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An electromagnetic micrometer to measure the wire centering in high-resolution aluminium drift tubes

The muon detector of the Atlas experiment at LHC (CERN) consists of 370000 high-resolution drift tubes assembled in multi-layer chambers. To take full advantage of the single-tube resolution, the high-precision external surface of tube end-plugs is used for positioning a layer before gluing. The wire position in the tube is defined at construction time by means of a locator inserted in the end-plug. To check that the wire is in the nominal position after the tube assembly an electromagnetic micrometer (EMMI) has been developed. EMMI detects the wire position by measuring the electromotive force induced in two sensors symmetrically placed on both sides of the tube when a small sinusoidal current circulates on the wire. An accuracy of two microns on the wire centering has been obtained. The simplicity of this method and the short measuring time allow every drift tube to be checked, thus guaranteeing the precision requested for the Atlas muon detector.