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 way out East, in either a disused military complex in Borogoditsk, Russia, or the Shanghai Institute of Ceramics (SIC) in China, that 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 matt 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.
“You end up with the very fine transparent surface that is essential for light collection in the crystals. The process also controls the size of the crystals, giving micrometre precision,” says Lebeau.
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 turn the crystals into their sleek experiment form must also be fine-tuned and precise. After extensive research and development, Michel’s team designed the machines that could give these perfect finishes, had them fabricated locally and finally shipped, installed and ran them at production sites in Russia and China.
Into the detector
For the crystals arriving at the regional centres in CERN and Rome, the welcome is warm but testing is thorough, as each must be vetted before becoming part of the detector.
The arriving crystals are first unpacked and identified with a barcode before a visual inspection checks for any visible defects Finally they are registered into a database so they can be followed throughout production and their lifetime in CMS.
Then the dimensions, light transmission and scintillation properties of the crystals are 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 prove unsuitable for use
The vast majority of crystals make it through and are carefully glued with a photodetector, which will collect and amplify their light and turn it into an electrical signal. Each photodetector is housed in one of 22 slightly different varieties of capsule 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 are 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’ is then constructed out of 40 or 50 sub-modules.
These modules are then joined by those assembled at the INFN/ENEA Laboratory near Rome, which also carries out the crystal tests and the gluing of photodetectors, in tandem with CERN. These are 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 will fit around the beam allowing easy installation.
Finally the monitoring, cooling system and final electronics are added to the supermodules before they enter the experiment cavern. The cooling system is a record-breaking achievement that keeps the 75,648 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 Geneva’s fountain output.
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 account itself for just 10% of the total weight - like a car that carries four passengers weighing just 30kg!