§ OVERVIEW

The LHC at CERN and its detectors have ushered in a new era in particle physics, colliding protons at a centre-of-mass energy up to $\rts = \SI{13}{\TeV}$ and a peak instantaneous luminosity of . The ATLAS detector, which was installed at the LHC IP 1 during the period $2000-2008$, has performed extremely well during and of the LHC, recording 5;21;147 of proton-proton collision data at $\rts = $7;8;13, respectively. The highlight of this period was the discovery of the Higgs boson at a mass of 125, which was announced by the ATLAS and CMS collaborations (each comprising nearly 3000 scientific authors) on July 4th, 2012 <cit.>. The excellent performance of the ATLAS detector and of the LHC has enabled over 1000 publications exploring the nature of the Higgs boson, searching for new phenomena, and continuing to probe the Standard Model.

The ATLAS detector was designed for a peak instantaneous luminosity of $\luminominal$ at a proton-proton () centre-of mass energy of $\rts=\SI{14}{\TeV}$ with about 25 interactions per bunch crossing, and of $\lumiHInominal$ of heavy ion (lead-lead: , and proton-lead: ) collisions at 5.5 per nucleon pair. The original configuration of the detector, as it was built for the start of the LHC, is described in Ref <cit.>. However, the LHC operating conditions differed from expectations; in particular, the instantaneous luminosity surpassed design, reaching a maximum value of  during . There were on average 33.7 interactions per bunch crossing during , with a peak value of over 60 interactions per bunch crossing recorded in 2017 and 2018.
The ATLAS detector performed well despite these harsher conditions; performance and analysis techniques were adapted and improved to maintain the experiment's excellent physics reach during . A substantial upgrade to the ATLAS detector, the “Phase-I Upgrade”, has consisted of improvements to the detector subsystems and their electronics in order to withstand the expected conditions of
(and at maximum 60)
interactions per bunch crossing, and maintain the lowest achievable trigger thresholds, enabling the continued broad physics program planned for .
The ATLAS detector configuration during of the LHC is described in this paper.

§.§ Brief history of the ATLAS detector performance and LHC roadmap

The ATLAS detector has been in operation since autumn 2009. During this period, the LHC and ATLAS have alternated between distinct running periods and long shutdown periods:

, $2009-2013$: ATLAS recorded 5 of collision data at $\rts = \SI{7}{\TeV}$ and 21 at $\rts = \SI{8}{\TeV}$;  of collisions were recorded in 2010 and 2011;  of collisions were recorded in 2013.

LS1, $2013-2015$: This shutdown was used to consolidate the LHC machine elements (repairing the magnet splices and upgrading the collimation scheme) to increase the centre-of-mass energy and reach the design luminosity.
Upgrades to ATLAS installed and commissioned during LS1 are referred to as “Phase-0” Upgrades, and are described in this document; the most significant Phase-0 upgrade was the addition of the innermost layer of silicon pixel detectors described in Section <ref>.

, $2015-2018$: ATLAS recorded 147 of proton-proton collision data at $\rts = \SI{13}{\TeV}$;  of collisions were recorded in 2016 and  of collisions were recorded in 2018. The LHC reached a peak instantaneous luminosity of  and, on average, delivered 33.7 interactions per bunch crossing.

LS2, $2019-2022$: During this shutdown, a new linear accelerator (LINAC-4) was connected into the injector complex <cit.>, and
the injection beam energy of the PS Booster was upgraded in order to reduce the beam emittance. New cryogenics plants were installed to separate the cooling circuits of the superconducting RF cavities from those of the superconducting magnets. The ATLAS Phase-I Upgrade was installed and commissioned during LS2 and its description constitutes the bulk of this document.

, $2022-2025$: The LHC is expected to deliver a peak instantaneous luminosity of approximately $\lumirunthree$ and an integrated luminosity of 250 of proton-proton collision data at a centre-of-mass energy of $\rts = \SI{13.6}{\TeV}$.

LS3, $2026-2028$: The LHC will undergo a major upgrade of its components (e.g. low-$\beta$ quadrupole triplets, crab cavities at the interaction regions). The ATLAS Phase-II Upgrade will be installed and commissioned.

HL-LHC, $2029$ and beyond: The LHC complex is expected to deliver a levelled instantaneous luminosity of $\lumihllhc$ and an annual integrated luminosity of approximately 250 to reach a total dataset of 3000.

Figure <ref> presents the distributions of the delivered luminosity for each production year between 2011 and 2018 and the distribution of the ($\mu$, the number of collisions per LHC bunch crossing) for the four data-taking years of the LHC .

figOv:pplumi Cumulative luminosity versus day delivered to during stable beams for high energy collisions. figOv:PbPblumi Cumulative luminosity versus day delivered to during stable beams for and collisions (indicated on the left and right vertical axes, respectively). figOv:muRun2 Integrated luminosity as a function of the mean number of interactions per crossing, $\langle\mu\rangle$, for the 2015–2018 collision data at $\rts=\SI{13}{TeV}$. All data recorded by ATLAS during stable beams are
shown, including the integrated luminosity and the mean $\mu$ value for each year.
The mean number of interactions per bunch crossing, $\langle\mu\rangle$, corresponds to the calculated mean of the Poisson distribution
of the number of inelastic interactions per bunch crossing.

Over the ten years since the start of LHC, the ATLAS detector has operated reliably under the challenging conditions provided by the excellent performance of the LHC. Three indicators illustrate the detector performance:
the fraction of operational channels, the data taking and the data quality efficiencies. The fraction of operational channels at the start of LS2 was above 99.5% for calorimeters and about 95% for tracking detectors. By the start of , some faulty channels had been repaired, while some detectors that had undergone extensive modifications were still being commissioned. The RPC required flushing with argon to remove debris that had accumulated during LS2, after which the operational fraction improved.
The full details are shown in Table <ref>. ATLAS data-taking efficiency (the fraction of the time ATLAS collects data while
LHC delivers collisions in stable conditions) improved from 90 to 95% in the course of the eight years of data taking. Over the same period, the fraction of collected data declared good for physics analysis has increased from 88.8 to 97.5% <cit.>.

The number of channels vs. the approximate operational fraction for ATLAS subdetectors with more than 100 channels as of the end of (March 2019) compared with the start of (May 2022). The RPC operational fraction is expected to improve as commissioning continues,
and compares favourably with the operational fraction at the start of . None of the 8704 RPC channels of the BIS78 were operational at the start of .

2c|     2c
Subdetector     Number of     Operational      Number of     Operational

Channels     Fraction      Channels     Fraction
Pixel IBL     12M     $99.3\%$     12M     96.7%

Pixel outer 3 layers     80M     $94.8\%$     80M     96.7%

SCT     6.3M     $98.6\%$     6.3M     98.3%

TRT     350k     $97.2\%$     350k     96.6%

LAr Calorimeter (EM)     170k     $100\%$     170k     100%

Tile Calorimeter     5200     $99.5\%$     5200     99.2%

LAr Calorimeter (HEC)     5600     $99.7\%$     5600     99.9%

LAr Calorimeter (FCAL)     3500     $99.8\%$     3500     99.8%

L1Calo Legacy Trigger     7160     $99.9\%$     7160     99.9%

L1Calo SC Trigger     not present     –     34k      100%

L1Muon RPC Trigger     383k     $100\%$     392k     97.5%

L1Muon TGC Trigger     320k     $99.9\%$     312k     100%

MDT     367k     $99.7\%$     344k     $99.7\%$

CSC     31k     $93.0\%$     not present     –

RPC     383k     $93.3\%$     392k     85.8%

TGC     320k     $98.9\%$     312k     99.4%

sTGC     not present     –     358k     99.2%

MM     not present     –     2.1M     98.0%

ALFA     10k     $98.9\%$     10k     100%

AFP     430k     $97.0\%$     430k     100%

§.§ LHC Performance and Status

The LHC and its injector chain have undergone multiple major
repairs, consolidations and upgrades since as detailed in Ref. <cit.>.
These changes facilitated the increase in collision energy and luminosity during and are expected to provide further increases for and beyond.

During LS1 the more than 10000 high current splices between the
LHC superconducting magnets were repaired and consolidated, 18 dipole
magnets replaced and new safety systems added in order to safely
increase the beam energy for . The LHC collimation
system, injection kicker magnets and injection protection system were
upgraded to handle long trains of bunches with 25 bunch
spacing and higher bunch brightness. Additional improvements to the
injection and beam dump systems were done in winter shutdowns during
to be able to further increase the beam intensity.

High-luminosity proton-proton collisions in were delivered at
$\rts = \SI{13}{\TeV}$, while Pb-Pb collisions were delivered at $\rts
= \SI{5.02}{\TeV}$ per nucleon-pair. Multiple improvements were
deployed during the running period to increase the instantaneous
luminosity a factor two beyond the increases brought by the higher
beam energy and the larger number of bunches from shorter bunch
spacing. As the beam optics and the understanding of collimation
tolerances were improved, the interaction optics (), which determines the transverse beam size at the interaction point, was gradually squeezed more and more. In 2016 brighter bunches were introduced
through a change to the bunch-production scheme in the PS called Batch Compression Merging and Splitting
which gave smaller transverse beam size <cit.>.  In 2017 crossing angle
anti-levelling was introduced, where the crossing angle in the
IP was gradually reduced during each fill as bunch
intensity decreased. In 2018 this was augmented with a anti-levelling
where a 15 additional squeeze at the IP
was done towards the end of each fill. Both anti-levelling schemes
increased the integrated luminosity without increasing the peak
instantaneous luminosity, by running near the peak instantaneous luminosity for a much larger fraction of the total running time.

In LS2 a major consolidation campaign was carried out for the
bypass diodes of the LHC superconducting dipole magnets in order
to more safely condition the magnets for highest beam energies. A
large set of upgrades was carried out in the injector chain, including
the connection of the new LINAC-4 to the injector complex and an
increase of the PS Booster beam energy. This will
allow very low emittance, high intensity bunches to be produced for
the HL-LHC. The full intensity beams of the injector will only
be usable after LS3, but upgrades and modifications during
LS2 to the LHC collimators, injection kicker magnets and
the beam dump will allow the LHC to use up to 60% more intense
beams in . In the SPS, upgrades to the RF
system will be used to produce ion beams with 50ns bunch
spacing using a technique called slip-stacking <cit.>, thus enabling a
larger beam intensity to be injected in the LHC for Pb-Pb collisions.

In , the proton-proton collision energy will increase to
13.6TeV, but the peak instantaneous luminosity will remain limited
to about in ATLAS. The main
limitation is luminosity-induced heating of the inner triplet
magnets that provide the final focus before the beams reach the IP. The higher beam brightness from the LS2 injector
upgrade will instead be used to provide extended periods of luminosity
leveled at for as long as 10 hours per LHC fill. The corresponding peak pileup
level is expected to be between 52 and 57 collisions per crossing
during the luminosity levelling period. This is expected to give a
yearly integrated luminosity of more than 80 in the latter
part of and potentially more than for the full
run. For
ions, more than 6^-1 of Pb-Pb collisions is expected in .

§.§ Brief overview of the ATLAS detector configuration for

The ATLAS detector <cit.> at the LHC covers nearly the entire solid angle around the collision point[ATLAS uses a right-handed coordinate system with its origin at the nominal IP in the centre of the detector and the $z$-axis along the . The $x$-axis points from the IP to the centre of the LHC ring, and the $y$-axis points upward. Cylindrical coordinates $(r,\phi)$ are used in the transverse plane, $\phi$ being the azimuthal angle around the $z$-axis. The pseudorapidity is defined in terms of the polar angle $\theta$ as $\eta=-\ln\tan(\theta/2)$. The two ends of the detector are labelled A for $+z$ and C for $-z$, with B used for elements at $\eta=0$.].

It consists of an inner tracking detector surrounded by a thin superconducting solenoid, electromagnetic and hadronic calorimeters,
and a muon spectrometer incorporating three large superconducting air-core toroidal magnets. The dimensions of the detector are 25 in height and 44 in length; the overall weight of the detector is approximately 7000.

The ID system is immersed in a 2 axial magnetic field
and provides charged-particle tracking in the range \(|\eta| < 2.5\).
The high-granularity silicon Pixel detector covers the vertex region and typically provides four measurements per track,
the first hit normally being in the IBL installed before  <cit.>.
It is followed by the SCT, a silicon microstrip tracker that usually provides eight measurements per track.
These silicon detectors are complemented by the TRT,
which enables radially extended track reconstruction up to \(|\eta| = 2.0\).
The TRT also provides electron identification information
based on the fraction of hits above a higher energy-deposit threshold corresponding to transition radiation.

The calorimeter system covers the pseudorapidity range \(\abseta < 4.9\).
Within the region \(\abseta< 3.2\), electromagnetic calorimetry is provided by barrel and
endcap high-granularity lead/LAr calorimeters,
with an additional thin LAr presampler covering \(\abseta < 1.8\)
to correct for energy loss in material upstream of the calorimeters.
Hadron calorimetry is provided by the steel/scintillator-tile calorimeter,
segmented into three barrel structures within \(\abseta < 1.7\), and two copper/LAr hadron endcap calorimeters.
The solid angle coverage is completed with forward copper/LAr and tungsten/LAr calorimeter modules
optimised for electromagnetic and hadronic energy measurements respectively.

The MS comprises separate trigger and
high-precision tracking chambers measuring the deflection of muons in a magnetic field generated by the superconducting air-core toroidal magnets.
The field integral of the toroids ranges between 2.0 and 6.0
across most of the detector.
Three stations of precision chambers, each consisting of layers of MDT, cover the region \(\abseta < 2.7\),
except in the innermost station of the endcaps, in the range $\abseta>1.3$, where the NSW detectors, described below, have replaced the detectors used in .
The muon trigger system covers the range \(\abseta < 2.4\) with RPC in the barrel  \(\abseta < 1.0\) , TGC in the endcap  \(\abseta > 1.0\) regions outside the toroids, and the NSW between the cryostats of the endcap calorimeters and the endcap toroids.

