is presented for boundary jets that are
separated by up to six units of rapidity and with mean transverse momentum 50 <
pT(avg) < 500 GeV. The mean multiplicity of jets above the veto scale in the
rapidity interval bounded by the dijet system is also presented as an
alternative method for quantifying perturbative QCD emission. The data are
compared to a next-to-leading order plus parton shower prediction from the
POWHEG-BOX, an all-order resummation using the HEJ calculation and the PYTHIA,
HERWIG++ and ALPGEN event generators. The measurement was performed using pp
collisions at sqrt(s)=7 TeV using data recorded by the ATLAS detector in 2010.
