Drift Chamber Performance in a Strong Magnetic Field: Measurement of the Drift Angle up to 4.5 T

We present the results of the first measurements in a study of drift chamber performance in very strong magnetic fields. The angle of the electron drift has been measured as a function of electric and magnetic field intensity for an argon-iso-butane-methylal gas mixture. Fields up to 5500 V/cm and 4.5 T have been studied. 1. Introduction The next generation of high energy accelerators (ISABELLE, Energy Doubler/Saver, VBA, etc.) opens new regions of available energy and particle momenta. The need for ever stronger magnetic fields for track separation and momentum analysis is evident. Increasingly, physicists are resorting to large, high field superconducting magnets in their plans for the future. Especially at storage rings, where the measurement is made in the reaction center of mass frame, large solid angle detectors with high fields and track detection within the field are most attractive. The highest spatial resolutions are obtained using drift chambers. To date, however, no use has been made of such chambers in more than moderate fields. Careful studies have been made by Charpak and co-workers 1-3) of drift chamber performance in magnetic fields up to 1.6 T. They have shown that by appropriately tilting the electric field which causes the electron drift, the Lorentz force due to the magnetic field can be compensated for, with no significant degradation of chamber resolution or efficiency. Most features of drift chamber performance in fields up to 2 T seem to be consistent with the classical theory of electrons in gases4,5). In higher fields, 3-6 T, there appears to be no data assuring the success of the drift chamber technique. We have therefore begun direct measurements of the various detector properties in the new regime. The most disturbing feature introduced by a strong local magnetic field occurs when a field component is parallel to the sense wire. The drifting electron swarm may then be swept away from the sensitive ceil. The relationship between the pulse arrival time and the particle track position is strongly distorted and the chamber efficiency is re