718
D. Froidevaux
• the total power required for the tracker front-end electronics will increase from
approximately 62 to 85 kW from initial operation to high-luminosity operation
after irradiation. Bringing this amount of power to the detector requires large
amounts of copper; the resulting heat load is very uniformly distributed across
the entire active volume of the tracker and has to be removed using innovative
techniques (fluor-inert liquids to mitigate the risks from possible leaks, thinwalled pipes made from light metals, evaporative techniques for optimal heat
removal in the case of the silicon-strip and pixel detectors). There is also
considerable heat created by the detectors themselves: the silicon-strip modules
will dissipate about 1 W each from sensor leakage currents at the end of their
lifetime, and the highest-occupancy TRT straws dissipate about 10 mW each at
the LHC design luminosity.
• for all of the above reasons, it has been well known since the early 90’s in
the LHC community that the material budget of the tracker systems as built
would pose serious problems in terms of their own performance (see Sect. 16.8.1)
and even more so in terms of the intrinsic performance of the electromagnetic
calorimeter and of the overall performance for electron/photon measurements
(see Sect. 16.8.2). Despite the best efforts of the community, the material budget
for the tracker has risen steadily over the years and reached values of two
radiation lengths (X 0 ) and close to 0.6 interaction lengths (λ) in the worst regions
(see Sect. 16.3.2.1 for more details and plots).
The high-radiation environment imposes stringent conditions on the innerdetector sensors, on-detector electronics, mechanical structure and services. Over
the 10-year design lifetime of the experiment, the pixel inner vertexing layer must
be replaced after approximately 3 years of operation at design luminosity. The
other pixel layers and the pixel disks must withstand a 1 MeV neutron equivalent
fluence F neq [18] of up to ∼8 × 10 14 cm −2 . The innermost parts of the SCT must
withstand F neq of up to 2 × 10 14 cm −2 . To maintain an adequate noise performance
after radiation damage, the silicon sensors must be kept at low temperature
(approximately −5 to −10 ◦ C) implying coolant temperatures of ∼−25 ◦ C. In
contrast, the TRT is designed to operate at room temperature.
The above operating specifications imply requirements on the alignment precision which are summarised in Table 16.7 and which serve as stringent upper
limits on the silicon-module build precision, the TRT straw-tube position, and the
measured module placement accuracy and stability.
This leads to:
(a) a good construction accuracy with radiation-tolerant materials having adequate detector stability and well understood position reproducibility following
repeated cycling between temperatures of −20 and +20 ◦ C, and a temperature
uniformity on the structure and module mechanics which minimises thermal
distortions;
(b) an ability to monitor the position of the detector elements using charged tracks
and, for the SCT, laser interferometric monitoring [19];
D. Froidevaux
• the total power required for the tracker front-end electronics will increase from
approximately 62 to 85 kW from initial operation to high-luminosity operation
after irradiation. Bringing this amount of power to the detector requires large
amounts of copper; the resulting heat load is very uniformly distributed across
the entire active volume of the tracker and has to be removed using innovative
techniques (fluor-inert liquids to mitigate the risks from possible leaks, thinwalled pipes made from light metals, evaporative techniques for optimal heat
removal in the case of the silicon-strip and pixel detectors). There is also
considerable heat created by the detectors themselves: the silicon-strip modules
will dissipate about 1 W each from sensor leakage currents at the end of their
lifetime, and the highest-occupancy TRT straws dissipate about 10 mW each at
the LHC design luminosity.
• for all of the above reasons, it has been well known since the early 90’s in
the LHC community that the material budget of the tracker systems as built
would pose serious problems in terms of their own performance (see Sect. 16.8.1)
and even more so in terms of the intrinsic performance of the electromagnetic
calorimeter and of the overall performance for electron/photon measurements
(see Sect. 16.8.2). Despite the best efforts of the community, the material budget
for the tracker has risen steadily over the years and reached values of two
radiation lengths (X 0 ) and close to 0.6 interaction lengths (λ) in the worst regions
(see Sect. 16.3.2.1 for more details and plots).
The high-radiation environment imposes stringent conditions on the innerdetector sensors, on-detector electronics, mechanical structure and services. Over
the 10-year design lifetime of the experiment, the pixel inner vertexing layer must
be replaced after approximately 3 years of operation at design luminosity. The
other pixel layers and the pixel disks must withstand a 1 MeV neutron equivalent
fluence F neq [18] of up to ∼8 × 10 14 cm −2 . The innermost parts of the SCT must
withstand F neq of up to 2 × 10 14 cm −2 . To maintain an adequate noise performance
after radiation damage, the silicon sensors must be kept at low temperature
(approximately −5 to −10 ◦ C) implying coolant temperatures of ∼−25 ◦ C. In
contrast, the TRT is designed to operate at room temperature.
The above operating specifications imply requirements on the alignment precision which are summarised in Table 16.7 and which serve as stringent upper
limits on the silicon-module build precision, the TRT straw-tube position, and the
measured module placement accuracy and stability.
This leads to:
(a) a good construction accuracy with radiation-tolerant materials having adequate detector stability and well understood position reproducibility following
repeated cycling between temperatures of −20 and +20 ◦ C, and a temperature
uniformity on the structure and module mechanics which minimises thermal
distortions;
(b) an ability to monitor the position of the detector elements using charged tracks
and, for the SCT, laser interferometric monitoring [19];