An alignment method for the ATLAS end-cap TRT detector using a narrow monochromatic X-ray beam

The end-cap Transition Radiation Tracker (TRT), consisting of 36 modules (wheels), is being constructed as a part of the ATLAS Inner Detector at the CERN LHC. This paper describes a method for determining the wire positions inside the straw proportional tubes (SPT), which are the basic building blocks of the ATLAS TRT, with an accuracy of better than 10μm. The procedure involves moving a narrow monochromatic X-ray beam across the straw and measuring the counting rate as a function of the position of the X-ray beam in the straw. To achieve this goal, a Beam Directing Device (BDD), providing the possibility to direct the X-ray beam in a chosen direction within some solid angle and supplying an accurate angular measurement system, has been constructed. The results of the wire position measurements performed using this BDD on a full-scale mechanical prototype end-cap wheel of the TRT are presented in this paper.