Interesting events are selected by the first-level trigger system implemented in custom hardware,
followed by selections made by algorithms implemented in software in the high-level trigger <cit.>.
The first-level trigger accepts events from the 40 bunch crossings at a rate below 100,
which the high-level trigger further reduces in order to record events to disk at about 3.

An extensive software suite <cit.> is used in the reconstruction and analysis of real
and simulated data, in detector operations, and in the trigger and data acquisition systems of the experiment.

Figure <ref> illustrates the  configuration of the ATLAS detector; the main modifications implemented to the detector, its electronics, and the trigger and data acquisition system as well as their expected impact on the radiation levels are summarised here and will be described in detail in the following sections of this document.

Cut-away view of the configuration of the detector indicating the locations of the larger detector sub-systems.

§.§.§ Radiation and shielding

At the LHC, the primary source of radiation at full luminosity
comes from collisions at the IP. In the ID, charged hadron
secondaries from inelastic proton-proton interactions dominate the radiation
backgrounds at small radii while at larger radii other sources,
such as neutrons, become more important for the calorimeters and the MS. The expected background radiation levels for are presented in Section <ref>, based on measurements from the LHC and and the modified and improved detector geometry.

In ATLAS, most of the energy from collisions at the IP is dumped into two regions: the
collimators protecting LHC quadrupoles and the FCAL.
The beam vacuum system spans the length
of the detector, and in the forward region it is a major source of radiation backgrounds.
Primary particles from the IP strike the  at very shallow angles,
such that the projected material depth is large. Studies have shown that the beam-line
material contributes more than half of the radiation backgrounds in the muon system.

Details of the predicted radiation levels and methods for monitoring and simulating the radiation in the ATLAS cavern are described in Section <ref>.

§.§.§ Tracking

The layout of the ID is illustrated in Figure <ref> and detailed in
Chapter <ref>. Its
basic parameters  are summarised in Table <ref>.
The ID is immersed in a 2 magnetic field generated by the
central solenoid, which extends over a length of 5.3 with
a diameter of 2.5.

The precision tracking detectors (Pixel and SCT) cover
the region $\abseta<2.5$. In the barrel region, they are arranged on concentric
cylinders around the beam axis while in the endcap regions they are located
on discs perpendicular to the beam axis.
The highest granularity is achieved around the vertex
region using silicon   detectors. The Pixel detector consists of three barrel layers and three discs on each side, and has
approximately 80 million readout channels.  The  layers are segmented in $r\phi$ and $z$ with typically
three   layers crossed by each track. For these three outer layers, all pixel sensors are identical, with a pixel size
of [parse-numbers=false]50 x 400. The intrinsic accuracies in the barrel
are 10 ($r\phi$) and 115 ($z$) and in the discs
are 10 ($r\phi$) and 115 ($r$). A fourth inner layer, the Insertable $b$-layer or IBL, was installed during LS1 and started to be operational at the start of data taking. The IBL sensors have [parse-numbers=false]50 x 250 pixels and are at an average radius of 33.4, adding an additional 12 million readout channels to the system (for a total of 92 million pixel channels).
For the SCT, eight strip layers (four space points) are crossed by each track.
In the barrel region, this detector uses small-angle (40) stereo strips
to measure both $r\phi$ and $z$, with one set of strips in each layer parallel
to the beam direction, measuring $r\phi$.
They consist of two 6.4 long daisy-chained sensors with a strip pitch
of 80. In the endcap region, the detectors have a set of strips running radially and a
set of stereo strips at an angle of 40.  The mean pitch of the strips is also
approximately 80. The intrinsic accuracies per module in the barrel
are 17 ($r\phi$) and 580 ($z$) and in the discs are
17 ($r\phi$) and 580 ($r$).
There are approximately 6.3 million readout channels in the SCT.

The TRT is the outermost of the three tracking subsystems of the ID, and comprises several layers of gas-filled straw tubes interleaved with transition radiation material. The 300000 thin-walled proportional-mode drift tubes provide on average 30 $(r,\phi)$ points with 130 resolution for charged particle tracks with $|\eta| < 2$ and $\pT > \SI{0.5}{\GeV}$, contributing to the combined tracking system  resolution. Along with continuous tracking, the TRT provides electron identification capability through the detection of transition radiation X-ray photons.

Cut-away view of the ID, which is designed to provide a high-precision reconstruction of charged-particle trajectories. The ID covers the pseudorapidity range $|\eta| < 2.5$ and has full coverage in $\phi$. It consists of a silicon Pixel detector at the innermost radii surrounded by a silicon microstrip detector (SCT) and a straw-tube detector, the TRT, that combines continuous tracking capabilities with particle identification based on transition radiation.

Main parameters of the Inner Detector system.

Item         Radial extension ()    Length ()

Overall ID envelope                               $0<r<1150$               $0<|z|<3512$

Beampipe                                              $ 23.5 <r<30$

Pixel (IBL included)                    Overall envelope        $ 31 <r<242$                $0<|z|<3092$

4 cylindrical layers           Sensitive barrel          $33.5<r<122.5$            $0<|z|<400.5$

$2\times 3$ discs             Sensitive endcap       $88.8<r<149.6$                $495<|z|<650$

SCT                    Overall envelope      $255<r<549$ (barrel)     $0<|z|<805$

$251<r<610$ (endcap )   $810<|z|<2797$

4 cylindrical layers          Sensitive barrel       $299<r<514$              $0<|z|<749$

$2 \times 9$ discs            Sensitive endcap     $275<r<560$              $839<|z|<2735$

TRT                      Overall envelope      $554<r<1082$ (barrel)        $0<|z|<780$

$617<r<1106$ (endcap )     $827<|z|<2744$

73 straw planes               Sensitive barrel          $563<r<1066$                    $0<|z|<712$

160 straw planes              Sensitive endcap      $644<r<1004$                    $848<|z|<2710$

The updated ID material distribution as a function of pseudorapidity is presented in Figure <ref>.
Compared with the equivalent distribution for the detector, there is considerably more material at \(\abseta > 3.5\), due to the additional services for the IBL, but almost no change in the region covered by tracking.

Radiation length ($X_0$) as a function of $|\eta|$ (averaged over $\phi$) for the and different ID components as implemented in the ATLAS geometry model describing the and configuration. “Services” refers to supports, cooling infrastructure and cabling.

§.§.§ Calorimetry

The ATLAS calorimeters measure the energies and positions of charged and neutral electromagnetically or strongly interacting particles. They are designed to absorb most of the particles coming from a collision, forcing them to deposit all of their energy and stop within the detector. The ATLAS calorimeters are sampling calorimeters, in that they consist of layers of “absorbing" high-density materials that stop incoming particles, interleaved with layers of “active" media that measure the particle energies.

ATLAS uses two sampling calorimeter technologies: LAr <cit.> for the electromagnetic calorimeters and all of the endcap and forward calorimeters, and scintillating Tiles <cit.> for hadron calorimetry in the central region. The calorimeters are highlighted in Figure <ref>.

Cut-away view of the  calorimeter system that measures the energies and positions of charged and neutral particles through interleaved absorber and active layers out to $|\eta| < 4.9$. LAr is used as the active medium for the electromagnetic calorimeters and all of the endcap and forward calorimeters; scintillating Tiles are used for hadron calorimetry in the central region.

The LAr Calorimeter system consists of several subsystems, namely the
EMB, the EMEC, the HEC,
and the FCAL. The ATLAS Tile Calorimeter covers the pseudorapidity region $\abseta<1.7$ using pseudo-projective calorimeter towers composed of scintillating tiles in a steel matrix, read out by wavelength-shifting fibers. The pseudorapidity coverage, granularity, and longitudinal
segmentation of the calorimeters are summarised in Table <ref>.
These calorimeters cover the range  $\abseta<4.9$, using different
geometries and absorber materials suited to the widely varying requirements of the physics processes of interest
and of the radiation environment over this large $\eta$-range.
Over the $\eta$ region for which the inner detector provides tracking, the fine granularity of
the EM calorimeter is ideally suited for precision measurements of electrons and photons.
The coarser granularity of the rest of the calorimeter is sufficient to satisfy the
physics requirements for jet reconstruction and missing transverse momentum () measurements.

The ATLAS calorimeter detectors <cit.> require very few changes to run at luminosities substantially higher than the original design, and are expected to last the entire lifetime of the LHC and HL-LHC. The main calorimeter upgrades were performed on the LAr calorimeter electronics. A new digital trigger path provides finer granularity inputs to the upgraded trigger system and aims to better control the trigger rates by improving the selectivity of electron, photon, and tau lepton objects, the resolution of jets and  trigger signatures, and the discrimination power against background emerging from both out-of-time and in-time pileup. These upgrades as well as improvements to Tile calorimeter cryostat scintillation counters and Minimum Bias Trigger Scintillators are described in Chapter <ref>.

Main parameters of the calorimeter system.

2c|Barrel      2c|Endcap

5|c|EM calorimeter

5|c|Number of layers and $\abseta$ coverage

Presampler      1      $\abseta<1.52$     1     $1.5<\abseta<1.8$

Calorimeter     3     $\abseta<1.35$               2     $1.375<\abseta<1.5$

2     $1.35<\abseta<1.475$         3     $1.5<\abseta<2.5$

2     $2.5<\abseta<3.2$

5|c|Granularity $\Delta\eta\times\Delta\phi$ versus $\abseta$

Presampler      $0.025\times 0.1$     $\abseta<1.52$      $0.025\times 0.1$

Calorimeter 1st layer      $0.025/8\times 0.1$     $\abseta<1.40$
$0.050\times 0.1$     $1.375<\abseta<1.425$

$0.025\times 0.025$     $1.40<\abseta<1.475$
$0.025\times 0.1$     $1.425<\abseta<1.5$

$0.025/8\times 0.1$     $1.5<\abseta<1.8$

$0.025/6\times 0.1$     $1.8<\abseta<2.0$

$0.025/4\times 0.1$     $2.0<\abseta<2.4$

$0.025\times 0.1$     $2.4<\abseta<2.5$

$0.1\times 0.1$     $2.5<\abseta<3.2$

Calorimeter 2nd layer        $0.025\times 0.025$     $\abseta<1.40$
$0.050\times 0.025$     $1.375<\abseta<1.425$

$0.075\times 0.025$     $1.40<\abseta<1.475$
$0.025\times 0.025$     $1.425<\abseta<2.5$

$0.1\times 0.1$     $2.5<\abseta<3.2$

Calorimeter 3rd layer
$0.050\times 0.025$     $\abseta<1.35$      $0.050\times 0.025$

5|c|Number of readout channels

Presampler     7808         1536 (both sides)

Calorimeter     101760         62208 (both sides)

5|c|LAr hadronic endcap

$\abseta$ coverage             $1.5<\abseta<3.2$

Number of layers             4

Granularity ${\Delta\eta\times\Delta\phi}$
$0.1\times 0.1$     $1.5<\abseta<2.5$

$0.2\times 0.2$     $2.5<\abseta<3.2$

Readout channels             5632 (both sides)

5|c|LAr forward calorimeter

$\abseta$ coverage             $3.1<\abseta<4.9$

Number of layers             3

Granularity ${\Delta x\times\Delta y }$ (cm)            FCAL1:  $3.0 \times 2.6$      $3.15<\abseta<4.30$

FCAL1: $\sim$ four times finer      $3.10<\abseta<3.15$,

FCAL2:  $3.3 \times 4.2$      $3.24<\abseta<4.50$

FCAL2:  $\sim$ four times finer      $3.20<\abseta<3.24$,

FCAL3: $5.4 \times 4.7$      $3.32<\abseta<4.60$

FCAL3: $\sim$ four times finer      $3.29<\abseta<3.32$,

Readout channels             3524 (both sides)

5|c|Scintillator tile calorimeter

Barrel         Extended barrel

$\abseta$ coverage     $\abseta<1.0$         $0.8<\abseta<1.7$

Number of layers     3         3

Granularity ${\Delta\eta\times\Delta\phi}$     $0.1\times 0.1$         $0.1\times 0.1$

Last layer                           $0.2\times 0.1$         $0.2\times 0.1$

Readout channels      5760          4092 (both sides)

§.§.§ Muon system

The muon spectrometer forms the large outer part of the ATLAS detector and
detects charged particles exiting the barrel and endcap
calorimeters, measuring their momentum in the pseudorapidity range
$\abseta<2.7$; the layout of the muon spectrometer is shown
in Figure <ref>. The muon system is based on the
magnetic deflection of muon tracks in the large superconducting
air-core toroid magnets, instrumented with separate trigger and
high-precision tracking chambers. Over the range $\abseta<1.4$,
magnetic bending is provided by the large barrel toroid. For
$1.6<\abseta<2.7$, muon tracks are bent by two smaller endcap toroid magnets
inserted into both ends of the barrel toroid. In between these two regions, $1.4<\abseta<1.6$, magnetic deflection is provided by a combination of barrel and endcap fields. This magnet configuration provides a field which is mostly
orthogonal to the muon trajectories, while minimising the
degradation of resolution due to multiple scattering. The muon detector is included in the trigger system in the region $|\eta| < 2.4$.

Cut-away view of the  muon system that detects charged particles exiting the barrel and endcap
calorimeters and measures their momentum in the pseudorapidity range
$\abseta<2.7$. The muon system is built around an air-core toroidal magnet system. The main muon detector components upgraded for  (e.g., the NSW) are visible.

