  A network of 16 Medipix-2 (MPX) silicon pixel devices was installed in the
ATLAS detector cavern at CERN. It was designed to measure the composition and
spectral characteristics of the radiation field in the ATLAS experiment and its
surroundings. This study demonstrates that the MPX network can also be used as
a self-sufficient luminosity monitoring system. The MPX detectors collect data
independently of the ATLAS data-recording chain, and thus they provide
independent measurements of the bunch-integrated ATLAS/LHC luminosity. In
particular, the MPX detectors located close enough to the primary interaction
point are used to perform van der Meer calibration scans with high precision.
Results from the luminosity monitoring are presented for 2012 data taken at
sqrt(s) = 8 TeV proton-proton collisions. The characteristics of the LHC
luminosity reduction rate are studied and the effects of beam-beam (burn-off)
and beam-gas (single bunch) interactions are evaluated. The systematic
variations observed in the MPX luminosity measurements are below 0.3% for one
minute intervals.
