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CMS Detector at the LHC: Calorimetry (EM, Had, Forward)

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Published: Tue, 30 Jan 2018

CMS structure:

  • Solenoid: Most particles are stopped by the detector except for few, such as muons, neutrino. Main difference between ATLAS and CMS is that CMS has solenoid on the outer layer, so it bends the trajectory of the muons again in an opposite direction (opposite pointing magnetic field). Depending on how much the trajectory is bent, we can deduce the momentum of the particle. Tracking system and both EM and Hadronic calorimeters fit inside the superconducting CMS solenoid, which generated Magnetic Field of 3.8 Tesla (100 000 that of the Earth).
  • Tracking detector (measures momentum, charge, decay) – silicon detector is the inner most layer. The CMS tracker records the paths taken by charged (not neutral) particles by registering their positions at various key points. The tracker can detect the paths of high energy muons, electrons and hadrons, as well as tracks coming from decays of very short lived particles such as b quark used to study the differences between matter and antimatter. (WEB: http://cms.web.cern.ch/news/tracker-detector). The tracker is very lightweight and precise, so it has minimal effect on the paths the particles take. Each position measurement is accurate to 10 micrometers. The tracker material is selected to withstand high levels of radiation, since it is the inner most layer and so receives the highest volume of particles.

CMS uses silicon strip sensors (detectors) in shape of rods, covering area of 206 sq.m. (wiki), adding up to 25000 silicon sensors. Also used silicon pixel detectors, which are in principle very similar to silicon strip sensors, but have a segmentation of pixel diodes instead of strip diodes. The 65 million pixels (each generating ~50 microwatts) are mounted on the cooling tubes form the 3 inner most layers. Silicon microstrip detectors then stretch out in a 130 cm combined radius barrel with inner and outer endcaps to close off the tracker.

  • Calorimeter (an apparatus to measure energy of the particle AND particle identification) – scintillating crystal (EM calorimeter made of lead tungsten, a very dense material that produces light when hit), and then sampling calorimeter for hadrons. The ECAL is sandwiched inside the solenoid after the tracking system and before the HCAL. EM calorimeter is used to measure energies of electrons and photons, because they are likely to be produced in reactions for Higgs and other new physics. LHC collides bunches of high energy protons every 25 ns, so the calorimeter material is required to have very specific properties. PbWO4 – lead tungstate is the crystal of choice for the following reasons: 1. the material is high density and has heavy nuclei (explain why is this good); 2. the oxide crystal is transparent and “scintillates”, emits a small flash of light(well-defined photon bursts), when electron or photon pass through it. This means the calorimeter system is very precise and very compact; 3.lead tungstate is relatively easy to manufacture from readily available raw materials. Each crystal is equipped with a photodetector (specially designed to work in a high radiation levels and strong magnetic field) that registers the scintillation light which is converted into an electric signal, amplified, and sent for analysis.

The ECAL made in a barrel shape (to fit inside the solenoid, of course) with two flat endcaps (one closing off each side of the barrel). The barrel part consists of 36 “supermodules”, each containing 1700 crystals, adding up to 61,200 crystals in total. The endcaps are made up of almost 15000 crystals. There are 75,848 crystals in ECAL. Each crystal (volume 2.2×2.2×23 cm in the barrel; 3x3x22 cm in the endcaps) weights 1.5 kg, each crystal took 2 days to grow, in total it took 10 years to grow all crystals. The crystals were manufactured in Russia and China, where appropriate facilities already existed.

Issues: The yield of light in the crystal depends strongly on temperature, so a sophisticated cooling system is required to keep the crystals at constant temperature. Also, the light signal needs to be converted into an electrical signal (via photodetectors) to be recorded, and since the initial signal is relatively weak, amplification is required. Photodetectors: Avalanche photodiodes (APD) for the barrel and vacuum phototriodes (VPT) for endcaps (because the radiation is too high to use silicon photodiodes), as these can operate in strong magnetic field and high radiation. Lead tungstate crystals (though fairly radiation resistant) suffer limited radiation damage – the crystal structure is disturbed, hence the optical transmission decreases. This effect is accounted for during the operation of the detector and appropriate corrections are included in the data analysis. The crystals are probed by “light monitoring system” to register the optical transmission. The radiation damage can be reversed (anneal) when CMS is not operating. In room temperature the atoms within the crystal return to orderly positions.

