1. Aras Papadelis NIKHEF Vertex 2005, Nikko, Japan
Observations of anomalous(?) bias dependent cluster centroid shifts in the LHCb VELO detector.
Aras Papadelis NIKHEF
Vertex 2005, Nikko, Japan

2. Outline Silicon sensors for the LHCb VErtex LOcator. Beam test results
Sensors and FE electronics.
Silicon specs.
Beam test results
Motivation
Observed bias dependency of reconstructed cluster position.
Observation of possible ballistic deficit in VELO.
Anomalous centroid shifts.

3. The sensors in the VELO detector
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.
25 ns peaking time
128 strips read out in parallel.
7mm
42mm

4. Some silicon specifications
Non-irradiated sensor from Micron, R-geometry, n+-on-n with p-spray
Bulk thickness
200 µm
300 µm
Depletion Voltage
34V
55V
Drift time at 100V
~4 ns
~8 ns
Doping concentration
1012 cm-3
Resistivity
~ 5 kΩ cm
Strip resistance
~ 50 Ω (Strip + routing line ≈ 3.1 cm)
Bias resistors
1.8 MΩ (110 x 16.5 kΩ)
Crystal orientation
<100>
Strip pitch
µm (with increasing radius)
Strip width
0.4 * pitch
VELO sensor is of ”normal” design, no surprises are expected.
Drift times well below 25 ns.

5. Background
Sept 2004: Steve Biagi (LHCb Liverpool) suggests that ballistic deficit in non-irradiated sensors can give offsets in reconstructed cluster position for angled tracks.
Subsequently investigated in VELO beam tests of Nov

6. Test beam setup CERN X7 SPS 120GeV pions. sensor Beam beam
10° incidence angle
Beam
Took data at 10°, bias voltages 100V, 200V and 300V.
(Vfd is 34V and 55 V)
Hamamatsu R sensor (Telescope)
Hamamatsu Phi sensor (Telescope)
PR micron R sensor
PR micron R sensor

7. Bias dependent mean residual shift
We are looking for a shift in the mean value of the track residual distribution.
Align system at 100V
Study mean residual at different bias voltages.
200 µm
300 µm
100 V
200 V
300 V
Residual (cm)
Residual (cm)

8. Residual shift vs radius
200 µm
300 µm
9m
8m
pitch
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)
”Banana bend” for 200 µm at high pitch, possibly due to warped sensor.
Looks like sensors are misaligned!

9. Further checks...
Widths of residual distribution more or less independent of bias voltage.
-distributions look as expected for a misaligned detector.
300 µm
200 µm

10. VERY CLEAR: No time dependence big enough to cause misalignment
Safety check
Data taking was done in one sequence during 10 hours.
200 µm
300 µm
Time 
VERY CLEAR: No time dependence big enough to cause misalignment

11. Ballistic deficit? Signal increases between 100V and 300V.
Is this ballistic deficit?
Also:
300V arrives about 2 ns before 100 V.

12. Publications on ballistic deficit
”Similar” unirr. sensor (thickness, strip pitch, Vfd ) and FE electronics.
Result: Small ballistic deficit for sensor bias above 100V is observed.
Study of the performance of ATLAS prototype detectors using analogue front end electronics. (Riedler, Dabrowski, Kaplon, Weilhammer)
NIM A 477 (2002) 104–109
Unirradiated
p-on-n
Charge collection efficiency studies with irradiated silicon detectors. (Allport et al.)
NIM A 501 (2003)
Similar results, the ballistic deficit at 100 V is small.

13. Signal loss due to timing
*Simplified* way of viewing ballistic deficit. Assume:
no charge trapping (unirradiated)
homegenous E-field
same µ everywhere in sensor.
Electrons in the grey region do not contribute fully to the signal.
 shift in the cluster centroid!
For a 300 m sensor, a 8 m shift corresponds to a ~30 % signal loss between 300V and 100V.
This is not what we observe!
Sensor thickness dependence should be seen!
Simulation needed!
Cluster centroid shift n+ w
n-bulk p+
100V
300V
NB: Proportions are not correct!

14. 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
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)
Timing effect
Is the 2 ns peak shift big enough to cause 10 µm shift?.

15. Suggestions are very welcome!
Conclusion
We see a distinct and systematic shift in mean residuals at 300V compared to 100V bias voltage.
We have not understood it yet, despite a lot of effort.
The question remains open…
Suggestions are very welcome!

16. “Oh no, not homework again”

17. …
RESERVE SLIDES

18. Residuals vs inter strip position
Simulation Perfect alignment 10 m misalignment
 in high pitch region
=QL/(QL+QR)