CMS uses lead tungstate (PbWO4) for the almost 80,000 crystals: a material with high density that produces scintillation light in fast, small, well-defined photon showers. This means the calorimeter system can be very precise and very compact, fitting within the CMS superconducting magnet. Lead tungstate is also relatively easy to produce from readily available raw materials, and the experience and capability to manufacture them already existed in China and Russia.

Of course, the material also has drawbacks: firstly that the yield of light depends strongly on temperature, a problem given how much heat is released by the readout electronics. To solve the cooling problem a custom-built system maintains the temperature of the 100 tonnes of crystal to within 0.1^oC.

Additionally, because the yield of light output is low,the scintillation light [see explanation below]  must be captured by photodetectors, converted to an electrical signal, and then amplified. The stronger and now digitised electrical signals are then transmitted through optic fibres. Because the photodetectors also need to be radiation hard and operate within a strong magnetic field, avalanche photodiodes or APDs [see explanation below] were selected for the crystal barrel, and vacuum phototriodes (VPTs) for the endcaps.

Finally, though already fairly radiation resistant after intense research to purify the material, researchers found that lead tungstate would still suffer limited radiation damage inside CMS. However the R&D programme led to a much better understanding of how this damage occurs and found that  its main effect is to colour the crystal, affecting how light travels through it.

Knowing this, physicists can ensure that they allow for it in their analysis using a “light monitoring system” during operation that sends pulses of light through each crystal to measure its “optical transmission”. The crystals can reverse radiation damage, or “anneal”, when the accelerator stops (when they are at room temperature) as the warmer temperature naturally shakes atoms back into their ordered structure.

Getting the material right was only one of the challenges for the ECAL team; each crystal had to be cut, machined, polished, tested,and attached to a photodetector. Groups of crystals were then assembled side-by-side in glass-fibre or carbon-fibre “pockets” to form larger structures known as “supercrystals”, “modules”, and “supermodules”.

Crystal Facts

• Crystals measure 2.2 x 2.2 x 23 cm in the barrel and 3 x 3 x 22 cm in the endcaps
• There are exactly 75,848 crystals in the ECAL
• The density of lead tungstate is 8.3 g/cm³

What is scintillation?

When an atom is excited, that is, given energy, an electron goes into a higher orbit and then when it falls back, it releases this energy as a photon. In CMS, when a high-energy electron or photon collides with the heavy nuclei of the ECAL crystals, it generates a shower of electrons, positrons, and photons, and atoms in the material take energy from the passing particles to excite their electrons. The atoms quickly ‘relax’ and the electrons each emit the extra energy as a photon of blue light. A photo device [see below] then picks up these ‘scintillation’ photons and the amount of light generated is proportional to the energy that was deposited in this crystal. This tells us the energy of the incoming electron or photon.

What are photodetectors?

Avalanche photodiodes (APDs) are photodetectors made of semiconducting silicon with a strong electric field applied to them. When a scintillation photon strikes the silicon and knocks an electron out of an atom, the electron is accelerated in the electric field, quickly striking other atoms and knocking electrons off those too. As these are also accelerated, this method will produce an avalanche of electrons with their numbers increasing exponentially. Through this method, APDs are able to produce a very high current in a short time, which is necessary as the lead tungstate crystals give a relatively low light yield for each incident particle. The signal is then amplified, digitised and immediately transported away by fibre optic cables, so that the analysis can be done away from the radiation area.

Crystals in the endcaps use vacuum phototriodes (VPTs) because here the radiation is too high to use silicon photodiodes. As opposed to an APD, a VPT contains three electrodes within a vacuum. Initially, electrons are released when the light strikes atoms in the first electrode in the normal way. Then the voltage difference between electrodes accelerates the electrons into the second electrode (the anode), producing several more electrons, that are all then accelerated to the third (the dynode), releasing a second batch of electrons. Thus this also creates a large current from the initial tiny amount of scintillation light, which is turned into a digital signal and sent along the optic fibres to the upper level readout.

All the photodetectors were glued to the crystals here at CERN, but the finished products are the sum of contributions made by several hundred people and many institutes.

 

One way the Higgs boson might decay is into high-energy photons and detecting them is one of the ECAL’s primary functions. However, short-lived particles called neutral pions, also produced in collisions, can inadvertently mimic high-energy photons when they decay into two closely-spaced lower energy photons that the ECAL picks up together.

In the endcap regions, where the angle between the two emerging photons from the decay of a neutral pion is likely to be small enough to cause this problem, a preshower detector sits in front of the ECAL to prevent such false signals. The preshower has a much finer granularity than the ECAL with detector strips 2 mm wide, compared to the 3 cm-wide ECAL crystals, and can measure each of the pion-produced particles as a separate photon.

The preshower is made of two planes of lead followed by silicon sensors, similar to those used in the CMS tracker. When a photon passes through the lead layer it causes an electromagnetic shower, containing electron-positron pairs, which the silicon sensors then detect and measure. From this we get a measure of the photon’s energy, while having two detector layers gives us two measurements, allowing us to pinpoint the particle’s position.

When seemingly high-energy photons are then found in the ECAL, we can extrapolate their paths back to the centre of the collision and look for their “hits” in the preshower along the way, adding the energy deposited there to the total energy from the ECAL, and deducing if they really were individual high-energy photons or photon pairs.