—Each crystal is identified with a unique barcode, registered in a database, and measured (light transmission and scintillating properties in ACCOS machine). Cut to micrometer precision. Getting the material right was only one of the challenges for the ECAL team; each crystal had to be cut, machined, polished, tested and given 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”. The crystals aren’t pure, but doped to improve their properties. Each crystal is cut and polished to a precise size, so that all pass the light the same way. There are 34 categories of crystal, 22 slightly different varieties of capsules with an attached photodetector. For barrel the crystals are first grouped into sub-modules: 10 crystals per lightweight glass fibre box. 40-50 “sub-modules” then make up a module, and 4 “modules” make up one of the 36 “supermodules”. Endcaps are constructed from 25 (5×5) crystal blocks, or “supercrystals”. Monitoring and cooling systems as well as final electronics are added to the supermodules before they are placed inside the experimental cavity. To ensure stable and equal operation of the crystals, the cooling system keeps all crystals within 0.1 oC of the optimum temperature.

What is scintillation? Scintillation detectors are one of the most often used particle detection devices (Leo 157). Scintillators are made of specific materials that emit a flash of light when struck by a particle or radiation. The emitted light signal is amplified by photomultipliers and converted into an electrical signal which is then analysed. In ECAL electron or photon collides with the heavy nuclei of PbWO4, generating a shower of electrons, positrons and photons. These shower particles penetrate the scintillator further, colliding with more nuclei and producing more shower particles. Atomic electrons take fraction of energy from the passing particles and enter excited states. When they de-excite back into a ground state, the atomic electrons emit a photon of blue light, i.e. a scintillation. The blue light is picked up by photodetectors. The lead tungstate crystals produce a relatively low yield for each incoming particle, so the signal needs to be amplified. (transmitted to the photomultiplier, converted into a weak current of photoelectrons, and further amplified by an electron multiplier system LEO 158). The total generated light signal is linearly proportional to the energy of the incident particle.

Photodetectors? All photodetectors are glued to the crystals.

  1. Avalanche Photodiodes (APDs) are made of silicon with a strong electric field applied to them. Scintilation photons knock an electron out of an atom, and the electron accelerates in the E field, striking more electrons from silicon atoms. The latter also accelerate and knock out more electrons (the number increases exponentially), hence creating an “avalanche”. This method allows producing a high current in a short period of time. The amplified and digitized signal is transported away by fibre optics cables away from the radiation area for analysis.
  2. A different kind of photodetectors is used in the endcaps due to much higher radiation levels than in the barrel. Vacuum Phototrides (VPTs) contain three electrodes within a vacuum (hence the name). When the scintillating photon strikes atoms in the first electrode, released electrons accelerate towards the second electrode (positive anode) and knock out more electrons. The latter accelerate towards the third electrode (dynode with a higher electric potential than the anode) and again knock out more electrons. This method also produces a strong current form a weak light signal, which is carried away from the high radiation zone via optic fibre cables (what kind of optic fibre cables?????????). http://cms.web.cern.ch/news/crystal-calorimeter

The region in the endcaps must was designed to distinguish between closely spaced particle pairs (such as for example in case of a short lived neutral pion decaying into two closely spaced low energy photons that might be mistaken for one high energy photon from Higgs decay). A special ECAL preshower is located in the endcaps before the EM calorimeter crystals. It is the made of two lead panels followed by silicon sensors (6.3cm x 6.3cm x 0.3mm). Each silicon sensor is divided into 32 strips each 2mm wide. Compared to 3cm wide scintillator crystals the preshower sensor resolution is better (5% precise energy measurement Ph.Bolch). The photon passing through lead sheet produces a shower containing e e+ pairs which are measured by the silicon detector strips. The silicon detectors are kept at temperatures between -10 oC and -15 oC for optimal and long-term performance. The outside of the preshower is heated to temperatures of the ECAL, since the crystals’ performance

  • Muon chamber, muon detectors which are inside the return yoke of the magnet (Track, muons identification). To identify muons and measure their momenta CMS uses three types of detectors: Drift tubes DT (in barrel – position measurement), cathode strip chambers CSC (in endcaps – position measurement), and resistive plate chambers RPC (in barrel and endcaps – trigger).

Energy measurement: calorimetry- by creation and total absorption of showers, either EM (light ammount) or hadronic (penetration depth).


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