trakcing systems with silicon with special reference to atlas-sct some generalities about tracking...
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Trakcing systems with Siliconwith special reference to ATLAS-SCT
• Some generalities about tracking• Special requirements in LHC environments• About silicon • About the ATLAS-SCT
Some general considerations
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qBLqBrp
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Tracking measures particle 3-momenta
Lever arm, L Sa
gitta
, s
r
Particle
track
Precision of sagitta measurement: hits N 3
Interaction point
(N position measurements)
Requirements for good resolution
• Lever arm as long as possible (large plarge detector)
• Measurement of sagitta as precise as possible• Magnetic field as large as possible
ssp
qBL
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A ‘typical’ event Granularity is essential
Requirements to LHC tracker
• FAST (40MHz)• Excludes ‘standard’ Drift Chambers due to large drift times
• Spacial resolution a few tens of microns• High granularity• Radiation hard• Must minimize material
• Drift Chambers would be optimal from this point of view
• Silicon is a good choice
Does very precise tracking give very precise momentum estimates?
• Not necessarily due to Multiple (coulomb) scattering
• Direction change due to a concentrated scatterer:
msc
msc
Ls
XxXxcp
MeV
))/ln(038.0.1(/6.13
00
x/X0 is the amount of material traversedin units of radiation lengths
Example: Atlas SCT, 3% X0 /layer, and pixel measurements inside (probably also about 3% X0
per layer).
Displacement of tracks in different tracker layers as function of momentum due to MSC.
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Excellent resolution never harms, but is sometimes useless….
The silicon strips of the ATLAS-SCT has a pitch of 80 µm
• …. So position resolution is 23 µm per hit….
• Standard deviation of a flat distribution =width/(√12)
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P(x)
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Uniform
Gauss
Basics of Silicon detectors
• ‘Simple’ p-n junctions• Reverse biased• Junctions can be segmented into strips
From L.G. Johansen, Thesis (Bergen)
~300um
thickness
Charge density
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Potential
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Electric field
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ld
A model for the diode: Constant charge density in the depletion region of the n-bulk, and heavily doped p-side
deN
xxEdx
dE
)(
2
2)( x
eNVx
eNxE
dx
dV dd
Nd is the donor concentration
Setting a reverse potential across the diode depletes it to a depth given by (about):
VeN
ld
2
How to measure the depletion depth?
• Charge stored in bulk: (charge density x volume (Al))
• Capacitance:
VeNAVeN
AeNAleNQ dd
dd
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VC
l
A
V
eNA
dV
dQlC d
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2)(
The formula works!
20 detectors from Hamamatsu (L.G. Johansen, thesis)
Charge collection
• Typical detector thickness is 300 µm.• Bethe-Bloch equation + Landau fluctuations
gives a most probable energy loss of about 80keV
• To create a free electron-hole pair you need 3.6eV
Signal is about 22000 electrons ( 3.5 fC)
Energy loss distributions for 2 GeV electrons,pionsand protons, broader than Landau. (Bak et al , NPB288:681,1987)
The ATLAS SCT barrel detector
• Pitch 80 µm (resolution 23 µm)• 768 strips per detector• Thickness 285 µm• Size 6.36x6.40 cm2
• Should biassed to 350V• p strips on n material
Read-out electronics
• Must have low noise• Noise scales with capacitance Size limitations• microstrip detectors: inter-strip capacitances dominate.
• Must be compact ASICs• For ATLAS: Must operate at 25 Mhz• For ATLAS: Must be radiation tolerant
Signal from electrons from a Ru-106 source (mostly minimum ionisingparticles). Fluctuations are dominated by Landau fluctuations in the deposited energy. Spectrum collected with fast analog electronicsChip SCT128A (from B.Pommersche, thesis, Bergen)
N/bin
Signal/noise
Charge collection vs bias, 25 ns collection time
Signal/Noise as a function of Detector Voltage
56789
10111213141516171819202122
0 20 40 60 80 100 120 140 160 180 200Detector Bias [V]
Sig
nal
/No
ise
#10
000
Detector nr 108oxygenated
Detector nr 7unoxygenated
From B. Pommeresche, Cand. Scient thesis, (U of Bergen)We must over-deplete the detectors to collect all the charge in time
Difference in signalcould be explained bydifferences in detectorthickness
What do we look for to assess the quality of a detector?