The MS comprises a “barrel”, consisting of three concentric, roughly cylindrical, stations (the inner, middle and outer barrels), and two endcaps, each consisting of three discs, referred to as the inner, middle, and outer endcap “wheels”, supplemented by an “extended” endcap ring of detectors positioned outside the radius of each endcap toroid cryostat.
The anticipated high level of particle flux has had a major impact on
the choice and design of the spectrometer instrumentation and its upgrade, affecting
performance parameters such as rate capability, granularity, ageing properties, and radiation hardness.

The majority of the barrel detectors are unchanged from the original configuration described in Ref. <cit.>:
all three stations use multilayered MDT chambers for the precision measurements in the bending coordinate,
and the outer and middle stations are also equipped with RPC for triggering and to measure the azimuthal coordinate of the tracks. The middle and outer wheels are unchanged from . The middle wheels are located on the far side of the endcap toroid cryostats and the outer wheels are mounted on scaffolding attached to the end-walls of the ATLAS cavern. The “extended” endcap rings provide a third measurement station between the inner and middle wheels for tracks with $1.05 < \abseta < 1.3$, which are outside the acceptance of the outer wheel.
The outer wheels contain only MDT, while the middle wheels have both MDT for precision tracking in the bending coordinate, and TGC for triggering and for measuring the azimuthal coordinate. The inner wheels (often nicknamed “small wheels”), which are the main focus of the Phase-I upgrade, sit between the calorimeters and the endcap toroid cryostats, inside the barrel toroids. These inner wheels have been completely replaced by NSW occupying the same position and providing tracking over the same polar angle range: $1.3 < \abseta < 2.7$. The NSW use two chamber technologies: sTGC and MM detectors, both fast enough for Level-1 trigger functionality, and both designed for precision tracking in the bending direction, as well as improved resolution in the azimuthal coordinate. These NSW allow for improved  resolution in the trigger and increased background rejection, allowing for a low muon threshold and manageable Level-1 trigger rate, thus maintaining the acceptance for many interesting physics processes.

The main parameters of the muon chambers are listed in Table <ref>; the upgrade of the muon system for is described in detail in Chapter <ref>. The asymmetry of the RPC coverage is due to the installation of BIS78 detectors on only the $\eta>0$ side of ATLAS for .

Main parameters of the  muon spectrometer.
$\eta$ coverage                     $\abseta<2.7$ (innermost layer: $\abseta<1.3$)

Number of modules           1098

Number of channels

Function                     Precision tracking
$\eta$ coverage                     $1.3<\abseta<2.7$ (2.4 for trigger)

Number of quadruplets        192

Number of gas volumes             768

Number of channels

Function                     Trigger, precision tracking, 2nd coordinate
$\eta$ coverage                     $1.3<\abseta<2.7$ (2.4 for trigger)

Number of quadruplets        128

Number of gas volumes             512

Number of channels

Function                     Precision tracking, trigger, 2nd coordinate
$\eta$ coverage                     $-1.05<\eta<1.3$

Number of modules           652

Number of channels

Function                     Trigger, 2nd coordinate
$\eta$ coverage                     $1.05<\abseta<2.7$ (2.4 for trigger)

Number of modules           1530

Number of gas volumes           3492

Number of channels

Function                     Trigger, 2nd coordinate

§.§.§ Forward detectors

Four smaller detector systems cover the forward region (see Chapter <ref>).
At [parse-numbers=false]±17 from the IP lies LUCID, which detects inelastic proton-proton scattering in the forward direction, and is the main online and offline luminosity monitor for . The second detector is ALFA. Located at
[parse-numbers=false]±240, it consists of scintillating fibre trackers housed inside Roman pots which are designed to approach as
close as 1 to the beam. ALFA is used in dedicated low luminosity and high running of the LHC and can provide a measurement of the total cross section for proton-proton interactions.
The third system is the ZDC, which plays
a key role in determining the centrality of heavy-ion collisions. It is located at
[parse-numbers=false]±140 from the IP, just beyond the point  where the common straight-section
vacuum-pipe divides back into two independent beampipes. The ZDC modules consist of layers of alternating quartz
rods and tungsten plates which measure neutral particles at pseudorapidities $\abseta \ge 8.2$.
The fourth system, which was installed in 2015 and 2016 is the AFP, which consists of two arms, each with two stations, that are [parse-numbers=false]±210 from the IP. The AFP is designed to study soft, hard, and central (exclusive) diffractive events at low luminosities using a silicon-based tracker for momentum measurements and a time-of-flight system to match protons from the two arms to a single inner detector vertex, thereby reducing the background from multiple proton-proton collisions.

§.§.§ Trigger and Data Acquisition System

The ATLAS TDAQ system selects events with distinguishing characteristics (such as the presence of energetic leptons, photons, hadronic jets, $\tau$ leptons, or large missing energy) that make them interesting for physics analyses, and reads them out for further offline processing. It is based on a two-level event selection system: the L1 system,
which consists of custom-built electronics, and the HLT, which is a software-based system implemented on commercial computers. Interwoven with these levels is the DAQ system, which transports data from custom subdetector electronics through to offline processing, according to the decisions made by the trigger. A diagram of the complete TDAQ system in is shown in Figure <ref>.

The L1 trigger uses reduced-granularity information from the calorimeters and muon system to search for signatures of these events.
The maximum L1 accept rate supported by the detector readout systems is 100, and a share of this rate budget is allocated to each underlying physics object according to the physics goals of ATLAS. All processing for an event must be completed within the time window (latency) permitted by the detector electronics. This latency is 2.5 per event.

The HLT software is designed to reproduce the offline selection as closely as possible, a philosophy that will be taken one step further in through the use of AthenaMT (see additional details in Section <ref>). On average, the event processing time at the HLT in 2018 was approximately 400 for runs with a peak luminosity of 2e34. The mean HLT event processing time in  is expected to be larger due to widespread use of full-detector HLT track reconstruction for hadronic signatures. Upgrades to the HLT farm, and algorithmic improvements to tracking will keep the HLT resource usage within the limits of the deployed system. The HLT reduces the event rate from 100 after the L1 selection to approximately 3 (averaged over the course of an LHC fill), after which the events are stored for offline analysis.

The DAQ system has also undergone an upgrade in order to read out the full detector during . The ROS must support the L1 accept rate of 100 plus $20\%$ contingency. The throughput of the ROS will increase in  due to the increased request rate at which the HLT will request data and a $30\%$ increase in the average event size (the event size at $\langle\mu\rangle \approx 60$ is 2.1MB).
The system must accommodate an average rate of 3 of physics events to mass storage with the flexibility to handle variations in such rate. The maximum throughput is 8GB/s, which offers sufficient margin for these variations in rate. For example, at $\langle\mu\rangle \approx 60$, assuming a maximum physics rate to disk of 3.3 and the above event size, the system throughput is 6.9GB/s. This represents nearly a factor of two increase in the required performance compared to .  In order to store the raw data volume for continuous operation at 3 average output, guaranteeing 24 hours of storage in case of downtime of the CERN mass storage services, the contribution of file transfer and deletion latencies, and the margin needed to guarantee the required file system throughput characteristics, at least 1.4PB of effective storage volume is available at Point 1.

Schematic overview of the Trigger and DAQ system in .

§.§ Physics and performance goals for

The ATLAS physics goals for will build on the successful discovery of the Higgs boson and take advantage of the 250 proton-proton collisions planned by the LHC. The exploration of the mechanism of electroweak symmetry breaking includes an exploration of the phenomenology of the Higgs boson, precisely measuring its mass and width, the couplings to both fermions and bosons, and the observation of rare decay modes. Furthermore, ATLAS will continue to exploit the unique access to the energy frontier offered by the LHC through the study of rare Standard Model processes, flavour physics, and searches for new phenomena such as SUSY and exotic BSM scenarios that may reveal the nature of dark matter.

The Phase-I upgrades of the ATLAS detector have focused on the following improvements to the detector and trigger system, which are required to remain effective not only for , but throughout the lifetime of ATLAS. The first overall objective is the preservation, and in some cases improvement, of the low transverse-momentum electron and muon trigger thresholds that enabled the successful  and  physics program through the collection of a rich dataset of electroweak boson ($H, \Wboson$, and $\Zboson$) decays. The second objective is to maintain sensitivity to electroweak-scale particles that produce hadronically-decaying tau leptons, jets and missing transverse momentum, the magnitude of which is referred to as . These objectives are accomplished through the detector upgrades described in this paper: calorimeter electronics upgrades that provide finer granularity and higher energy resolution to the trigger system,  a new endcap muon detector that can tolerate the high background radiation environment that will be present in the HL-LHC era, and TDAQ system upgrades that take advantage of the calorimeter and muon upgrades while rejecting background in a high pileup environment.

In addition to the Phase-I upgrades, the physics performance of the ATLAS detector in critically depends on its ability to measure charged particle tracks in order to reconstruct primary and secondary vertices; this performance was significantly enhanced by the installation of the IBL described above.

The techniques for the measurement of the instantaneous and integrated luminosities developed in will be refined further in to ensure a continuously high-performant luminosity measurement. provides, moreover, a testing ground for any new luminometer envisaged for the HL-LHC era to gain experience at moderate number of simultaneous particle interactions, before that number increases further for the HL-LHC. The interplay of different independent luminometers, carefully calibrated and together covering the full range in the number of simultaneous interactions over up to four orders of magnitude, is of key importance to achieve the targeted precision in the integrated luminosity in of $1\%$ or below.

The final  ATLAS detector configuration will enable the broad physics goals of the experiment described above through the enhanced selection of the signatures relevant to the ATLAS physics programme, including electrons, photons, muons, $\tau$-leptons, jets, $b$-jets, $B$ mesons, and . The trigger menu developed for translates the physics priorities of the experiment into allocations of the total L1 and HLT rates. At  these rates are about 95 and 3, respectively[These include $10\%$ ($13\%$) of the rate at L1 (HLT) for support triggers, as was done in .]. The trigger menu comprises a list of trigger chains used for data-taking, where a chain consists of a L1 trigger item and a series of HLT algorithms that reconstruct physics objects and apply kinematic selections to them. Roughly equal shares of the overall rate are given to electron and muon trigger chains, with a large share of bandwidth reserved for jets, , taus, and multi-object triggers; rates for single electrons and muons are each limited to approximately 25 or less. Improvements in the trigger performance allow  for lower single-lepton   thresholds, yielding increased physics acceptance for a given trigger rate.

The expected single-lepton trigger performance is demonstrated in Figure <ref>. Figure <ref> demonstrates the expected performance of the  single-electron trigger using $\Zee$ Monte Carlo simulation: a lower rate and improved efficiency is achieved compared to the  electron trigger. A threshold of 22 is used for the  (black) and uncalibrated  (red) electron and photon trigger. The  isolation thresholds were tuned to give the lowest rate while introducing only a $2\%$ inefficiency for electrons passing the L1 energy threshold; this isolation requirement is not applied for clusters with $\ET > 50 (60)\,\GeV$ in the  () trigger. A layer- and $\eta$-dependent calibration is introduced (blue) to compensate for the varying detector response. The threshold on calibrated cluster energy is chosen to produce the same rate as the uncalibrated trigger, resulting in an improved efficiency. For muon triggers, the NSW reduce the trigger rate through the use of trigger chambers that provide up to 16 space points for muons, allowing a constraint on both the position and direction of the candidate muon at the location of the NSW through a pointing segment matched to the primary trigger from the TGC of the middle station of the MS (see also Section <ref>); this rate reduction is illustrated in Figure <ref>. In this figure, the expected muon trigger performance is emulated by using the offline muon segments reconstructed by the MDT and CSC in the old Small Wheels on so-called “enhanced bias” data[To assure statistical sensitivity in the most relevant kinematic regions, a mix of events is selected by the L1 trigger system that emphasises higher energies and object multiplicities. This sample, which is taken from Run 360026, is constructed in such a way that the selection bias is removable with event weights.]. The NSW will provide even more important rate reductions in the HL-LHC era.

Expected fig:ElectronTriggerPerformance electron trigger efficiencies and fig:MuonTriggerPerformance muon trigger rate reduction after the Phase-I upgrades.

§.§ Outline of this Paper

This paper describes the  configuration of the ATLAS detector. The background radiation environment and shielding are described in Section <ref>. Then the ID, LAr and Tile calorimeters, and Muon systems are described in Sections <ref>, <ref>, and <ref>, respectively. The upgraded Forward Detectors are presented in Section <ref>, followed by a description of the TDAQ System in Section <ref>. Finally, an outlook to the Phase-II upgrades planned to prepare ATLAS for the HL-LHC are provided in Section <ref>.

§ BACKGROUND RADIATION AND SHIELDING

The radiation field within and around the ATLAS detector is almost entirely created by the collisions,
occurring at a rate of more than e9
at the IP. The secondary particles produced in
these collisions distribute the total collision energy into different parts of the detector
and the LHC ring.

About 50 of the particles produced are emitted into the acceptance of the central calorimeters
($|\eta| < 3$) but, on average, they carry only about 1 of the total energy. Around 5 of the collision energy
is deposited in the forward calorimeters ($3 < \abseta < 5$). Roughly a third is dissipated in the LHC machine elements within the
experimental cavern, carried by 20 of the particles produced.
The remainder escapes into the LHC ring and is of no consequence for the

Without any shielding the secondary particles interacting with the beam-line
elements and other material in the experimental area would result in prohibitively high radiation
levels in the ATLAS cavern and detectors. Even with dedicated and carefully optimised shielding
the radiation damage to sensors, electronics and power supplies is a serious concern and necessitates
a careful evaluation of radiation tolerance and in many cases recourse to dedicated radiation-hard
components. The margin between radiation exposure accumulated over the lifetime of the experiment and
the maximum tolerated by the components is often narrow. Accurate predictions of radiation
levels were therefore imperative for the original design of ATLAS and are ever more important for future upgrades.

