The Tracker Alignment System (TAS) checks the stability of the Tracker alignment.
Why do we need TAS?
When you measure the length of an object, you must know the position corresponding to the beginning of the measuring tape. Otherwise, you will make a systematic error, i.e., a biased measurement.
This is also true for high-energy physics tracking detectors. A tracking device describes the particle trajectory by means of several position measurements on different modules at different heights. The particle trajectory follows from the computation of the best curved line that passes through the measurements. Knowing the exact position of each module is of fundamental importance for the trajectory determination. We call the procedure that determines the modules misplacements alignment.
The AMS Tracker was aligned in 2010 using a straight proton beam at SPS (CERN). The proton beam was used as reference. Translation and rotation constants for all Tracker modules have been determined with respect to it.
Thermal condition may change rapidly in space. This fact can introduce some mechanical deformation and misalignments, that will affect the rigidity measurement. The TAS system will provide a fast and reliable monitoring of the Tracker geometrical stability during the AMS-02 mission. Then systematics due to misalignments can be monitored and corrected.
How does the TAS work?
The TAS provides laser beams that mimic straight tracks. The position of a laser beam can be reconstructed with more precision that the position of a single particle crossing the Tracker. Indeed we are able to trace a Tracker geometry change with accuracy better than 5 µm.
How is the TAS built?
Ten pairs of alignment control laser beams equip the AMS-02 Tracker. Laser diodes mounted outside of the inner Tracker volume generate the photons beam. The wavelength of these beams, 1082 nm (infrared bandwidth or IR), has been chosen such as to penetrate all 7 inner Tracker Silicon detector layers at once. Indeed, the Tracker sensors placed along the alignment beams have an anti-reflective coating (SiO₂ and Si₃N₄) optimized for the chosen wavelength (residual reflectivity ~ 1%). This coating reduces the strong attenuation caused by the high refractive index of Silicon.
The AMS Silicon Tracker alignment control based on IR laser beams fulfills the requirements of a space borne experiment. It is light weight (3kg), and very low power (1 mW). Based on the AMS experience, a similar system has been developed for the largest Silicon Tracker ever built, the CMS Tracker at LHC.