Lawrence Berkeley National Laboratory

The Insertable B-Layer (IBL) is the innermost layer of the ATLAS tracking system. It consists of planar pixel modules in the central region and 3D pixel modules at the two extremities. We use the longitudinal cluster-size distributions in 3D modules of the IBL to determine the number of pixel clusters per bunch crossing produced by primary charged particles in randomly triggered collision events, and to suppress the associated backgrounds. This Pixel-Cluster-Counting algorithm can provide both bunch-integrated and bunch-by-bunch relative-luminosity measurements, and thereby contribute independent constraints to the understanding and the evaluation of the systematic uncertainties that dominate the luminosity determination at the ATLAS experiment.