The use of O2 in gas mixtures for drift chambers

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. However, it is important to note that, in the case of ternary or quaternary mixtures (as in the case of MEG II), it is not possible to provide Garfield++ with just the ingredients of the mixture used to obtain correct attachment values. Instead, the code needs to be properly modified and recompiled.