Performance and Radiation Damageof the ATLAS Semiconductor Tracker
during LHC Run 226 December 2019, KEK seminar

Shigeki Hirose (Uni-Freiburg)

2/25■ ATLAS Experiment
44 m

25 m

Large Hadron Collider (LHC)• Circumference = 27 km• 𝑝𝑝 collision at 𝑠 = 13 TeV• Bunch per 25 ns• ATLAS detector– Targets high-𝑝T objects from decays of massive particles– Messy environment of 𝑝𝑝 collisions due to QCD→ Track finding performance of ID is essential for all physics analyses• ID consists of three different subsystems

■ SemiConductor Tracker (SCT)

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Barrel module
768 strips on each side

doublesided

6x2 ondetector chips

SCT (barrel)
2 x datatransmission line

SCT (endcap)• 4 layers (barrel) and9×2 disks (endcaps)• > 6M channels– Maintaining a stable system isimportantS-link

ATLAS
Trigger
SystemFormattedtrigger

6 cm

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■ Silicon Sensor
p+ strip

Depletion zone

Innermost barrel
p- bulk
typeinv.
n- bulkn+ electrode• 𝑝+ -on-𝑛 type sensor– Depletion zone grows from 𝑝+ side
– Full-depletion voltage 𝑉FD = 65 V without irradiation• Type-inversion around 1013 1-MeV 𝑛-eq/cm2– Full depletion is necessary to keep good performance■ LHC Run 2: 2015-2018

𝐿peak = 8×1033 /cm2/s (max)‫ = 𝐿 ׬‬26.4 fb-1 (7 or 8 TeV)

𝐿peak = 21×1033 /cm2/s (max)‫ = 𝐿 ׬‬147 fb-1 (13 TeV)

• LHC Run 2 was a productive period– ×1.6-1.9 higher energy, ×5.6 more data thanthat in Run-1
65 interactions
E.g. # of Higgs bosons: 5×105 → 80×105– However, pileup (number of interactions
per bunch crossing; 𝜇) increased by 60%Detectors had to endure high data rate
and radiation!

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■ Keys of SCT Run-2 Operation

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1. DAQ system– Enough data bandwidth– Tolerance to single event upsets

2. High hit efficiency and low noise– Low efficiency of SCT makes track finding difficult in high pileup events– Too many noise hits may cause many fake tracks
busy on DAQ due to enlarged data size

• SCT performance in Run 2– SCT joined the ATLAS data-taking for 99.85% of the stable beam time
– 99.7% of the SCT data was good quality for physics– As of end of Run 2, 98.6% of elements are active

SCT has maintained high performance in the Run 2 condition!

7/25■ Data Bandwidth• Expanding bandwidth– Increased the readout drivers (90 → 128)

• Data size reduction– Read only one bit per strip (“no hit” is required on the previous bunch
to avoid contributions from signal leakage)– Further reduction by more advanced data compression
25 ns

Run-1 scheme ...

Triggered...

Read data from three bunches

Run-2 scheme ...

Triggered...

Run-2 scheme
Run-2 max. →Use neighbour bunches only formaking decision to read data

Read data from only one bunch

All S-links are within the bandwidth up to 𝜇 = 60

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■ Single Event Upset
Noise hit map (measured using empty bunch crossings)

Recovered by GR

Quiet chips

Noisy chip

• Single-event upsets can give rise to configuration bit-flips– Chips suddenly become noisy or quiet• Global reconfiguration (GR)– Reconfigures all modules every 90 mins. (at cost of ~1 sec. deadtime)

GR is effective to clear problematic chips during a run

9/25■ Hit Efficiency (1)• SCT definition of hit efficiency𝑁hit
𝜀=A track creates a hit
𝑁hit + 𝑁hole• Typically the efficiency is > 99%

A track passes the sensor
but no hit is found
Dead modules and chips (~2%)
are excluded (also from tracking)– The intrinsic efficiency can be measured using the leading bunch– “No hit in the previous bunch“ is required→ Efficiency from all bunches is lower by ~1%(negligible impact for tracking)Leading bunch
...

...

