The crystal story

The crystal story lucas

A crystal’s journey from creation to its place within the CMS electromagnetic calorimeter is an arduous one: its trials include travelling thousands of miles, being sawed and polished, as well as being heated to 1,165oC and cooled to 18oC.

The journey starts  in either a disused military complex in Bogoroditsk, Russia, or the Shanghai Institute of Ceramics (SIC) in China. These two institutions together took on the job of producing the 78,000 crystals to stringent specifications. Crystals are grown around a tiny “seed” - an existing piece of the crystal with the required properties - from a 1,165oC melt of tungsten oxide, lead oxide and “doping” materials, small amounts of other materials that refine the crystals’ properties.

In the Russian method, the seed is pulled from the scalding mixture at a very slow, precise speed that allows the atoms of the melt to arrange into a crystal structure around it. “In 2 or 3 days you have your crystal, which may seem slow but it’s quick compared to geology,” explains Michel Lebeau, an engineer in the crystals team.

Dusting with diamond

But the raw form of the crystal still needs a makeover before it can become one of the sparkling gems we see in the experiment. First it must be cut, ground and polished with diamond. Diamond is the hardest naturally occurring material and can scratch or cut through other crystals, allowing them to be shaped.

A disc covered with fine diamond grains first cuts the crystals’ prismatic accurate shape from its original round form. The second stage, called lapping, involves grinding the rough sides of the crystal as if with diamond sandpaper - a sprayed diamond suspension coats a rotating table, similar to a potter’s wheel, and as the crystals pass over it their surfaces are sanded to a fine matte finish, the flatness of which is controlled to less than a fraction of the width of a human hair.

Finally, to replace the crystals’ cloudy surfaces with a transparent finish, they must now pass over an even finer suspension of diamond, as finer abrasive gives an even smoother surface. The grains used here are in fact so tiny that the polished surface pattern is smaller than the wavelength of light.

To ensure a perfect match with the neighbouring crystals, and to give accurate measures of particle energies, each crystal must comply with very accurate dimensions so that they react in exactly the same way to the light. To make sure of this, the machines that transform the crystals into their sleek final form must also be fine-tuned and precise. After extensive research and development, machines that could give these perfect finishes were designed,  fabricated locally and finally shipped, installed and ran at production sites in Russia and China.

Into the detector

For the crystals arriving at the regional centres in CERN and Rome, the welcome was warm but testing was thorough, as each had to be vetted before becoming part of the detector.

The arriving crystals were first unpacked and identified with a barcode before a visual inspection checks for any visible defects. Finally they were registered into a database so they could be followed throughout production and their lifetime in CMS.

Then the dimensions, light transmission and scintillation properties of the crystals were measured, 30 at a time, in an automatic machine (ACCOS) developed in both regional centres. “Usually with a crystal, the beauty of it is that it is coloured. But in our case if crystals are coloured would mean defects and reduced performance,” explains Etiennette Auffray, leader of the crystal assembly at CERN. But in fact only a very small number (less than 1%) of the 1200 crystals processed every month proved unsuitable for use

The vast majority of crystals made it through and were then carefully glued with a photodetector, which will collect and amplify their scintillation light and convert the light into an electrical signal. Each photodetector was then housed in one of 22 slightly different varieties of capsules and glued to one of 34 categories of crystal, in total making 146 different possible combinations: “So there’s a lot of scope to go wrong! We have to check it very carefully after each stage,” explains Auffray.

Then, like building blocks, the crystals were grouped in increasing numbers – for the barrel section they are first packed into lightweight glass-fibre boxes in groups of ten, making a ‘sub-module’. A ‘module’ was then constructed out of 40 or 50 sub-modules.

These modules were then joined by those assembled at the INFN/ENEA Laboratory near Rome, which also carried out the crystal tests and the glueing of photodetectors, in tandem with CERN. These were packed in groups of four, making one of the 36 “supermodules”, each of which weighs 2.5 tonnes.

For the endcaps, 25 crystals are grouped in five-by-five blocks named “supercrystals” and inserted into one of four “Dees”, structures named for their resemblance to the letter D, that fit around the beam allowing easy installation.

Finally the monitoring, cooling system and final electronics were added to the supermodules before they entered the experiment cavern. The cooling system is a record-breaking achievement that keeps the 75,848 crystals within 0.1°C of their optimum temperature to ensure they give a stable and equal response. The flow of water needed to achieve this, recycled through the system, equals 1/10th of the output of Geneva’s Jet d’Eau (Geneva’s fountain).

Inside CMS the supermodules are supported on a strong but lightweight system, designed to add only a minimum amount of material in front of the detector. The structure and crystals are so densely and precisely packed that the remaining space is only 1% of the total. The 2.5-tonne weight of each supermodule is cantilevered from one end so that two-thirds of it is taken at the back and the remaining third on a slender arm of aluminium and glass fibre composite. The maximum expected sag, including in the event of (very improbable) earthquakes, is about half a millimetre. This structure accounts itself for just 10% of the total weight - like a car that carries four passengers weighing just 30kg!


The CRISTAL database

The CRISTAL database achintya

Each CMS crystal is unique even though they are all made from the same material. Its physical characteristics therefore needed to be measured and recorded, not only for the construction phase but also for data collecting itself. Every crystal was labelled with a barcode to identify it and track it through the production and operation stages.

The long time-frame of producing the crystals meant that the means of production as well as the associated electronics evolved over time as improvements were made. Themethods of characterising the crystals themselves and the data formats used for storage also evolved with time. The database into which the information was to be stored would itself need to adapt to these evolving workflows. At the same time, the database must be able to fetch all of the data ever recorded –– i.e. it must be capable of handling legacy datasets –– and must be in a situation to do so in the future when the implementation might potentially be very different. It would also have to handle frequent changes, and so be flexible in design.

No such system existed in the mid-'90s, and so a consortium of scientists from CERN, CNRS (the French National Centre for Scientific Research) and UWE (the University of the West of England) set about developing their own database, giving birth to CRISTAL.

CRISTAL stands for Cooperative Repositories and Information System for Tracking Assembly Lifecycles, and was developed at CERN to manage the data of all CMS ECAL components at each stage of construction.

The CRISTAL database and its underlying technology is extremely novel, and has applications far beyond the world of tracking detector components. CRISTAL is description-driven; that is, the database structures can be modified at any time without affecting any of the collected information. The structures themselves are stored as data too, meaning they can be copied between different databases with the data they define. As a result, any large-scale project with rapidly evolving workflows can benefit from using CRISTAL. It was thus one of the first knowledge-transfer projects undertaken by CERN, and was spun-off into Agilium, a private company that handles business-process management. A second spin-off company called Technoledge is providing database solutions for e-governance, accounting firms, and more.

CRISTAL is another excellent example of technology developed in the course of particle physics research that has since found wide-spread use in industry and elsewhere.