More details on the ALICE ITS

The Inner Tracking System (ITS) of the ALICE experiment is made out of six layers of silicon detectors exploiting three different technologies (pixel, drift and strip). It covers the central pseudorapidity range of |η| < 0.9 and its distance from the beam line ranges from r = 3.9 cm for the innermost pixel layer up to r = 43 cm for the outermost strip layer. The main tasks of the ITS are to reconstruct the primary and secondary vertices, to track and identify charged particles with a low pt cutoff and to improve the momentum resolution at high pt.

Each layer has hermetic structure and it is coaxial with the beam pipe. The ITS covers the pseudorapidity range |η|≤ 0.9 and the distance from the nominal beam line ranges from 3.9 cm for the innermost layer up to 43 cm for the outermost. The two innermost layers are made of Silicon Pixel Detectors (SPD), the two central layers of Silicon Drift Detectors (SDD) and the two outermost layers of double sided Silicon Strip Detectors (SSD). The ITS has the main purposes of providing both primary and secondary vertices reconstruction and of improving the ALICE barrel tracking capabilities in the vicinity of the interaction point. Furthermore, as a standalone tracker, the ITS recovers particles which do not reach or are missed by the external barrel detector, due to acceptance limitations and momentum cutoff. 

The Silicon Pixel Detector (SPD)

The Silicon Pixel Detector (SPD) is part of the Inner Tracking System (ITS) of the ALICE experiment. The purpose of the SPD is to provide ALICE with adequate secondary vertexing capability for charm and beauty detection in such a high multiplicity environment (charged particle multiplicities of up to 8000 per unit of rapidity have been predicted for head-on Pb-Pb collisions at the LHC). The pseudorapidity coverage of the inner layer is |η| < 1.95. The two SPD layers allow to achieve a track impact parameter resolution in the plane perpendicular to the beam axis better than 50 μm for pt > 1.3 GeV/c.

The SPD, wrapped in a brown Kapton foil, ready to be inserted in the rest of the ITS surrounded by the TPC (in blue).

The two layers of the SPD are located at 39 and 70 mm from the beam-pipe and are equipped with hybrid silicon pixel detectors. The basic detector module is called half-stave and each contains two pixel detectors, called ladders, that are made of 5 readout chips. The readout chips are arranged in a matrix of 32 columns and 256 rows of pixels. Each pixel has a size of 50 x 425 μm2 and detects the passage of charged particles providing a binary output when the signal is above a set threshold

The Silicon Strip Detector (SSD)

The Silicon Strip Detector constitutes the two outermost layers of the Inner Tracking System of the ALICE experiment. The SSD plays a crucial role in the tracking of the particles produced in the collisions connecting the tracks from the external detectors (Time Projection Chambers) to ITS. The SSD also contributes to the particle identification through the measurement of their energy loss.

 

The Silicon Drift Detector (SDD)

Each SDD is made of a 12.7 cm diameter and 0.3 mm thickness silicon wafer. A total of 260 SDD with 133120 read out electronic channels. The active area is 7.0 x 7.5 cm2. When a particle crosses the thickness of SDD electrons are released. They drift under the effect of an applied electric field towards and array of 256 anodes (channels).

In this way the y coordinate of the particle is given by the measurement of the drift time. The x coordinate is obtained from the centroid of the electrons along the anodes.
 

Click here to read the Technical Design Report for ITS

The performance is crucial to measure the vertices of secondary tracks originatins from the weak decays of strange, charm and beauty particles, which are located close to the collision vertex. Secondary vertices separated by at least 100 μm from the primary vertex can be distinguished. 

The SPD trackls selected within a window are combined to extract the vertx position. This method is used to monitor the interaction position quasi-online at the early stage of the tracking procedure and to measure important first-physics observables, as for example the charged particle multiplicity density at midrapidity. A second method - that reaches a better resolution - is applied at the end of the tracking procedure; it is based on the straigh line approximation of the reconstructed tracks in the vicinity of the vertex. 

Reconstruction of the primary interaction vertex (in combination with other detectors): 

- Each dot corresponds to the position in the XY plane, perpendicular to the crossing beams, of a head-on collision between two beam particles. 

- The primary vertex position is determined by tracing back the reconstructed tracks. The SPD information is crucial to achieve an adequate space resolution, 10 um in central Pb-Pb collisions, being the first detector crossed by the  particles that fly out of the QGP. 

The ITS identifies low momentum particles in the non-relativistic region measuring the energy loss in the SDD and SSD silicon. The four analogue measurements of the charge collected in these four layers are corrected for the path length after the tracking and the truncated mean method is applied to account for the long tail towards high energy loss values. The particle with known pt is identified. The ITS achieves and energy resolution of 10 - 15% and allows separating kaons from protons up to 1 GeV/c and pions from kaons up to 450 MeV/c covering also the region below 100 MeV/c outside of TPC and TOF particle identification capability. 

 

The countries that were involved in the construction and operation of the detector are: Czech Republic, Finland, France, Italy, Russia,  Mexico, Poland, Russia, Slovakia, The Netherlands, USA, Ukraine, Switzerland. 

A key element of the ALICE upgrade in the second LHC long shutdown (2018-19) is the construction of a new, ultra-light, high-resolution Inner Tracking System (ITS). With respect to the one currently installed in ALICE, the new ITS will significantly improve the tracking precision, the tracking efficiency at low-transverse momenta and the readout rate capabilities. The new ITS will consists of seven concentric detector layers based on a 50-μm thick CMOS pixel sensor, with a pixel pitch of about 30 x 30 μmfor a total of about 12.5G pixels covering an active surface of about 10m2. A key feature of the new ITS is the very low mass of the three innermost layers, which have a material thickness of 0.3% X0 per layer. 

The countries that participate in the construction of the new ITS are: China, Czech Republic, France, Italy, Netherlands, Pakistan, Republic of Korea, Russia, Slovakia, Switzerland, Thailand, Ukraine, United Kingdom and the United States. 

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