For liquid argon calorimeters electronegative impurities dissolved in the medium degrade the detector response and deteriorate the energy resolution, especially at high energies. A concept for a purity monitoring system for liquid argon calorimeters has been developed and is presented here. Special combined monitors of 241Am- and 207Bi-cells are used to monitor the concentration of impurities. The working principle as well as results from test measurements are discussed.

In this note we present the two types of liquid argon purity monitors, the basic monitor using radioactive sources and the laser chamber monitor, which will be implemented in ATLAS. The monitors operate at a purity in the order of 1 ppm oxygen equivalent with a resolution and stability of < 0:1 ppm. The positions of the basic monitors in the ATLAS cryostats and the locations for the laser chamber monitors in contact with the liquid argon of the cryostats are presented. The numbers of cables and feedthrough connections for the 10 basic monitors per end cap cryostat and for the 9 basic monitors in the barrel cryostat are given.