Pre zaujimavost - detektor Alice zaznamenal prve castice (z vesmiru)

[Daniel Valuch]

preposielam clanok, v texte su dve linky na celkom zaujimave fotky...
b.



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.

http://cdsweb.cern.ch/search.py?recid=967177
    Peter Glassel, technical coordinator for the ALICE time projection 
chamber, sits in the middle of the completed TPC.

http://doc.cern.ch//archive/electronic/cern/others/PHO/photo-bul//bul-pho-2006-015.jpg
    One of the first cosmic-ray events recorded and reconstructed in two 
sectors of the TPC. A shower induced by cosmic rays is seen on this 3-D 
view.

    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.