§.§ LHC luminosity evolution and expectation

Figure <ref> shows the luminosity which the LHC has delivered to ATLAS
during proton-proton operation in the years 20112018. The 28 (at 8) from and 157 (at 13)
from add up to 185. This was sufficient to cause non-negligible damage in
detectors, especially the silicon sensors of the ID.

In Section <ref> the observed radiation damage, accumulated between 2010 and 2018, is compared with simulation results
in order to estimate the uncertainty of the simulation. For this the annealing (recovery) of the damage over
the 7-year period, including the end-of-year technical stops and the long shutdown between and ,
has to be corrected for.

The luminosity to be delivered during might be as high as , but since this value
is uncertain, the radiation values in this section will be kept generic by normalising them to 1.
These values can be converted to instantaneous rates at, e.g. e34, by multiplying them by

§.§ Radiation shielding

ATLAS forward radiation shielding, as described in the $\phi$-symmetric geometry. All dimensions are shown in . The $z$ dimensions (horizontal axis) are measured from the nominal IP, which is to the left of the left-hand side of the drawing. The $r$ dimensions (vertical axis) are measured from the centre of the (which is shown in the horizontal centre of the drawing). The role of the TAS absorber
is to protect the focusing magnets, e.g. Q1, against particle debris from collisions at the IP. The shielding around the TAS, here labelled “TX1S”, is also referred to as “JN” in some chapters of this document. The newJD shielding that supports the NSW of the MS is located to the left of the toroid (outside the region shown in the drawing); the tip of its central hub can be seen projecting into the central opening in the toroid at the far left.

The high energy particles emitted in the forward direction cross the LHC at very shallow
angles and a significant fraction of them interact in the thin wall. The particle showers
resulting from these interactions dissipate some of the energy locally while most continue into
the dedicated absorbers (TAS) at $|z|=\SI{19}{\m}$ from the IP. These absorbers protect the superconducting final focus
quadrupoles, e.g. Q1, of the LHC against the heat load due to the collision secondaries emerging from the IP.
These TAS absorbers, two 1.8 long copper cylinders with $r=\SI{17}{\mm}$ apertures for the beam, are
the most intense sources of background radiation in the ATLAS cavern.
They are therefore surrounded by shielding made of cast ductile iron of up to 1.8 thickness, lined
with a layer of boron-loaded polyethylene, as shown in
Figure <ref>.
In order to suppress the 470 photons from thermal neutron capture on boron, but also for fire protection, the polyethylene is
enclosed in a steel cover. Because boron-10 (which makes up 20 of natural boron) has a thermal neutron capture cross-section
of 3800barn, a boron concentration of as little as a few percent is sufficient to remove thermal neutrons and to prevent them from being captured on other elements that would emit more energetic capture photons.

The within ATLAS represents a distributed source of radiation. Since calorimeters and
muon stations are extended to high pseudorapidities, space left for dedicated shielding around the
is very limited. In addition, any shielding has to be placed such that it does not intercept particles
directed towards the TAS absorbers. Since the latter are embedded deep inside the shielding they are the preferred
element to deposit waste energy. This is achieved by a conical inner bore of the shielding, as shown in
Figure <ref>.

The purpose of the calorimeters is to measure the energy of particles,
so the radiation load on them cannot be reduced by shielding and sufficiently
radiation hard technologies must be applied. The liquid argon used in the ATLAS electromagnetic calorimeters,
the hadronic endcaps and the forward calorimeters is intrinsically radiation hard. The scintillators in the Tile calorimeter
are subject to radiation damage, but their dose is reduced because they are shielded by the electromagnetic calorimeter.
The electronics and power supplies are the most vulnerable parts and have to be protected
by shielding, and by optimising their location.

The albedo from the calorimeters, thermal neutrons emitted back towards the IP, is a significant
contribution to the radiation load of the inner tracking detector, adding to the irreducible
particle flux emerging from collisions at the IP. The absorber material
used in the ATLAS electromagnetic calorimeters is lead, which results in copious neutron emission
upon hadronic interactions. The neutron albedo is, however, reduced significantly by
polyethylene moderators lining the sides of the calorimeters which face the IP.

§.§ Characterisation of the radiation environment

The radiation environment, being composed of several particle types, all with a wide energy spectrum,
cannot be described by a single quantity. On the other hand it is not generally feasible – and usually not
necessary – to consider detailed particle spectra at all locations of interest.
In most cases the characterisation of the radiation environment can be based on a few quantities that have proven to
be related to the effects caused in various types of detectors or microelectronic circuits.
The three generic damage mechanisms and the associated radiation quantities are:

* Bulk damage in silicon is usually assumed to be proportional to the 1MeV neutron equivalent
fluence () <cit.>, where fluence is defined as total track length per unit volume.
For silicon this equivalence is defined through energy-dependent hardness factors <cit.>, which depend on particle type and are not applicable to any
other material[Hardness factor compilations also exist for other semiconductor materials, such as GaAs and diamond.].
These factors have been experimentally
determined only over limited energy ranges for neutrons, protons and pions. For other particles
they are extrapolated by theoretical calculations <cit.>.
In addition, different electrical properties, e.g. increase of leakage current and changes of the effective doping concentration, exhibit differences in particle type and material
dependence <cit.>.
For typical particle spectra encountered in ATLAS a systematic uncertainty of about 30 has to
be assigned to the characterisation of the leakage current increase in silicon through .
For estimates of the change of effective doping concentration,
where the particle type and silicon properties (e.g. doping level and impurities) play a non-negligible role, it is better to consider
divided into two components:  neutrons and other particles.

* TID, measured as the amount of energy deposited via ionising processes
per unit mass of material, leads to damage in electronics through charge trapped in oxide layers.
For instance in transistors, the accumulation of this trapped charge can induce a shift of the threshold voltage needed to switch the state
or increase lateral leakage current in the oxide layer. Scintillating materials and optical
fibres also suffer from damage that to good approximation is proportional to the TID <cit.>.
The damage manifests itself mainly as a reduction of the light transmission, but sometimes also as induced
phosphorescence or changes of the scintillating properties.

* SEE in electronics circuits are caused by large energy depositions
close to sensitive regions of the chips. The released charge can be sufficient to flip the logic state
of a transistor or, in the worst case, permanently damage the component.
The amount of ionisation needed to cause a SEE can only be deposited by slow heavy ions.
At the LHC such slow ions are nuclear fragments from hadronic interactions within the chip itself. The
generally adopted characterisation of the radiation field for SEE rate estimates is to use the total
flux of hadrons with energies above 20 <cit.> (). Contrary to the two first
mechanisms, which depend on the cumulative radiation exposure, the probability of a SEE occurring in
any given time interval can, to a good approximation, be assumed to be independent of the irradiation

In addition to the characterisation by the damage potential of the radiation a further quantity
is needed to describe the potential impact on detector performance: the instantaneous rate of charged
particles at peak luminosity. These charged particles create hits in detectors and lead to an increase of occupancy and possibly spurious
triggers which can compromise the available bandwidth and increase the dead-time. The muon system, which
is designed for much more modest hit rates than the inner detector, is particularly sensitive to
the detailed kinematics of the background which gives rise to fake muon trigger signals (see Section <ref>).
Since low-energy charged particles have short ranges in matter, an accurate
estimation of the hit rate in the sensitive volumes of the muon detectors requires a very detailed description
of the detector and its shielding. General fluence simulations, as presented here, give only an indication of the
hit rates to be expected.

§.§ Radiation monitoring

In view of the significant role which radiation damage was expected to play for ATLAS components, a monitoring
of the radiation environment was foreseen early on. The RadMon system <cit.> consists of sensors at 14 locations
inside and around the ATLAS detector. Each RadMon station comprises several RadFet with varying oxide thicknesses such that they
together cover a range from few  up to 100 of TID.
In order to account for the fact that the radiation field around ATLAS is composed of many particle types and energies,
the RadFet are calibrated at several facilities using different particle types: protons of different energies, X-rays,
$\gamma$-rays and neutrons, allowing for an assessment of the differences in response for TID from different particle types.
The uncertainty of the dose measurement with the RadFet was estimated at 20 <cit.>.

The RadMon system also includes two different kinds of PIN-diodes which provide two ranges of sensitivity to measure : through
an increase of the leakage current under reverse bias or through the resistivity change by monitoring the voltage under forward current.
The diodes have different sensitivity ranges, such that the high-sensitivity ones provide a linear response
from about e9 up to few times e12, where the
low-sensitivity ones pick up and can measure  up to around e15. The diodes are also calibrated at various facilities,
using protons, pions and neutrons, and no significant particle type dependence was uncovered. They are thus
well suited for the mixed radiation field present in ATLAS.

In addition to the dedicated RadMon system, the radiation damage in the detectors themselves can be used to determine their total exposure.
In particular the silicon sensors of the ID are ideal for such a study. Their irradiation and temperature (annealing)
histories are recorded accurately and their leakage currents are constantly monitored. Since sensors are distributed over the entire
Pixel and SCT volumes, they provide an excellent means to determine the spatial variation of in the ID <cit.>;
however, these sensors are not suited to measure the TID.

The scintillators of the Tile calorimeter have also suffered from radiation damage that has been compared with the simulated TID <cit.>.
However, the radiation-induced light loss at the end of was still very small, with large uncertainties, so
the Tile scintillator data are not yet sufficient to constrain the simulation.

§.§ Radiation simulation codes and methods

The radiation environment in ATLAS has formerly been studied with the  <cit.>
and  <cit.> Monte Carlo simulation programs.
Recent simulations, however, have been done with and 4 v10.4 <cit.>. While the
first has a long history as a dedicated radiation simulation program, 4 has only recently been
employed for such tasks. The advantage of the 4 simulation is that it benefits from a very
detailed three-dimensional model of the ATLAS detector, built for physics performance simulations.
Having two different simulation packages, utilising independent geometry models, provides a
means to assess the systematic uncertainties due to geometry description and accuracy of the
physics modelling.

The description of the ATLAS detector is much simpler than that of 4 and, with a few
exceptions, $\phi$-symmetric.
Since, however, the simulations are specifically targeted at
estimating the radiation levels, great attention has been paid to the modelling details and material
composition of the shielding. This is especially true for the ID, while for the calorimeter and cavern
regions, where the geometry lacks many details, the 4 results are considered more reliable.

During the simulations several generic maps are generated, which illustrate the spatial variation of the intensity of the
most relevant radiation quantities.
All these maps are averaged over $\phi$ even if the underlying geometry is not perfectly
$\phi$-symmetric[In a few situations, where the $\phi$-structure has a significant
impact on the radiation levels, the 4 results have been averaged over restricted $\phi$-ranges.]
and mirror-symmetry with respect to $z=0$ is assumed.
The entire ATLAS cavern is covered by maps with [parse-numbers=false]10 x 10
cell size in $r$ and $|z|$.
For the calorimeter regions a finer binning with [parse-numbers=false]4 x 4
cell size is used. In the ID
the radial gradient is steep and detector elements are thin. Here the radial bin size is 2 up to $r=\SI{20}{\cm}$ and
2 beyond that. In $|z|$ the bin size in the ID region is 2.

It is expected that the LHC will operate at a collision energy of 13.6 throughout . Since an
increase to 14 is anticipated after LS3, the radiation simulations have been performed assuming the ultimate design energy of 14.
The effect of this difference in the radiation levels is about 2.
The primary collisions were
simulated with 8 <cit.> and results were normalised assuming an
inelastic cross section of 79.3mb.

§.§ Comparison of observed radiation levels with simulations

Prior to LHC operation very few comparisons of radiation damage and simulations were available, so
the initial radiation predictions <cit.> relied heavily on simulations and only
rather vague estimations of the uncertainties could be given.

During the ten years of LHC operation the radiation field in and around ATLAS has been measured and monitored by
the methods described in section <ref>. A comparison of these measurements allows for better estimates of the uncertainties associated with the simulations.
The simulation models themselves have been further refined over the past decade and, most importantly, the
available computing power has increased dramatically. These developments have made it possible to
reduce the statistical uncertainties of the simulations to an extent that makes them negligible in almost
all cases. The only exceptions are regions with very low radiation levels where the statistical fluctuations from individual energy depositions
may be very large in a fully simulation.

§.§.§ Inner detector

Figure <ref> shows the comparison of the RadMon measurements  with and 4
simulations of the TID inside the ID volume. The contribution of has been
subtracted from the measurements in order to compare only the dose, received at 13.
The simulations have a tendency to overestimate the TID. The difference with respect to measurements is
30100 with the largest deviation on the cryostat wall where the dose is lowest.

Figure <ref> shows a comparison of the leakage current measured in SCT sensors with and 4 predictions.
The simulated values are converted to a prediction of the leakage current using the Hamburg annealing model
<cit.>, which takes into account the detailed irradiation and temperature histories of the silicon
modules. The agreement is better than about 20, i.e. within the uncertainty assumed for the silicon hardness

fig:Radmon_ID_dose Comparison of measured and simulated TID at several
locations inside the ID volume. One set of monitors is close to the IP, fixed on the PST.
Two sets are at different radii on the IDEP next to the endcap calorimeter, and a fourth set on the
wall of the cryostat of the solenoid. The error bars reflect the statistical uncertainty of the simulations and
the variation related to a position uncertainty of the RadMon sensors.
fig:SCT_sidam Comparison of measured and simulated leakage current in ID sensors as a function of radius <cit.>.
The error bars on the symbols include various uncertainties affecting the measurement while the widths of the simulated bands
reflect the statistical uncertainty and the variation as a function of sensor radius. Uncertainties of the silicon damage
factors, discussed in Section <ref>, are not included.

