OPERATIONAL ISSUES

OF PRESENT ATLAS

STRIP DETECTOR

SAHAL YACOOB (ON BEHALF OF THE ATLAS COLLABORATION)

UNIVERSITY OF KWAZULU-NATAL

ATLAS

9/17/2012

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ATLAS STRIP DETECTOR

(SEMI-CONDUCTOR TRACKER)

•C3F8 Cooling (-7oC to +4.5oC

silicon) to limit radiation

damage

• Radiation hard: tested to

2x1014 1-MeV neutron

equivalent/cm2

• Lightweight: 3% X0 per layer

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The SCT Sensors

• 8448 barrel sensors

• 64.0 x 63.6mm

• 80mm strip pitch

• all supplied by

Hamamatsu

• Single sided p-on-n

• mostly <111> substrate, 285mm thick

• 768+2 AC-coupled strips

• Polysilicon (1.5MW) Bias

• Strips reach-through protection 5-10mm

• Strip metal/implant widths 22/16mm

• 6944 wedge sensors

• 56.9-90.4 mm strip pitch

• 5 flavours

• 82.8% Hamamatsu

• 17.2% CiS (some (inner EC)

oxygenated)

THE SCT MODULES

• Back-to-back sensors

• glued to highly thermally conductive substrates

for mechanical thermal stability,

wire-bonded to form ~12cm long strips

• 40mrad stereo angle between strips on

opposite sides

• 1536 channels (768 on each side)

• 5.6W/module (rising to ~10W after 10 yrs LHC)

• up to 500V sensor bias (nominal 150V)

• 1976 end-cap modules

• 3 shapes

• 2112 barrel modules

• one shape

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• 128 channel ASIC with binary architecture

• Radiation-hard DMILL technology

• 12 chips per module (6 each side)

• glued to hybrid (Cu/polyimide flex circuit)

• 40MHz (25ns) clock

• 20ns front end shaping time

DAC

Binary Pipeline (132 deep)

Comparator

PreAmp+Shaper

Threshold Voltage

Edge-Detect circuit

Readout Buffer

Test-Input

Data Compression

Circuit

t

t

v

“Shaped” input pulse to

Comparator

“Logic” output of

comparator

• 3 pipeline bins read out,

centred on L1A trigger

• Hits contained in 1 or 2 bins

• Timing optimised using

pattern of hits in the 3 time

bins

THE ASICS (ABCD CHIPS)

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DATA TAKING Several enhancements to

DAQ during 2010/11 to

maximise data taking

efficiency:

“Stopless” reconfiguration

and re-integration of RODs

in case of (rare) BUSY

Auto reconfiguration &

recovery of modules which

have non-zero errors

Auto reconfiguration of

entire SCT to counter SEUs

Typical data link error rate

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HIT EFFICIENCY

• #Hits/#Possible hits on tracks

• Require PT>1GeV/c

• Require ≥7 hits for SCT

standalone

• Require ≥ 6 hits for ID combined

Hit efficiency well above 99%

design requirement

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ALIGNMENT

Alignment performed using a track based algorithm (minimise c2 of track-

hit residuals). Initial alignment from survey and cosmic ray data, then

isolated high Pt tracks for collision data. Continues to improve and

approach design values. 9/17/2012

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CALIBRATION

• Charge injection circuitry in ABCD

• Measures hits vs threshold (S-

curve)

• Fit by complementary error function

• Noise parameterised by width

SCT noise < 1500 electrons

(Hit threshold ~6000 electrons)

• Online method

• Counts hits in empty

bunches

Noise occupancy ~10-5

Design < 5x10-4 9/17/2012

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OPTICAL LINK REDUNDANCY SCHEMES

VCSEL VCSEL P-I-N

VCSEL VCSEL P-I-N

VCSEL VCSEL P-I-N

VCSEL VCSEL P-I-N

VCSEL VCSEL P-I-N

All chips, VCSELs and fibres ok

bypassed

All fibres ok

(for barrels, lose master chip of lost

link)

Clock and control from neighbouring

module

Typical snapshot in SCT

Link0 Data

Link1 Data TTC

Link0 Data

Link1 Data TTC

Link0 Data

Link1 Data TTC

Link0 Data

Link1 Data TTC

Link0 Data

Link1 Data TTC

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TX VCSEL ISSUES

• SCT has experienced poor reliability

and frequent failures of the VCSEL

arrays in the TX optical transmitters

• Initially attributed to poor ESD

precautions at factory

• New batch (2009) manufactured with

improved ESD precautions

• improved lifetime, but again

started to fail

• attributed to exposure to

humidity

• TXs with VCSELs from new vendor

with improved resistance against

humidity (2012)

Use of has minimised

impact on SCT operations, and

BOCs now operate in lower

humidity environment 9/17/2012

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RADIATION DAMAGE

• Radiation damages sensors and

components. Effects are monitored

constantly to predict future

performance.

• Fluence is measured on-detector

• Observe excellent agreement

between measured leakage current

and predictions from MC based on

measured fluence

Operationally, we see a gradual

and continuous increase in

leakage current for each module,

both at 50V standby and 150V

operation.

