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1.5 Constraints

The detector and its components are optimized and arranged to obtain the most data about the particles spawned by an event. This produces special constraints for the read-out electronics, the transmission links, and thus the measures of EMC.
material budget

The high rates at the LHC demand to design the tracking detectors with their electronics in close proximity to the active elements. This leads to a significant amount of material being placed in the tracking volume - such as electronics, power cabling, cooling, and monitoring cabling. In addition the high momenta of the tracks necessitate a very high level of precision in the detector and so the support structure requires mechanical stability at the level of a few tens of microns over distances of metres. This requirement can only be met by a rigid structure which inevitably increases the material budget.

The effect of the material decreases the performance of the detector. For instance, photons can convert in matter and create tails in the energy measurement. Electrons traversing the material provoke bremsstrahlung and affect both - the energy measurement in the calorimeter and the track momentum measurement.

By a careful design of the active detectors and by the use of materials with low conversion probability - such as aluminium for the power cables, and carbon-fibre reinforced plastic for the support structures - every effort has been made to keep the material in the tracking volume to a minimum.

Putting additional material like shields for EMC measures inside the detector reduces the performance and must be done only as absolutely required. 
radiation environment 

There are three major sources of radiation at the LHC: particle production at the interaction point, local beam losses, and beam-gas interactions. The dominant radiation source for ATLAS will descend from particles produced in proton-proton collisions at the interaction point. The enormous rate of these collisions produces radiation damage - see Table 1.1 - and activates the parts inside the detector. This influences not only the general design of the electronics which has to use radiation-hard or at least radiation-tolerant components, but also EMC. For example, electrolytic capacitors cannot be used for the TRT-front-end boards, because the radiation would activate the electrolyte.

Table 1.1 Maximum integrated dose and neutron fluence seen by the TRT front-end electronics during ten years of operation at the LHC [7].
Dose [kGy] Neutron Fluence [cm -2 ]
Barrel TRT 33 10 14
End-Cap TRT 17 10 14
static magnetic field

A solenoidal magnet creates a static magnetic field of 2 Tesla inside the Inner Detector to curve the paths of charged particles. Ferromagnetic materials are saturated at this high magnetic flux density which eliminates the use of ferromagnetic components inside the TRT. 
confined space

As mentioned before, the detector and its components are optimized to obtain the most possible data for physics. This means that the available space for the on-detector electronics, which is a dead area for physics, is minimized. Due to a dense package, the most powerful measure for EMC - the separation of sensitive and noisy parts - does not work inside the detector. 

Last but not least finances play an important part. ATLAS is financed by institutes of several universities all around the world. The project funding is limited and does not allow expensive custom solutions.

February 9, 2000 - Martin Mandl
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