Cluster ions in gas-based detectors

Avalanches in gas-based detectors operating at atmospheric pressure and using Ar−CO(2) or Ne−CO(2) as drift medium produce in a first instance mainly Ar(+), Ne(+) and CO(2)(+) ions. The noble gas ions transfer their charge to CO(2) in a few ns. A few ns later, the CO(2)(+) ions have picked up CO(2) molecules, forming cluster ions, in particular CO(2)(+)·(CO(2))n. Since the cluster ions are slower than the initial ions, the signals induced by ion motion are altered. The effect is shown to be present in constant-field detectors and TPC readout chambers, and is expected to affect devices such as Micromegas and drift tubes.