Anne Willimann  |  05/03/2009

CERN: Where Particles Collide

Aligning CERN collimators with two articulating arms





Particle Physics

Physicists use the LHC to re-create the conditions that may have existed just after the Big Bang by colliding the two beams head-on at very high energy. Teams of physicists from around the world will analyze the particles created in the collisions using special detectors in a number of experiments dedicated to the LHC.


Particle physics is a key component in searching for new methods for achieving high energy levels. What is the origin of particle mass? Do neutrinos really weigh nothing? The new CERN particle accelerator will help in the search for answers to those questions, and offer insight into the very nature of matter. The LHC is the most powerful instrument ever built for the purpose of researching the characteristics of elementary particles.


There are many theories as to what will result from these collisions, but a brave new world of physics will emerge from the new accelerator, as knowledge in particle physics goes on to describe the workings of the universe. For decades, the Standard Model of particle physics has served physicists well as a means of understanding the fundamental laws of nature, but it does not tell the whole story. Only experimental data using the higher energies reached by the LHC can push knowledge forward, challenging those who seek confirmation of established knowledge, and those who dare to look beyond it.


Hollywood and More at CERN

CERN recently hosted actors Tom Hanks, Ayelet Zurer, and director Ron Howard as they unveiled select footage from their new film adaptation of Dan Brown’s novel Angels & Demons, due to hit the big screen in May 2009. When Sony Pictures first contacted CERN early in 2007 about filming part of Angels & Demons there, the laboratory was excited to participate.


Understanding why nature prefers matter to antimatter is the main thrust of CERN’s antimatter research. Scientists hypothesize that when the universe was born some 13.7 billion years ago in the Big Bang, matter and antimatter would have been created in equal quantities, and as Brown correctly points out, when matter and antimatter meet, they annihilate each other, leaving only energy behind. One of the great mysteries of the universe today is how enough matter has survived to provide the building blocks for stars, planets, and even us.


Antimatter has practical uses, too. The medical imaging technique of positron emission tomography, or PET, scanning uses antimatter to help
doctors visualize the functioning of the human body. The scanners owe much to techniques developed for particle physics research. In the future, antimatter might also be used to treat cancer. Preliminary experiments carried out at CERN have shown that antimatter particle beams could be very effective at destroying cancer cells.


How does the LHC work?

Inside the accelerator, subatomic particles called “hadrons”—either protons or lead ions—travel at close to the speed of light with very high energies before colliding with one another. The beams travel in opposite directions in separate beam pipes—two tubes kept at ultrahigh vacuum. They are guided around the accelerator ring by a strong magnetic field, created by huge superconducting electromagnets. These are built from coils of special electric cable that operate in a superconducting state, efficiently conducting electricity with almost no resistance or loss of energy. This requires chilling the magnets to almost absolute zero, about –271°C—a temperature colder than outer space. For this reason, much of the accelerator is connected to a distribution system of liquid helium, which cools the magnets, as well as to other supply services.

Thousands of magnets of different varieties and sizes are used to direct the beams around the accelerator. These include 1,232 dipole magnets 15 meters long, which are used to bend the beams, and 392 quadrupole magnets, each five to seven meters long, to focus the beams. Just prior to collision, another type of magnet is used to “squeeze” the particles closer together to increase the chances of collisions. The particles are so tiny that the task of making them collide is akin to firing two needles at each other from more than six miles apart with such precision that they meet point to point at the halfway mark.

Precision metrology is needed

The efficiency of the LHC is primarily based on its extremely intense beam (i.e., its densification to energy and its fineness). The challenge lies in keeping this high beam intensity within the accelerator and the storage ring all the way to the collision points where the scientific experiment are being conducted—all this despite the enormous energy levels achieved in the particle rays. To guarantee protection from any kind of path deviation and dimensional variation, CERN planned to install a total of 125 collimators in two of the most radioactive areas of the accelerator ring. No quality, high-capacity beam and, consequently, no usable scientific results are possible without extremely robust collimation.

CERN approached Service de Mécanique Nucléaire (SMN) from the Society for the Development and Production of Nuclear Fuels (CERCA), a part of the AREVA group, with the task of producing the 125 collimators. SMN has its headquarters in Romans, France, and employs 850 people. The company specializes in manufacturing sophisticated electromechanical components in the field of nuclear physics, covering applications ranging from research and development to industrial production. A second field of work involves electron-beam welding machines and laser welding, as well as performing installations in nuclear facilities. SMN is a recognized leader in research and development, as well as for manufacturing a small series of sheathed nuclear fuels.