~100 bunches

10/25■ Hit Efficiency (2)Pb-Pb collisions

LHC technical stops & developments• Hit efficiency >99% was kept for almost entire Run 2• Gradually drops over the time
– Caused by baseline shift due to charge up from radiation effects→ Recovered by resetting the threshold(More will be shown later)

11/25■ Noise: Occupancy
2018
2018

• NO: probability of noise hit with 1 fC = 6250 𝑒 −– SCT is required to be < 5×10-4• Measurement from empty bunch crossings in a low-mu 𝑝𝑝 run– Strip-by-strip difference is large due to baseline shift

NO is sufficiently low in all barrel layers(Similar results in endcaps)■ Noise: Threshold Scan
A. Abdesselam et al., JINST 3 P01003 (2008)

Input = 1 fC

50% occ.

𝜎 = noise• Noise can be estimated from threshold scan with a fixed
input charge– Extract noise from 𝜎 of an error function– More stable estimation → Better for monitoring time dependence• 1600～1800 𝑒 − at the end of Run 2

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■ Noise: Time Evolution

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LHC shutdown• Noise increased by 10-15% during Run 2– Doesn’t impact data quality (NO was low enough)• Somewhat unstable during the data-taking periods– Under investigation

14/25■ Detector Calibration

Run 1Run 2

Many noisy strips
are recovered• Electrical calibration is performed every ~1 month– Reset threshold etc.
– Disable too noisy elements• E.g. number of noisy strips increases with accumulatingradiation effects→ Recovered by calibration
Periodical calibration is essential to maintain performance!

15/25■ Radiation Damage on Sensors
Barrel layers

Endcap disks

ATLAS Collab., JINST 9 P08009 (2014)
ATLAS Simulation𝑟 (cm) 60

SCT

5.6×1013 𝑛/cm2
4020

4.5×1013 𝑛/cm20050100150200250300350• SCT has been heavily damaged by radiation– Exposed up to 5.6×1013 1-MeV 𝑛-eq/cm2• Potential crucial problems in Run-3 (2021 - 2023)– Too large leakage current (𝐼leak )– Too high full-depletion voltage (𝑉FD )• Periodical measurements of 𝐼leak and 𝑉FD– Not only Run 2, but also during LS2 (2019-2020)

400𝑧 (cm)16/25■ Measurements during LS2
SCT turned on Room temp. (for ID endcap opening)
~-7℃
~-17℃

LS2 temperature
Nominal temperature

JanFebMarAprMayJunJulAug• Perform IV scan etc. every two months– 5-10 people staying at CERN contributeSep

OctNovDec

History of 2019■ Leakage Current• 𝐼leak is monitored since 2010
– It increased by a factor of 106!
– Annealing is very visible every year-end• Our measurement agrees with the models– Radiation flux is well understood

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18/25■ I-V Measurement
𝑉FD ?

𝑉FD ?

High temperature• Measure leakage current and noise vs HV– After reaching HV = 𝑉FD , leakage current and noise become plateau– However the transition is not very sharp• 𝑉FD estimation from the I-V measurement– Rough estimate is 50-100 V■ Evolution of 𝑉FD𝑽𝐤𝐧𝐞𝐞• The “knee” structure appears after type-inversion– All modules have been type-inverted• Evolution of 𝑉FD can be tracked using 𝑉knee– Annealing is very clear in 2019
– Reverse annealing is suppressed thanks to cooling

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20/25■ Efficiency vs HV

~20 V

𝑉FD ?
We’re here~ 20 V

• Efficiency curve is another measure of 𝑉FD– Efficiency is significantly lower if HV < 𝑉FD– But the transition is not sharp...

• Model prediction with the Hamburg model– Currently 𝑉FD = 50 V → Will be up to 150 V– All measurements consistently indicate much higher 𝑉FD ...
– Even +50 V is not worrisome; the system limit is 480 V

21/25■ Towards Run 3: 2021-2024
We’re here

= 8×1033 /cm2/s (max)𝐿peak‫ = 𝐿 ׬‬26.4 fb-1 (7 or 8 TeV)

𝐿peak = 20×1033 /cm2/s (max)‫ = 𝐿 ׬‬220-230 fb-1 (13 or 14 TeV)

= 21×1033 /cm2/s (max)𝐿peak‫ = 𝐿 ׬‬147 fb-1 (13 TeV)

Details: N. Karastathis’ reportat Evian Workshop 20192024

One-year extension was recently decided:
https://home.cern/news/news/accelerators/newschedule-lhc-and-its-successor• Conditions will be similar to Run 2• No major upgrade is planned for SCT– The very robust and stable system has been already establishedbecause of a number of improvements– Need a solid prediction for Run 3 operation

22/25■ Projection to Run 3

Up to ~250 V(Limit: 480 V)
Up to ~4 mA(Limit: 5 mA)
60 fb-1

80 fb-1 @ 14 TeV• Projection until the end of Run 3– 𝑉FD : enough headroom until it reaches 480 V– 𝐼leak : safety margin of only 1 mA is a little worrisome?
→ 𝐼leak can be further suppressed by lowering temperature23/25■ Thermal Performance

Outermost: (intentionally) higher 𝑇Sensor
(~0℃)

2021

2023

Rapid increase(Thermal runaway)

Thermal impedance
Cooling (~-17℃)• Sensor temperature evolution

Innermost:highest radiation
Power dissipation on sensor [μm/mm2]– Higher 𝐼leak → More heat generation → Further 𝐼leak → ...

