The Use of O2 in Gas Mixtures for Drift Chambers

TL;DR

This paper demonstrates that Oxygen, traditionally avoided in drift chamber gas mixtures due to electronegativity concerns, can be effectively used alongside Isopropanol without significant electron attachment issues, supported by experimental and simulation data.

Contribution

It shows that Oxygen can be incorporated into drift chamber gas mixtures without impairing performance, challenging conventional discouragement and supported by empirical and simulation results.

Findings

Oxygen presence does not significantly reduce electron survival.

The gas mixture with Oxygen operates effectively in the MEG II detector.

Simulation results align with experimental observations after proper modeling.

Abstract

The use of Oxygen in gas mixtures for drift chambers is highly discouraged because Oxygen, being strongly electronegative, is generally believed to lead, even in very small quantities, to extremely reduced drift electron survival probability, thus preventing the detector's operation.The drift chamber of the MEG II experiment at PSI has been operating for several years with a gas mixture that mainly contains He:Isobutane in relative proportions of 90:10% by molar concentration, in addition to 1.5% Isopropanol and 0.5% Oxygen. Oxygen and Isopropanol are essential for the proper functioning of the chamber. The electron attachment in the mixture used has proven negligible for the proper operation of the chamber and agrees well with the Garfield++ simulation after correctly accounting for the three-body attachment simulation.