Additives that prevent or reverse cathode aging in drift chambers with helium–isobutane gas

Noise and Malter breakdown have been studied at high rates in a test chamber with the same cell structure and gas as the BaBar drift chamber. The chamber was first damaged by exposing it to a high source level at an elevated high voltage, until its operating current at normal voltages was only ∼0.5 nA/cm. Additives such as water or alcohol allowed the damaged chamber to operate at 25 nA/cm, but when the additive was removed, the operating point reverted to the original low value. However, with 0.02–0.05% oxygen or 5% carbon dioxide, the chamber could operate at more than 25 nA/cm and continued to operate at this level even after the additive was removed. This shows for the first time that running with an O2 or CO2 additive at high ionization levels can cure a damaged chamber from breakdown problems.