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Vidyo meeting
17.3.2014
Central and total currents in the
irradiated ATLAS-12 sensors
A.Chilingarov and H.Fox
Lancaster University, UK
Contents
1. Introduction
2. An example
3. IV for different fluences
4. Currents at 600 V vs fluence
5. Conclusions
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A.Chilingarov and H.Fox, 17.3.2014
1. Introduction.
The sensors have no contactable guard ring. Therefore only the total current,
Itotal, can be measured for them by the usual methods. However from the
interstrip resistance measurements the voltage drop at the bias resistor, U0,
due to the dark current can be found for both active (“master”) and passive
(“slave”) strips. Dividing U0 by the value of Rbias found in the same
measurements gives the dark current for the strip, I0. Multiplying the latter by
the number of strips, 104, estimates the current flowing through the central
area of the sensor, Ic.
For the master strip U0 can be found as the master strip potential, Vm, at
which the current from the master source-meter is equal zero. For the slave
strip U0 is the voltage Us measured at it by voltmeter for the Vm = U0.
All measurements reported here were made at -25oC.
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2. An example. Sensor w627-bz3f-p5 irradiated by 1015 cm-2
The apparent
bias resistor
Ra=0.69 MW is
significantly
lower than the
reconstructed
Rbias=2.12 MW.
The Us/Vm
slope for the
second next
strip S2=0.261
is close to the
expected value
(S1)2=0.5042=
0.254.
Resulting
Ris=1.0 MW.
The master current line crosses the x-axis at Vm= -83.6 mV. At this Vm the voltage
at both the slave strip next to the master one and that over one strip is -83.9 mV.
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3. IV for different fluences
Standard IV and the currents (total and central) from the Ris measurements
made at 400 and 600V for the sensors irradiated by 1014cm-2. The central
currents are significantly lower than the total ones and agree well with the
expectations.
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A.Chilingarov and H.Fox, 17.3.2014
The same data for the sensors irradiated by 1015cm-2 fluence. The central currents
follow closely the total ones apart from the surge at high bias for bz3c sensor. The
Itotal surge in the Ris measurements is shifted to higher voltages compared to the
standard IV data.
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A.Chilingarov and H.Fox, 17.3.2014
The same data for the sensor irradiated by 5∙1015 cm-2. The Ris measurements were
made at 300, 400 and 600V. The central current is about a half of the total one.
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A.Chilingarov and H.Fox, 17.3.2014
4. Currents at 600 V vs fluence
The total and central currents measured at 600V and expected for a fully depleted
sensor. Note that for the fluence above 1014cm-2 the sensors are not fully depleted
at 600V. For the same fluence the central currents are closer than the total ones.
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5. Conclusions
1. The currents at three central strips extrapolated to the
whole sensor surface are always lower than the total
currents. It can be due to the edge strips operating as the
grounded guard ring that picks up part of the current
flowing over the sensor edge.
2. At 600 V and the same fluence the central currents in
different sensors are closer to each other than the total
ones.
3. The central currents at 600 V agree better with the
expectations than the total ones.
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Backup slides
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A.Chilingarov and H.Fox, 17.3.2014
The ratio of the central current to the total one for different sensors at -25oC.
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The same at -35oC.
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Current normalised by its value at full depletion in the sensors irradiated by the
fluences from 1013 to 1016 neq/cm2. For details see A.Chilingarov, 2013, JINST 8
P10003.
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A.Chilingarov and H.Fox, 17.3.2014