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The ALICE Inner Tracking System:
present and future
Vito Manzari – INFN Bari
(vito.manzari@cern.ch)
on behalf of the ITS Collaboration in the ALICE Experiment at LHC
Outline
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 ALICE

Inner Tracking System



experiment
Detector overview and Performance
ITS Upgrade:

Physics motivations

Upgrade strategy

R&D activities

Timeline
Conclusions
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A Large Ion Collider Experiment
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 ALICE is the dedicated heavy ion experiment at LHC

Study of the behavior of strongly interacting matter under extreme conditions of
compression and heat in heavy-ion collisions up to Pb-Pb collisions at 5.5 TeV

Proton-proton collisions:
•
Reference data for heavy-ion program
•
Genuine physics (momentum cut-off < 100 MeV/c, excellent PID)
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The ALICE detector
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Central Barrel
2 p tracking & PID
Dh ≈ ± 1
Detector:
Size: 16 x 26 meters
Weight: 10,000 tons
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Central Barrel
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 Tracking
 Pseudo-rapidity coverage |η| < 0.9
 Robust tracking for heavy ion environment
• up to 150 points along the tracks

Wide transverse momentum range (100 MeV/c –
100 GeV/c)
• Low material budget (13% X0 for ITS+TPC)
 PID over a wide momentum range

Combined PID based on several techniques: dE/dx,
TOF, transition and Cherenkov radiation
Inner Tracking System (ITS)
 Rate capabilities

Interaction rates: Pb-Pb < 8kHz, p-p < 200 kHz (~30
events in the TPC)

Multiplicities: central Pb-Pb events ~2000, Pb-Pb
MB ~ 600
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The Inner Tracking System (ITS)
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 The ITS plays a key role for the study of yields and spectra of particles
containing heavy quarks
 The ITS tasks:

Secondary vertex reconstruction (c, b
decays) with high resolution
•
Good track impact parameter resolution
< 60 µm (rφ) for pt > 1 GeV/c in Pb-Pb

Improve primary vertex reconstruction,
track momentum and angle resolution

Tracking and PID of low pt particles, also
in stand-alone

Prompt L0 trigger capability (FAST OR)
with a latency <800 ns (SPD)
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The “current” Inner Tracking System
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
ITS requirements

Good spatial precision

High efficiency

High granularity (≈ few % occupancy)

Minimize distance of innermost layer
from beam axis (mean radius ≈ 3.9 cm)

