The ALICE Inner Tracking System: commissioning and running experience

The ALICE Inner Tracking System: commissioning and running

experienceV. Manzari / INFN Bari

on behalf of the ITS project in the ALICE Collaboration

V. Manzari - ALICE ITS 2

Outline

The Inner Tracking System Pixel, Drift and double-side Strip detectors Commissioning and Operation with cosmics The Pixel L0 trigger Alignment and Calibration First experiences with LHC Conclusions

Vertex '09 / Putten (Nl)


ALICE (A Large Ion Collider Experiment) at LHC

Size: 16 x 26 metersWeight: 10,000 tonnes

Ultra-relativistic nucleus-nucleus collisions- study behavior of strongly interacting matter under extreme conditions of compression and heat

Proton-Proton collisions- reference data for heavy-ion program- unique physics (momentum cutoff <100MeV/c, excellent PID, efficient minimum bias trigger)



What makes ALICE different? With respect to ATLAS, CMS and LHCb.....and complementary to them• Experiment designed for Heavy Ion collisions (Pb-Pb @ 2.75+2.75 TeV per nucleon)

- only dedicated experiment at LHC, must be comprehensive and be able to cover all relevant observables

• Extreme track densities dNch/dh ~ 2000 – 8000- at r = 4 cm (1st pixel layer) up to 80/cm2 (x500 compared to pp @ LHC)- high-granularity detectors with many space points per track- very low material budget and moderate magnetic field- very robust tracking

• Hadrons, Leptons and Photons PID over a large pT range- from very soft (0.1 GeV/c) to fairly hard (100 GeV/c)

• Very low pT cutoff• Excellent vertexing capability• Modest luminosity and interaction rates

- 10 kHZ (Pb-Pb) to 300 kHZ (pp) (< 1/1000 of pp@1034)• Irradiation levels at the innermost SPD layer:

- 10 years standard running (108s pp + 5x106s Pb-Pb + 106s Ar-Ar)TID ≈ 2.5kGy, F ≈ 3•1012 (1MeV neq)/cm2

• The price to be paid slow detectorsVertex '09 / Putten (Nl)

central Au-Au event @ ~130 GeV/nucleon CM energy

STAR


ALICE Inner Tracking System 6 barrel layer 3 different silicon detector technologies, 2 layers each, as seen by produced particles:

- Pixels (SPD), Drift (SDD), double-side Strips (SSD)

Size: 16 x 26 metersWeight: 10,000 tonnes



The ITS role in ALICE- Improve primary vertex reconstruction and momentum resolution- Secondary vertexing capability (c, b and hyperon decays)- Track impact parameter resolution- Tracking and PID of low pT particles- Prompt L0 trigger capability (<800 ns)- Charged particle pseudorapidity distribution (First Physics measurement both in p-p and Pb-Pb)

Detector requirements- 2D detectors - High spatial precision- High efficiency- High granularity (≈few% occupancy)- Minimize distance of innermost layer from beam axis (<r> ≈ 3.9 cm)- Limited material budget- dE/dx information in 4 layers at least for particle ID in 1/b2 region

The Inner Tracking System

< 60 mm (rf)for pt > 1 GeV/c

Central Pb–Pb

Trac

k im

pact

par

amet

er r

esol

utio

n [m

m]



Detector parameters


Layer Det. Radius (cm)

Length

(cm)

Surface(m2)

Chan. Spatial precision

(mm)

Cell(μm2)

Max occupancycentral PbPb

(%)

Power dissipation(W)

rf z barrel end-cap

1SPD

3.9 28.20.21 9.8M 12 100 50x425

2.11.35k 30

2 7.6 28.2 0.63

SDD15.0 44.4

1.31 133K 35 25 202x2942.5

1.06k 1.75k4 23.9 59.4 1.05

SSD38.0 86.2

5.0 2.6M 20 830 95x400004.0

850 1.15k6 43.0 97.8 3.3

Integral of material thickness traversed by a perpendicular track originating ad the primary vertex versus radius

