ALICE joins the cosmic ballet
ALICE has detected its first real events. On 16 June, the time projection chamber started to record and reconstruct the tracks of cosmic rays.
In its quest for the quark-gluon plasma, a state of matter that is believed to have existed just after the Big Bang, ALICE will use a very accurate tracker system. The major part of this system is the time projection chamber (TPC), wherein the trajectories of electrically charged particles are reconstructed and their identity is determined. The ALICE TPC, a cylinder of 5 metres in diameter and 5 metres in length, is the largest of its kind worldwide. Nearly completed, it now has all read-out chambers installed with the custom electronics complete for the approximately 560000 read-out channels.
Before being lowered to the underground experimental area, commissioning the TPC has begun in the large clean room on the surface, where it has been assembled. The team involved in this operation, with around 20 people, began to put the readout electronics into service at the beginning of March. The gas and high voltage systems were then commissioned (see Bulletin No. 21-22/2006). This continuous effort was rewarded on 16 May when the first cosmic rays were detected and their tracks reconstructed in the TPC.
The TPC consists of a field cage, made of carbon-fibre composites, containing 95 m3 of a mixture of gas, across which an electric field is applied. The read-out system is partitioned into 18 sectors on either side, each sector comprising an inner and outer read-out chamber based on the principle of multiwire proportional chambers with pad read-out.
Due to the ultra-low power consumption of the electronics, the whole TPC needs about 25 kW of electrical power for full operation. Only a fraction of that power and of the corresponding water-cooling plant is available in the clean room. That is why commissioning the read-out system currently proceeds with two sectors at a time, using the ALICE cosmic-muon trigger detector ACORDE, as well as a specially designed UV laser system for the generation of tracks in the detector.
Preliminary analysis of the cosmic events recorded and of laser-induced tracks show that the drift velocity and diffusion of electrons liberated by traversing charged particles, as well as spatial resolutions, are very close to the design values.
All of the 36 sectors will be turned on and their performance studied during this commissioning phase, which will last until October 2006. The TPC will then be transported into the ALICE underground area later this year, where it will be put into service in the spring of 2007.
Did you know?
In a time projection chamber (TPC), an electric field is applied across a large volume of gas. When a charged particle traverses the TPC, it ionizes the gas and the liberated electrons drift in the electrical field to the endplates. The position on the endplate gives two coordinates. The third is given by the time of arrival of the electrons- hence the name of time projection chamber.