Limited material budget

Analogue information in 4 layers for
particle identification via dE/dx
 The ITS (Inner tracking System) consists of 6 concentric barrels of silicon detectors
based on 3 different technologies
•
2 layers of Silicon Pixel Detector (SPD)
•
2 layers of Silicon Drift Detector (SDD)
•
2 layers of Silicon double-sided microStrip Detector (SSD)
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The Inner Tracking System in numbers
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 Radial distance defined by beam-pipe
(inwards) and requirements for track
matching with TPC (outwards)
 Inner layers: high multiplicity environment
(~100 tracks/cm2)  2 layers of pixel
detectors
Layer
Det.
1
Radius
(cm)
3.9
Length
(cm)
3
5
7.6
28.2
15.0
44.4
23.9
59.4
38.0
86.2
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100
barrel
end-cap
9.8M
50x425
1.35k
30
133K
35
25
202x294
1.14
1.13
1.06k
1.75k
1.0
1.26
4.0
2.6M
20
830
95x40000
97.8
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0.83
850
3.3
Material
budget
(% X/X0)
1.14
2.5
5.0
43.0
z
Power dissipation
(W)
0.6
1.31
SSD
6
rf
Max
occupancy
central PbPb
(%)
2.1
0.21
SDD
4
Ch.
Cell
(μm2)
28.2
SPD
2
Surface
(m2)
Spatial
precision
(mm)
1.15k
0.86
8
PbPb event @ 2.76 A TeV
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Online SPD Vertex
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 SPD Vertex built out of tracklets
 Same algorithm in pp and PbPb
with different configuration
parameters
• e.g.: cut on # clusters on SPD
 Vertex diamond information
delivered to LHC
 SPD vertex used as input for
offline reconstruction
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Track impact parameter
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 The transverse impact parameter in the bending
plane d0(rφ) is the reference variable to look for
secondary tracks from strange, charm and beauty
decay vertices
Few hundred micron
 Impact parameter resolution is crucial to reconstruct
secondary vertices : below 60 µm for pt > 1 GeV/c
 Good agreement data-MC (~10%)
 The material budget mainly affect the
performance at low pt (multiple
scattering)
 The point resolution of each layers
drives the asymptotic performance
Pb-Pb
 ITS standalone enables the tracking for
very low momentum particles (80-100
MeV/c pions)
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Particle IDentification
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 dE/dx measurement
• Analogue read-out of charge deposited in 4 ITS
layers (SDD & SSD)
• Charge samples corrected for the path length
• Truncated mean method applied to account for the
long tails in the Landau distribution
 PID performance
• PID combined with stand-alone tracking allows to
identify charged particles below 100 MeV/c
• p-K separation up to 1 GeV/c
• K-p separation up to 450 MeV/c
• A resolution of about 10-15% is achieved
p-p
Pb-Pb
p-p
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Intermediate Summary
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 The ITS performance are well in agreement with the design values
 ALICE has collected p-p and PbPb data at the various energies and the data analysis is
progressing very well
 Many papers are being published containing very relevant results
 Then …..
Why do we want to upgrade the ITS?
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ITS Upgrade Motivations
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 Extend ALICE capability to study heavy quarks as probes of the QGP in
heavy-ion collisions
 Main Physics Topics and Measurements of interest
 Study the quark mass dependence of the energy loss
• Measure the Nuclear Modification factor RAA vs pT, down to
low pT, of D and B mesons
 Study the thermalization process of heavy quarks in the hot and
dense medium formed by heavy ion collisions
• Measure the baryon over meson ratio (Λc / D or Λb / B)
• Measure elliptic flow of charged mesons
 Exploit the LHC luminosity increase improving the readout capabilities,
now limited to ≈1 kHz
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Upgrade Strategy
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 Improve the impact parameter resolution by a factor 2÷3
 How:
 Reduce of the radial distance of the innermost layer (closest to the IP)
 Reduce of the material budget
 Reduce of the pixel size
 Physics reach:
 Low pT heavy-flavour mesons
 v2 of charmed hadrons
 Heavy flavour baryons (Λc, Λb, …)
 Better identification of secondary vertices from decaying charm and
beauty and increase statistical accuracy of channels already measured by
ALICE (e.g. displaced D0, J/Ψ, etc.)
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Upgrade Strategy
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 Improve trigger capabilities
 How:
 Improve standalone tracking efficiency and pT resolution
 Selection of event topologies with displaced vertices at Level 2 (~100 μs)
 Physics reach:
 Strong enhancement of relevant signals
 Exploit luminosity increase
 How:
 Improve readout time and standalone tracking capability
 Physics reach:
 Strong enhancement of relevant signals
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Improve the impact parameter resolution
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 Get closer to the IP
 Radius of innermost Pixel layer is defined by central beam pipe radius
•
•
Present beam pipe: ROUT = 29.8 mm, ΔR = 0.8 mm
New Reduced beam pipe: ROUT = 19 mm, ΔR = 0.5 mm
 Reduce material budget (especially innermost layers)
 reduce mass of silicon, electrical bus (power and signals), cooling,
mechanics
•
•
Present ITS Pixel layers: X/X0 ~1.14% per layer
Target value for new ITS: X/X0 ~0.3 – 0.5% per layer
 Reduce pixel size
 Reduce size of interconnect bumps, monolithic Pixels
•
currently 50μm x 425μm
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Improve tracking, triggering and pT resolution
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 Higher standalone tracking efficiency

Increase granularity

Increase number of layers in the outer region (seeding) and inner region
(occupancy)
 Extended trigger capabilities
 High standalone tracking efficiency
 Low readout time < 50μs for Pb-Pb, ~μs for p-p (current ITS ~1ms in both cases)
 Increase momentum resolution
 increase track length
 increase spatial resolution
 reduce material budget
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Upgrade Scenario
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





7 silicon layers (r = 2.2 ÷ 45 cm) or more to cover the region from IP to TPC
3 innermost layers made of pixels, 3 outer layers either pixels or double sided strips
Pixel size ~ 20-30 µm (rφ), rφ resolution ~ 4 ÷ 6 µm
Material budget 0.3 ÷ 0.5% X0 per layer
Power consumption 250-300 mW/cm2
Innermost pixel layer: ultra-light high-resolution high-granularity as-close-as-possible
to IP (r ≈ 2.2 cm)
• Hit density ~ 100 tracks/cm2 in HI collisions
• Radiation tolerant design (innermost layer) compatible with 2 Mrad / 2 x1013 neq
over 10 years (safety factor ~2 included)
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Upgrade Scenario
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
The new ITS will be based on Pixel and Strip detectors
• The innermost layers should be mounted on an insertable mechanics and should
be served from one side only for a fast replacement in case of reduced
efficiency
Current
Upgrade
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Impact parameter resolution
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
An additional innermost pixel layer would achieve already a substantial
improvement of the pointing resolution (factor 3 at 200 MeV/c)
ITS standalone tracking

