12 - Ageing and radiation effects

Summary

Life would be infinitely happier if we could only be born at the age of eighty and gradually approach eighteen.

Mark Twain

Ageing effects in gaseous detectors

Ageing processes in gaseous detectors are about as complicated and unpredictable as in humans.

Avalanche formation in multiwire proportional or drift chambers can be considered as a microplasma discharge. In the plasma of an electron avalanche, chamber gases, vapour additions and possible contaminants are partially decomposed, with the consequence that aggressive radicals may be formed (molecule fragments). These free radicals can then form long chains of molecules, i.e., polymerisation can set in. These polymers may be attached to the electrodes of the wire chamber, thereby reducing the gas amplification for a fixed applied voltage: the chamber ages. After a certain amount of charge deposited on the anodes or cathodes, the chamber properties deteriorate so much that the detector can no longer be used for accurate measurements (e.g. energy-loss measurements for particle identification).

Ageing phenomena represent serious problems for the uses of gaseous detectors especially in harsh radiation environments, such as at future high-intensity experiments at the Large Hadron Collider at CERN. It is not only that gas mixtures for detectors have to be properly chosen, also all other components and construction materials of the detector systems have to be selected for extraordinary radiation hardness.