Tracking

Tracking lucas Wed, 11/23/2011 - 14:52

Momentum of particles is crucial in helping us to build up a picture of events at the heart of the collision. One method to calculate the momentum of a particle is to track its path through a magnetic field; the more curved the path, the less momentum the particle had. The CMS tracker records the paths taken by charged particles by finding their positions at a number of key points.

The tracker can reconstruct the paths of high-energy muons, electrons and hadrons (particles made up of quarks) as well as see tracks coming from the decay of very short-lived particles such as beauty or “b quarks” that will be used to study the differences between matter and antimatter.

The tracker needs to record particle paths accurately yet be lightweight so as to disturb the particle as little as possible. It does this by taking position measurements so accurate that tracks can be reliably reconstructed using just a few measurement points. Each measurement is accurate to 10 µm, a fraction of the width of a human hair. It is also the inner most layer of the detector and so receives the highest volume of particles: the construction materials were therefore carefully chosen to resist radiation.

The final design consists of a tracker made entirely of silicon: the pixels, at the very core of the detector and dealing with the highest intensity of particles, and the silicon microstrip detectors that surround it. As particles travel through the tracker the pixels and microstrips produce tiny electric signals that are amplified and detected. The tracker employs sensors covering an area the size of a tennis court, with 75 million separate electronic read-out channels: in the pixel detector there are some 6000 connections per square centimetre.

 

For a more detailed account of the tracker see:
CMS: The Tracker Project Technical Design Report
(please be aware that elements of the detector may have changed since the report's writing in 1998)

Some design history ...

In the very first design sketch of CMS, the tracker section was left blank because it was thought that with the intensity of particles experienced in the LHC it would be impossible to make a tracker that could withstand it. In particular, making readout electronics that could work in this radiation would be a challenge. Hints that the US military, needing such electronics for space travel and nuclear warheads, might have solved some of the potential problems encouraged the team to explore this field.

However, the technology actually came from a most unexpected source; as the military market shrank at the end of the cold war and commercial electronics began to excel, the team took a gamble on a very fine feature manufacturing process, produced for a commercial market, which was not formally in the realm of “radiation hard electronics”. With some modifications, a few simple design tricks and selection of the right technology, it could be as radiation hard as needed. And it was a remarkable success. Eighteen months later the teams had rebuilt the old chips. They had low power consumption, low noise, and high overall performance and on top of this could be easily produced on a large scale, were relatively cheap and thoroughly tested. It was a big breakthrough. The same technology was then also used for the ECAL as well as CMS and ATLAS pixels.

Silicon Pixels

Silicon Pixels lucas Wed, 11/23/2011 - 15:00

 

The pixel detector, though about the size of a shoebox, contains 65 million pixels, allowing it to track the paths of particles emerging from the collision with extreme accuracy. It is also the closest detector to the beam pipe, with cylindrical layers at 4cm, 7cm and 11cm and disks at either end, and so will be vital in reconstructing the tracks of very short-lived particles.

However, being so close to the collision means that the number of particles passing through is huge: the rate of particles received 8cm from the beam line will be around 10 million particles per square centimetre per second. The pixel detector is able to disentangle and reconstruct all the tracks they leave behind, and withstand such a pummeling over the ten-year duration of the experiment.

Each layer is spilt into segments like tiny kitchen tiles, each a little silicon sensor, 100µm by 150µm, about two hairs widths. When a charged particle passes through it gives enough energy for electrons to be ejected from the silicon atoms, creating electron-hole pairs. Each pixel uses an electric current to collect these charges on the surface as a small electric signal. A electronic silicon chip, one for each tile is attached, using an almost microscopic spot of solder using the so-called bump bonding technique, which amplifies the signal.

Knowing which pixels have been touched allows us to deduce the particle's trajectory. And because the detector is made of 2D tiles, rather than strips, and has a number of layers, we can create a three-dimensional picture.

Because there are 65 million channels, the power for each pixel must be kept to a minimum. Even with each only generating around 50 microwatts, the total power output is around the same as the energy produced by a hot plate. So as not to overheat the detector, the pixels are mounted on cooling tubes.

Silicon Strips

Silicon Strips lucas Wed, 11/23/2011 - 15:04

 

After the pixels and on their way out of the tracker, particles pass through ten layers of silicon strip detectors, reaching out to a radius of 130 centimetres.

The tracker silicon strip detector consists of four inner barrel (TIB) layers assembled in shells with two inner endcaps (TID), each composed of three small discs. The outer barrel (TOB) consists of six concentric layers. Finally two endcaps (TEC) close off the tracker. Each has silicon modules designed differently for its place within the detector.

This part of the tracker contains 15,200 highly sensitive modules with a total of 10 million detector strips read by 80,000 microelectronic chips. Each module consists of three elements: a set of sensors, its mechanical support structure and readout electronics.

Silicon sensors are highly suited to receive many particles in a small space due to their fast response and good spatial resolution. The silicon detectors work in much the same way as the pixels: as a charged particle crosses the material it knocks electron from atoms and within the applied electric field these move giving a very small pulse of current lasting a few nanoseconds. This small amount of charge is then amplified by APV25 chips, giving us “hits” when a particle passes, allowing us to reconstruct its path.

Due to the nature of their job, the tracker and its electronics are pummeled by radiation but they are designed to withstand it. To minimise disorder in the silicon this part of the detector is kept at -20oC, to “freeze” any damage and prevent it from perpetuating.

The charge on each microstrip is read out and amplified by an Analogue Pipeline Voltage (APV25) chip. Four or six such chips are housed within a “hybrid”, which also contains electronics to monitor key sensor information, such as temperature, and provide timing information in order to match “hits” with collisions. The APV25 stores the signals in a memory for several microseconds and then processes them before sending to a laser to be converted into infrared pulses. These are then transmitted over a 100m fibre optic cable for analysis in a radiation-free environment. The tracker uses 40,000 such fibre optic links providing a low power, lightweight way of transporting the signal. Much of the technology behind the tracker electronics came from innovation in collaboration with industry.