16 Integration of Detectors into a Large Experiment: Examples from ATLAS. . .
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over the past 15 years to validate the design choices involving each and every one
of the delicate components composing these tracking detectors. Only a few of the
most prominent examples are quoted below:
• all the front-end electronic designs had to be submitted to stringent specifications
in terms of survival to very high ionisation doses and neutron fluences and of
robustness against single-event upsets. The performance of fully irradiated and
operational modules equipped with the latest iteration in the design had to be
repeatedly measured and characterised in laboratory tests and particle beams of
various types and intensities [20].
• each component in contact with the active gas of the ATLAS TRT straws has
had to be validated in a well-controlled set-up over many hundreds of hours
of accelerated ageing tests using the gas mixture chosen for operation in the
experiment. This was necessary because impurities of only a few parts per billion,
picked up somewhere in the system, could be deposited on the wires and thereby
destroy the gas gain in an irrecoverable way [21]. One critical component in
the barrel TRT modules, a glass bead serving as wire joint to separate the two
halves of each wire, actually failed the ageing tests with the originally chosen
gas mixture (Xe–CO 2 –CF 4 ) and the collaboration had to eventually change the
gas mixture to the current one (Xe–CO 2 –O 2 ), in which the fluorine component
has been removed. This gas mixture reduces the direct risk to the wire joints, but
is somewhat less stable operationally and does not have the same self-cleaning
properties as the original one.
16.3.2.3 Pixel Detectors
The ATLAS pixel detector has been one of the last elements installed in the
experiment, in great part for practical reasons, but also because this is the detector
which has undergone the most difficult development path. It can perhaps be
considered as the most striking example of the marvels achieved during the long
and painstaking years of research and development: the pixel detector will survive
over many years in the most hostile region of the experiment and deliver some of the
most important data required to understand in detail what will be happening within
a few tens of microns from the interaction point.
Fifteen years ago, at the time of the ATLAS Technical Proposal, very few
physicists believed that these detectors could be built within the specifications
required in terms of radiation hardness and of readout bandwidth and speed. Today,
the data collected using cosmic rays (in 2008 and 2009) and early collisions (end
of 2009) have demonstrated that the pixel detector works as expected. The future
will tell how long the innermost layer will survive, but the collaboration is already
proceeding towards a strategy of “replacement” of the innermost pixel layer on the
timescale of 2015. This innermost layer is not expected to survive over the full timespan of the operation of the experiment, which should lead to integrated luminosities
723
over the past 15 years to validate the design choices involving each and every one
of the delicate components composing these tracking detectors. Only a few of the
most prominent examples are quoted below:
• all the front-end electronic designs had to be submitted to stringent specifications
in terms of survival to very high ionisation doses and neutron fluences and of
robustness against single-event upsets. The performance of fully irradiated and
operational modules equipped with the latest iteration in the design had to be
repeatedly measured and characterised in laboratory tests and particle beams of
various types and intensities [20].
• each component in contact with the active gas of the ATLAS TRT straws has
had to be validated in a well-controlled set-up over many hundreds of hours
of accelerated ageing tests using the gas mixture chosen for operation in the
experiment. This was necessary because impurities of only a few parts per billion,
picked up somewhere in the system, could be deposited on the wires and thereby
destroy the gas gain in an irrecoverable way [21]. One critical component in
the barrel TRT modules, a glass bead serving as wire joint to separate the two
halves of each wire, actually failed the ageing tests with the originally chosen
gas mixture (Xe–CO 2 –CF 4 ) and the collaboration had to eventually change the
gas mixture to the current one (Xe–CO 2 –O 2 ), in which the fluorine component
has been removed. This gas mixture reduces the direct risk to the wire joints, but
is somewhat less stable operationally and does not have the same self-cleaning
properties as the original one.
16.3.2.3 Pixel Detectors
The ATLAS pixel detector has been one of the last elements installed in the
experiment, in great part for practical reasons, but also because this is the detector
which has undergone the most difficult development path. It can perhaps be
considered as the most striking example of the marvels achieved during the long
and painstaking years of research and development: the pixel detector will survive
over many years in the most hostile region of the experiment and deliver some of the
most important data required to understand in detail what will be happening within
a few tens of microns from the interaction point.
Fifteen years ago, at the time of the ATLAS Technical Proposal, very few
physicists believed that these detectors could be built within the specifications
required in terms of radiation hardness and of readout bandwidth and speed. Today,
the data collected using cosmic rays (in 2008 and 2009) and early collisions (end
of 2009) have demonstrated that the pixel detector works as expected. The future
will tell how long the innermost layer will survive, but the collaboration is already
proceeding towards a strategy of “replacement” of the innermost pixel layer on the
timescale of 2015. This innermost layer is not expected to survive over the full timespan of the operation of the experiment, which should lead to integrated luminosities