However, a completely new ITS is mandatory to improve the standalone
tracking efficiency at low pT and cope with the increased LHC luminosity
• New detector technologies for a faster readout
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Standalone Tracking Efficiency
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
A factor 2 gain in tracking efficiency at 200 MeV is achieved with the
configuration under study

Tracking efficiency and an improved d0 resolution allow to detect
charmed and beauty hadrons below 2 GeV/c
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Physics signal benchmark
D0  Kπ
Increase of the statistical significance 
 reduction the statistical uncertainty!
pT range not accessible with the current ITS
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Physics signal benchmark
Λc
New measurement!
Important physics reach:
barion over meson ratio in heavy-quark sector
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R&D activities
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
Pixel detectors
• Hybrid pixels with reduced material budget and small pitch
• Monolithic pixels rad-tolerant

Double-sided strip detectors (outer layers)
• Shorter strips and new readout electronics

Electrical bus for power and signal distribution
• Low material budget

Cooling system options
• air cooling, carbon foam, polyimide and silicon micro-channels structure,
liquid vs evaporative
• low material budget
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Monolithic Pixel R&D
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
State-of-the-art architecture (MIMOSA family) uses rolling-shutter readout

Pixel size ~20 µm possible

Target for material budget < 0.3 % X0 (50 µm thick chip)
•

STAR HFT Monolithic: 0.37% X0
Ongoing developments:
• Evaluation of properties of a quadruple well 0.18 CMOS
•
radiation tests structures
• study characteristics of process using the MIMOSA architecture as reference
• design of new circuit dedicated to ALICE (MISTRAL)
• investigation of in-pixel signal processing using the quadruple-well approach
• Novel high resistivity base material for depleted operation (LePix)
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Hybrid Pixel R&D
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
Pixel size limit due to flip chip bonding technology (~30 µm)

Target for overall material budget < 0.5 % X0, about 1/3 of silicon
(100 µm sensor, 50 µm front-end chip)
•
Present SPD 1.14% X0, silicon 0.38% X0
(200 µm sensor, 150 µm front-end chip)

Edgeless sensors to reduce insensitive overlap regions

High S/N ratio, ~ 8000 e-h pairs/MIP

Power/Speed optimization

Proven radiation hardness

Ongoing developments:
•
Thin and Edgless detectors (FBK, VTT, IZM)
Low cost bump bonding, Lower power FEE
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Double-sided Micro-strip R&D
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 Sensor layout
• Strip detector technologies are rather mature
• Optimize the design to cope the expected higher multiplicity at smaller
radius and nominal LHC energy
•
Optimize stereo angle to limit ambiguities in track reconstruction
•
Smaller “virtual” cell to reduce occupancy
 Front-end electronics
• Low-momentum PID requires a wide dynamic range
• Data digitization directly on front-end chip
 Ongoing developments
•
Sensor layout
•
Fully differential front-end chip
•
ADC or ToT for the digitization of the analogue information
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ITS Upgrade Timeline
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 The upgrade should target the 2017-18 shutdown (Phase I)
• Decisions on the upgrade plans in terms of physics strategy, detector
feasibility and funding availability will be taken in 2012
• The global upgrade may require a two-stage approach with a Phase II in
2020 and beyond.
 end 2011: Preparation of a Conceptual Design Report
 2011-2014: R&D for Phase I
 2014-2016: Production and pre-commissioning for Phase I
 2017-2018: Installation and commissioning for Phase I
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Conclusions
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 The current Inner Tracking System performance is well in agreement with the
design requirements and expectations
•
The achieved impact parameter resolution allows to reconstruct the secondary
vertices of charm decays
•
Standalone capability allows to track and identify charged particles with momenta
down to 100 MeV/c
 An upgraded ITS will extend the ALICE physics capabilities:
•
Strong increase of the statistical accuracy in the measurements of yields and spectra
of charmed mesons and baryons already possible with the present detector
•
A significant extension of the present physics programme with new measurements
that at present are not possible
 Several options for the detector technology implementation are being investigated
and developed
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Back-up slides
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“Russian Doll” Installation
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SDD barrel
SSD barrel
 Inserting the SDD barrel inside
the SSD barrel
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“Russian Doll” Installation
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SPD half-barrels
mounted face to face
around the beam pipe
 Moving of the SDD+SSD barrel
over the SPD
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“Russian Doll” Installation
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 Moving the TPC over the ITS barrel, i.e. SPD+SDD+SSD
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SPD L0 trigger
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 The SPD is made of 120
modules, called half-staves
SPD Half Stave
 Pixel chip prompt Fast-OR
• Active if at least one pixel hit in
the chip matrix
1
• 10 signals in each half-stave
(1200 signals in total)
• Transmitted every 100 ns
Pixel chips
 Overall latency constrain 800 ns (Central Trigger Processor)
 Key timing processes are data deserialization and Fast-OR extraction
• Algorithm processing time < 25 ns
 10 Algorithms provided in parallel
• Detectors commissioning, p-p and PbPb physics
• Cosmic, minimum bias and multiplicity algorithms
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Tracking strategies
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 “Global”
1.
2.
3.
4.
Seeds in outer part of TPC (lower track density)
Inward tracking from the outer to the inner TPC radius
Matching the outer SSD layer and tracking in the ITS
Outward tracking from ITS to outer detectors  PID
ok
5. Inward refitting to ITS  Track parameters OK
 “ITS stand-alone”
 Recovers not-used hits in the ITS
layers
 Aim: track and identify particles
missed by TPC due to pt cut-off,
dead zones between sectors, decays
• pt resolution ≤ 6% for a pion in
pt range 200-800 MeV/c
• pt acceptance extended down to
80-100 MeV/c (for p)
TPC-ITS track matching
pt resolution
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Vertexing
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
Primary vertex reconstructed with tracklets

