Cluster ions in gas-based detectors

Abstract

Avalanches in gas-based detectors operating at atmospheric pressure and using Ar−CO2 or Ne−CO2 as drift medium produce in a first instance mainly Ar+, Ne+ and CO2+ ions. The noble gas ions transfer their charge to CO2 in a few ns. A few ns later, the CO2+ ions have picked up CO2 molecules, forming cluster ions, in particular CO2+·(CO2)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.

Experimental ion mobility measurements in Xe-CO2

“…In this work, the mobility of ions in Xe-CO 2 gas mixtures was measured at pressures in the 6 to 8 Torr (8 to 10.6 mbar) range and for reduced electric fields commonly used in gaseous detectors, 15 Td (3.6 kV·cm −1 ·bar −1 ), extending previous studies developed in our group for other gases [10][11][12][13][14][15][16][17][18][19][20].…”

“…Although the Langevin limit applies rigorously for real ion-neutral systems only in the double limit of low E/N and low temperature, it predicts the low-field mobility at room remperature with relatively good accuracy [24], which is the case in our experimental conditions. Although generally accepted, Langevin theory, has some known limitations in its application, namely with CO 2 , where it fails to provide correct values for the ion's mobility [11].…”

“…To address the lack of experimental values for Xe + and Xe + 2 in pure CO 2 an alternative method was devised using the data in paper [11] for pure CO 2 .…”

“…The values were obtained using the ion mass-mobility data for CO 2 in [11] and the masses of Xe + and Xe + 2 yielding mobility values of 1.15±0.05 cm 2 ·V −1 ·s −1 and 1.05±0.10 cm 2 ·V −1 ·s −1 , respectively. A similar analysis of the ion mobility-mass data in Xe, obtained from [36] showed that in pure Xe the ion mobility follows fairly well the values obtained from the Langevin polarization limit.…”

Experimental ion mobility measurements in Xe-CF4 mixtures

“…Although the Langevin limit is meant to be applied for real ion-neutral systems in the double limit of low E/N and low temperature, it predicts the low-field mobility at room temperature with relatively good accuracy [20]. Although generally accepted, Langevin theory has some known limitations in its application, namely with ions that may interact by resonant charge transfer, in which case it fails to provide correct values for the ion's mobility [12].…”