§.§.§ Calorimeter and muon regions

Figures <ref> and <ref> show the comparison of the RadMon measurements
with TID and predictions of and 4 at the locations of calorimeter front-end electronics
and power supplies.
The agreement between the simulated and measured TID is worse and less consistent than for the ID regions. At some
locations the simulations are in satisfactory agreement with the measurements, but there are regions where the
predictions are five times higher than the RadMon devices indicate. All locations shown in
Figure <ref> are behind a significant amount of material with respect
to the IP. Some are close to, or even within, service channels where the exact amount of material is difficult to
describe accurately in the simulation models. It is therefore very likely that the large deviations, which are all
overestimations by the simulations, are caused by an underestimate of cables and other services.
in many cases, TID predictions of and 4 are more consistent mutually than with data. It is possible that
this indicates an issue with the calibration transfer of the RadFet devices, i.e. a systematic effect between the
spectra at the calibration facilities and the wide particle spectrum at the location where the sensors are installed in ATLAS.
This possibility needs further investigation and cross checks with additional monitors in .

For the agreement is better with maximum discrepancies not exceeding a factor of two. This is quite plausible
since the TID is more sensitive to the accurate description of material in the immediate vicinity of the RadMon
detectors. Even small inaccuracies in the description of these immediate surroundings can result in a large deviation
of the TID estimate while the other radiation quantities are less affected.

Comparison of measured fig:Radmon_LAr_dose TID  and fig:Radmon_LAr_sidam in different locations around the calorimeters and the ATLAS muon system. The error bars reflect the statistical uncertainty of
the simulations and the variation related to a position uncertainty of the RadMon sensors. Some of the very large error bars for TID
are due to the fact that the quantities are recorded in a small well-shielded volume. Here a single deposition by an energetic
electron can dominate and cause a large fluctuation.

§.§.§ Simulation safety factors

Figures <ref> and, especially, <ref> indicate a that the simulations can deviate from
measurements by up to a factor of five. The larger deviations, however, are systematically overestimates of the simulations. All of these
appear in regions behind a significant amount of material, and most likely indicate that some material is missing in the simulation models.
In some regions of the inner detector the simulations underestimate the measurements, but by not more than 20%.

These comparisons are crucial input for defining the safety factors to be applied on top of the simulations, when designing the detectors and
their electronics. Since the largest deviations systematically correspond to an overestimation of the simulations, ATLAS has adopted a
unique simulation safety factor of 1.5 for all radiation quantities and detector regions.

§.§ Predicted radiation levels

The radiation levels are mapped separately for each of the three main detector subsystems, as the very different sizes of the ID, the Calorimeters and
the MS make different scales appropriate.

§.§.§ Radiation levels in the Inner detector

In the inner detector region the background originates from two main sources, of those discussed in Section <ref>:

* particles produced in the proton-proton collisions at the IP and secondaries from their
interactions in the walls or the material of the ID.

* albedo from the electromagnetic calorimeters.

The first comprises a mix of all particle types, but for silicon bulk damage, charged pions are
the most significant component.
While the fluence of charged hadrons decreases rapidly with radius, the neutron fluence, which is
dominated by calorimeter albedo, is more uniform throughout the Pixel and SCT volumes.

Figure <ref> shows that, in terms of , the contributions of both generalised particle categories (neutrons, and all other particles)
are equal at radii of  20 and 15 at $z=0$ (in the central transverse plane of the detector) and at
$|z|=\SI{272}{\cm}$ (at the extremities of the SCT envelope), close to the endcap calorimeters,
The bulk damage in the pixel detector is dominated by charged pions, but in the SCT by neutrons. Figure <ref> shows that the neutron
component is more significant closer to the endcap calorimeters. This is due to the intense neutron albedo from the lead absorbers, which cannot be
entirely suppressed by the neutron moderators on the calorimeter face.
While for older technologies, including all those
installed for , provides a sufficiently accurate parametrisation of the bulk damage in silicon, an additional
particle-type dependence has been observed in some more recent devices.

In the absence of scattering and a magnetic field, it can be shown that particles emerging from the IP with a flat $|\eta|$-distribution
would result in a fluence independent of $|z|$ and dropping as $r^{-2}$.
Figure <ref> shows that this
radial dependence models the data well up to $r\sim$20 but in the SCT region the fluence drops somewhat more slowly
due to particle production in inelastic interactions and possibly the curling up of tracks in the 2 axial field.
Comparing Figures <ref> and <ref> also reveals a small $|z|$-dependence of the
1 neutron-equivalent fluence.
Besides the fact that the $|\eta|$-distribution is not perfectly flat, this increase of with
$|z|$ may also be due to secondary production and the magnetic field.
The increase of the neutron contribution closer to the endcap calorimeter, clearly seen when comparing the two plots,
shows the impact of the calorimeter albedo, which is the main source of neutrons.

Radial dependence of at fig:rDepIDz0 $|z|=0$ and fig:rDepIDz272 $|z|=\SI{272}{\cm}$ in the ID, as obtained from simulations. The total is divided into the contribution by neutrons and by all other particles.
The $r^{-2}$ dependence is normalised at $r=\SI{3.1}{\cm}$ to the `By others' value. The lower panels show the ratio
of and 4 results for the total.

This $r$- and $|z|$-dependence of can also be appreciated qualitatively from Figure <ref>.
Table <ref> lists the three main radiation quantities averaged over the regions indicated by
rectangles and labelled with letters in Figure <ref>. These areas are selected to cover
representative regions of the Pixel, SCT and TRT detectors. The $r$ and $|z|$ positions given in Table <ref>
correspond to the centre of the rectangle. The statistical uncertainties of the simulations are typically less
than 1 but if the last digit given in Table <ref> is affected, this is indicated.
Systematic uncertainties are not included and, as discussed before, can amount to several
tens of percent.

figureRadiation levels in the inner detector region. The contours show obtained from simulations. The radiation levels, averaged over the labelled volumes are given in Table <ref>
The Pixel and SCT layers are indicated by grey lines; the TRT and adjacent calorimeter and associated neutron moderator
volumes are indicated by boxes.

tableTID and characteristic particle fluences in the inner detector volume, obtained from simulations at the locations indicated in Figure <ref>.
The statistical uncertainty is in the last digit given, or smaller.

Region         1c||z| [cm]         1c|r [cm]        TID [Gy/fb$^{-1}$]       [cm$^{-2}$/fb$^{-1}$]       $\Phi_\mathrm{20}^\textrm{had}$ [cm$^{-2}$/fb$^{-1}$]
IBL e           29          3     3.423     5.3712     9.0812

IBL c           4          3     3.123     6.1912     8.4912

PIX D3           65          9     $821$     1.1312     1.4512

PIX B3           4          12     $377$     7.8511     7.2211

SCT D6           178          30     $171$     3.5811     2.4911

SCT B1           71          30     $118$     2.9311     2.0011

SCT D9           273          46     $99.1$     3.3611     1.5811

TRT EC low           266          68     $65.0$     2.3411     8.5010

SCT B4           71          52     $46.0$     1.6811     8.1210

TRT B low           69          61     $36.7$     1.3411     6.0910

TRT EC up           266          94     $35.2$     1.8611     5.1010

TRT B up           69          103     $12.9$     8.2710     2.3710

§.§.§ Radiation levels around the calorimeters

Figure <ref> and the associated Table <ref> show the distribution in the
calorimeter regions and the volume-averaged main radiation quantities, respectively. The regions for which the averages
have been calculated correspond to the location of front-end and power supply electronics of the calorimeters, i.e. to
equipment susceptible to suffer from radiation damage.
In Figure <ref> the streaming of radiation through the gaps between the calorimeter parts, especially at
$|z|\approx 320$ cm, is clearly visible.
In reality these gaps are filled with electrical cables, optical fibres and cooling pipes following complicated routing.
These can be described only in an approximate and average way in the simulation codes, which is the possible cause for the
overestimation of the radiation levels, seen in Figure <ref>.
Although no direct measurements are available to support the predictions of , high-energy hadrons are less sensitive
to material than the other two radiation components. Therefore it can be assumed that the uncertainties are bounded by the
uncertainty on , seen in Figure <ref>.

figureRadiation levels at the locations of calorimeter electronics. The contours show obtained from
4 simulations. The radiation levels in the indicated volumes are given in Table <ref>. The faint grey lines are
density contours from the 4 geometry model and only serve to give an indication of the detector geometry.

tableTID and characteristic particle fluences obtained from 4 simulations at the locations of calorimeter
electronics and power supplies, i.e the regions indicated in Figure <ref>.

Region         1c||z| [cm]         1c|r [cm]        TID [Gy/fb$^{-1}$]       [cm$^{-2}$/fb$^{-1}$]       $\Phi_\mathrm{20}^\textrm{had}$ [cm$^{-2}$/fb$^{-1}$]
Barrel FE crates           318          314     1.42-1     3.809     6.508

HEC Electronics           516          206     8.00-2     7.159     4.448

Barrel LVPS           302          406     1.97-2     1.009     1.458

EC FE crates           648          314     1.88-2     3.408     4.147

EC LVPS           634          406     5.9-3     1.318     1.337

§.§.§ Radiation levels in the muon system and cavern

Since the cavern is efficiently shielded with respect to the beam-line, the available simulation samples have too much
statistical fluctuation between bins to produce a contour plot of TID or charged particle fluences in the entire ATLAS
experimental cavern. Instead Figure <ref> shows the distribution of the photon fluence.
This quantity is chosen for the illustration, since the photons are generating the e$^{\pm}$, which dominate the
charge particle fluence and TID in the muon spectrometer region.

figureContours of constant fluence for photons with $E>\SI{10}{\keV}$ ($\Phi^\gamma_\textrm{0.01}$) in the ATLAS cavern and muon spectrometer regions.
The black boxes indicate regions over which the main radiation quantities given in Table <ref> are averaged. More detailed explanations of the
detectors in these regions can be found in Section <ref>.
The faint grey lines are density contours from the 4 geometry model and only serve to give an indication of the detector geometry.

tableTID and obtained from 4 simulations in the ATLAS cavern and muon spectrometer
as averages over the volumes indicated in Figure <ref>. The rightmost column gives the
fluence of photons with $E>\SI{10}{\keV}$ ().
$r$ and $|z|$ refer to the centre of the given region.
The statistical uncertainty is in the last digit given, or smaller.

Region         1c||z| [cm]         1c|r [cm]        TID [Gy/fb$^{-1}$]       [cm$^{-2}$/fb$^{-1}$]       $\Phi_\mathrm{20}^\textrm{had}$ [cm$^{-2}$/fb$^{-1}$]       $\Phi^\gamma_\mathrm{0.01}$ [cm$^{-2}$/fb$^{-1}$]
LUCID           1705          15     2.913     2.5712     4.8311     5.7813

NSWH           725          95     4.40-1     1.7910     1.869     3.0910

EM-1           1325          240     7.6-3     3.038     6.457     6.178

NSWL           735          400     6.9-3     2.028     4.817     9.038

BI-7           625          450     4.4-3     9.057     1.137     3.428

EE-1           1055          660     2.43-3     1.108     2.307     2.058

BM-6           870          795     1.63-3     4.637     1.137     1.568

EO-1           2165          325     1.3-3     3.197     3.996     9.587

EM-5           1325          1025     1.15-3     3.217     8.346     9.327

BO-5           945          1045     1.07-3     2.367     6.946     7.987

BM-1           95          795     8.1-4     1.527     2.436     8.877

BI-1           50          450     7.3-4     2.707     7.05     8.557

BO-1           120          1045     6.4-4     1.037     2.596     5.697

EO-6           2165          1090     5.9-4     1.367     3.236     5.827

All major radiation quantities  are collected in Table <ref> as averages over representative volumes.
The dose rates in the muon system vary from 0.59 in the central barrel to 44 at the innermost radius of the NSW
region. At an instantaneous luminosity of 2e34 these correspond to 12/ and 0.9/, respectively. Assuming
all particles to be minimum ionising, these doses can be converted to rough charged particle flux estimates of
about 50
and 5000,
respectively. In reality the TID has contributions from
particles with higher ionisation potential, which means that those estimates serve only as rough upper limits.

As shown in Figure <ref>, the LUCID detector is situated in the immediate proximity of the where it is
also subject to intense albedo from the TAS regions. A comparison of the radiation levels in that region with those of the cavern
gives a good indication of the performance of the forward shielding of ATLAS: the particle fluences are reduced by about four
orders of magnitude and the TID by almost 6 orders of magnitude (e.g. comparing regions `LUCID' and `EM-1'). This larger shielding efficiency
for TID is due to the fact that charged particle showers, except for muons, developing around the beam-line all get
suppressed and only neutrons with an associated photon component penetrate the shielding.
Thus the radiation field in the cavern is almost entirely due to neutrons, photons from neutron capture and e$^\pm$ from
interactions of the photons.
One notable exception in the radiation exposure of the muon system is the high-$\eta$ edge of the NSW
(NSWH in Table <ref>); here the space available for shielding is limited by the required acceptance and
the clearance needed with respect to the . Even though dedicated shielding materials are employed, the radiation levels at the
smallest radii of the NSW are almost two orders of magnitude higher than anywhere else in the muon system.

§ INNER DETECTOR

The ID is the primary tracking device for measuring the paths of all charged particles in ATLAS. It has been designed to provide hermetic and robust pattern recognition, excellent momentum resolution and both primary and secondary vertex measurements for charged-particle tracks within the pseudorapidity range $\abseta<2.5$. It is contained in a cylindrical envelope
long and 1150 in radius, immersed in the 2 field of a solenoidal magnet.

As described in Section <ref>, the ID consists of three complementary sub-detectors arranged coaxially around the beam line (see Figure <ref>): a high-resolution silicon Pixel detector <cit.> ($r< \SI{122.5}{\mm}$), the SCT <cit.> relying on stereo micro-strips ($299<  r< \SI{514}{\mm}$) and the TRT <cit.> comprising several layers of gaseous straw tubes interleaved with transition radiation material ($563<  r< \SI{1066}{\mm}$).

