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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 2 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. 3 A.Chilingarov and H.Fox, 17.3.2014 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. 4 A.Chilingarov and H.Fox, 17.3.2014 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. 5 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. 6 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. 7 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. 8 A.Chilingarov and H.Fox, 17.3.2014 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. 9 A.Chilingarov and H.Fox, 17.3.2014 Backup slides 10 A.Chilingarov and H.Fox, 17.3.2014 The ratio of the central current to the total one for different sensors at -25oC. 11 A.Chilingarov and H.Fox, 17.3.2014 The same at -35oC. 12 A.Chilingarov and H.Fox, 17.3.2014 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. 13 A.Chilingarov and H.Fox, 17.3.2014