A system to monitor possible displacements of the inner tracking system of ALICE

In a large collider experiment, the precision of the measurements is often limited by the knowledge of the location of the different sub-detectors which make up such an experiment. In A Large Ion Collider Experiment (ALICE), two such critical sub-detectors are the time projection chamber and the inner tracking system. Here is described a system which can monitor the relative position of these two sub-detector systems with the needed accuracy and within the space available. This system utilizes off the shelf USB web-cameras, diode laser light sources, and spherical mirrors. Tests have shown that it can provide a resolution of 9.4 ± 0.3 μ m for displacements in the plane of the camera and about 78 ± 2 μ m perpendicular to it.