The ALICE Silicon Pixel Detector

Gianfranco Segato Dipartimento di Fisica Università di Padova and INFN for the ALICE Collaboration

A barrel of two layers of Silicon Pixel Detectors (SPD) constitutes the

innermost part of the Inner Tracking System of ALICE, an experiment

on ultra-relativistic heavy-ion collisions at CERN LHC.

SPD will have to track particles in a high multiplicity environment close

to the interaction point; being SPD fundamental in determining the quality

of the vertexing capability of ALICE, high precision and granularity are

necessary requirements for the detector.

The basic building block of the ALICE SPD is the ladder, consisting of a pixel

silicon detector matrix of 256 160 cells, each measuring 50 m in the r

direction and 425 m in the z direction parallel to the beam pipe; the ladder

measures 13.9 mm (r) 70.7 mm (z).

Five front-end chips, each containing the electronics for the readout of a

sub-matrix of 256 (r) 32 (z) detector cells, are flip-chip bonded to the ladder.

Four ladders are aligned in the z direction to form a stave.

Six staves, two for the inner layer and four for the outer layer, are mounted on

a carbon fiber support and cooling sector. The staves of the inner (outer) SPD

layer are located at an average distance of 4 cm (7 cm) from the beam axis.

Ten such sectors are mounted together around the beam pipe to close the barrel.

In total, there are 60 staves, 240 ladders, 9.8 M pixel cells.

Schematic of the electronics readout circuit in each pixel cell.The front-end chip is fabricated in a commercial 0.25µm CMOStechnology and transistors are designed with a radiation-tolerantgeometry. The flip-flops of each pixel cell in the same column are

connected to form a shift-register; the 32 columns are

read out in parallel, using 10 MHz clock frequency.

Cross section of the SPD barrel. The detectors

are distributed in 2 layers of 20 and 40 staves

respectively, mounted on 10 carbon fiber

support sectors, 200 m thick.

Carbon fiber sector with 4 staves of the

outer detector layer. The sector is equipped

with cooling squeezed ducts running under

the staves. Fluorocarbon is used as coolant

fluid. A power dissipation of about 0.2 kW

is foreseen for each sector.

Photograph of two carbon fiber sectorprototypes (left) and of a sector fullyequipped with cooling system for tests.

2 ladders are glued and wire-bonded on a multi-layer thin carrier (half stave)

which contains the bus and power lines. One Pilot Chip located at the extremity

of the half stave performs the readout and control functions and transmits the

binary data from the pixel cells to a remote Router.

425m

50m

FRONT-END CHIP

The data from the addressed de-randomizing buffer (FIFO)

are shifted out of the front-end chip into the Pilot Chip and

serialized for transmission over an optical fiber link to the

Router Module, located outside the ALICE detector. Two

other optical links connect at lower speed the Pilot Chip to

the Router to supply the control and trigger information.

The 6 half staves of the same sector side are connected

with the same Router Module by 18 optical links.