Tracker/Hodoscope

A hodoscope is an instrument used in particle detectors to determine the trajectories of charged particles crossing the detector, for this reason they are commonly known as particle trackers.
In high energy physics experiments, trackers are usually employed in vertex reconstruction and momentum measurement.
A particle tracker is made by position sensitive device, in order to provide a precise reconstruction of the particle path. Each position sensitive device is made either by a scintillator material optically coupled to a photodetector, or directly by a solid-state detector called a Silicon Strip Detectors.

In the former case, when a charged particle passes through the scintillating material, it transfers part of its energy to the scintillator. This is in turn emits an amount of light that is proportional to the transferred energy. The light is detected by the photodetector and is converted into an electrical signal. A threshold is usually set for each photodetector, therefore when a light signal over this threshold is detected it implies that a particle passed through the photodetector.
In the latter case, charged particles collide with silicon atoms, liberating electrons and creating an electric current, which indicates the path of the original particle. The spatial resolution of the particle trackers is always limited by the spatial resolution of the position sensitive device.

FIG. 1 CMS Inner tracker. It is possible to see the silicon strips with blue-violet colour.

In high energy physics experiments, trackers are crucial in obtaining quantitative information on particle collisions. For this reason, they are usually located as close as possible to the interaction point. They are used for the reconstruction of the vertex events and of the particles momenta. The momentum can be measured by exploiting the Lorentz force, acting on charged particles moving in a magnetic field.
Starting from these considerations, all trackers must have the following characteristics: sensitivity to charged particles, good spatial resolution, small size to reduce the effect on the particle trajectories being measured, fast response to ease the particle identification through the signal timing and finally, low dead time to track high number of particles.
CMS (Compact Muon Solenoid) is one of the detectors of the Large Hadron Collider (LHC) accelerator located at CERN. The key feature of this experiment is the precise measurement of muons and consists of several shells of different detector elements. Particles created in the high-energy collisions in the very centre of the detector will first traverse the Inner Tracker (FIG. 1), a system of silicon sensors designed to detect charged particles.

SSD by Hamamatsu for CMS Inner tracker, the adopted series are S9153 and S9154
FIG. 2 SSD by Hamamatsu for CMS Inner tracker, the adopted series are S9153 and S9154

Outside of the Inner Tracker there are four more shells: the electromagnetic calorimeter, the hadronic calorimeter, a superconducting solenoid magnet and finally the muon chamber system, a gas detector system designed to measure the tracks of muons that pass through. Combined with the muon tracks measured in the Inner Tracker, the momentum of the muons can be determined with high precision.
The overall length of the inner tracker will be 5.4 m with a diameter of 2.4 m and the total weight is about 3 tons. It comprises 66 million pixels and 9.6 million silicon strips, each strip detectorhaving p+ strips on n-type bulk material. The strip pitch varies from 80 mm up to 205 mm.
The realization of the silicon strips for the inner part of the tracker was made by Hamamatsu Photonics (FIG. 2).

An example of a hodoscope obtained by coupling scintillators and photodetectors is provided by the upgrade of LHCb experiment. LHCb is a specialized b-physics experiment at LHC, that measures the parameters of CP violation in the interactions of b-hadrons (heavy particles containing a bottom quark). It is set up to explore what happened immediately after the Big Bang which allowed matter to survive and build the Universe we inhabit today.
Its tracking system consists of four rectangular stations, each covering an area of about 40 m². The tracking detectors provide a high precision estimate of the momentum of charged particles which leads to a precise mass resolution for decayed particles.
Currently two different detector technologies are employed: the Silicon Tracker, which is placed close to the beam pipe, uses Silicon Strip Detectors to detect passing particles, whereas the Outer Tracker, which is situated further from the beam pipe, is made up of thousands of gas-filled straw tubes.


FIG. 3 SciFi Tracker module. TOP: In the upper part of the picture there is a section of the tracker module with the scintillating fibres matrix. BOTTOM: An overview of the multichannel detector array.

In 2018, the tracking stations will be replaced by a Scintillating Fibre Tracker (SciFi Tracker/SFT) which will cover the full acceptance after the magnet. The detector modules will have 2.5 m long scintillating fibres with a diameter of 250 μm. The fibres will be read out by Silicon Photomultipliers (MPPCs, also known as SiPMs).

Principle of operation of the SciFi tracker module.
FIG. 4 Principle of operation of the SciFi tracker module.

The multi-channel detector arrays are designed for a channel pitch of 250 μm and an height that can cover the stack height for six layers of fibres. The number of channels per detection module is 128 which is built of two 64 channel MPPCs.

These modules will use the Hamamatsu MPPC array model S13552. The photons produced along the trajectory of the particle are propagated to the fibre end and further to the detector. Each pixel of the detector can detect one photon and the signal is proportional to the total number of pixels. The signal (coloured pixels) is the signal amplitude per channel illustrated in the top part of the figure 4. The particle position can be calculated with a weighted mean value of the channel signal. In order to suppress thermal noise (single fired pixel), hits will be considered as clusters of at least two fired pixels.

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