Alice

ALICE (A Large Ion Collider Experiment) is a large detector built to study the products of heavy ions collisions at the CERN LHC accelerator - starting in year 2005.

Pb²⁰⁸ atoms, 27 times ionized, with an energy of 5.5 TeV/ionized nucleon, will interact at a rate of 10⁴ collisions per second; of these, a few per cent ( ~ 100 events/s) are head-on collisions, with maximum secondary particle production.

At these energy densities, the formation of a new phase of matter, the quark-gluon plasma (QGP), is expected, a transition which took place in the early universe some 10^-15s after the Big Bang and which might still play a role today in the core of collapsing neutron stars.

The behaviour of QGP will be studied by comprehensive detection of the hadrons, electrons, muons and photons produced in the collision of heavy nuclei.
If the present estimates are correct, the QGP could produce more conclusive evidence for the onset of quark deconfinement (isolated quarks have not been observed yet).

The ALICE project is a collaboration of about 560 physicists from 63 institutions.

In addition to Pb-Pb, the ALICE detector will study collisions of one or two lighter ion species, which will provide the necessary means to vary the energy density. Initial runs with protons will allow to commission the detector and to provide reference data for the nuclear program.
 

High Momentum Particle Identification System (HMPID)

A sub-system of the apparatus is specifically devoted to the detection of high transverse momentum particles.

The HMPID will be located away from the interaction vertex, at r = 4.7 m, in order to reduce the particle density and hence improve the performance. It will consist of  7 modules of about 1.7 m² each (see mechanical layout).

The current choice is a proximity-focusing RICH detector with liquid freon radiator, solid photocathode and pad pattern readout.
 
RICH (Ring Imaging Cherenkov) detectors have been used for particle identification over the last decade; the special feature of the ALICE RICH is identification of particles reaching the detector with high-multiplicity (50 particles per m²) and in a broad range of momenta.

Cherenkov radiation consists of light emitted at a very precise cone-angle around the trajectory of a fast particle which crosses a transparent medium at a speed higher than the speed of light in the medium (equal to c/n, i.e. the light speed in vacuum divided by the refractive index of the material).
The angle of light emission is related to the particle speed, hence to the particle (linear) momentum, and can help in identifying the particle.
The transparent medium, here liquid freon C₆F₁₄ delimited by a quartz window, is also called a radiator, to indicate a substance which produces the particular type of Cherenkov light radiation .

A novel photoconverter has also been investigated, consisting of a thin film of CsI  on large area photocathodes with high quantum efficiency.

 
 
Figure 1: Schematic layout of the fast CsI-RICH.
 

The fast RICH layout, as sketched in Fig. above, consists of:

- A radiator array using liquid C₆F₁₄ with a quartz window for containment.
- A distance frame creating the proximity gap necessary to obtain suitable ring radii (100-150 mm),
- A symmetric MWPC (Multi Wire Proportional Chamber) as photodetector, with 2 mm anode to cathode distance, 4 mm wire spacing and pad size of  8x8 mm  . The CsI film is deposited on the cathode pads.
 

The radiator

For the momentum range of interest, the radiator showing the best performance is liquid C₆F₁₂ with a thickness of up to 12 mm. It has a good UV transparency and a low chromaticity, permitting a satisfactory Cherenkov angle resolution. The radiator is closed by a quartz window for containment. This compound is widely used in existing large RICH systems, like DELPHI and CRID, where good experience has been gained.
 

Detector overview

 

Figure 2: Principal layout of the forward muon spectrometer, including absorbers, dipole magnet, chambers, and the muon identifier. (img43.gif)

 
Figure 3: Conceptual layout of the muon arm integrated into ALICE. (img44.gif)
 
 
 

Engineering design

The HMPID will be located a long way from the vertex, at r = 4.7 m, in order to reduce the particle density and hence improve the performance.
To maintain an incident angle of 10-12 deg. over the polar emission angle range, the HMPID is segmented into 7 modules, tilted according to their distance from the interaction vertex.

The layout of the RICH module is shown in Fig. 4. Its size is defined by the maximum feasible size of the quartz window containing the liquid radiator (about 50x50 cm²). The fact that rings are truncated close to the chamber edges leads to a mean area per module of about 1.7 m2.
 

The radiator

The radiator design is based on the technology in use at the DELPHI liquid barrel RICH. Succesfully operated for several years, it permits a safe choice of materials guaranteeing long-term operation with stable and optimal UV transparency.

The radiator array is based on a stiff composite panel supporting three independent cavities of about 35 x 140cm² circulated with C₆F₁₄. The window is assembled by gluing three quartz plates following the DELPHI procedure. The plate is supported by several inserts, enabling it to withstand the liquid weight and the hydrostatic pressure with a sagitta < 0.1 mm. Inefficient zones at the joins between cavities and the inserts amount to 2% of the sensitive area.

Engineering studies and tests are in progress in order to optimize the inserts' locations for a minimum thickness of the quartz plate. In order to guarantee a long-term UV transparency performance, the liquid has to be circulated according to the scheme in use at DELPHI.

Slow control

Control and associated safety systems are planned for

• chamber gas circulation (automatic valve operations, inlet and outlet flow control, chemical elements trace monitoring, temperatures);
•  radiator array (temperature and differential pressure);
•  low-voltage supplies and electronics;
•  high voltage;
•  alignment.

R&D programme

The principle of operation of large-area fast CsI-RICH, including basic production technologies, has been proved after three years of R&D. However, the long-term stability of large detectors using this new technology under the expected operating conditions has still to be demonstrated. Two areas call for further R&D. First, any possible improvement in the light conversion performance to improve the separation power in the critical low momentum range; and second, a physical understanding of the ageing mechanisms. The pattern recognition capability has been extensively simulated, but the results require confirmation for heavy-ion beams at large multiplicities.

A 1 m² prototype is under construction and will be exposed next year to high multiplicity events in a heavy-ion line and to complete a programme to study ageing under real conditions. Then, a full-size module and complete electronics could be developed in 1997. Engineering studies should be pursued at the same time on the radiator and the large-scale production of the photodetectors.