Silicon Pad Detectors for tracking and particle identification in heavy ion collisions

1
Silicon Pad Detectors for tracking andparticle
identification in heavy ion collisions

• Heinz Pernegger for the Phobos collaboration
• Massachusetts Institute of Technology
• March 31, 2000

2
Outline

• Brief introduction of the PHOBOS experiment at
  RHIC
• Layout of silicon pad detectors
• Measurement signal pad detectors
• Wafer parameters
• signal response in beam tests and lab
  measurements
• Measurement of energy loss in silicon pad
  detectors
• Measurement of energy loss for low and high
  momentum
• Comparison to simulation
• Test of particle identification capabilities

3
The PHOBOS Collaboration
A small experiment with 72 collaborators
from Argonne National Laboratory Brookhaven
National Laboratory Institute of Nuclear Physics
Krakow Jagiellonian University Krakow Massachusett
s Institute of Technology National Central
University, Taiwan University of Rochester
University of Illinois Chicago University of
Maryland
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1.) Phobos at RHIC

• Aim of Phobos Search for Quark-Gluon Plasma in
  Au-Au collisions
• Phobos searches for events with very high charged
  multiplicity and will study them with the
  spectrometer
• particle multiplicity over full solid angle
• reconstruct tracks in mid-rapidity range with low
  Pt threshold and identify them
• Allows to measure particle spectra, particle
  correlation

Au 100 GeV/n
Au 100 GeV/n
5
Layout of the Experiment
1m

• 1 layer Silicon multiplicity Vertex Detector
  (20,000 readout channels)
• 14 layer Silicon Spectrometer Arms (60,000
  readout channels/arm)
• Time of Flight Wall

Magnet (top half not shown)
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2 Silicon detectors with different
aimsMultiplicity Detector - Spectrometer

• Measure charged multiplicity in 4p
• study event with high multliplicity

• Full track reconstruction and particle
  identification in mid rapidity
• reconstruction efficiency 85
• Pt Threshold 50MeV for p and 200 MeV p

dE/dx 650 MeV/c p /K 1200 MeV/c K/p TOF 1200
MeV/c p /K 2000 MeV/c K/p
Single event multiplicity
Si TOF
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What do our silicon detectors do

• The octagon and ring multiplicity detector
• 1 large layer of silicon around the beam pipe
• Measuring total multiplicity for charged
  particles
• expect very high occupancy (90-100 ?)
• cover pseudorapidity of -5.5 to 5.5 and full
  phi coverage
• The vertex detector
• Determining the interaction point with an
  accuracy of 50 um in a range of /- 10 cm around
  the nominal interaction point
• uses 2 layers of silicon on each side of the
  interaction region
• The Spectrometer
• does 3d-tracking of charged particles (for about
  1 of full solid angle)
• operates inside a 2 T magnetic field
• uses 14 layers of highly segmented silicon
  detector for tracking
• uses the silicon signal for particle
  identification for pions, kaons and protons

8
The Spectrometer Detector
66 sensors x 256 ch
21 sensors x 256 ch.
28 sensors x 512 ch.
18 sensors x 500 ch.
1x1mm to 0.7x19mm 57896 channels/arm
8 sensors x 1536 ch.
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A central event in the spectrometer
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Silicon Pad Sensor
Double Metal, Single sided, AC coupled,
polysilicon biased produced by ERSO, Taiwan
PN junction
Thin ONO
Thick ONO
Polysilicon
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Double metal layer for readout of pad detectors

• Use a metal 1 layer as electrode and the metal 2
  layer to route signals to the detector edge

AC coupled Pad (p-implant metal
1pad) polisilicon bias resistor metal 2 readout
line contact hole metal 1- metal 2
Advantages of this readout scheme readout on
detector edge minimizes multiple scattering (no
extra material in active area) simplifies
the readout different pad geometries can be
routed to a single type bond pad array can
use conventional Si-strip detector readout chips
on pad detectors Disadvantages - double metal
structure adds to the total detector capacitance
- increases capacitance between pads
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Module Assembly
SMD/conn to hybrid
Chip to hybrid
Hybrid bonding
Test of - VA chips - flex cables - hybrids
Find electronic problems
backpl. contact
MS rework
Module gluing
Module bonding
(fast) Module function test
Module rework
Hybrid calibration
Sensor tests
Module SOURCE test
Module calibration
SURVEY measurement
Mounting on frames
Determine number of defect channels with
channel/channel calibration good if lt 5
Scan the full module surface with Sr90 source on
automated test station good if peak S/N gt 101
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The Spectrometer Modules
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Sensor Testing and Module Assembly
Hughes 2470-V bonder
Inspection stations
Probe Stations
Clean Rooms
Gluing Station
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Test results of detector properties

