ALICE TPC control and read-out system

Abstract

ALICE is a dedicated heavy-ion experiment at CERN LHC aiming to study the properties of the quark–gluon plasma. A lead– lead collision might produce several ten thousand new particles. Detailed study of the event requires precise measurements of the particle tracks. A 90 m Time Projection Chamber (TPC) with more than 500 000 read-out pads was built as the main central barrel tracker. Collisions can be recorded at a rate of up to about 1 kHz. The front-end electronics, designed from FPGAs and custom ASICs, performs shaping, amplification, digitisation and digital filtering of the signals. The data is forwarded to DAQ via 216 1.25 Gb/s fibre-optical links. Configuration, control and monitoring is done by an embedded Linux system on the front-end electronics. First results on the performance of the front-end electronics and the distributed detector control system are presented. I. TIME PROJECTION CHAMBER (TPC) The A Large Ion Collider Experiment (ALICE) [1] is using a TPC [2] as the main track-finding detector. A TPC is a gaseous detector. It is shaped like an horizontal barrel and positioned in the same direction as the beam pipe, which is passing through the centre of the barrel. The overall length is 500 cm, divided by a 100 kV Central Electrode (CE) into two identical drift volume. The diameter is 494 cm, though the innermost 170 cm is not part of the TPC to make room for the beam pipe and inner tracking detectors. A schematic view of the TPC can be seen in Figure 1. Collisions will take place in the beam pipe in the centre of the TPC, allowing the particles produced to traverse the TPC and leave tracks of ionised gas along their paths. A strong electric field of from the CE will make the electrons drift towards the end planes, where data read-out is performed. Each end plane is divided into 18 azimuthal sectors, which again are divided into two Multi-Wire Proportional Chambers (MWPC), the Outer and Inner Read-Out Chamber (OROC/IROC). The OROC has four Read-out Partitions (RPs); the IROC two. A RP is an electronic entity for reading out data from read-out pads. The ionistic signal will be amplified by the space charge around the wires of the MWPC. The induced charge on the read-out pads is forwarded to the read-out electronics. In total for both sides there are 557568 pads. The drift volume is filled with counting gas composed of 85.7 % Ne, 9.5 % CO2 and 4.8 % N2. A cold, light gas is used to assure low diffusion and low multiple scattering. Field distortions are minimised because of the high ion mobility and few ionisation electrons per unit length. The electronics design noise figure is 1000 RMS e− (700 actually achieved); not limiting the position resolution will require a signal/noise ratio of at least 20. Apart from tracking—measuring the charged particle momentum and having a good two-track separation—it also provides Particle IDentification (PID). The TPC is expected to perform well at multiplicities as high as dNch/dη=8000 in the particle momentum range [0.1, 100] GeV/c and within |η| 90 %, and the dE/dx resolution better than 10 %. Further, the TPC alone will have a momentum resolution of about 1 % at 2 GeV/c and 10 % at 50 GeV/c. For p–p collisions a read-out rate of ≈1 kHz is expected, while for central Pb–Pb collisions ≈0.2 kHz. Figure 1: Schematic view of the TPC. To the left a singe Read-out Partition (RP) is enlarged for visibility. The support for the sectors is shown on the two end planes. Between them is the Central Electrode (CE). The TPC allows space around the centre of the length axis for beam pipe and inner silicon detectors. II. DATA READ-OUT DESIGN As already mentioned, each sector has six RPs. A RP consists of a Read-out Control Unit (RCU) with up to 25 Front-End Cards (FEC), depending on the radial location. The innermost RPs have the highest number of FECs, as a smaller size for the readout-pads is used to increase the resolution to take into account the higher track density close to the collisions. The electronics for one RP, as well as its connection to the central systems is shown as a block diagram in Figure 2. Eight ALICE TPC Read-Out (ALTRO) [3] chips are mounted on a FEC, each capable of reading out 16 read-out pads. The FECs are attached to the RCU via buses; one for data transfer and one for control/monitoring. Once on the RCU, the data is forwarded to Data Acquisition system (DAQ) and the High Level Trigger (HLT) via a 1.25 Gb/s optical fibre. A Detector Control System (DCS) board equipped with an embedded