The layout of the ATLAS ID, including the IBL detector <cit.>.

For many physics channels, particularly those involving relatively long-lived particles such as $B$-hadrons that decay and produce a secondary vertex inside the , the performance of the ATLAS experiment depends critically on the innermost layer of the Pixel detector. For this reason,
during LS1, the detector underwent a major upgrade. During this period a fourth pixel layer, the IBL <cit.>,
was added to the Pixel detector between a new, narrower beryllium and the previously existing innermost pixel layer (Pixel $B$-Layer). Figure <ref> shows the transversal layout of the upgraded ATLAS ID during and . A description of the main concepts and characteristics of the IBL is given in Section <ref>.

Transverse view of the ATLAS ID including the IBL detector.

At the same time, part of the Pixel services was replaced by the nSQP, allowing for the optical-to-electrical converters to be moved into an area accessible for service by extending the
electrical readout cables
A brief overview of the nSQP  installation together with a description of the Pixel off-detector readout upgrades in is given in Section <ref>.

The readout systems of the SCT and TRT were also upgraded during , to cope with the increasing requirements coming from the LHC performance. A brief description of these upgrades is presented in Sections <ref> and <ref>.

§.§ Insertable $B$-layer

The driving motivation for the IBL detector was the consolidation and enhancement of the ID tracking performance in high luminosity scenarios. The reduced distance of the IBL from the beam axis (3.3, compared to 5.0 for the Pixel $B$-Layer) increases the resolution of track impact parameters and thus enhances the vertex reconstruction and flavour-tagging performance of the tracking system.  An additional layer of highly segmented pixel sensors helps to mitigate inefficiencies of the Pixel detector caused by overall radiation damage and irreparable module failures, and reduces the occurrence of fake tracks due to the combinatorics of high pileup backgrounds.

The reduced distance from the beam axis, combined with the increased luminosity of the machine, set stricter radiation hardness requirements.
Simulations <cit.> predicted that IBL sensors  and electronics would be subject to an overall fluence of 3.3e15
for an integrated luminosity of 550, which was the original design requirement for .
A planar technology adapted according to the Pixel experience was used for the sensors in the central part while a new 3D technology was developed for the forward region. Section <ref> describes these two technologies.
For the electronics, a new front-end chip was developed on the basis of the  Pixel experience to cope with the more stringent
requirements foreseen in future data taking scenarios.

IBL layout <cit.>: (a) Longitudinal layout of planar and 3D modules on a stave. (b) An $r-\phi$ section showing the , the IPT, the staves of the IBL detector and the IST, as viewed from the C-side. (c)  An expanded $r-\phi$ view of the corner of a 3D module fixed to the stave.

The IBL layout is shown in Figure <ref>. The IBL consists of a single layer of pixel sensors assembled with their readout chips (modules) and arranged on 14 longitudinal supports (staves) fastened onto a high precision carbon fibre tubular structure, the IPT (outer radius of 29.3), via the stave support ring. The staves surround a 23.5-radius beryllium . The outer envelope is defined by a second carbon fibre cylinder, the IST (inner radius of 42.5), fastened to the Pixel detector support structure.

The staves are tilted by 14 in a classic turbine design in order to ensure complete azimuthal coverage for high $\pT$ tracks and to partially compensate for the Lorentz angle affecting the trajectory of the charges drifting in the planar sensors. With this arrangement, the average distance of the innermost pixel sensors from the beam-axis was reduced from 5.05 of the Pixel $B$-Layer down to 3.35. Each stave hosts 20 modules aligned along the beam axis (z-direction): 12 two-chip modules covering the central part and eight single-chip modules dedicated to the high-regions extending to $\abseta=3.0$. The full detector coverage is effectively $\abseta=2.58$ if the 122 two-standard-deviation spread of the primary vertex distribution is taken into account. The main layout parameters of the IBL are presented in Table <ref>.

Main layout parameters of the IBL. The nominal $\eta$ coverage of the detector (assuming no spread on the longitudinal position of the IP)  is $\abseta < 3.0$ but a spread of the vertex position on the $z$-coordinate reduces the detector coverage down to $\abseta < 2.58$.

Item     Value

Number of staves     14

Number of modules per stave     12 planar $+$ 8 3D

Number of front-end chips per stave     32

Coverage in $\eta$ – no vertex spread     $\abseta<3.0$

Coverage in $\eta$ – $2\sigma$ (122) vertex spread

Active $|z|$ stave length      330.15

Stave tilt in $\phi$     14

Overlap in $\phi$     1.82

Center of the sensor radius      33.5

Limited IBL radial clearance prevents sensor shingling along a stave; for this reason, in order to maximise the coverage, thin-edge sensors were developed for the detector. An effective inactive edge width of 215 (175) was measured for planar (3D) sensors, considerably reduced with respect to the 1100  of the Pixel detector in . An air gap of 205 is maintained between contiguous modules to provide adequate electrical insulation.

Cooling is achieved by means of a ${\text{CO}}_2$ two-phase system where the coolant is circulated within titanium pipes embedded in the stave structure; in order to optimise the thermal contact between the active components and the pipes, staves are filled with carbon foam, which contributes to the global stiffness of the mechanical supports. A detailed description of the cooling system is given in Section <ref>.

§.§.§ New

The ATLAS had to be replaced to allow sufficient radial clearance for the insertion of the IBL detector and its mechanical structure. The new , characterised by an inner radius of 23.5 (reduced from the previous 29), consists of a 7100-long beryllium section with an average wall thickness of approximately 870; its extremities are welded to aluminium flanges, the size of which allow its insertion through the IPT.

The inner surface of the is treated with a NEG coating to improve the vacuum quality by bonding to gas molecules remaining within the partial vacuum;
this getter coating had to be activated with a bake-out procedure using heaters wrapped around the surface. To mitigate the effect of extreme heat on the silicon sensors, a layer of aluminium was interleaved between the and the IBL to reduce the infra-red emissivity of the . The heater temperature was carefully monitored during the process.

§.§.§ Sensors

Two sensor technologies are used in the IBL: an improved version of the ATLAS Pixel planar sensors <cit.>, able to
cope with unprecedented operational conditions, and an innovative design of 3D devices <cit.> a first-ever application in high energy physics.

The two sensor implementations share a common footprint in order to be interfaced to the same readout chip, with two-chip
planar sensor tiles having an equal longitudinal dimension of two single-chip 3D sensors in order to fit a common mechanical
layout. Both technologies are characterised by the same granularity, with [parse-numbers=false]250 x 50
pixels organised in [parse-numbers=false]80 x 336 arrays (to be compared with the [parse-numbers=false]18 x 160
matrix configuration
[parse-numbers=false]400 x
50 pixels of the three outermost Pixel layers). Pixels of sizes different from the nominal are also present in order to fill the gaps between front-ends. A comparison of the main characteristics of the Pixel and IBL detectors is shown in Table <ref>.

Comparison of the main characteristics of the IBL pixels with the original Pixel detector layers.

Technical Characteristic     Pixel     IBL

Active surface ()     1.73     0.15

Number of channels ( $\times \num{e6}$)     80.26     12.04

Pixel size () - nominal/long
[parse-numbers=false]50 x 400/600     [parse-numbers=false]50 x 250/500

Pixel array (columns$\times$rows)
[parse-numbers=false]160 x 18     [parse-numbers=false]336 x 80

Front-end chip size ()
[parse-numbers=false]7.6 x 10.8     [parse-numbers=false]20.2 x 19.2

Active surface fraction ($\%$)     74     89

current (A)     26     10

Digital current (A)     17     10

voltage ()     1.6     1.4

Digital voltage ()     2.0     1.2

Data out transmission (Mbps)
$40-160$     160

Sensor type     planar     planar/3D

Sensor thickness ()     250     200/230

Layer thickness ($X_0$)     2.8     1.9

Cooling fluid     ${\text{C}}_3{\text{F}}_8$     ${\text{CO}}_2$

IBL planar sensors are fabricated with a n-in-n design on a 200-thick substrate, thinner than the 250 devices used in the rest of the Pixel detector.
Slim inactive edges (200 wide) are achieved by shifting the guard rings on the p-side underneath the pixel implantations.
Planar sensors are produced in two-chip tiles of an overall dimension of
[parse-numbers=false]18.59 x 41.32;
the operational voltage is expected to evolve during the device lifetime from 80 up to 1000.

IBL 3D sensors rely on a 230-thick p-type substrate and are fabricated with a process in which 10-diameter columnar electrodes are implanted by double-sided DRIE – i.e. n-type and p-type columns penetrate the substrate from opposite sides.
Columnar electrodes are either passing-through or 210 deep according to the different designs implemented by the vendors (FBK[Fondazione Bruno Kessler, Trento, Italy] and CNM[Centro Nacional de Microelectronica, Barcelona, Spain]) at the time of production; see Figure <ref> for a visual comparison of the two designs.
The pixel layout consists of two n-type readout electrodes connected at the wafer surface and surrounded by six p-type ohmic electrodes  which are shared with the neighbouring cells. Edge isolation is achieved by a fence of ohmic electrodes corresponding to an inactive area of 200 deep.
Due to the characteristic design, which decouples the sensor thickness from the drift distance, 3D sensors are expected to be particularly radiation tolerant and for this reason suitable to equip detectors exposed to high irradiation levels.
The operational bias of these sensors started with 20 at the beginning of and is expected not to exceed 200 during the device lifetime.

Design of the columns of (a) FBK and (b) CNM 3D sensors, including the location of the bump used to bond the sensor to the front-end chip <cit.>. This sketch is for illustration only and is not to scale.

§.§.§ Modules and staves

The connection between sensor and front-end chip is achieved by means of fine-pitch bump-bonding and flip-chip technology.
Due to more stringent radiation and occupancy requirements, the front-end electronics of the IBL has been completely redesigned with respect to the integrated circuit used in the rest of the Pixel detector in (the FE-I3).
The new front-end chip, the FE-I4B <cit.>, was manufactured in 130 CMOS technology, featuring a large footprint
([parse-numbers=false]20.3 x 19.2)
and 26880 readout channels replicating the pixel matrix. Each cell comprises an independent amplifier with adjustable shaping and a discriminator with an individually adjustable threshold. The threshold (typically 1500-2500 electrons) sets the minimum collected charge to be processed (and converted to ToT) by the front-end chip; the digital threshold (in units of ToT)
sets the minimum digital collected charge amplitude to be transmitted by the module and is typically $2$BC. The collected charge amplitude is measured as ToT in units of the 25 LHC bunch crossing period with a four-bit resolution.
The FE-I4B chips (see Section <ref> for details about the readout architecture) are slimmed to 150 in order to reduce the material budget.

A double-sided, flexible printed circuit (known as the module flex hybrid) is glued to the back side of the sensor (see Figure <ref>) and connects the module to external electrical services.
Read-out and power lines connections between the module flex hybrid, the FE-I4B chips and the sensors are wire-bonded.
The module temperature monitoring and interlock is achieved with a NTC thermistor mounted on the module flex hybrid.

IBL modules are fastened by two glue dots to their supporting staves. Staves are carbon foam structures glued to V-shaped carbon fiber laminates; the carbon foam provides efficient thermal coupling to the titanium cooling pipes that are embedded within the staves.
Cooling is obtained by a ${\text{CO}}_2$ evaporative system, capable of maintaining the silicon sensors at an operating temperature of -15; the minimum value achievable for future operation is -35. Power, data acquisition and module configuration are routed through a multi-layer bus (the stave flex hybrid) laminated on the back of the staves, from which they are distributed to modules by means of the module flex hybrids.

Photograph of an IBL planar module <cit.>. The full size of the silicon sensor is

During the first year of the IBL operation in 2015, a significant increase of the low voltage current of the front-end chip and a detuning of its parameters (threshold and ToT) were observed in relation to the received TID. The increase of the low voltage current  of the FE-I4B chip and the drifting of its tuning parameters were traced back to the generation of a leakage current in NMOS transistors induced by radiation. The radiation induces positive charges that are quickly trapped into the shallow-trench-isolation oxide at the edge of the transistor. Their accumulation builds up an electric field sufficient to open a source-drain channel where the leakage current flows. If the accumulation of positive charges is relatively fast, the formation of interface states is a slower process. The negative charges trapped into interface states start to compete with the oxide-trapped charges with a delay. This is what gives origin to the TID effect at low dose <cit.>.

In order to study the dependence of the low voltage current increase on temperature and dose rate, several irradiation tests were performed by setting one of those variables and changing the other.
Given the results of these tests, it was decided to increase the IBL operational set point from -10 to [retain-explicit-plus]+15 for a few months in early 2016 to limit the TID effects during the first radiation exposure, before reducing it to -20 for the remainder of .
In addition, the digital supply voltage (V$_{\mathrm{D}}$) was lowered from 1.2 to 1 to decrease the low voltage current.

§.§.§ Integration and installation of the *IBL in the *ID

A total of 20 staves were assembled during the production phase. An issue concerning wire-bond corrosion <cit.> was identified on most staves mid-way through the production.
This resulted from a combination of extreme susceptibility to corrosion due to chemical contamination on the flex and accidental exposure to humidity during the temperature cycling tests after stave loading; all but two staves were fully repaired.
Because the wire bonds of the IBL were left unprotected (potting the bond foot or the use of spray coatings such as polyurethane were initially considered as options to protect them), it was crucial that the temperature be maintained well above the dew point at all stages of the integration and installation, and it remains crucial for operation.