Material budget


Pixel

Half-stave

Outer surface: 80 half-staves

Beam pipeOuter layer

Inner layer

13.5 mm

15.8 mm

~1200 wire-bonds

• ALICELHCb1 readout chip• mixed signals• 8192 cells• 50x425mm2


Unique L0 trigger capability• Prompt FastOR signal from each chip• Extract and synchronize 1200 FastOR

signals from the 120 half-staves• User defined programmable algorithms

Inner surface: 40 half-staves

Minimum distance inner layer-beam pipe 5 mm

• 2 layer barrel• Total surface: ~0.24m2

• Power consumption ~1.4kW• Evaporative cooling C4F10

• Operating at room temperature• Material budget per layer ~1% X0

MCM

5 Al layer bus + extender

Ladder 1 Ladder 2 MCM + extender + 3 fiber link

Grounding foil


Drift

Central Cathode at -HV

Edrift

Edrift

Voltage divider

vd (e-)

vd (e-)

Anodes

HV supply

LV supplyCommandsTrigger Data

Modules mounted on

ladders Carbon fiber support

Cables to power supplies and DAQ

SDD layers into SSD

Cooling (H2O) tubes70

.2 m

m

Layer # ladders Mod./ladder # modules3 14 6 844 22 8 176


Front-end electronics (4 pairs of ASICs)-> Amplifier, shaper, 10-bit ADC, 40 MHz sampling-> Four-buffer analog memory


double-side Striprf- overlap:

z - overlap:

L5: 34 laddersL6: 38 ladders

L5: 22 modulesL6: 25 modules

Ladder

End ladder electronics

Sensor:double sided strip:

768 strips 95 um pitchP-side orientation 7.5 mrad

N-side orientation 27.5 mrad

Hybrid:identical for P- and N-sideAl on polyimide connections6 front-end chips HAL25water cooled

•carbon fibre support•module pitch: 39.1 mm •Al on polyimide laddercables



“Russian doll” installation



120 SPD modules, each contains 10 readout pixel chips Pixel chip prompt trigger signal (Fast-OR)• Active if at least one pixel hit in the chip matrix• 10 bits on each of 120 optical links (1200)• Transmitted every 100 ns

Prompt Pixel Trigger


SPD Half Stave

Half stave

Sensor

Pixel chips

Readout MCM

Sensor141 mm

1


Pixel Trigger System

Optical splitters

ALICE Central Trigger

Processor

SPDLTU

Clk40 &Serial

C side

A side

Data

TTC

TTC

36.6±0.2 m

107.6±0.15 m

38.5±0.2 m

60C.R.

PixelTrigger main outputs L0 in

TTC



Pixel Trigger System

PROC

CONTROLDDL SIU SRAM

Router + Link-Rx (SPD readout)• Fast-OR signals in the data stream

OPTIN board• Fast-OR extraction and syncronization

BRAIN board• Pre-defined algorithm processing

OPT

IN

BR

AIN


Pixel Trigger crate

Optical splitters

C22 rack Pixel Trigger System

Vertex '09 / Putten (Nl) 15V. Manzari - ALICE ITS


Pixel trigger algorithms

1 Minimum Bias (I+O)thIO,mb and IthI,mb and OthO,mb

2 High Multiplicity 1 IthI,hm1 and OthO,hm1




6 Past Future Prot (I+O)thIO,pfp and IthI,pfp and OthO,pfp

7 Background(0) I O+ offsetI

8 Background(1) O I+ offsetO

9 Background(2) (I+O) th(I+O),bnd

10 Cosmic Selectable coincidence

I/O = number of active FastOr on Inner/Outer layer

Cosmic algorithm can be selected from Control Room out of the following :• TOP_outer and BOTTOM_outer• OR_OUTER and OR_INNER• DLAYER (2 FOs in the INNER and 2 FOs in the OUTER)• TOP_outer and BOTTOM_outer and TOP_inner and BOTTOM_inner• TOP_outer and BOTTOM_outer and OR_INNER• GLOBAL_OR



ITS commissioning with cosmics Detector installation Jun ‘07

• Completion of service connections Nov ’07 1st Cosmic Run Dec’07

• First acquisition tests on a fraction of modules 2nd Cosmic Run Feb÷Mar

‘08• ≈ 50 % of the ITS operable (cooling and power supply availability)• Calibration tests + first cosmic muons seen in ITS

