bias dependent cluster centroid shi

•Charge collection efficiency studies with irradiated silicon detectors. (Allport et al.)
NIM A 501 (2003) 146-152
Similar results, the ballistic deficit at 100 V is
position. – Observation of possible ballistic deficit in VELO. – Anomalous centroid shifts.
Aras Papadelis. NIKHEF. Nov 9. 2005, Vertex 2005
2/16
The sensors in the VELO detector
to cause misalignment
Aras Papadelis. NIKHEF. Nov 9. 2005, Vertex 2005
300 µm 10/16
Ballistic deficit?
R300 R200
R200
• Signal increases between 100V and 300V. • Is this ballistic deficit?
Cluster centroid shift n+
– no charge trapping (unirradiated) – homegenous E-field – same µ everywhere in sensor. ● Electrons in the grey region do not contribute fully to the signal.
● Sept 2004: Steve Biagi (LHCb Liverpool) suggests that ballistic deficit in non-irradiated sensors can give offsets in reconstructed cluster position for angled tracks.
6/16
Bias dependent mean residual shift
• We are looking for a shift in the mean value of the track residual distribution.
1) Align system at 100V
2) Study mean residual at different bias voltages.
• Clear systematic shift in residuals between different voltages. Same size for both 200 and 300 µm thick
sensors. (Effect first observed by J. Palacios, CERN)
● Timing effect
– Is the 2 ns peak shift big enough to cause 10 µm shift?.
Aras Papadelis. NIKHEF. Nov 9. 2005, Vertex 2005
Bulk thickness Depletion Voltage Drift time at 100V Doping concentration Resistivity Strip resistance Bias resistors Crystal orientation Strip pitch Strip width
• Sensor thickness dependence should be seen!
3100V
p+
• Simulation needed!
NB: Proportions are not correct!
Aras Papadelis. NIKHEF. Nov 9. 2005, Vertex 2005
● Looks like a surface effect
– Caused by changes in drift field when going from 100V to 300V? – Effects related to shape of weighting field?
– But! Why is no pitch dependence seen? (Remember: strip width/pitch=constant)
200 µmBiblioteka 300 µm34V
55V
~4 ns
~8 ns
1012 cm-3
~ 5 kΩ cm
~ 50 Ω (Strip + routing line ≈ 3.1 cm)
1.8 MΩ (110 x 16.5 kΩ)
<100>
40 - 100 µm (with increasing radius)
0.4 * pitch
Quick reminder
(for more details, see talk of L.Eklund )
● R and measuring sensors,
2048 strips per sensor
● Analogue FE read out done by the Beetle chip.
VELO sensor is of ”normal” design, no surprises are expected. Drift times well below 25 ns.
Aras Papadelis. NIKHEF. Nov 9. 2005, Vertex 2005
4/16
Background
Observations of anomalous(?) bias dependent cluster centroid shifts in the
LHCb VELO detector.
Aras Papadelis NIKHEF
Vertex 2005, Nikko, Japan
Aras Papadelis. NIKHEF. Nov 9. 2005, Vertex 2005
small.
Unirradiated p-on-n
Aras Papadelis. NIKHEF. Nov 9. 2005, Vertex 2005
12/16
Signal loss due to timing
● *Simplified* way of viewing ballistic deficit. Assume:
200 µm
100 V 200 V 300 V
300 µm
Residual (cm)
Aras Papadelis. NIKHEF. Nov 9. 2005, Vertex 2005
Residual (cm)
7/16
Residual shift vs radius
200 µm
300 µm
9m
8m
pitch
13/16
So, whodunnit?
● Slowly moving sensors? NO! ● Sensor movement due to change in voltage? VERY UNLIKELY! ● Ballistic deficit? NO!
– Unclear if the signal loss that can be observed is ballistic deficit. – Signal loss observed is not big enough to cause observed shift. – No width dependence can be seen
● Subsequently investigated in VELO beam tests of Nov 2004.
Aras Papadelis. NIKHEF. Nov 9. 2005, Vertex 2005
5/16
Test beam setup
Took data at 10°, bias voltages 100V, 200V and 300V. (Vfd is 34V and 55 V)
shift in the cluster centroid!
w
For a 300 m sensor, a 8 m shift corresponds to a ~30 % signal loss
between 300V and 100V.
n-bulk
This is not what we observe!
– ”Banana bend” for 200 µm at high pitch, possibly due to warped sensor.
• Looks like sensors are misaligned!
Aras Papadelis. NIKHEF. Nov 9. 2005, Vertex 2005
Outline
● Silicon sensors for the LHCb VErtex LOcator.
– Sensors and FE electronics. – Silicon specs.
● Beam test results
– Motivation – Observed bias dependency of reconstructed cluster
Aras Papadelis. NIKHEF. Nov 9. 2005, Vertex 2005
9/16
Safety check
● Data taking was done in one sequence during 10 hours.
200 µm
Time
VERY CLEAR: No time dependence big enough
8/16
Further checks...
● Widths of residual distribution more or less independent of bias voltage.
200 µm
300 µm
● -distributions look as expected for a misaligned detector.
NIM A 477 (2002) 104–109
•”Similar” unirr. sensor (thickness, strip pitch, Vfd ) and FE electronics. •Result: Small ballistic deficit for sensor bias above 100V is observed.
– 25 ns peaking time – 128 strips read out in parallel.
Aras Papadelis. NIKHEF. Nov 9. 2005, Vertex 2005
3/16
Some silicon specifications
Non-irradiated sensor from Micron, R-geometry, n+-on-n with p-spray
Aras Papadelis. NIKHEF. Nov 9. 2005, Vertex 2005
CERN X7 SPS 120GeV pions.
sensor
beam 10° incidence angle
Hamamatsu R sensor (Telescope) Hamamatsu Phi sensor (Telescope) PR04 200 micron R sensor PR04 300 micron R sensor
Also: • 300V arrives about 2 ns before 100 V.
Aras Papadelis. NIKHEF. Nov 9. 2005, Vertex 2005
11/16
Publications on ballistic deficit
•Study of the performance of ATLAS prototype detectors using analogue front end electronics. (Riedler, Dabrowski, Kaplon, Weilhammer)