The TAS

Motivation:

TAS-M.jpgWhen measuring the length of an object, the position corresponding to the beginning of the measuring tape must be known. Otherwise, a systematic error, i.e. a biased measurement, will be the result.

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. The procedure that determines the modules misplacements is called alignment.

The AMS-02 Tracker was aligned in 2010 using a straight proton beam at SPS (CERN). The proton beam was used as a reference. Translation and rotation constants for all Tracker modules have been determined with respect to it.

However, thermal conditions may change rapidly in space. This fact can introduce some mechanical deformation and misalignments, that will affect the rigidity measurement. The Tracker Alignment System (TAS) will provide the means for a fast and reliable monitoring of the Tracker geometrical stability during the AMS-02 mission. Systematic errors due to misalignments can then be monitored and corrected.

Background:

The TAS provides laser beams that mimic straight tracks. The position of a laser beam can be reconstructed with a higher precision than the position of a single particle crossing the Tracker. Indeed, a change in the tracker geometry can be determined with accuracy better than 5 µm.

Construction: 

CIMG3605_0.jpgTen pairs of alignment control laser beams are installed beneath the AMS-02 Tracker. Laser diodes mounted outside the inner tracker volume generate a beam of photons. The wavelength of these beams, 1082 nm (infrared bandwidth or IR), was chosen in order to penetrate all 7 inner tracker detector layers at once. 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.

ams-02_tas_lfcr_vibration_2.jpgThe AMS-02 tracker alignment control based on IR laser beams fulfills the requirements of a space borne experiment. It is light weight (3kg) and has a very low power consumption (1 mW). Based on the AMS-02 experience, a similar system has been developed for the largest silicon tracker ever built, the CMS tracker at LHC.