Of the 18 staves available for the integration, 14 were selected on the basis of quality criteria and installed in the detector.
These criteria rely on a weighted pixel failure fraction aimed at minimising the geometrical inefficiencies and clusterisation of defects.
The average bad pixel fraction for the integrated IBL staves at the time of installation amounted to 0.09 (0.07 for $\abseta<2.5$).

The IBL was integrated as a single package that was lowered into the ATLAS experimental cavern and finally inserted inside the Pixel detector.
Once all staves were loaded, an annular support was installed and clipped to each stave center in order to provide additional radial stiffness.
This constraint did not eliminate other degrees of freedom, most notably a rotation around the beam axis arising from the mismatch of thermal expansion coefficients of the stave and stave flex and their asymmetric assembly; this resulted in a distortion of the staves of the order of a few  /  that required an enhanced temperature stability at the level of 0.2 and the development of in-run alignment correction procedures <cit.>.

§.§.§ Readout of the *IBL

The IBL readout system was designed to be fully efficient at a few times the nominal LHC peak luminosity
(up to [range-phrase=–]23e34)  with a L1 trigger rate of 100kHz.
The readout can be naturally divided into two parts, the on-detector and the off-detector systems, which communicate via optical fibres in both directions.

The on-detector system consists of the FE-I4B front-end readout chips and the service electronics, which include the electro-optical converter boards (optoboards).  The optoboards are connected to the front-end chips by the FE-I4B input and output lines.

The building block of the readout system, the IBL DAQ module, is formed by two neighbouring front-end chips.
Each IBL DAQ module shares the same clock and command lines (including the L1 trigger signals) that are distributed to the pair of front-end chips.

The FE-I4B input lines run at 40Mbps. When charge deposition is detected by the discriminator in each pixel, the hit and its timestamp are briefly buffered in the pixel cells. The FE-I4B pixel array is organised in double-columns like the FE-I3 chip, but the readout architecture is very different. Instead of moving all hits from the pixel array to a global shared memory structure for later trigger processing, the FE-I4B double-columns are further divided in $2\times 2$ pixel regions where hits are stored locally. This results in an enhanced capability to cope with high hit rates and reduced inefficiency. Each region contains four identical pixels and one shared memory and logic block called the Pixel Digital Region. This memory can store up to five events.

When the L1A arrives, any event for which the timestamp matches is read out via a serial LVDS output
that operates at 160Mbps. There is one LVDS output line per front-end.

The optoboards are responsible for translating the optical signals received from the off-detector electronics into electrical signals for the FE-I4B inputs; vice versa they translate the electrical signals received from the FE-I4B outputs to optical signals before transmitting them to the off-detector electronics.
PIN diodes are used to convert optical into electrical signals. VCSEL arrays convert electrical into optical signals.

The off-detector electronics consists of commercial optical Rx plugins and Tx plugins, used to interface the readout hardware with the optical fibres. The plugins are mounted in the BOC cards, used to transmit  the clock, triggers and commands to the modules after BPM encoding, and to receive and decode 8b/10b-encoded data from the front-ends. The BOC communicates through the VME backplane to the ROD cards. The ROD has several tasks: it distributes the L1 trigger upstream, along with signals received from the ATLAS TIM, reformats the data received from the front-ends (via the BOC), and finally transmits the generated event fragments to the ATLAS ROS <cit.> via CERN s-link <cit.>.
This transmission is done by forwarding the reformatted data back through the backplane to dedicated optical transceivers (QSFP <cit.>) on the BOC.
Each s-link has a bandwidth of 1.28Gbps. As shown in Figure <ref>, a single IBL ROD/BOC pair hosts four s-link
for a total output bandwidth towards the ATLAS ROS units of 5.12Gbps.
The ROD is also used to calibrate and tune the IBL detector.
A schematic of the entire readout chain is shown in Figure <ref>.

The IBL ROD/BOC readout cards <cit.> (used also for the Pixel readout upgrades during three consecutive winter shutdowns) are located at the off-detector side of the optical link. (a) The BOC card and (b) the ROD card are paired in a VME crate via its back-plane.

Schematic of the IBL read-out system <cit.>. The IBL Stave is the only on-detector component in this diagram.

Each IBL ROD/BOC pair reads out an entire IBL stave via 32 FE-I4B chips. Two Tx plugins are used to communicate with 16 double-chip modules via 16 independent serial lines, eight lines for each Tx plugin. Four Rx plugins read back the data from each of the 32 front-ends individually; each Rx plugin receives data from eight front-ends. A total of 14 IBL ROD/BOC pairs have been equipped in a 9U VME crate located in the ATLAS service cavern (USA15).

The same type of readout board was used to upgrade the outer Pixel layers during .  A detailed description of these upgrades will be given in Section <ref>.

§.§.§ Off-detector services and detector power supplies

The basic element for the segmentation of the IBL electrical services is the half stave.
The end of each half-stave is connected to a set of corrugated flex PCB which on the other side connect to a PCB called the cable board as is schematically shown in Figure <ref>. The cable board is part of a cable harness, called a type-1 cable, which makes the connection out to the ID endplate region. Each harness is divided into two individual sub-cables, one for data transmission and one for power and the DCS.

The type-1 cable transmission bundle consists of 24 polyimide-clad copper twisted pairs.
For the data from the front-ends 28AWG wire is used due to the higher rate of 160Mbps, while for
the 40 clock and the 40Mbps command transmission 36AWG wire is used. Each command and clock line
is shared by two frontend chips. The standard for all signals is LVDS. Four 36AWG ground wires in each
bundle ensure a common ground level between transmitters and receivers. The transmission
cable is approximately 5 long and terminates at the optobox in the outer part of the ID endplate region.

The second cable in the type-1 cable harness contains 61 polyimide-clad copper wires: 32 low voltage 24AWG wires,
eight wires ([parse-numbers=false]24/28AWG) for low-voltage sensing, 16 32AWG DCS wires,
four 32AWG wires for high voltage and
one 26AWG drain wire. The cable is 3.5 long and terminates in a custom 67-pin AXON33 connector of 21

In the ID endplate area the type-1 power sub-cables connect to 9-long type-2 cables which end in the PP2
situated inside the MS volume. For the entire IBL there are four PP2
crates, two per detector side.
The PP2 contains regulators for the low voltage and is passive for HV and DCS. Type-3 cables, 75 long, connect the PP2 with the HV and LV power supplies and the DCS electronics inside the counting room.

Commercial power supplies provide high and low voltage for the detector. The maximum high voltage for planar sensors is 1000 and for 3D sensors 500, at a maximum current of 8 (planar) and 10 (3D). The low voltage from the power supplies is regulated down to around 2 by the PP2 regulator boards (see Table <ref>). Four front-ends chips share one HV channel, as well as one LV channel on a regulator board. Each PP2 regulator board, supplying one half-stave, is fed by a single primary voltage supply channel.

The optoboard LV supply chain replicates the frontend LV chain design from power supply to the detector-side end of
the type-2 cable at which point a direct connection to the optobox is made.
In addition to LV, the optoboards require a bias voltage for the PIN diodes of 5 and a control voltage for the VCSELs of about 0.8 (see Table <ref>).
All voltages, as well as a reset signal, are provided by custom SCOL modules in the counting room.
Each optoboard which reads out half a stave is powered separately.

The DCS lines coming from each half stave are used to monitor the temperature on every fourth frontend chip, on the cable board,
on the cooling pipe, and for a fraction of the staves on the type-1 bundle while for the rest of the staves this temperature sensor is replaced by a humidity sensor on the cable board.
The temperature monitoring uses NTC sensors. The humidity sensors were not radiation hard; for this reason they were only used during the installation phase.
In the counting room the DCS lines are connected to a custom crate that processes the signals.

Table <ref> summarises the power requirements for Pixel and IBL. The power consumption of the IBL was about 1.6 for the low voltage at the end of ,  while for the high voltage it was about 15. The power consumption is expected to increase further towards the end of life due to a rise in leakage current (HV) and LV current caused by radiation damage.

Block diagram of on-detector and off-detector electrical services for one half-stave of the IBL detector <cit.>.

Summary of the silicon sensor bias voltage and front-end electronics requirements and granularity. The different front-end electronics supply voltages are
described in the text. The power consumption expected during initial operation is also shown. Following irradiation, the power consumption increases significantly in the sensors, front-end electronics and cables.

max width=0.9

Pixel     IBL Planar (3D)

5*Bias voltage supplies     Voltage (maximum)          600     1000 (500)

Voltage (start of )       250     80 (20)

Voltage (end of )        400      400 (40)

Current (maximum)            4       8  (10)

Segmentation          one per module     one per 4 FE-I4B
60.3Front-end electronics low-voltage supplies     Voltages     1.72.1     1.22.2 feeding both

2.12.5 digital                     and digital

0.8;2.5;5 opto-device      0.8; 2.5;5 opto-device

Current      3.7     209

Segmentation of bulk supply     One per 67 modules     One per 4 FE-I4B

Segmentation of regulated supplies     One regulator per module     One regulator per 4 FE-I4B

4*Power     Front-end electronics power
$\sim\SI{6}{\kW}$          $\sim\SI{0.3}{\kW}$

Cables plus regulators        $\sim\SI{18}{\kW}$        $\sim\SI{0.8}{\kW}$

Total power (start of )      $\sim\SI{24}{\kW}$       $\sim\SI{1.1}{\kW}$

Total power (end of )     $\sim\SI{32}{\kW}$       $\sim\SI{1.6}{\kW}$

§.§.§ Detector control and interlock systems

The IBL DCS provides monitoring, control, and safety for the IBL. Its hardware encompasses the sensors (humidity and temperature), the monitoring crates, and the interlock crates. From an operational point of view, the power supplies and the regulator stations at PP2 also belong to the DCS. Table <ref> gives an overview of the DCS hardware components while their interconnections and functional dependencies are shown in Figure <ref>. The heart of all custom built crates is an ELMB128 <cit.>.

Main IBL DCS hardware components.

max width=1.0

Crate     Location     Task     Communication     Vendor

Beam-building module     PP3, UX15     Monitoring of environmental temperature and humidity     CAN     custom built

HV     USA15    Power Supply: sensor depletion voltage for planar and 3D modules    CAN    ISEG

LV     USA15    Power Supply: Front-end Electronics     TCP/IP   Wiener

LV-PP4     USA15     Distribution of LV and current monitoring per module     CAN    custom

SCOL     USA15     PS: Optical Link    CAN    custom

Regulator Station     PP2, UX15     LV regulation     CAN    custom

IMC     USA15     HW Interlock. Temperature monitoring of interlock controlled devices     CAN    custom

Schematic <cit.> of the IBL DCS control and monitoring functions on the sensor (HV), front-end electronics (LV) and optoboard (SCOL) power supplies, as well as temperature and humidity monitoring. The schematic also indicates an independent hardware interlock system used for detector and operational security.

The DCS software is based on the WinCC OA SCADA system <cit.> which is used throughout the LHC experiments. The data transmission and communication between DCS hardware and software is mainly done via CAN bus and an OPC client-server architecture, where the servers are provided by the vendor, and the client provided by WinCC OA. External information (for example, for the ${\text{CO}}_2$ cooling system) can be accessed via distributed systems such as the ATLAS-wide DSS <cit.> or the information published by the LHC regarding the machine and beam state.

The IBL-specific DCS is organised in three layers: the lowest level implements the control at a hardware based layer and provides the necessary setup and configuration. The middle level maps the hardware channels to the detector units. The smallest units that can be controlled independently are groups of four front-ends, or a DCS module, and the optoboards each serving four DCS modules. At the top level, detector oriented control and monitoring is provided by a FSM hierarchy implemented in a framework provided by the CERN JCOP and ATLAS groups. Thanks to this tree structure, shifters can easily monitor the detector state by observing the changes of the upper level of the tree; the detector can be controlled via FSM commands by non-DCS-experts. The lowest level of the FSM hierarchy is based on modules and optoboards and follows the detector geometry of readout units and staves building the innermost Pixel layer.

For detector safety, a dedicated hardware interlock system is installed. It is fully independent from the DCS software,
but its input and output signals are monitored by the DCS. The interlock system protects the detector and certain electronics against overheating by generating interlock signals from temperature monitoring with a high granularity. In addition, it receives signals from DSS and distributes the corresponding interlocks, and protects the IBL from dangerous beam conditions by receiving the Stable Beams signal from the LHC and using it to provide a software-generated beam injection permit signal.

§.§.§ *IBL ${\text{CO}}_2$ cooling system

The IBL is cooled by a two-phase ${\text{CO}}_2$ system which guarantees an operational temperature in the range between [retain-explicit-plus]-35+15, thanks to a total design cooling power of about 3. The cooling prevents thermal runaway of the irradiated sensors. Figure <ref> shows a simplified schematic view of the system.

Schematic representation of the IBL ${\text{CO}}_2$ cooling system <cit.>. The numbers indicate the loop direction.

The ${\text{CO}}_2$ is cooled by commercial chillers based on a hydrofluorocarbon mixture (R404A) and pumped to the experimental cavern through a concentric transfer line of a length of about 100.  The liquid supply line is inside the two-phase return line to pre-heat the sub-cooled liquid from the plant to approximately the same temperature as the returning two-phase fluid.
This also protects the supply liquid from possible ambient heating. The transfer line is vacuum insulated with a stationary vacuum shield of 63.5.
At the end of the transfer line the fluid is distributed to the detector staves through a manifold box (see the left side of Figure <ref>) where 14 concentric vacuum flex lines (seven for each side) bring the fluid to the detector through the same routing as the cables on the ID end plate (see the red arrows in Figure <ref> on the right).
The flex line vacuum is obtained thanks to an active vacuum pump system installed in UX15 and monitored by a dedicated
Next to the manifold box there is a junction box where temperature and pressure sensors are installed to monitor the
${\text{CO}}_2$ conditions closest to the detector. Also a 3 heater is present in order to simulate the detector load running in bypass mode for
commissioning or test of the system.