Completion of Power Supply deployment May ‘08 3rd Cosmic Run Jun÷Oct

‘08• Subdetector specific calibration runs

– Maps of dead and noise channels, gain, drift speed, …• Cosmic runs with Pixel trigger

– First alignment of the ITS modules + test TPC/ITS track matching– Calibration of the charge signal (dE/dx) in SDD and SSDVertex '09 / Putten (Nl)


Pixel: operation and calibration 106/120 modules stably running

• Dead+noisy pixels < 0.15%• Typical threshold ≈ 2800e-• Operating temperature ≈ design value• Average leakage current @ ≤50V ≈ 5.8 µA• Average Bus current (≈ 4.4 A)• Detector readout time: ≈ 320 ms• Detector dead time:

- 0% up to ≈ 3kHz (multi-event buffering)- ≈ 320 ms at 40 MHz trigger rate

• max readout rate (100% dead time ) ≈ 3.3 kHz

Temperature (°C)

Leakage current (µA)


Prompt L0 trigger with ≈800 ns latency

SPD Online Event Display - Cosmic RunSelf-triggered coincidence of top outer and bottom outer layer

V. Manzari - ALICE ITS

Drift: operation and calibration 247 out of 260 modules in DAQ

Calibration quantities monitored every ≈ 24h

• Fraction of bad anodes ≈ 2%• <Noise> ≈ 2.5 ADC counts

- Signal for a MIP on anodes ≈ 100 ADC • Drift speed from dedicated runs with

charge injectors

19

Display of 1 injector event on 1 drift side of 1 module

Drift speed on 1 drift side from fit to 3 injector points

vdrift = mE T-2.4

Lower e- mobility / higher temperature on the edges

Drift speed on 1 anode during 3 months of data taking

Measurement of vdrift vs. anode and vs. time crucial to reach the design

resolution of 35 mm along rf



Charge ‘ratio’

Clu

ster

cha

rge

N-s

ide

Cluster charge P-side

Strip: operation and calibration

1477/1698 modules in DAQ• ≈86% of the surface • Fraction of bad strips ≈ 1.5 %

11 %

Charge matching between p and n sides• Relative calibration from 40k

cosmic clusters• Important to reduce noise and

ghost clustersVertex '09 / Putten (Nl)


Cosmic Runs Pixel Trigger:

• coincidence between Top Outer Layer AND Bottom Outer Layer• rate: 0.18 Hz

Statistics from 2008 cosmic runs: ≈105 good events (no B field)• 65000 events 3 clusters in SPD• 35000 events 4 clusters in SPD



Alignment methods Dedicated talk by A. Dainese in session “Alignment” on 15/09 Two track-based methods to extract the alignment parameters (translations and

rotations) of the 2198 ITS modules:• Global minimization with Millepede (default method)• Iterative approach

Strategy:• Use geometrical survey data as a starting point

- Measurements of sensor positions on ladders during SDD and SSD construction• Hierarchical approach:

- Start with SPD barrel: 10 sectors 120 half staves 240 sensors - Align SSD barrel w.r.t. SPD barrel- Internal alignment of the SSD barrel: 72 ladders …- Align SDD barrel (longer time for calibration) w.r.t. SPD+SSD

• Include SDD calibration parameters:- Non-constant drift field due to non-linear voltage divider- Parasitic electric fields due to inhomogeneities in dopant concentration



Alignment with cosmics

After realignment with cosmics using SPD triggered data and Millepede:• Effective rf resolution ~14 mm (nominal detector position resolution r 12 µm)

= 20 μm (vs 15 μm in simulation without misalignment)


ITS Standalone tracker adapted for cosmics• Pseudo-vertex = point of closest approach between two “tracklets” in top and bottom SPD half-barrels• Search for two back-to-back tracks starting from this vertex

Track-to-track (top vs bottom) distance in transv. plane

= 48 μm (vs 40 μm in simulation without misalignment)

Track-to-“extra clusters” distance in transv. plane (sensor overlap)

after alignment

before alignment

after alignment

before alignment

SPD only, 2008 B=0 data SPD only, 2008 B=0 data

preliminary preliminary

track-to-track Dxy [cm]


Drift: calibration


Interplay between alignment and calibration• Space coordinates depends on T0 and drift speed, calibration is needed• With cosmics the resolution along drift direction is affected by the jitter of

the Pixel trigger (at 10 MHz 4 SDD time bins) with respect to the time when the muon crosses the SDD sensor

Geometry only Geometry + calibration


Strip and Drift: energy lossSimulations

The four outermost layers of the ITS (2xSDD + 2xSSD) contribute to the energy loss measurements by providing dE/dx values.

