The design, construction and testing of a 2.1m long straw detector prototype operated in vacuum for the NA62 experiment

We have designed, built and tested a full-length prototype (2.1m) for the straw tracker in NA62 (rare Kaon decay). To minimize the multiple scattering of the outgoing pion, it is proposed to operate the magnetic spectrometer in vacuum, upstream of the decay region. This means that the decay region and the spectrometer are inside the same volume and share a common vacuum. In this paper we present some of the considerations, which led to the present design in terms of detector layout, material choice and mechanical design. Very tight requirements on straw straightness, wire positions and leak tightness put great demands upon design and assembly. In order to verify the design, a 64-straw prototype was built. The straws have a diameter of about 10 mm and are filled with a gas mixture of 90% CO₂, 5% CF₄ and 5% isoC₄H₁₀. The overall mechanics as well as the detailed design of individual components are presented. In particular, the measured mechanical characteristics of the straws tubes are shown. Furthermore, the principles of dedicated assembly and quality control tooling, e.g. straw straightness and leak rate are discussed.