A new design of wire locators for drift tubes

Every position-sensitive wire detector needs to solve the problem of wire positioning with a defined accuracy all over its sensitive volume. In particular thin-walled drift tubes (" straws"), which are nowadays being assembled to large detector units of several 10 m² of surface, need to he equipped with wire locators along the signal wires. Especially for medium-sized drift tube systems a wire locator has been developed together with an insertion device, which significantly reduces the production time and avoids the danger of applying epoxy glue to the signal wire. The wire locator is being inserted in one time-saving production step together with the signal wire itself. The pre-stretched polyurethan wire locator automatically centers the signal wire when it is released and then mechanically sticks to its location. An inert-gas cushion prevents the insertion device from causing harm to the possibly carbon-coated cathode surface of the drift tube. The proposed design has been compared to the rigid wire-locators in use in the COMPASS straw tracking system at C.E.R.N., which consists of drift tubes of 6 and 10 mm diameter. The investigation comprises both wire-centering capability and influence on the efficiency of adjacent detector regions. Its sufficient mechanical reliability could be demonstrated by means of an X-ray tomography, the competitively small influence on the drift field was determined in a test-beam measurement using a silicon tracking system.