CMS End Cap Hadron Calorimeter
The CMS detector, presented in fig. 1, is aimed at study of wide range of fundamental problems. In order to cleanly detect the diverse signatures from new physics the identification and precise measurements of final state are needed. A final state of interest may contain all kinds of particles such as muons, photons, electrons, hadrons (or jets) and neutral particles (such as neutrino) which are detected by missing energy.
Fig. 1. General view of CMS. Dark color marks the end cap calorimeters.
For example, Higgs identification involves many different final states and branching ratios depending on its mass. Clean detection of all of these signals will imply that the detector must be capable of detecting almost any unexpected phenomenon.
For reliable identification of such events CMS is equipped with different detectors among which is the End cap hadron calorimeters - îå, marked on fig.1 by dark color. The solid angle of îå is only 13,2%, but the range of pseudorapidity covered by the calorimeter is from 1.3 to 3 that is very substantial part of all pseudorapidity range covered by calorimeters (). The particle density is proportional to pseudorapidity range, that is îå will contain about 34% particles in an event. The presence of îå is extremely important for events containing hadrons and jets in a final state, particularly for events with missing energy. High luminosity of LHC, which is required for realisation of the physical program, corresponds to large number of particles per event in the solid angle of HE. As a consequence a high counting rate and high radiation tolerance is a primary concern (calorimeter elements at small angles must withstand irradiation about 10 Mrad after 10 years of operation).
Because the calorimeter is inserted into 4 ô magnet the absorber must be made from a nonmagnetic material but with maximum absorption length, good mechanical properties and minimal cost. A "cartridge brass", 70% brass and 30% zinc was chosen.
Fig. 2. Generic view of the hadron calorimeter moved out of the magnet.
The HE is attached to the muon end cap. Only a small part of the calorimeter structure can be used for the fixation because the major part of the space between HE and muon absorber is occupied with proportional chambers. To the forward part of HE a 10 tons electromagnetic calorimeter is attached to 2 tons preshower fixed to the front part of the electromagnetic calorimeter. Taking into account the HE weight (about 300 tons) and a strict requirement to minimise quantity of dead materials on the particle path the design of the HE is an unprecedented challenge to engineers.
The calorimeter structure, presented in fig. 3, was proposed for HE and adopted also for the barrel calorimeter. The design provides a self supporting construction without "dead" zones and can be several times assembled (for control) and disassembled for transportation. The total length of the calorimeter (including electromagnetic calorimeter) is about 10. The spacing between the gaps is 8 cm of brass and the gap width is 9 mm. This sampling corresponds to the energy resolution that was verified by measurements with a prototype. Because the energy resolution of HE will be limited by jet algorithm, fragmentation, magnetic field effects and energy pileup at high luminosity minimisation of dead areas and cracks and the absence of tails in the jet energy distribution are more important than a low value for the energy resolution. The absorber is produced in Minsk, MZOR from the rolling made in St. Peterburg, Krasnyi Vyborzhets.
Fig. 3. Mechanical structure of HE absorber.
In the absorber gap active elements, detecting the energy of shower particles, are inserted. As the active elements scintillators are used the light from which is collected by wavelength shifting fibres. The method possesses a number of advantages in comparison with the other types of calorimeters: no "dead" zones, the light can be fed to any place (where photodetectors can be located), possibility to make absorber as solid piece without supporting structures that is very important for collider facilities. The method was proposed for the first time in IHEP. As shown in fig. 4 the trapezoid scintillator 4 or 9 mm thick has a grove with cross section of keyhole, in which a wavelength shifting fibre is inserted. The ends of the fibres are machined with a diamond flaying cutter and one end is covered with aluminium to increase the light collection. The other end is spliced to a clear fibre, which is terminated in an optical connector. The connector with the glued fibres is machined with flying diamond cutter. A bundle of such fibres with the optical connector is tested on a stand presented in fig. 5. The fibres are inserted into grooves machined in aluminium and fixed by pumped out air. Along the fibres UV lamp is moving exiting light in fibres, which is transmitted to PIN diodes. The current from the diodes and UV lamp coordinate are recorded in PC.
