720
D. Froidevaux
16.3.2 Construction Experience
16.3.2.1 General Aspects
The ATLAS tracker system has evolved considerably since the submission of the
Technical Proposal in 1994 and even since the corresponding Technical Design
Reports in 1997/1998. The evolution was dictated by many factors, some of which
have already been alluded to in Sect. 16.2.3 and some of which are related to the
specific design challenges posed:
• the rapid development of radiation-hard silicon sensors and of their front-end
electronics led many physicists and engineers in the community to focus for a
long time on the single module scale and, as a consequence, to perhaps address
some of the systems issues, especially for the readout and cooling aspects, too
late.
• the legitimate concerns throughout the collaborations about the material budget
of the tracker systems resulted in huge pressures on the engineering design effort
in terms of materials at a very early stage. This effort has been largely successful
in terms of mechanics, as can be seen from the very light and state-of-theart structures used to support and hold the detector components in the tracker
system. The already considerable experience from the space industry across the
world turned out to be invaluable, including in terms of thermal behaviour and of
resistance to radiation and to moisture absorption.
• the tracker macro-assemblies, once completed as operational devices, are the sum
of a large number of diverse and tiny components. Many of these components
were not built into the design from the very beginning and only general
assumptions based on past experience were made concerning their manufacture.
Several of these assumptions turned out to be incorrect: for example, the use of
silver in the electrical connections and cables has had to be minimised because of
activation issues. The pressure on the material budget led to the choice of risky
technical solutions for cooling and power, involving hard-to-validate thin-walled
Aluminium, copper/Nickel or Titanium pipes and polyimide/Aluminium tapes
rather than the less risky but heavier stainless steel pipes and polyimide/copper
tapes.
• many of the systems aspects were discovered as the detailed design progressed,
rather than foreseen early on, and this has led to difficult retrofitting exercises and
sometimes to technical solutions more complex and risky than those which would
be devised from a clean slate today. Some of the substrates for the electronics of
the silicon modules barely existed in terms of conceptual design at a time when
the front-end electronics chip was ready for production. This is one example of
a specific and critical component, which was not always incorporated into the
detailed design of the system from the very beginning.
Another more general example stems from the engineering choices made for
the implementation of the on-detector and off-detector cooling systems: there
are as many on-detector cooling schemes and pipe material choices as there are
D. Froidevaux
16.3.2 Construction Experience
16.3.2.1 General Aspects
The ATLAS tracker system has evolved considerably since the submission of the
Technical Proposal in 1994 and even since the corresponding Technical Design
Reports in 1997/1998. The evolution was dictated by many factors, some of which
have already been alluded to in Sect. 16.2.3 and some of which are related to the
specific design challenges posed:
• the rapid development of radiation-hard silicon sensors and of their front-end
electronics led many physicists and engineers in the community to focus for a
long time on the single module scale and, as a consequence, to perhaps address
some of the systems issues, especially for the readout and cooling aspects, too
late.
• the legitimate concerns throughout the collaborations about the material budget
of the tracker systems resulted in huge pressures on the engineering design effort
in terms of materials at a very early stage. This effort has been largely successful
in terms of mechanics, as can be seen from the very light and state-of-theart structures used to support and hold the detector components in the tracker
system. The already considerable experience from the space industry across the
world turned out to be invaluable, including in terms of thermal behaviour and of
resistance to radiation and to moisture absorption.
• the tracker macro-assemblies, once completed as operational devices, are the sum
of a large number of diverse and tiny components. Many of these components
were not built into the design from the very beginning and only general
assumptions based on past experience were made concerning their manufacture.
Several of these assumptions turned out to be incorrect: for example, the use of
silver in the electrical connections and cables has had to be minimised because of
activation issues. The pressure on the material budget led to the choice of risky
technical solutions for cooling and power, involving hard-to-validate thin-walled
Aluminium, copper/Nickel or Titanium pipes and polyimide/Aluminium tapes
rather than the less risky but heavier stainless steel pipes and polyimide/copper
tapes.
• many of the systems aspects were discovered as the detailed design progressed,
rather than foreseen early on, and this has led to difficult retrofitting exercises and
sometimes to technical solutions more complex and risky than those which would
be devised from a clean slate today. Some of the substrates for the electronics of
the silicon modules barely existed in terms of conceptual design at a time when
the front-end electronics chip was ready for production. This is one example of
a specific and critical component, which was not always incorporated into the
detailed design of the system from the very beginning.
Another more general example stems from the engineering choices made for
the implementation of the on-detector and off-detector cooling systems: there
are as many on-detector cooling schemes and pipe material choices as there are