Energy of Hadrons (HCAL)

Energy of Hadrons (HCAL) lucas Wed, 11/23/2011 - 16:58

 

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 record 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 casements 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 HCAL Technical Design Report
(please be aware that elements of the detector may have changed since the report's writing in 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.

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 Hybrid Photodiodes (HPDs) . Special electronics then integrates and encodes these signals and sends them to the data acquisition system for purposes of event triggering and event reconstruction.

Hybrid Photodiodes (HPDs)

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

The light-sensitive surface of a HPD is called the photocathode, which converts light into electrons by the photoelectric effect. Inside the HPD, these low-energy electrons are quickly accelerated across a narrow gap of a few millimetres onto a silicon diode target. The target is divided up into 19 pixels each of which can generate its own amplified electronic signal when the accelerated electrons strike it. This allows the detection and amplification of up to 19 separate calorimetry signals with one HPD.  The electronic signals are then sampled for each collision, digitised using special HCAL-designed integrated circuits called QIE chips (Charge Integration and Encode) and sent to the trigger and data acquisition system for analysis. The HPDs amplify the calorimetry signals approximately 2000 times, and 420 of these devices are used in CMS.

Using Russian navy shells

Using Russian navy shells Anonymous (not verified) Wed, 11/23/2011 - 17:48

 

Constructing the endcap HCAL (HE) was the responsibility of the Russian and Dubna (RDMS) groups. The endcaps consist of 17 layers of dense absorber plates that take energy from quarks and muons emerging from collisions and, together with plastic scintillator plates, give a measure of the particles’ energies.

Because the plates are so large and heavy the teams knew that within the experiment they would be under high levels of stress; yet, to maintain their position relative to the beam pipe for as long as 15 years, they couldn’t bend. The material eventually chosen for the plates was brass, in layers 50 mm thick.

But they needed brass of such high quality that it was practically impossible to find. And brass is an expensive material - about twice as expensive as aluminium and 1.5 times that of steel. When Russian engineers discussed the question, one remembered a study carried out on the properties of brass made for military artillery use that sounded promising.

In Russian military storage there were thousands of shells made of brass that would fit such stringent requirements – all 50 years old, made by the Navy and designed to stand internal high stress and sea storage aboard a 1940s Navy Vessel. The shells could be melted for use in the HCAL.

The first hurdle involved obtaining permission of the Commander of the Navy, never easy, but who in fact agreed fairly quickly. Fifteen navy arsenals began recycling their shells, many being off-loaded from battleships in Murmansk to begin their new scientific life as absorber plates within CMS.

First the shells were sent to a plant in the North of Russia to be disarmed of their explosive contents. The shell casings were then stored at JSC Krasnyj Vyborzhets in St. Petersburg before being melted down and rolled into plates, a difficult process given that the brass was already semi-processed.

Over a million WW2 brass shell casements were melted down to make the detector components. But when even this fell short of the massive 600 tonnes of brass needed to make the endcaps, the US agreed to provide a further $1 million in copper (brass being an alloy of copper and zinc) to complete the endcaps. As an example of the international cooperation and mutual trust fostered through this work, even this deal was agreed to on an amicable "handshake" basis.

In Minsk, Belarus, the brass was machined into absorber plates and finally preassembled into parts of the endcaps, weighing 300 tonnes each, before being sent to CERN. Dollezhal Research and Development Institute of Power Engineering (NIKIET) coordinated the efforts of the Northern Navy and the companies involved, and ran 1400 tests to ensure the material was of high quality and suitable for constructing the endcap HCALs.

The shells and additional copper became a total of 1296 brass pieces, which were delivered to CERN in 2002 and 2003 and have since been installed and tested within CMS.

This project was part of an international agreement to convert Russian military industry into peaceful technology. The recycling of stocks of ship artillery into materials for fundamental research was also symbolic; instead of being used to destroy, the weapons could actively benefit humankind through their contribution to enhanced knowledge and technology.