• Depletion voltage• Inter-strip capacitances
• Radiation hardnesswill require biassing to high volts
• Leakage currents must under control at high volts
The leakage current nightmare
• Current through the bulk: No problem• The Problem: Currents on detector surface, around
corners and who knows from where..….• High currents into the readout destroys the
electronics, to avoid it we take the following measures:• Capacitively (AC) coupled aluminium readout strips • Guard ring structures around active detector area (connect
to ground to suck out current)
The nightmare (part II)
• Large currents result in high power dissipation and heating of the detector system.
• Must be able to control the current, if not fully understood, it should at least be stable with time!
• Must test all detectors and detector modules for leakage current.
Careful detector design is required!
A detail of the detector for ATLAS-SCT
(picture from L.G. Johansen, thesis)
Leakage currents for some SCT detectors
(From L.G.Johansen, thesis (Bergen))
The detector modules must be radiation hard
• All components tested in a proton beam to a fluence of 3 x 1014 protons/cm2
• This is 50% more than expected for ten years of LHC operation
A few words on radiation damage
• The two most important effects are:1: Crystal defects are created in such a way that the effective
doping gets more p-like with fluence (dose). Vdep decreasesType inversionVdep increases
2: Leakage current increases increase in noise
3: Depletion from ‘below’n+ doping of back sidepreserves diode junction
Development of leakage current with time
L.G. Johansen, thesis
Signal/noise of an irradiated detector
Plot from L.G. Johansen (of course….)
Leakage current doubles for a temperature increase of 7 degrees
• ATLAS-SCT will be operated at about -10 degrees C
• Detector modules to be in thermal contact with cooling agent.
ATLAS-SCT readout electronics
• Digital readout (hit/no hit)• Pros and cons.of digital electronics
• Rad. Hard.• 128 readout channels per chip.
Schematic of the ABCD3T chip
The ALTAS SCT module
ATLAS-SCT barrel module
• 4 detectors• 1 baseboard (Patented TPG solution)
• Must be thermally conductive
• Hybrid with 12 chips, wraps around the sensor-baseboard.
• Strips are bonded together in pairs, to form 12 cm long strips.
• About 3000 wire bonds per module
A drawing of the module
Production of about 2000 modules at 4 university ‘clusters’ around the world
• Necessary for efficient use of small resources at each university.
• Production clusters:• Japan• US• UK• Scandinavia
Equipment needed or developed
• Cleanrooms• Tools for precision mounting (motorized jigs etc)• Microscopes• Metrology equipment (‘smart microscope’)• Bonding machines• Setups for electric tests
Module production
• Detector testing• Glueing of detectors to baseboard (5 um
precision)• Testing (IV), metrology• Hybrid testing and glueing • Bonding• Testing
To mount to 5 micron precision is not trivial!
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Noise occupancy must be under control!
Average noise/channel
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Module noise occupancy
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Occupancy
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Some module IV curves (nightmare, part III)
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Volts
uA
M51M52M53M54M56M57M58M59M60M61M66M67M68M69M70M71M73M74M75M76M77M78M79M80*M81*M82*M83*M84M85M86M87M88M89M90M91M93M94M95
But, in the end, the project seems to have been successful
• Yield factor OK (above 85 %)• Module mounting on barrels in Oxford• Transfer to CERN OK• Cosmic tests OK• Now, the barrel is in the ATLAS pit to be cabled
and tested further……
Conclusions
• Silicon tracking is very attractive in HEP• But not at all trivial to make….
• Very cost and manpower intensive…