The detection gas

 The transition radiation created by the radiator has to be detected. Radiation in the keV range is best absorbed by an element with high atomic weight which has low lying atomic energy levels. Xenon is such an element and as a gas it can be implemented as detection gas in proportional chambers.

The detection gas registers both the energy from the absorbed photons and the ionisation from the primary particle. The particle identification is based on the difference between ionisation from a primary hadron and the sum of transition radiation and ionisation from a primary electron. Thus a correct understanding of the ionisation in the detection gas is just as important as the understanding of the creation and absorption of transition radiation to evaluate the performance of a detector.