ALICE INDUSTRIAL AWARD
To NORTH CRYSTALS
For their collaboration with the PHOS project

ALICE is going to study matter at extreme conditions of temperature and density colliding heavy nuclei at the ultra-relativistic energy at the CERN LHC collider.

At temperatures more than 100 000 times that of the sun nuclei melt into their most elementary components, the quarks and gluons, creating, for a brief instant of time, a state of matter called the Quark Gluon Plasma

Such a very hot and dense mixture of quarks and gluons existed a few microseconds after the Big Bang and studying it at the laboratory will contribute to better under­standing of the evolution of matter from the early universe to today.

The ALICE detector will track and identify the thousands of particles that will be produced in each collision.

The North Crystals company in Apatity,
Murmansk region, Russia, is a large facility specialized in the production of heavy monocrystals.

Among their facilities they have

• 150 modern crystal growth furnaces based on the Czochralski technique
• annealing furnaces with a temperature gradientcontrol better than 0.5 degrees
• machines for cutting and polishing the crystals

Hence, by allocating a part of their facilities to ALICE, they can achieve a production rate of about 300 crystals per month.

PHOS

From the particles reaching the detectors the physicists have to infer what the tem­perature was at the initial state of the collision.

The Photon Spectrometer (PHOS) has been designed to measure the temperature of the collision by detecting photons emitted from it and determining with high preci­sion their quite high energy.

The challenge

Therefore the PHOS has to be made out of very special material that should

• be very dense to stop all the photons
• scintillate (glow) proportionally to the energy deposited by the photons
• be very transparent to allow the collection of the light produced in it
• be not too expensive so as to allow production of a rather large device

The solution:

A very special combination of lead and tungsten, namely the lead tungstate (PbWO₄) produces crystals that fulfil these requirements; they are denser than iron yet com­pletely transparent.

The PHOS spectrometer will be made of five modules with a total of 18 000 lead tungstate (PWO) crystals of size 2.2 x 2.2 x 18 cm.

Collaboration of company with ALICE/PHOS

Since 2000 the ALICE PHOS project and the North Crystals company have been cooperating for the development of the technology needed for large-scale production of lead tungstate crystals.

The production of the crystals is done at the company's factory.

The measurement of the characteristics of each crystal (i.e. degree of transparency, how accurately it can measure the energy of the photons, how precise are its dimensions etc.) is performed at the Kurt ha to v Institute in Moscow.

The results

The measurements of their characteristics

• the geometry of the crystals
• the light yield
• the transmission and emission spectra

show that they are of excellent quality complying with the requirements specified for the detector.

All the 3584 crystals needed for the PHOS module have been produced, tested, and delivered to CERN.

A prototype module has been put in a test beam and the results show that the photon signals can be mea­sured with high precision. This was done by measuring the energy resolution in a broad energy range, as well as by reconstructing the decay of particles like pi zero, eta and omega into two and three (for omega) photons. The peaks of the plots show that these particles can be recognized with good precision.

The ALICE Collaboration awarded the North Crystals company for their contribution and excellent collaboration on this demanding project.

The lead tungstate crystals are the mast delicate components in the PHOPS spectrometer. They are grown in furnaces with temperatures higher then 1000 degrees. At this temperature, they are slowly stretched out of a platinum crucible. It takes about 70 hours to grow a crystal to the requisite size. Each crystal is then annealed in another furnace before being cut to shape with great precision and then polished. These operations require a high degree of care: the temperature and speed of the growing process must both be constant. The crystals are rigid and fragile. Therefore, the cutting and polishing are quite complicated processes. Each of these technical problems is tendered all the more complex through the requirements of series production.

In quite a short time the North Crystals company, in close cooperation with the ALICE PHOS Collaboration, developed a high technology for the lead tungstate crystal series production, and thus ensured the construction of a true quark gluon plasma thermometer: the PHOS spectrometer of the ALICE experiment.