operational issues of present atlas strip detectorvertex2012.knu.ac.kr/slide/s2/operational issues...
TRANSCRIPT
OPERATIONAL ISSUES
OF PRESENT ATLAS
STRIP DETECTOR
SAHAL YACOOB (ON BEHALF OF THE ATLAS COLLABORATION)
UNIVERSITY OF KWAZULU-NATAL
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
9/17/2012
3
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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5
• 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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6
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
9/17/2012
7
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
9/17/2012
8
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
9
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
9/17/2012
11
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
12
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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13
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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14
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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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
9/17/2012
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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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