Trips limits incremented (from 5mA

to 50mA) appropriately as

required. So far expect negligible

shift in depletion voltage. Typical module current evolution at 50V

and 150V

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ATLAS preliminary

THE CIS LEAKAGE

CURRENT PROBLEM

It become clear that the CIS SCT sensor performance was very sensitive to humidity

• A significant subset displayed poor IV and early (<150V) breakdown in dry conditions

• Need humidity to maintain a ‘healthy looking’ IV

• Problem identified as micro-discharge from strips, due to lack of field plate (strip metal narrower

than implant)

As this became an issue rather late in the delivery program, SCT adopted a pragmatic strategy:

• Only accept sensors with no sign of breakdown below 150V in dry air

• OK for the short term, and then strip micro-discharge becomes less relevant after type inversion

Approximately 100 modules have developed anomalously high leakage currents this year

Almost all were constructed with CIS sensors, and showed IV breakdown above 150V during

production QA tests

• Remedy:

• Decreasing HV and increasing current limits means we can keep operating these devices

with full efficiency so far

• Changing the STANDBY voltage to 5V has decreased the current rise

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OCCUPANCY AND RATE

LIMITATIONS

• We expect around 1% peak SCT

occupancy for 23 interactions per BX at

14TeV

• Rate limit at 1% occupancy is ~90kHz,

comfortably above ATLAS nominal peak

trigger rate of 75kHz

(*) Complex Dead-Time: Maximum number of triggers

within a given number of bunch crossings

-> Imposed by ABCD 8-deep event buffer

(*ATLAS 7/415)

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SUMMARY

The SCT has enjoyed an outstanding first two years of LHC physics

93.6% overall data taking efficiency (All of ATLAS) for 2012

99.3% of the 6 million channels are operational

Fulfilled design requirements for noise, hit efficiency, tracking and alignment

Significant effects of radiation damage are in very good agreement with expectations

Only significant operational issues have been related to TX VCSEL deaths which we may have overcome.

Minimal effect on acceptance due to availability of redundancy

Replacement program underway with improved resistance to humidity (and now operate in lower humidity environment)

We look forward to many more years of successful tracking at higher energy and luminosity

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BACK-UP

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RO

D

RX chans

TX Chans

RO

D

BOC

RO

D

RX chans

TX Chans

RO

D

BOC

RO

D

RX chans

TX Chans

RO

D

BOC

RO

D

RX chans

TX Chans

RO

D

BOC

RO

D

RX chans

TX Chans

RO

D

BOC

RO

D

RX chans

TX Chans

RO

D

BOC

RO

D

RX chans

TX Chans

RO

D

BOC

RO

D

RX chans

TX Chans

RO

D

BOC

RO

D

RX chans

TX Chans

RO

D

BOC

RO

D

RX chans

TX Chans

RO

D

BOC

RO

D

RX chans

TX Chans

RO

D

BOC

OPTICAL COMMUNICATION & POWER

SUPPLIES

DATA

LV:

- Vdd

- Vcc

- Vvcsel

- Vpin

HV:

- Vbias

TTC

VCSEL VCSEL P-I-N

RO

D

RX chans

TX Chans

RO

D

BOC

SCT Module

48 modules

per

ROD/BOC

ROD crate (DAQ)

DCS

8 ROD crates (90 ROD/BOC pairs) 88 Power Supply Crates

P-I-N receives Timing, Trigger & Control

VCSEL* for each link (side) returns data

* VCSEL=Vertical Cavity Surface Emitting Laser 9/17/2012

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TIMING

The ABCD chip is a binary chip - “hit” or “no hit” above 1fC threshold. It

samples hits in 3 consecutive time bins (25ns LHC clock cycles), and is

configured to flag a “hit” in the readout depending on the pattern of hits in

those 3 bins

Three bin sampling provides means to

time in the SCT, and provide rejection

of ghost tracks from hits associated

with collisions 25ns or 50ns earlier.

• XXX for timing in, cosmic rays and

≥75ns bunch trains

• X1X used currently for 50ns bunch

trains

• 01X will be used for 25ns bunch trains

Dedicated timing scans in first low lumi

pp collisions of the year, with each of

the 4088 modules optimised for 01X

(1ns precision)

Mean of 3bit hit pattern across

SCT 9/17/2012

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COOLING & ENVIRONMENT

C3F8 Compressor Issues

• Worries about long term reliability have motivated the

Thermosiphon project

• Gravity fed C3F8 system, to be commissioned in long 2013/14

shutdown

Cooling temperatures

• Pressure gradient in long delivery lines means the SCT silicon

cannot reach the design operating temperature of -7oC

• Fluorocarbon blends (C3F8-C2F6) will allow is to reach target

temperature

Thermo-heater Pad issues

• Some non-operational heater pads (between SCT and TRT)

requires us to run barrel 6 (outermost barrel) at elevated

temperature

The C3F8 evaporative cooling system has operated very reliably all year.

But….

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FREQUENCY SCANNING

INTERFEROMETRY

• Optical alignment system to

monitor the long term SCT

mechanical stability

• 842 fibre coupled

interferometers

• Typical deviations

associated with solenoid

cycle:

- before ~11nm

- during <3mm

- after ~49nm

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The SCT CiS Sensors – “Same spec,

different species”

Hamamatsu CiS

Bias Resistors (1.5MW) Polysilicon Implant

Strip metal/implant widths

(mm)

20/16 16/20

Guard design Single floating Multi-guard

Barrels supplied 8448 0

Wedges supplied 6944 1196 9/17/2012

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Comparison of 1MeV neutron equivalent fluences determined

from SCT leakage current measurements with simulated FLUKA Snapshot corresponding to 2010 7TeV data with integrated luminosity of 48.6pb-1

Excellent agreement for barrels. Reasonable agreement for end-caps 9/17/2012

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