This article describes the design, construction and use of a calibration and monitoring system, based on movable 137Cs gamma-ray sources, for the ATLAS Tile Calorimeter (TileCal). The sources, propelled by a water-based liquid through tubes that traverse all the calorimeters cells, produce signals that precisely characterise the response of each tile, thereby providing very granular and accurate data on the response of TileCal to particles. The system has been used to guide and control the quality of the optical instrumentation of all TileCal modules, to set and equalise the dynamic range of the response to physics data, and to set the energy scale of the readout system. In the ATLAS cavern, periodic measurements of the whole detectors response to 137Cs sources allow monitoring the uniformity and stability of all the calorimeters cells as well as maintaining precise knowledge of its energy calibration. The design of the source hydraulic drive systems hardware and software, the data acquisition system and the data processing algorithms are described. Finally, the results of this two-decade program are shown.
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