Under strong international competition, the decision to go with CERCA was in part based on positive experiences during the 1990s, when CERCA delivered parts for the super-conducting accelerator for the large electron positron collider ring. CERCA was entrusted with the job of artisan serial production of an array of high-tech prototypes.

“Collimators are similar to energy-absorbing joints that purify (or collimate) particle beams, thus controlling the power output of the accelerator by capturing particle clusters with the superconducting magnets,” explains Pierre Maccioni, director of the nuclear mechanics department at CERCA. “The beam hits parts made of carbon composite materials, an idea that came from the development of fusion receptacles and the divergent cones of rocket propellers. The collimator holders are made of aluminum-oxide-enriched copper and exhibit extreme mechanical and thermal resilience. The assembly is cooled by a water flow running at 20 bar and is then set into motion in a stainless steel container by means of a highly developed, highly precise drivetrain.”

The technical specifications for the collimators designated by CERN require rigorous inspection for accuracy. Therefore, a solution had to be found to conduct part inspections that took into consideration the needs of both the physicists involved and the realities of industrial manufacturing. A portable articulating arm quickly emerged as the ideal instrument for this important task, setting itself above traditional measurement systems that are far less flexible and require much higher investments of capital.

Articulating arms fit the bill

To fulfill the exacting CERN accuracy requirements, two articulating arms were installed at the site. ROMER, a Hexagon Metrology Co., manufactures the ROMER seven-axis articulated arms (a popular type of portable coordinate measuring machine), made of advanced carbon fiber and aluminum. The arm has a 4-foot to 12-foot hemispherical measuring envelope and operates much like a human arm. Holding the probe in hand, the operator can gather 3-D data from a surface of a part, the motion being similar to holding a pencil and touching its point to a piece of paper.

“The decision to go with ROMER arms was unanimous,” says Alain Morin, director of nondestructive control methods at CERCA. “We went with them because they were able to offer us a measurement instrument with the best performance-to-cost ratio. In addition, CERN already owns several ROMER articulating arms.”

CERCA produced 125 collimators for the Large Hadron Collider. Each collimator was rigorously inspected using a ROMER articulating arm to ensure each part fulfilled CERN accuracy requirements.


From start to finish, CERCA used the articulated arms for the complete inspection and adjustment of 125 collimators. Today, CERN technicians continually use two arms with a measurement volume of 2.2 meters, and the included measurement software for fine-tuning the collimators. CERN specifications make it necessary for the technicians to wear protective gloves at all times to avoid introducing even the slightest contamination with organic materials that would influence the scientific experiments once the parts are hermetically closed and heated in a vacuum.

CERN’s inspection routine itself is not merely for performing a 3-D check. A collimator consists of a complex array of many parts that have to be perfectly aligned to one another. After each measurement, the technicians perform an adjustment based on a highly specific process designed for each step. The accuracy requirement for different parts is about 20 µm.

ROMER also provided skilled technicians who were in charge of designing a measurement procedure for successfully adjusting the collimators. By developing a repeatable, nearly automated measurement procedure that was totally compatible with the macros in the control module, a significant time savings was realized. The inspection time for a single collimator was reduced to two working days.

Dimensional measurement at CERN

For more than 20 years, CERN has closely collaborated with various measurement companies. During this time, the highly skilled team of metrology engineers and technicians at CERN relied on a variety of instruments for surveying and metrology—from optical and digital levels; Leica theodolites, total stations, and laser scanners; to Leica laser trackers, and ROMER articulated arms, and associated software.

Surveying in the LHC tunnel using a Leica HDS 3000 high-density, large-scale laser scanner ensured that all services and infrastructure were installed in accordance with specifications. The process ensured that there was always adequate space for successive installations of equipment and accelerator elements. Precision laser trackers were used for dimensional control of the dipole and quadrupole magnets, and levels and theodolites were used to accurately align the magnets along the curved path of the LHC. In addition to the articulating arms, CERCA also utilizes several gauging cylinders made by TESA.



About The Author

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Anne Willimann

Anne Willimann works as a marketing manager for Leica Geosystems (MetrologyProducts) and Hexagon Metrology Portable Group of Europe. After getting an associate degree in advertising and communications and a bachelor’s degree in international management and marketing in France, she joined a Swiss company as an advertising consultant and later as an area sales manager for Western and Southern Europe. Later, she worked as a communications manager in Sankt Gallen, Switzerland, before joining Leica Geosystems in December 2005.