On 5 October, technicians at NIKHEF, the Dutch National Institute for Nuclear Physics and High Energy Physics, glued the last layer of aluminium drift tubes on the 101st and final ATLAS precision muon chamber to be assembled in Amsterdam. Ninety-six of these chambers make up the full set of the “Barrel Outer Large” chambers, which comprise the major part of the third and outer layer of the ATLAS barrel muon system.
During three years of chamber assembly, following many years of R&D work, some 42 000 “monitored drift tubes” 30 mm in diameter and 5 m long have been produced. A semi-automatic wiring machine equipped the tubes with a total of 200km of tungsten sense wire, as well as 84 000 end plugs.
The tubes were then subjected to a number of tests on the precision of the wire location and wire tension, leak tightness under 3 bar pressure, and the ability to stand a high voltage on the sense wire with small dark current.
Tubes passing the quality test were glued into layers of up to 72 tubes in parallel; the full muon chambers consist of two sets of three layers of tubes each, separated by a spacer of precise dimensions.
In order to measure high-momentum muons accurately at the LHC, precision and control have been the key words in chamber assembly. Within the chamber dimensions of up to 5x2m, the tubes needed to be mounted with a precision of 20μm. In order to achieve this, precision jigs were used on a granite table inside a temperature- and humidity-controlled clean room. The positions of the tubes were constantly monitored during assembly, exploiting the NIKHEF RASNIK alignment system.
The muon chambers are currently being equipped with gas-distribution services and electronics. As from October, sets of five chambers are being routinely tested in a dedicated cosmic-ray test stand, into which the ATLAS muon-detector control system, the RASNIK alignment system and the ATLAS read-out electronics are also integrated.
From December onwards, the chambers will be shipped to CERN, where they will be mounted together with trigger chambers (resistive plate chambers) in a common support. The first of these assemblies will be mounted in ATLAS in mid-May 2005.
