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30 th March 2010

IoP HEPP/APP Annual Conference 2010 UCL. Charge Collection Annealing in ATLAS SCT Silicon Sensors Craig Wiglesworth. 30 th March 2010. The ATLAS Experiment and the SCT. The ATLAS semiconductor tracker (SCT) is a silicon microstrip detector.

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30 th March 2010

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  1. IoP HEPP/APP Annual Conference 2010 UCL Charge Collection Annealing in ATLAS SCT Silicon Sensors Craig Wiglesworth 30th March 2010

  2. The ATLAS Experiment and the SCT The ATLAS semiconductor tracker (SCT) is a silicon microstrip detector. Provides discrete space points for tracking and vertexing. Exposure to high levels of radiation damages sensors. Subsequent changes in sensor properties continue to change after irradiation. Important to understand SCT performance over full 10 year lifetime of ATLAS. Craig Wiglesworth IoP HEPP/APP Annual Conference UCL March 2010 1

  3. Evolution of Sensor Properties After Irradiation Sensor leakage current (ILEAK), depletion voltage (VDEP) and charge collection efficiency (CCE) are all affected by irradiation. Evolution of VDEP shows dependence on both temperature and time spent after irradiation. Require full depletion of sensor in order to optimise CCE. ILEAK Shows strong temperature dependence but decreases with time. (Typical behaviour for a silicon microstrip sensor). Low temperatures will suppress ILEAK but risk of thermal runaway (ILEAK not discussed further). Need to avoid the sensors being warm (> 0 oC) for long periods of time! Craig Wiglesworth IoP HEPP/APP Annual Conference UCL March 2010 2

  4. Evaluating the Evolution of VDEP and ILEAK in the SCT In the ATLAS Inner Detector Technical Design Report (1997) VDEPand ILEAK were predicted for SCT sensors. Standard Access Procedure (SAP) Many of the inputs to these calculations have since changed. Assumed LHC luminosity profile: 3 years @ 1033 cm-2s-1 7 years @ 1034 cm-2s-1 (~ 1.4x1014 neq cm-2) ATLAS maintenance scenarios (days@temp yr-1): In addition, the radiation damage model itself has evolved. Predictions re-evaluated with new radiation damage model and updated inputs. Craig Wiglesworth IoP HEPP/APP Annual Conference UCL March 2010 3

  5. Re-Evaluating the Evolution of VDEP and ILEAK in the SCT An updated LHC luminosity profile now exists. Maintenance/shutdown time and cooling temperatures reviewed in line with: Achievable coolant temperatures. Insertable B-Layer installation. Possibly longer maintenance. VDEP predictions suggest 450 V (max for SCT) is sufficient for at least 10 years operation. Paul Dervan, Joost Vossebeld, Tim Jones (Liverpool), Taka Kondo (KEK), Graham Beck (QMUL), Georg Viehhauser (Oxford), Steve McMahon (RAL), Koichi Nagai (Brookhaven), Kirill Egorov (Indiana), Richard Bates, Alexander Bitadze (Glasgow). Craig Wiglesworth IoP HEPP/APP Annual Conference UCL March 2010 4

  6. Comparison of Hamburg Model and TDR Model Hamburg model is now believed to be the best model available to predict VDEP. However, large differences observed between the predictions of the TDR model and the Hamburg model. Origin is in reverse annealing contribution DNY to the predicted change in effective doping concentration DNEFF: TDR model parameterised reverse annealing as a second order process. Hamburg model parameterises reverse annealing as a modified first order process. Need high fluence + long annealing data to compare to predictions of both models. Craig Wiglesworth IoP HEPP/APP Annual Conference UCL March 2010 5

  7. Programme of Accelerated Annealing Measurements Sensor performance traditionally studied by determining VDEP from CV measurements. In Liverpool the focus has been on measuring the annealing of CCE. Much data available for n-side readout sensors. Less detailed information for p-in-n sensors. New programme of accelerated annealing measurements on ATLAS mini sensors. Pair of sensors irradiated with neutrons at Ljublijana (V. Cindro et al) to 2x1014 neq cm-2 (new prediction for SCT = 1.6x1014 neq cm-2). One sensor used for CCE measurements and one sensor used for VDEP measurements. Both sensors annealed together at same temperature for same length of time. All following plots by…. A. Affolder, H. Brown, G. Casse, P. Dervan, J. Vossebeld, C. Wiglesworth (Liverpool) Craig Wiglesworth IoP HEPP/APP Annual Conference UCL March 2010 6

  8. Experimental Setup and Analysis Procedure Charge Collection Measurements 90Sr fast electron source used to generate signal. Readout triggered by scintillator. Charge collection measured using analogue electronics chip (SCT128) clocked at LHC speed (40 MHz clock, 25 ns shaping time). System calibrated to most probable value of MIP energy loss in non-irradiated 300 m thick sensor (~ 23000 e). Depletion Voltage Measurements VDEP Determined by standard method of measuring the VBIAS at which 1/C2 saturates. Both sets of measurements performed in freezer at temperature of ~25 oC with N2 flush. @ 1 kHz Craig Wiglesworth IoP HEPP/APP Annual Conference UCL March 2010 7

  9. Experimental Setup and Analysis Procedure Hot channels are masked out in analysis (1 ADC ~ 22 e) Measure charge on strip (+ neighbours) with largest S/N ratio Noise (ADCs) Cluster Frequency Strip Number Channels with low hit frequency are masked out Cluster Frequency Cluster Charge (ADCs) A landau convoluted with a gaussian is fitted to the resulting distribution of the collected charge. Most probable valueis recorded. Strip Number Craig Wiglesworth IoP HEPP/APP Annual Conference UCL March 2010 8

  10. Results of Charge Collection Measurements No sharp drop in the collected charge Smooth fall off after ~ 100 days @ 20 oC SCT Default threshold 1 fC (~ 6.2 ke) Craig Wiglesworth IoP HEPP/APP Annual Conference UCL March 2010 9

  11. Results of Charge Collection Measurements Still collecting significant charge after 1000 days @ 20 oC SCT Default threshold 1 fC (~ 6.2 ke) Craig Wiglesworth IoP HEPP/APP Annual Conference UCL March 2010 10

  12. Results of Charge Collection Measurements Largest increase in charge collection ratio @ 300 V Above ~ 500 V collected charge remains almost constant in time Craig Wiglesworth IoP HEPP/APP Annual Conference UCL March 2010 11

  13. Comparison of Measured VDEP and Charge Collected Craig Wiglesworth IoP HEPP/APP Annual Conference UCL March 2010 12

  14. Comparison of Predicted VDEP and Measured VDEP Data shows a slower annealing effect than the two models Data looks closer to Hamburg model Craig Wiglesworth IoP HEPP/APP Annual Conference UCL March 2010 13

  15. Conclusions and Future Plans A programme for measuring the annealing of CCE in p-in-n microstrip sensors has started. No sharp drop in collected signal when sensor becomes under-depleted. Slow drop in collected signal with annealing time. Significant charge collected even when VBIAS is well below the VDEP predicted by the models. For ATLAS SCT this means operational range defined by VDEP prediction is pessimistic. Next…. Repeat measurements on pair of ATLAS mini sensors irradiated to 3x1014 neq cm-2. Make more detailed comparisons between results and predictions of the two models. (+ Perhaps an attempt at an updated model). Craig Wiglesworth IoP HEPP/APP Annual Conference UCL March 2010 14

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