• Statistics on Silicon parameters
• Tests on finished modules
• signal uniformity for different sensor geometries
• cross-talk between pads
• noise for different geometries
• Overall module performance
• Readout electronics
• VA-HDR1 chips in 64 and 128 channel versions
• Viking type electronics produced by IDE AS,
  Norway
• consists of preamp , RC-CR shaper, track-hold
  stage multiplexed analog output
• peaking time 1.0ms
• high dynamic range gt 100 MIP input signals

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Measurement of detector capacitances

• Metal 1 to Metal 2 capacitance (1.2mm
  oxide/nitride insulator layer)
• Backplane capacitance of a Type 5 pad

Neighbour columns grounded 1 MHz source
frequency line width 10mm line length
6cm Metal 1-Metal 2 capacitance from test
structure 4.5pF/cm from detector 4.7pF/cm
Neighbour pads grounded 1 MHz source
frequency pad width 0.667mm pad length 19
mm Backplane capacitance from p-n diode
5.3pF/pad from detector 5.4pF/pad
Vfd105V
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Readout line quality

• Detect broken readout lines by measuring C
  back-plane
• Other typical detector parameter
• leakage currents active area _at_ Vfd 3-5mA
• polysilicon resistors 5MW
• depletion voltage 100-110V

70 mm
Broken readout line
M1
p
Cb
22 mm
n
Currently typical 5 broken readout lines - work
on improvement to lt2
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Sensor parameters
Full Depletion Voltage V
Leakage Current uA
PolySilicon Resistance Mohm
Operational Range V
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Signal uniformity across sensors

• Relative signal across the pads (row-wise)

/- 2 full scale
row i-1 row i row i1
Relative signal
Row number
Very uniform signal response within a
sensor better than /- 1
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Measurement of cross talk on small pads

• Use reference system to predict hit position on
  Type 1 detector
• plot signal distribution for predicted pad and
  all neighbouring pads

bot
top
top
hit
left
right
C ltSpadgt/Scenter cross talk top lt0.1 bot
0.6 left lt0.1 right lt0.1
bot
20
20
-20
0
-20
0
left
right
center
crosstalk in analog readout chain
200
20
20
-20
0
-20
0
0
Pad signal (ADC)
Pad signal (ADC)
Pad signal (ADC)
(Detector related) cross talk less than 0.6 on
Type 1 detector
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Measurement of cross talk on large pads

• Expect largest cross talk of all Phobos detector
  types due to readout line to pad capacitance
  (9.4 pF)

Row 0
Row 1
C ltSpadgt/Scenter cross talk row 0 0.9 row 1
0.6 row 3 0.5
-5
0
5
-5
0
5
Row 3
Hit in Row 2
0
100
-5
0
5
Pad signal (ADC)
Pad signal (ADC)
Cross talk less than 1 on Type 5 detector
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Calculated and measured noise values
Noise largely dominated by constant part
preamp-only noise main detector noise source
bias resistor and pad to pad capacitance
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Assembled Modules
Peak Signal/Noise Number of defect channels ()

• Mean module S/N 161
• Mean number of detefect channels 1

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Measured energy loss for low momentumpions and
kaons

• Phobos lives on the Silicon analog signals
• The multiplicity is directly calculated from the
  analog signal
• The spectrometer needs it for particle
  identification and track reconstruction
• The aim of this measurement was
• to measure and understand the response of our
  detector for the low momentum pions and kaons
• measure the dE/dx loss and straggling for kaon
  and pions versus momentum
• This allows us to
• compare and tune our Geant simulation
• test the particle identification

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Test setup at AGS
TOF start (Degrader) Phobos 4 planes of TOF
stop Cerenkow Paddle (Trg) type 1 modules

• The Silicon detector
• use first 4 planes of the spectrometer (12k
  channels)
• small pads -gt good and full tracking
• high S/N -gt good energy loss measurement
• 8 sensors 96 chips -gt minimize systematic
  error, give redundancy and allow cross checks
• The TOF and Cerenkow
• provides pi/K separation and particle
  identification in the low p range
• suppress e- back ground of secondary beams

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How do we process the signal?