PYTHIA 6.214 p+p events at √s = 10 TeV

Cosmics During the cosmic run campaigns of 2009 (field of 0.5 T) SSD and SDD were active in the acquisition. Tracks reconstructed in TPC+ITS

Muon according to:• Atomic Data Tables 78, (2001) 183.• H. Bichsel, Rev. Mod. Phys. 60, (1988) 663• H. Bichsel, NIM A562 (2006) 154



First signs of life in LHC ITS succesfully commissioned with cosmics in Summer 2008 June 15, 2008: during the beam injection test in Tl2, the ITS pixel layers in self-

triggering mode detected the first “sign of life” of LHC


Longit

udina

l trac

ks al

ong

one

pixel

module

(14

cm)

During following injection tests more ITS layers were active In

ject

ion

even

t see

n by

the

Drif

ts

Injec

tion e

vent

seen b

y Pixe

ls


Beam induced background


Study of the LHC screen related background in ALICE• ITS pixel layers in self-triggering mode during the beam injection tests

provided relevant information on the background levels in ALICE


First collision


In Sept. 2008 the ITS was ready to record the first collisions in LHC «It's 9 a.m. and the Silicon Pixel Detector in ALICE lights up with particle “debris“ created as beam in the transfer line from the SPS hits the beam stop before Point 2.» From CERN Courier –Nov. ’08 – LHC focus: “LHC first beam: a day to remember”

First LHC beam-induced interaction was recorded by the ALICE ITS on 11 Sep ’08

Collision of beam-halo particle with the first pixel layer: 7 reconstructed tracks from common vertex.

• Pixel trigger• ITS standalone tracking


ALICE and ITS in 2009

Detector operations were resumed after the reconnection of all services in July ’09• Re-commissioning and optimization in progress

Cosmics run with magnetic field: B=0.5T & 0.2T, both polarities, ongoing


In Oct ‘08 ALICE has opted for a long shutdown to complete the installation of outer detectors and re-arrange all services (power, optical and cooling) on Side A of the central detectors, including the ITS, in order to allow an “easy” access to the TPC electronics.


The lesson learnt so far.... During the commissioning and the first operations we have learnt that:

• We have developed and built performing and robust detectors- Performance well in agreement with the design specs and goals- They survive also to “unforeseen treatments”

but....• we underestimated services and accessibility

- Optics require frequent check and cleaning- Power Supply- Cooling system

• SDD-SSD water system: it has undergone a substantial upgraded in May ‘08 to fulfil the requirements of the two detectors

• SPD evaporative system: the whole installation is undergoing a cleaning process to cure local inefficiencies which might be caused by lack of C4F10 flow.

- Air conditions in the innermost detector volume• Control and monitoring of temperature and humidity



Conclusions The ALICE Inner Tracking System was successfully commissioned with cosmics

during summer 2008 and was ready for the first collisions in September

• Integration and operating stability with ALICE central services (ECS, DAQ, CTP and DCS), Alignment studies and Calibration runs were performed over several months of data taking

Alignment is very well advanced

• Collected statistics of cosmic tracks allowed for:- Most of SPD modules alignement to 8 mm; 50% of SSD modules (the ones close to the vertical with higher statistics) also aligned. SDD on the way- dE/dx signal calibration in SDD and SSD

Cosmic runs with different magnetic fields are ongoing

Final optimization of the detector performance is well advanced• Activities tuned with the LHC schedule Vertex '09 / Putten (Nl)


Alignment Monitoring System Laser based system which uses spherical mirrors and CCDs to monitor the movements of

the ITS with respect to the TPC Any 3 mirror/camera pairs yield movement measurements for all 6 degrees of freedom. Resolution is limited by the CCD pixel size ~5μm square. Measured Resolutions are:

Δx and Δy~25μm

Δz~235μm

Δθx and Δθy

~0.30e-3 °Δθz

~1.75e-3 °


Documents

The ALICE Inner Tracking System: commissioning and running experience