The Silicon Tracker
The Silicon Tracker is the matter/antimatter hunter! Tracker is the only sub-detector able to separate positive particles from negative particles from the direct measurement of the trajectory deflection. The curvature measurement allows the particle momentum and incoming direction determination.
Why do we need the Tracker?
The Tracker is the main actor among the AMS detectors. It is able to precisely measure the curvature of the particles traversing the magnet. Higher is the particle energy lower is the curvature. We can say that the Tracker is able to measure the rigidity of a particle!
Tracker is the only detector able to distinguish directly among matter and antimatter by means of the charge sign determination. The directions of curvature of a positive and a negative are opposite.
The measurement of the particle trajectory also provides the incoming direction and the momentum (= mass multiplied by velocity) of a particle. From incoming direction knowledge, and the momentum measurement, it is possible to distinguish between low-energy particles trapped in the geo-magnetic field, of secondary origin, from the particle coming from outside the earth atmosphere, called galactic cosmic-rays.
Tracker is also one of the three sub-detectors – with ToF and RICH – able to evaluate the absolute charge (Z) of a particle, contributing to the chemical distinction capability of the AMS-02 spectrometer.
In Depth: What is Rigidity?
Rigidity is defined as the particle momentum divided by charge (R = p/Z). High-energy particles are more rigid than low-energy ones. Two particles with the same momentum could have different rigidities: the one with the higher charge is less rigid than the other.
The relation between rigidity (R), magnetic field (B), and curvature (r) is very simple: R = Br. Since Tracker measures the curvature, the corresponding rigidity can be immediately derived. If the charge Z is also measured, the particle momentum can be calculated.
In real life the rigidity computation is not an easy job. You have to take in account the magnetic field non-uniformities, particle scatterings on AMS detector, passive materials and many other things. In particular, in order to avoid undesired effects, the Tracker materials and its supporting structure are kept as light as possible.
How does the Tracker work?
The Silicon Tracker measures with a high precision (10 µm = 1/100 mm) the position of passage of a particle at 8 different position along the track. The radius of the best circular trajectory passing through the 9 points is the particle curvature.
When very high rigidity particles are only poorly described by arcs and appear to be straight lines, we say that the particle rigidity is near to the Maximum Detectable Rigidity (MDR). The AMS-02 MDR is about 2 TV, a very high value with respect to similar experiments.
How does a Silicon Sensor measure the position?
The basic element of the AMS Silicon Tracker is the double-sided micro-strip sensor. The sensor consists on a substrate of high purity doped silicon 300 μm thick. On the two sides of the substrate tiny aluminum strips are running in orthogonal directions (the typical inter-strip distance is 50 µm).
When a charged particle crosses the Silicon substrate about 24,000 electron/hole pairs are created. These charges are drifting in opposite directions within 10 ns (= 10-8 s) due to the electric field generated by the voltage bias applied between the two sides (80 V). Only strips near to the migrating charges will give signal. The charge center of gravity of these strips provides a position resolution of 10 µm. The sum of the electric signals on the hit strips is proportional to the square of the absolute charge of the particle.
How is the Tracker built?
With an effective sensible area of 6.2 m2 the AMS Silicon Tracker is the largest precision tracker ever built for space application. It is composed by 2,264 double-sided Silicon sensors (72×41 mm2, 300 µm thick) assembled in 192 read-out units, the ladders, totaling 200,000 read-out channels! In order to keep the event size at a manageable level, an early zero suppression is performed during the online processing of the Tracker data by dedicated Tracker Data Reduction (TDR) boards. The read out electronics is characterized by a very low power consumption (∼ 0.7 mW per channel), with low noise and a large dynamic range. The large number of channels of the Tracker generates about 200 W of heat which must be removed. This duty is fullfilled by the Tracker Thermal Control System (TTCS).
In Depth: The Tracker Thermal Control System (TTCS)
The 200,000 Tracker channels electronics produce around 200 W which need to be removed. Since in space there’s no atmosphere, we cannot use fans to cool it down. The best way to cool down an object in space is to transfer its heat to a radiator. Large radiators – able to radiate more than 2,000 W, – have been installed on the two sides of the experiment.
The coupling between Tracker and the AMS main radiators is realized by the TTCS. The Tracker front-end electronics is connected through thermal bars to two cooling loops filled with high pressure liquid CO₂. The CO₂ absorbs heat making a liquid/gas phase transition. Since the tube is thermally coupled to the radiators, they adsorb heat from CO₂ that returns to a fully liquid phase. The TTCS is able to create a stable set-point providing the necessary liquid CO₂ flow in the cooling loops.