  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 calorimeter's 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 detector's response to 137Cs sources allow
monitoring the uniformity and stability of all the calorimeter's cells as well
as maintaining precise knowledge of its energy calibration. The design of the
source hydraulic drive system's hardware and software, the data acquisition
system and the data processing algorithms are described. Finally, the results
of this two-decade program are shown.
