Secondary avalanches in gas mixtures

Avalanche development in gas-based detectors relies not only on direct ionisation but also on excitation of noble gas atoms. Some quencher molecules can be ionised when they collide with excited atoms, a process on which we reported earlier [1]. Alternatively, excited atoms can decay by photon emission. If these photons are insufficiently absorbed by the quencher, yet capable of ionising, then they may escape from the avalanche region and start secondary avalanches. This process, called photon feedback, leads to an over-exponential increase of the gas gain which limits the working range. In this paper, we derive photon feedback parameters from published gain measurements for several gas mixtures and fit these parameters in a model which describes their dependence on the quencher concentration and the pressure.