Tracks reconstruction starts from outside (TPC) towards ITS using the vertex as seed

TPC reconstructed tracks are matched with SSD outer layer

Once the reconstruction reaches the first SPD layer it is back-propagated

Re-fit from outside and the vertex is recalculated using the tracks
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Upgrade Simulation Tools
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 3 independent simulation tools have been developed
 Fast Estimation Tool: “Toy-Model” originally developed by the STAR HFT
collaboration which allows to build a simple detector model. The model featured
the calculation of the covariance matrix at each step of a measurement (e.g. layer
with radius r) including the multiple scattering.
 Fast MC Tool: Extension of the FET that allows to disentangle the performance of
the layout from the efficiency of the specic track finding algorithm.
 Full MC: Transport code (geant3) designed to be flexible : the detector
segmentation, the number of layers, their radii and material budgets can be set as
external parameters of the simulation.
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Upgrade Simulation Validation
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Fast Estimation Tool (pions)
Full MC
 Fast MC shows the same perfomance as the FET
 The 3 simulation tools reproduce the current ITS performance
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Pointing Resolution
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 Effects of the innermost layer L0
• No vertex resolution
Radial
Distance
Material
Budget
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Pointing Resolution
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Spatial
Resolution
 Configuration design for better pointing resolution performances:
 Improvement at low pT:
• Smallest radial distance to the beam line
• Smallest material budget
 Improvement at high pT:
• Smallest cell size
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Particle Identification
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
Different configurations are being studied
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Red : proton/Kaon
separation
Black : kaon / pion
separation
42
Hybrid pixel material budget
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 Each urrent SPD layer
• Carbon fiber support: 200 μm
• Cooling tube (Phynox): 40 μm wall thickness
• Grounding foil (Al-Kapton): 75 μm
• Pixel chip (Silicon): 150 μm  0.16%
• Bump bonds (Pb-Sn): diameter ~15-20 μm
• Silicon sensor: 200 μm  0.22%
• Pixel bus (Al+Kapton): 280 μm  0.48%
• SMD components
• Glue (Eccobond 45) and thermal grease
Two main contributors: silicon and interconnect structure (bus)
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How material budget can be reduced
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 How can the material budget be reduced?
 Reduce silicon chip thickness
 Reduce silicon sensor thickness
 Thin monolithic structures
 Reduce bus contribution (reduce power)
 Reduce edge regions on sensor
 Review also other components (but average contribution 0.1-0.2%)
 What can be a reasonable target
 Hybrid pixels: ~0.5% X0
•
silicon: 0.16% X0 (present SPD 0.38%)
•
bus: 0.24% X0 (present SPD 0.48%)
•
others: 0.12% X0 (present SPD 0.24%)
 Monolithic pixels: 0.37% X0 (as for STAR HFT)
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