Experience with the ZEUS central tracking detector

The central tracking detector of the ZEUS experiment has been in operation since 1992. It is a cylindrical drift chamber, approximately 2 m in length and 1 m in diameter, and has operated reliably in a magnetic field of 1.43 T using a gas mixture of 83:12:5 argon/CO2/ethane with a trace (∼0.5%) of ethanol. Initial studies conducted between 1995 and 2000 using dE/dx to search for a long-term loss of gain were inconclusive. During the course of 2000, operational problems caused by repeated trips of high-voltage in the outer superlayers of the detector were experienced. These were attributed to the Malter effect, which has shown to be alleviated by the addition of water to the gas mixture. Consequently, a trace of water (∼0.15%) was added to the gas and has, to date, proved to be a successful remedy. Changes in the chamber performance due to the additional water are covered and solutions to the problems encountered are detailed.