Tests of Silicon Sensors for the CMS Pixel Detector Andrei Dorokhov University of Zurich 22 October

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Tests of Silicon Sensors for the CMS Pixel
Detector Andrei DorokhovUniversity of
Zurich22 October 2001
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The Pixel Detector
150mm x 150mm pixel size thickness 250mm
56cm
Low luminosityR4.3cm and 7.2cm High luminosity
R7.2cm and 11.0cm

• The whole pixel system consists of about 50
  millions pixels
• Single pixel counting rate will be about 10kHz
• Estimated resolution is about 15mm

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Different sensor designs

• p-spray

• p-stop

• The inter pixel isolation must isolate the
  pixels, but it also must provide a stable very
  high resistive connection between them this is
  important when a pixel by an accident is not
  connected to the amplifier (missing bond).

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Test beam at CERN

• The purpose of the test beam in 2002 at SPS
  (pions 225GeV/c) is to characterize the pixel
  sensors
• Charge distribution,
• Spatial resolution,
• Lorentz angle,
• Depletion depth for the irradiated sensors
  (3x1014 pions/cm2 at PSI)

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The Test Beam Setup
PSI30 honeywell readout chip is used, it has
22x32 pixel readout area. The chip is used in a
full readout mode, VME and NIM modules are used
to control and readout the chip. DAQ system
written in LabView 5.1 running on PC with
Windows NT 4.0 installed.
In order to run the readout chip in full readout
mode a fast trigger system was developed. It uses
PIN diode as a particle detector with fast low
noise amplifier. Signals from the pions (225GeV).
4 from the amplifier and 2 from the comparator.
6mmx3mm silicon PIN diode is connected to the
input of the amplifier.
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The test beam setup
Beam telescope and pixel setup view.
X3,Y3 plane (50mm)
X2,Y2 plane (50mm)
X1,Y1 plane(50mm)
X4,Y4 plane (50mm)
Beam, Magnetic field
Pixel Array (22x32) of 125mmx125mm one pixel size
Trigger PIN diode(3x6mm)
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The test beam setup
Beam telescope and pixel installed between two
Helmholtz coils, maximum magnetic field 3T.
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Test Beam Runs Summary
In total we have collected 100GBytes of data in
341 runs. The measurements were performed with
all samples at different angles (150, 900)
between pixel plane and the beam. One sample was
measured at 650 between pixel plane and the
beam. When the angle was 150, the data taken also
in magnetic field (3T) parallel to the beam. For
the irradiated sensors Peltier cooler was
installed, and the data were taken at different
bias voltages (80V 300V).
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The charge diffusion
The xy pixel plane was set perpendicular to the
beam. The charge developed in the silicon spreads
over x and y directions due to diffusion. The
beam entry point was reconstructed from the beam
telescope.
Normalized amplitudes
mm
mm
The distribution of normalized amplitudes in the
pixels versus pixel position with respect to the
reconstructed beam entry point.
The reconstructed charge diffusion using a
de-convolution method.
mm
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Spatial resolution
Simple analog centroid method was used in order
to reconstruct beam position. In the presence of
the magnetic field the charge will experience
Lorentz force and spread over some adjacent
pixels. The deviation of the cloud by the
magnetic field can be used to determine the
Lorentz angle.
Entries
mm
Without any additional charge diffusion the
difference between the reconstructed beam entry
point and the entry point measured from pixels
has a typical box distribution shape.
Entries
In order to simulate the charge sharing due to
the magnetic field the pixel plane was tilted by
an angle angle 650 to the beam. A spatial
resolution of 11 microns is derived from the
residuals to the beam position from the beam
telescope.
mm
650
beam
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Lorentz angle
In order to measure the Lorentz angle the grazing
angle method was used. The beam enters the pixel
under a small angle a. The charge drifts in
direction of Lorentz angle QL yielding a
deflection of the angle b for the charge
measured in the pixels.
b
mm
tan(QL) tan(b )/sin(a )
The distribution of normalized amplitudes in the
pixels versus pixel position with respect to the
reconstructed beam entry point. One can clearly
see the deflection due to the Lorentz force.
mm
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Lorentz angle
When the magnetic field is off there is no
deflection. The distribution of normalized
amplitudes in the pixels versus pixel position
with respect to the reconstructed beam entry
point.
Normalized amplitudes
mm
mm
mm
mm
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Lorentz angle
mm
For each x slice of the two dimensional charge
distribution the center was determined. The line
fit is the direct measure of the angle b .
mm
With magnetic field. The Lorentz angle is
arctg(0.07924/sin(15 0)) 170 At 4 Tesla this
gives 22.2 0
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Total cluster charge distribution
Entries
In order to estimate signal to noise ratio the
cluster amplitude distribution fit with
convolution Landau and gauss. The signal to
noise ratio is 132.
Entries
Noise amplitude distribution
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Irradiated sensors
For the irradiated sensors the depletion depth
decreases. The beam comes through the tilted
plane and the street length defines the sensitive
region (depleted region).
Depleted region
beam
mm
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Outlook

• Analysis is ongoing,
• Next beam test at CERN more measurement with
  different samples, Lorentz angle for the
  irradiated sensors, charge collection efficiency
  for different sensor designs.