Fig. 4. Basic structure of scintillator ( tail) with a groove to fix wavelength fibre. Two layers of reflecting paint cover the side surfaces of the tile.
The measurements are written in database and are compared with standard values. If the measured values are within the allowed range () the fibres with the connector are accepted.
Fig. 5. Stand for quality control of optical fibres with a connector.
Otherwise a fibre or a connector is replaced and measured again. The scintillator produced in Japan, Kuraray (SCSN81) and experimental radiation hard scintillator produced in Kharkov, Institute of single crystal, are machined in Kharkov (cut along the contour and made a groove) and sent to Protvino where they are painted along the narrow edges and put into a frame of so called megatile. The total number of tiles for both HE calorimeters is 20916 and the number of megatiles is 1368. The design of a megatile is presented in fig. 6. As one can see from the cross section W-W, between duraluminum sheets 2 1 mm thick light insulating Tedlar film is placed then a sheet f reflecting Tyvek and then tiles 3.
Fig. 6. Structure of the calorimeter optical elements - megatiles. Green color denotes wave length shifting fibers spliced to clear fibers and glued into the optical connectors.
The tiles are covered by Tyvek with holes for fibres 1 terminated with optical connectors M. Above it other sheet of light insulating film is placed. The gap between the duraluminium plates is fixed by brass spaseres 5 screwed together. The granularity of the calorimeters is for and for .
The megatile design is very robust and reliable (damage of a tile does not lead to rejection of the whole megatile), relatively stiff that is very important for HE because the assemblage of megatiles into absorber will be at big height when HE is assembled.
To control megatile quality and the electronics UV nitrogen laser is used to exite the scintillators. The light is fed by quarts fiber to the connector N. The UV light is fan out by fiber bundle shown in fig. 6 by violet color. These fibres are terminated by aluminum reflectors and distribute the light to all tiles. The light signal produced by UV flash in the scintillator is the same as the signal incited by a charge particle. In this way the performance of all recording rout starting from scintillator (possible degradation of transparency due to radiation damage) up to a final modules of electronics located in control room is checked. For the same porpose a radioactive source moving in a stailess still tube 7 is used. It has additional capability to measure uniformity of scintillator with time and transfere calibration coefficients obtained with fixed target beam to CMS.
After assemblage the megatiles are tested with scanning radioactive source stand shown in fig. 7. Two optical cables are connected to megatile connectors and the other ends are coupled to phototubes situated in blue box. The light signal from each tile is fed to individual phototube (total number of phototubes is 20) and phototube current is measured by digital converter and the information is stored in PC together with radioactive source coordinates. All data are sent to database. The dispersion of light yield from tiles in a tower must be within 10%. Otherwise the megatile is disassembled and faulty element (as a rule it is a fibre or a connector) is replaced.
Fig. 7. Radioactive source scanner to test megatile quality. Tedlar covers a megatile for light isolation.
The megatiles are inserted into gaps in absorber as shown in fig. 8 and fixed by screws. At the backside of the calorimeter in the cut of absorber boxes with photodetectors and electronics are located. Optical cables transfer optical signals from megatiles to these boxes.
Fig. 8. Main elements of HE: absorber with shifted gaps for active elements - megatiles, megatiles and photodetectors connected with megatiles by optical cables.
As photodetectors multipixel Hybrid Photo Diodes are used due to low sensitivity to magnetic field, high gain and large dynamical range (>104).
Now (August 2001) the absorber of the first calorimeter is manufactured and made a control assemblage before transportation to CERN. All parts of megatiles are produced for both calorimeters, 400 megatiles are assembled and 38 megatiles tested and shipped to CERN for the calibration, which must be done before insertion into absorber next March.