The straw detector for the NA62 rare kaon decay experiment

The NA62 experiment at the CERN Super Proton Synchrotron (SPS) has been proposed with the purpose to measure the Branching Ratio for the ultrarare decay K⁺ →π⁺ vv. In order 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 share a common vaccum. In this paper is presented 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 full-length (2.1m) 64-straw prototype was built. The straws have a diameter of about 10 mm and two different gas mixture have been studied: 90% CO₂, 5% CF₄ with 5% isoC₄H₁₀ and 70% Ar with 30% CO₂. The overall mechanics as well as the detailed design of individual components are presented. Furthermore, results from performance studies (test beam) are discussed, e.g. space resolution and straw positions.