The Silicon Tracker

 

Motivation:

Tracker00-M.jpgOne of the unique characteristics of a particle - beside its mass, energy, charge and charge sign - is its momentum. The mass of an incoming particle can not be directly measured. However, if the momentum and energy are known, the mass can be calculated. These are two of the reasons why a momentum measurement is so important.

The easiest way to determine the momentum of a charged particle is the examination of its trajectory while it traverses a homogeneous magnetic field. When a particle passes through such a field, it is forced onto a circular path. By reconstructing this path and determining the circle's radius, the particles rigidity R - which is the particles momentum divided by its charge (R = p/Z) - can be calculated through the simple equation: R = Br, where r is the radius and B is the magnetic field strength. If the particle's charge can also be determined, the momentum is known. Of courcse, this is not as simple as it sounds. Many different things - for example irregularities in the magnetic field or particle scattering in the detector material - must be taken into account.

By measuring the incoming direction of a particle and its momentum it is possible to distiguish between low-energy particles trapped in the geomagnetic field - a so-called secondary origin - and particles coming from outside the earth's atmosphere: the galactic rays. Additionally, because matter and antimatter have opposite charge signs a homogeneous magnetic field will bend both in opposite directions. It is therefore possible to seperate matter from antimatter with the help of a tracker.

In order to measure a particle's track, it is sufficient to measure its path at a number of discrete points. This is where the tracker comes in: 

Construction:

The Tracker consists of 9 layers allowing for the measurement of 9 points along an incoming particle's trajectory with a single point precision of 10 microns. The original design of AMS-02 called for 8 tracker planes. However, in order to compensate for the weaker magnetic field resulting from the decision to exchange the superconducting magnet for the permant magnet, the tracker configuration was changed. One layer of the inner tracker was moved to a new position on top of the experiment and one of the other 7 layers was split into two parts, creating an additional ninth layer installed on top of the ECAL.

Each layer is made up of multiple ladders with the total number of 192 ladders. The ladders consist of a total of 2264 double sides silicon sensors, each 72x41 mm2 large and 300 microns thick. The operating principle of the sensors is described below. With a total area of 6.2 m2, the AMS-02 tracker is the largest precision tracker ever built for space flight.

In total, the tracker has about 200.000 readout channels. In order to reduce the amount of data, an early zero suppression is performed by the Tracker Data Reduction boards. The readout electronics were designed to have a small power consumption (0.7 mW per channel), low noise and a large dynamic range.

Although the power consumption per channel is low, the large number of channels lead to total power consumption (and therefore heat output) of 200 W. This heat must of course be removed. This is done by the Tracker Thermal Control System (TTCS). As AMS-02 operates in the vacuum of space, fans can not be used to cool the tracker electronics. The best way to remove heat in a vacuum is to transfer the heat to a radiator. The TTCS facillitates this transfer. The readout electronics are connected through thermal bars to two cooling loops which are filled with high-pressure CO2. The CO2 absorbs the heat, thereby undergoing a transistion from liquid to gas. The radiators are connected to the other side of the cooling loops, where the CO2 gives off the absorbed heat and returns to its liquid phase. 

Physics Background:

 

Tracker-Sensor-M.jpgThe basic element of the AMS-02 Silicon Tracker is the double-sided micro-strip sensor. The sensor consists of a substrate of high-purity doped silicon with a thickness of 300 microns. On the two sides of the substrate tiny aluminum strips run in orthogonal directions (the typical inter-strip distance is 50 microns).

When a charged particle crosses the Silicon substrate about 24.000 electron/hole pairs are created. These charges drift in opposite directions within 10 ns (=10-8 s) due to the electric field generated by the bias voltage applied between the two sides (80 V). Only strips near the migrating charges will give a signal. The charge center of gravity of these strips provides a position resolution of 10 ┬Ám. The sum of the electric signals on the hit strips is proportional to the square of the absolute charge of the particle.