Each endcap preshower uses 8 square metres of silicon (a material especially chosen for its accuracy, compactness, ability to deal with radiation, and easiness to handle). The silicon sensors, each measuring about 6.3cm x 6.3cm x 0.3mm and divided into 32 strips, are arranged in a grid in the endcaps to form an approximately circular shape covering most of the area of the crystal endcap. For optimal performance during the lifetime of the experiment, as in the tracker, the silicon detectors must be kept at a temperature of between -10^oC and -15^oC. However, the nearby ECAL is very sensitive and must be kept within precisely 0.1^oC of its (higher) optimal temperature. The preshower must therefore be cold on the inside but warm on the outside, achieved using both heating and cooling systems.

The complete preshower system forms a disc, about 2.5m in circumference with a 50cm diameter hole in the middle (through which the beam pipe passes). This disc is only 20cm thick but manages to squeeze inside two layers of lead, two layers of sensors (and their electronics) as well as the cooling and heating systems – another example of the “compact” nature of CMS.

 

The Hadron Calorimeter (HCAL) measures the energy of “hadrons”, particles made of quarks and gluons (for example protons, neutrons, pions and kaons). Additionally it provides indirect measurement of the presence of non-interacting, uncharged particles such as neutrinos.

Measuring these particles is important as they can tell us if new particles such as the Higgs boson or supersymmetric particles (much heavier versions of the standard particles we know) have been formed.

As these particles decay they may produce new particles that do not leave records of their presence in any part of the CMS detector. To spot these the HCAL must be “hermetic”, that is make sure it captures, to the extent possible, every particle emerging from the collisions. This way if we see particles shoot out one side of the detector, but not the other, with an imbalance in the momentum and energy (measured in the sideways “transverse” direction relative to the beam line), we can deduce that we’re producing “invisible” particles.

To ensure that we’re seeing something new, rather than just letting familiar particles escape undetected, layers of the HCAL were built in a staggered fashion so that there are no gaps in direct lines that a familiar particle might escape through.

The HCAL is a sampling calorimeter [see explanation below] meaning it finds a particle’s position, energy and arrival time using alternating layers of “absorber” and fluorescent “scintillator” materials that produce a rapid light pulse when the particle passes through. Special optic fibres collect up this light and feed it into readout boxes where photodetectors amplify the signal.   When the amount of light in a given region is summed up over many layers of tiles in depth, called a “tower”, this total amount of light is a measure of a particle’s energy.

As the HCAL is massive and thick, fitting it into “compact” CMS was a challenge, as the cascades of particles produced when a hadron hits the dense absorber material (known as showers) are large, and the minimum amount of material needed to contain and measure them is about one metre.   

To accomplish this feat, the HCAL is organised into barrel (HB and HO), endcap (HE) and forward (HF) sections. There are 36 barrel “wedges”, each weighing 26 tonnes. These form the last layer of detector inside the magnet coil whilst a few additional layers, the outer barrel (HO), sit outside the coil, ensuring no energy leaks out the back of the HB undetected.  Similarly, 36 endcap wedges measure particle energies as they emerge through the ends of the solenoid magnet.

Lastly, the two hadronic forward calorimeters (HF) are positioned at either end of CMS, to pick up the myriad particles coming out of the collision region at shallow angles relative to the beam line. These receive the bulk of the particle energy contained in the collision so must be very resistant to radiation and use different materials to the other parts of the HCAL.

The CMS HCAL…

• used over a million World War II brass shell casings from the Russian Navy in making some of its detector components;
• is made up of 36 wedges, each of which weighs as much as 6 African elephants;
• contains over 400 “optical decoder” units, all of which were made by American high school students through the QuarkNet programme.

For a detailed account of the HCAL detector see:

CMS Technical Design Report for the Phase 1 Upgrade of the Hadron Calorimeter https://cds.cern.ch/record/1481837/files/CMS-TDR-010.pdf 

CMS HCAL Technical Design Report (1997)

HCAL Sampling Calorimeter

The CMS barrel and endcap sampling calorimeters are made of repeating layers of dense absorber and tiles of plastic scintillator. When a hadronic particle hits a plate of absorber, in this case brass or steel, an interaction can occur producing numerous secondary particles. As these secondary particles flow through successive layers of absorber they too can interact and a cascade or “shower” of particles results.  As this shower develops, the particles pass through the alternating layers of active scintillation material causing them to emit blue-violet light. Within each tile tiny optical “wavelength-shifting fibres”, with a diameter of less than 1mm, absorb this light. These shift the blue-violet light into the green region of the spectrum, and clear optic cables then carry the green light away to readout boxes located at strategic locations within the HCAL volume.

A megatile is a layer of tiles whose sizes depend on their spatial location and orientation relative to the collision, chosen so that each receives roughly the same number of particles. Optic fibres fit into grooves cut into the individual tiles. Because the light picked up gives a measure of energy, the gaps between tiles must be filled with a reflective paint to ensure that light produced in each tile cannot escape into others and vice versa.

HCAL module showing sampling layers

The optical signals arrive at the readout boxes from megatile layers.  There, signals from successive tiles, one behind the other, are then added optically to form “towers”. This optical summation covers the path of the particle through the HCAL and is a measure of its energy and/or can be an indicator of particle type.

These summed optical signals are converted into fast electronic signals by photosensors called Silicon Photomultipliers (SiPMs) . Special electronics then integrates and encodes these signals and sends them to the data acquisition system for purposes of event triggering and event reconstruction.

SiPMs are photodetectors configured especially for CMS that can operate in any magnetic field and give an amplified response, in proportion to the original signal, for a large range of particle energies. The SiPMs are housed in special readout boxes within the calorimeter volume. Light signals from the calorimeter megatiles are delivered to the SiPMs by special fibre-optic waveguides.

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 1,165^oC and cooled to 18^oC.

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 1,165^oC 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!