The evaporation temperature in the detector is regulated by the pressure in the accumulator (${\text{CO}}_2$ tank providing coolant reservoir).
The return pressure (saturation temperature) is controlled by heating or cooling of the two-phase mixture inside the accumulator.
By controlling the accumulator it is possible to control in a very precise way the evaporation temperature in the detector and to provide the required cooling conditions and stability.
The diameter of the return lines is chosen to be slightly larger than that of the IBL cooling pipes, to ensure that the pressure drop along these lines remains sufficiently low that the accumulator pressure directly determines the evaporator pressure.

fig:iblCoolLines IBL cooling lines and fig:IDendPlateCool the ID end-plate region, with the path of the cooling lines highlighted.

In USA15, two redundant cooling units are installed and running, one providing cooling to the detector
with the other running cold in stand-by mode through a by-pass. The purpose of the stand-by mode is to keep the pump
of the inactive system cold to allow for a fast start in case it is required.
One unit is running with normal electrical power while the other is also supplied by UPS batteries:
in case of a failure of the normal power, there is an automatic swap between the two units which is
transparent for the detector, and only a very small and short temperature variation is visible.
The logic of installing two redundant units allows uninterrupted operation in case of
failure or servicing of one line. The two cooling units are controlled by two independent PLC.

Since the start of operation, the IBL cooling has operated at three different temperatures. In May 2015 the IBL was operated at -10, until the start of the study of the high current drawn by the readout chips discussed in Section <ref>,
when the cooling system was run at the much higher temperature of ([retain-explicit-plus]+15) until June 2016. From then on, the operational set point of the plant was reduced to -20.

The IBL cooling system has been shown to operate stably over the full range between room temperature and its lowest verified limit, which is -35.
Its reliability was demonstrated during external events like power cuts, glitches and primary cooling failures. No downtime occurred during data taking from the beginning of and, on the occasion of the only system failure, the stand-by unit kept the detector in the required conditions.

The temperature stability, needed to limit the detector bowing effect described in Section <ref>,
was better than 0.2  (with less than 0.05 RMS) and made it possible to correct for bowing through alignment corrections.

§.§ The ATLAS Pixel upgrade

§.§.§ Installation of new Service Quarter Panels

The installation of the nSQP during LS1 enabled the relocation of the optoboards outside the ID volume to an accessible area, making future repairs possible without extraction of the Pixel detector. All defects originating from broken data transmission lines and faulty optoboards were therefore repaired during LS1.
Additionally, all faulty connections outside the active Pixel detector volume were repaired during the process of reconnection after the nSQP installation. Faulty connections within the active volume were not accessible and thus could not be repaired.

The full detector package was removed, taken to the surface and tested there, before being re-installed in ATLAS in December 2013.
The refurbished three-layer Pixel detector was then reconnected and tested.
The number of disabled modules was decreased to 33,
$1.9\%$ of the total. The biggest improvement was achieved in the $B$-Layer, where the disabled fraction was reduced from $6.3\%$ to $1.4\%$, and Layer 2, where the $7.0\%$ faulty fraction was reduced to $1.9\%$.

The nSQP included additional data fibres dedicated to the readout of Layer 1 (and part of the Discs); this eventually resulted in doubling the bandwidth of the transmitted data once new IBL ROD/BOC card pairs were deployed at a later stage of the project (see Section <ref>).
For Layer 1 and part of the Discs, the readout upgrade increased the bandwidth from 80160Mbps, and for Layer 2 from 4080Mbps.
This increase of the bandwidth by a factor two would allow the detector to run without bandwidth limitation up to an instantaneous luminosity of . Further details of the readout upgrade are given in the Section <ref>.

Because of the easier maintenance afforded by the installation of the nSQP, another optoboard replacement campaign was conducted during LS2. The detector status at the start of was as follows: 10 modules ($3.5\%$) disabled in the $B$-layer, 14 modules ($2.8\%$) disabled in Layer 1, 32 modules ($4.7\%$) disabled in Layer 2, and 10 modules ($3.5\%$) disabled in the Discs. In addition, three IBL front ends were disabled.

§.§.§ Readout upgrades during the winter shutdowns

New readout components developed for the IBL (see Section <ref> for details) were used to upgrade the off-detector readout hardware of the outer Pixel layers, making use of the new fibre installation provided with the nSQP. The nSQP project was an opportunity to increase the bandwidth for some of the Pixel layers and remove some limitations that would have arisen with the expected increase of luminosity.
The strategy was to gradually upgrade the various layers during three consecutive winter shutdowns between 2015 and 2018. The timeline of these upgrades was as follows:

2015/16 Layer 2 + 12.5% of Layer 1, a total of 22 ROD/BOC card pairs

2016/17 Remaining part of Layer 1 (87.5%), a total of 42 ROD/BOC card pairs

2017/18 $B$-Layer and Disc, a total of 38 ROD/BOC card pairs

By the end of , in 2018, both Pixel and IBL finally shared the same type of readout system, making it easier to operate the detector and offering more powerful debugging capabilities. No further changes to the off-detector readout system are foreseen during operation.

§.§ Semiconductor Tracker readout upgrade

The SCT data acquisition system in comprised 90 ROD and BOC cards.  The BOC provides the optical interface between a ROD and up to 48 SCT modules; for each module the BOC transmits the clock and trigger via a single command  stream and receives data back via two optical links from the six ASIC on each side of the module. The ROD processes incoming data from the 96 data links at the L1 trigger rate, and combines those data into a single event fragment which is broadcast on a single optical s-link to the ATLAS DAQ. Figure <ref> shows a schematic of the SCT DAQ.

Schematic of the SCT Data Acquisition System.

The SCT was designed to operate with 0.20.5 occupancy from its 6.3 million sampled strips with an LHC pileup of up to about $\mu=23$ collisions per BC. The fundamental bottlenecks arising from higher pileup that restrict increasing occupancy are the bandwidth limitations of the data links from the modules (which transmit bit streams at 40Mbps) and the s-link (which transmit 32-bit words at 40, a throughput of 1.28Mbps). Extrapolations of occupancy with increasing pileup during suggested that the data flow in the data links and s-link would exceed bandwidth limitations at pileups of around $\mu=87$ and 33 collisions per BC respectively, at a L1 trigger rate of 100. These projections used the optimum on-chip data compression mode (where only hits exceeding a threshold from the in-time BC are read out, while simultaneously vetoing hits from the preceding BC), and the standard data packing schema of the ROD which included readout of the hits from each strip over three consecutive BC.

A number of mitigation steps were deployed to prepare the SCT for the anticipated higher pileup during . Furthermore, it was discovered early in that SCT hit occupancy had increased significantly compared to data, which is now attributed to secondary interactions with the newly-installed IBL services. The increased occupancy led to a revision of the pileup limits for SCT operation, leading to further mitigation steps during in order to achieve a comfortable margin for operation with $\mu\sim 60$. These mitigation steps

Expanded DAQ
To address the pileup limit imposed by the s-link, the numbers of ROD and BOC were increased in 2014, during LS1, thereby reducing the number of modules serviced by each ROD from 48 to 36. During LS2 the DAQ system was expanded from 90 to 128 cards (the full complement of cards possible in each ROD crate), increasing the total number of s-link from 90 to 128.

Cable remapping
In 2015, the cabling of the data links to the ROD was rearranged to harmonize the data load on each ROD. This resulted in a flatter occupancy distribution across the s-link, removing spikes from the highest occupancy s-link and thereby improving the pileup limit imposed by the s-link.

Data compression
A new highly efficient data packing scheme with compression on the ROD (“supercondensed mode”) was developed during LS1 and was deployed routinely from 2016.  The new scheme resulted in a roughly 25% data size reduction, at the expense of losing the 3-bit timing information.

Chip Masking
From 2017, the noisiest chips were masked to reduce data throughput on the data links. This worked by using a reference table of the noisiest chips as a function of pileup, and masking the 128 channels of those chips. The mechanism worked “on the fly”, gradually reducing to zero the number of masked chips as instantaneous luminosity decreased during a run. Masking was flagged in the data to avoid an arbitrary loss in hit efficiency. Typically, even at the highest pileup of $\langle\mu\rangle\sim 60$, well under 1% of chips were masked.

Figure <ref> shows the impact of the different mitigation steps related to s-link, showing the number of s-link which operate above a defined fraction of the available bandwidth (occupancy threshold)
as a function of that threshold. The plot illustrates, for example, that no s-link used more than about $90\%$ of the bandwidth from 2016.

Number of s-link that would be operating above the bandwidth occupancy threshold as a function of that threshold. The 2012 configuration was the original 90-ROD system, the 2015 configuration used 128 ROD. Neither of these configurations was suitable for conditions; however, the 2016 configuration incorporated supercondensed mode and the fibre reshuffling, and this allows all the s-link to operate below the bandwidth occupancy threshold. A further minor cable remapping was performed in 2019, improving matters even more.

Following these mitigation steps, pileup limits at 100 trigger rate are around $\langle\mu\rangle = 70$ for both the data links and the s-link, which is sufficient for and operation. Hot chip masking is not deployed routinely, but remains an option for future operation.

§.§ Transition Radiation Tracker upgrades

§.§.§ *TRT *DAQ upgrades in

The TRT <cit.> DAQ consists of custom /digital front-end electronics and the back-end ROD <cit.>.
An component of the front-end, the ASDBLR, detects signals from two drift straws using two different thresholds: a tracking threshold of about 200, the LT, and the transition radiation threshold of about 6, the HT.
During each 25 bunch-crossing, the digital component of the electronics, the DTMROC, samples the output of the ASDBLR eight times, creating one bit for each 3.125 indicating whether the straw signal exceeds the low threshold, and one additional bit to record whether the signal crosses the high threshold at any time during the 25 ns.
Upon receiving a L1A signal, the data from three bunch-crossings are sent to the ROD on the detector back-end.
These 27 bits of information constitute a “straw word”, which is the fundamental block of data from a readout and compression perspective.
Each ROD validates and formats the data from up to 120 DTMROC, building a “data fragment” by serialising the straw words.
The fragment is then compressed with an entropy-based Huffman algorithm <cit.> implemented in the ROD firmware and is sent to the ATLAS central ROS system via an s-link.

With this setup, the TRT DAQ system was capable of operating at L1 input rates up to 80 at the average pileup of $\langle \mu \rangle = 35$ in ; however,
as the LHC exceeded its initial design luminosity, the goal for the final years of (2017-18) and for became running at an L1 input rate of 100 with pileup up to $\langle \mu \rangle = 60 - 70$ collisions per BC.
This pileup corresponds to an average per-event straw hit occupancy of $75 - 85\%$ in the high-$z$ slices of the TRT endcaps, which have the highest occupancies due to their high-positions.
Under these more strenuous conditions, bottlenecks were observed when reading data both into and out of the ROD.
Together, these bandwidth saturations limited the system to a L1 rate of 90 and a maximum straw hit occupancy of 50%, for which the following DAQ upgrades were implemented between the end of and early (2015–16).

To alleviate the ROD input bandwidth saturation, the four trailing bits in the 27-bit straw word were truncated
off in the DTMROC. This corresponds to discarding hits with the longest drift time where the ionizing particles barely cross the edge of the straws, which are rather rare and are less important to tracking. This truncation reduces the straw word size sent to the ROD by 15% with no impact on tracking performance, enabling the TRT to run without saturating the ROD input bandwidth at up to 104 with evenly spaced triggers, and up to 102 in physics data taking where the rate can instantaneously exceed 104.

To cope with the s-link bandwidth saturation when reading data out of the ROD at high rates, a series of ROD firmware updates and hardware upgrades were performed. The firmware updates include the introduction of a validity gate to remove words leaving no LT hits within the 18.7556.25 time window.
Such hits are most likely to originate from a neighbouring bunch-crossing, and
only about 3% of the hits from the bunch of interest are lost to this validity gate.
Similarly, the HT bits from the first and third bunch-crossings were masked to zero to allow more efficient compression, which keeps 94% of HT hits from the bunch of interest.
Additionally, non-vital information was suppressed in the error blocks attached to the ROD data fragments.
An auxiliary word-caching hash table was also implemented to relieve the heavy use of the Network Search Engine ASIC on the ROD <cit.> during the Huffman compression.
Finally, the Huffman compression table was updated to maintain optimal compression performance in the harsher run conditions of late and .
Together, these firmware updates resulted in a 40% reduction of the event fragment size sent off the ROD, without significant loss of performance and operational stability.
On the hardware side, the HOLA s-link interface cards <cit.> used to transfer data from the TRT ROD to the ROS were upgraded with a faster clock crystal (60 vs 40) and a faster FPGA (2.5Gbps vs 2.0Gbps).
This created 50% additional bandwidth for the s-link.

Figure <ref> summarises the effect of the updated compression table and HOLA cards.
The projected maximum L1 rate at which the TRT can read out its data fragments without saturating the s-link is shown as a function of the detector occupancy.
These limits are calculated from the average Huffman codeword length for the hit straws, the number of empty straws, and the s-link bandwidth.
Only the high-$z$ endcap ROD are used in this calculation, as they have the highest occupancies and are closest to the bandwidth limit.
The bands represent the uncertainty in the fragment size due to the variable length of each fragment's error block, which can range from 0 to 60 32-bit words; the upper (lower) edge of the shaded area corresponds to the minimum (maximum) error block lengths.
Under the final conditions, the error block sizes were typically around 15 words or less.
After all the upgrades, the system can handle occupancies up to 76-86% at the target L1 rate of 100, depending on the error block size.

Projected maximum achievable L1 input rate as a function of TRT straw occupancy (dotted orange) before and (blue) after increasing the s-link bandwidth and reoptimising the compression table.