• Predicted sensor temperature indicates enough headroom• Uncertainty on the thermal impedance→ Important to carefully monitor sensor temperature in thefirst half of Run 3

24/25■ Preparation for DQ in Run 3

Efficiency recovered
after HV was increased

1.5 months(N.B. lower HV wasintentionally applied to thismodule)• Radiation damage will be more severe– Catch DQ degradation before it becomes intolerant• Efficiency rapidly drops within ~1 month– However, it’s too slow to be caught by run-by-run comparisons→Sophisticated monitoring & quick diagnosis tool isbeing developed■ Summary25/25• ATLAS SCT has been operated since 2010
– Successful operation with high efficiency + low noise in Run 2• SCT provides interesting opportunities to study radiationdamaged silicon sensors– Leakage current well agrees with model predictions– Full depletion voltage may be ~50 V higher than the prediction→ Complicated to predict 𝑉FD with the Hamburg model...

• For Run 3 in 2021-2024– No critical operational problems are foreseen– Total radiation dose will be more by a factor of ~2→ More careful performance monitoring will be important

SCT will continue providing high quality data in ATLAS Run 3...and will be replaced by a new tracker for HL-LHC26/25

■ Efficiency Drop in 2018
• Overall efficiency drop was observed on Barrel 3(innermost layer)– This wasn’t specific to some modules, so couldn’t be
attributed to too many noisy strips• Increase of HV helped recovering the efficiency– Clearly indicates the lower efficiency was due to radiation
damage27/25

28/25■ Global reconfiguration• Introduction of GR was effective to reduce the number ofproblematic chips– This is important not only in terms of data quality (efficiency etc.), butalso of DAQ; too many noisy chips may interrupt DAQ due to too highdata transmission rate

w/o GR

w/ GR29/25■ Type-inversion and “knee”• After type-inversion, the depletion zone grows from the backelectrode– Depletion zone doesn’t contact with strips before full depletion→ This may be a cause of “two-step” structure of noise (or capacitance)• The knee was reported by RD48 project in 1999

ROSE Collab. CERN/LHCC 2000-009

p+ strip

Depletion zone
p- bulktypeinv.

n- bulkn+ electrode* Exact mechanism of “knee”
is still under investigation

30/25■ Luminosity Levelling• Too high instantaneous luminosity is harmful...
– Detector: too high data rate

• The luminosity is intentionally reduced to the maximum
allowed range– Beam lifetime is extended

For HL-LHC
L. Rossi et al., IPAC
2019 proceedings

■ Number of Higgs Bosons

ATLAS lumi.
ggF
VBFWH
ZH
ttH
bbH
Total
# of Higgs bosons

7 TeV [1]4.57 fb-1
15.11
1.222
0.57700.33410.086110.155517.49
79,906

8 TeV
20.3 fb-1
19.24
1.579
0.70270.41420.12900.203022.27
452,040

13 TeV [3]139 fb-1
48.52
3.779
1.369
0.94040.50650.486355.60
7,728,539

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[1]:
https://twiki.cern.ch/twiki/bin/view/LHCPhysics/C
ERNYellowReportPageAt7TeV[2]:https://twiki.cern.ch/twiki/bin/view/LHCPhysics/C
ERNYellowReportPageAt8TeV[3]:https://twiki.cern.ch/twiki/bin/view/LHCPhysics/C
ERNYellowReportPageAt13TeV

■ Run-2 Summary

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K. Mochizuki, presented at workshop forradiation effects in the LHC experiments

■ Leakage current

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K. Mochizuki, presented at workshop forradiation effects in the LHC experiments

■ SEU without GR
Manually
recovered

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■ Lorentz Angle• Deflection of charge drift due to the Lorentz force• Important to correct in order to obtain precise points ofcharged tracks• Also a good indicator of the inner electric field• Study using 2018 is ongoing

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