• The basic step in the signal calibration
• calibrate the gain and linearity of on each
  channel
• convert the measured charge to energy deposited
  using a constant of 3.62eV for the creation of 1
  electron/hole pair
• correct for the measured detector thickness
• The intrinsic detector signal
• Landau part described by restricted Bethe-Bloch
• Intrinsic gaussian contribution to the energy
  loss due to variation of Ionization potential for
  e- in different Si- shell (Shulek et al.
  )
• electronic noise (5keV in our case)
• The measurements
• make a convolute LandauGauss fit to distribution
• determine the most probable signal of the Landau
  part to measure dE/dx loss
• use sigma of gaussian part and FWHM to
  characterize the energy straggling

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Pions at low momentum the measured signal
Peak at 80keV
500 MeV/c
1GeV/c
Peak at 150keV
130 MeV/c
285 MeV/c
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Most probable energy loss for high momentum pions
preliminary
Landau most probable energy loss keV
? Data
? Geant

• We measure a 4 logarithmic rise of dE/dx (0.5 -
  8GeV/c) for pions
• Geant agrees very well with our measurement

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Go to even lower momentum for pions 130 - 10
MeV/c
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Kaon on Pion at the same momentum

• Use the peak (Landau mp) to determine the dE/dx
• use the width to measure the straggling

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The measured dE/dx versus bg compare to scaled
Bethe-Bloch

• Scaling accounts for most probable to mean (as in
  BB) difference (determined at 1GeV)

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Putting it to work Particle Identification with
4 planes only?

• Test our particle ID capabilities with 4 of 14
  on mixed kaon pion data sample
• The particle momenta are nicely at the limit of
  our claimed pi/K separation (650MeV/c)
• Use the TOF measurement to determine efficiency
  and purity
• define
• efficiency e(pi) N(pi-gtpi)/N(pi)
• contamination c(pi) N(K-gtpi)/N(K)
• and vise-versa for Kaons

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First approach Truncated mean with 3 of 4
measurements
p
500MeV/c
620MeV/c
K

• Works up to 620 MeV/c but worsens at 750MeV/c
• requires very careful tuning of the cut
• cut strongly depends on relative fraction of p/K

750 MeV/c
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Second approach Using a Maximum-Likelyhood
estimation for pi/K

• based on calculated signal probabilities for p
  and K hypothesis Slog(f(Si)) max
• fprobability density function for pion or kaon
  at fixed momentum
• requires knowledge of signal distribution at
  different p

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The particle ID efficiency with 4 planes
likelyhood
likelyhood
Truncated mean
Truncated mean

• Good efficiency already with 4 planes in both
  cases
• eff (pi) gt 85 to 90 at 750MeV/c
• eff (K) 85 at 750MeV/c

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The assembled spectrometer

• Installed the fully assembled spectrometer in
  December in Phobos and made system tests during
  January

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Conclusion

• Phobos uses silicon pad detectors for
• reconstruction of low momentum proton and pions
  in a 14 layer spectrometer
• particle identification with dE/dx measurements
• The detectors
• the measured cross talk less than 1
• signal uniformity better than /- 1
• measured Signal / Noise 141 to 181
• The spectrometer
• Made detailed studies on the dE/dx for our
  particle ID
• Was installed in December
• Full system tests after installation showed gt98
  functional channels
• Detector noise in-situ is close to
  detector/electronics limit
• Stability tests in area showed excellent
  stability
• Looking forward to the first RHIC physics run in
  June!

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Defects associated with double metal structure
Non Func. Channels
Broken Signal Lines
Shorted Channels
Shorted Coupling Cap
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Spectrometer Acceptance
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Momentum Resolution
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Signal amplitude for different pad sizes
Smp21500 e- (78keV)
Small pads
Large pads

• Acquired with 90Sr b source
• Average source signal 21081 e-
• Signal MP agrees with capacitive loss calculation
  (charge sharing between detector and VA input
  capacities)

S/N mp 16.4
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... In 9 different configurations
MOD1 6 (9)
MOD6 5 (9)
MOD2 3 (5)
M4T4 4 (7)
M4T5 7 (13)
M7T3 5 (9)
M7T2 3 (5)
M5T5 16 (30)
M5T4 4 (7)
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Extracting the gaussian component of energy loss
(Shulek correction)
s/Smp0.078
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GEANT simulation versus DATACompare it for pions
at 285MeV/c (Phobos typical)
without gaussian addition
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The selection contamination with 4 planes

• Very little contamination already with 4 planes
  in both cases
• c (pi) lt15 at 750MeV/c and reaches levels of
  5 beyond 600MeV
• using Maximum Likelyhood produces slightly better
  purity

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Summary Measured Signals versus Geant and
Bethe-Bloch

• GEANT
• Geant reproduces the most probable energy loss
  extremely well!!!
• Geant has trouble with the straggling
  (distribution is too sharp)
• Adding gaussian componenet to account for the
  Shulek correction significantly improves the
  modelling of energy straggling
• Bethe-Bloch
• need to apply an restricted energy loss
  calculation due to escaping d electrons
• can reproduce the momentum behaviour quite well
  once is it normalized at one point.