Radiation Damage Effects in the NA60 Silicon Pixel Detectors

1. Radiation Damage Effects in the NA60 Silicon Pixel Detectors • The NA60 Experiment • The NA60 Silicon Pixel Detector • Predictions of Radiation Damage • Fluence in the 2003 physics run • Leakage Current • Depletion Voltage • Measurements in the 2003 Physics Run • Depletion Voltage • Leakage Current • Predictions for 2004 • Summary Markus Keil, CERN

2. NA50 melting of directly produced J/ ? Questions Left Open by Previous SPS Experiments • What is the origin of the intermediate mass dimuon excess ?Thermal dimuons produced from a QGP phase ? • Is the open charm yield enhanced in nucleus-nucleuscollisions ? How does it compare to the suppressionpattern of charmonium states ? • Which physics variable drives the onset of ’, c andJ/ suppression ? Energy density ? Cluster density ? • What is the normal nuclear absorption pattern of the c ? NA50 melting ofcc? New and better measurements are needed ! Markus Keil, CERN

3. Beam Tracker vertex tracker Pixels Overview of the NA60 Experiment Muon Spectrometer 2.5 T dipole magnet beam ZDC +Quartz Blade Interaction Counter Muon track matching through the absorber m vertex pm offset } < 1 mm Micro-Strips D m Muon track offset measurement :Separate charm from prompt dimuons Markus Keil, CERN

4. The Silicon Pixel TelescopePhysics Run 2003, 158 GeV/nucleon Indium Beam on Indium Targets • Beam tracker, target and silicon pixel telescope in the dipole magnet gap in front of the hadron absorber ×8 ×8 Markus Keil, CERN NA60 pixel detector planes.

5. The Silicon Pixel Detector • Single chip assemblies: • ALICE1LHCb readout chips • Deep submicron, radhard design • Tested up to 12 Mrad • ALICE Pixel sensors • p-on-n silicon • 15kWcm • Assemblies placed around beam hole ( 6 mm) Markus Keil, CERN

6. Physics Run 2003 • 158 GeV/nucleon Indium ions on Indium targets • Beam Intensity measured with beam tracker • In total ~ 5·1012 ions on 20% lint target Markus Keil, CERN

7. Simulation of the Radiation Dose • Back of the envelope calculation from simulation of charged tracks • Strong R-dependence • Weak Z-dependence • Fluence of several 1013 in the innermost pixels • Difference of a factor 10 within one sensor chip Markus Keil, CERN

8. Radiation Damage, Expectations • Leakage current: • Growing with accumulated beam intensity. Higher for pixels closer to the beam axis (like the fluence). • Depletion voltage: • Depletion voltage decreases; faster for pixels close to the beam axis. • After a certain fluence, bulk converted to p-type material. This will happen earlier for the pixels close to the beam axis while for the outer pixels it might not occur at all. • The depletion voltage of the type-inverted pixels will rise; again faster for the pixels close to the beam axis. Markus Keil, CERN

9. Damage Prediction: Reverse-bias Currrent Complete Plane: Single Pixels: • Leakage current should reach ~50µA per Plane, ~10 nA in the worst pixels (at 20°C) Markus Keil, CERN

10. Damage Prediction:Depletion Voltage Outermost pixels, not yet type-inverted Pixels that are just at the point of type-inversion Inner, type-inverted, pixels with rising depletion voltage Markus Keil, CERN

11. Fluence Measurement • Dosimeter plane placed inside the vertex telescope at Z=40 cm • Measurements done with help from M. Moll and M. Glaser (CERN PH/TA1) • Measurement with Al activation (22Na) • Agrees well with track simulation • Measurement with Si pin diodes • Much higher fluence measured • Slow neutrons? (not seen by Al measurement and Track simulation) • FLUKA Simulation • Includes slow, back-scattered particles • Much better agreement with pin diodes • Of course agreement to this level is coincidence, however, we can be confident that our estimates are in the right ball-park Markus Keil, CERN

12. 150 V 50 V 80 V 60 V 100 V 40 V 30 V Measurement of Depletion Voltage • Assumption: After type inversion only fully depleted pixels work (original p-on-n pixels  p-on-p after type inversion) • Simulation: Bias voltage should increase for smaller radii • Lowering the bias voltage should leave an ever larger area undepleted (i.e. not working) • Occupancy Measurements at decreased bias voltage Markus Keil, CERN

13. Normalised Occupancy • Occupancy divided by occupancy at full depletion (150V) • No change in the outer pixels • Close to the beam axis the occupancy drops Type inverted, undepleted pixels • The radius of the drop increases for lower bias voltage Depletion voltage depends on the radius To get a quantitative measure: Interpret 50% point (erf-fit) as radius where Udep = 40 V Markus Keil, CERN

14. Depletion Voltage vs. Radius • Bias voltage scan, radii extracted as described before • Qualitative behaviour Udep(R) agrees well with simulation • Quantitative disagreement similar to the one observed in the fluence measurement Markus Keil, CERN

15. Measurement of Leakage Current • Leakage current and temperature measured throughout the whole run • After temperature correction (I  T2 exp( -Egap / 2kT) ) the leakage current follows very nicely the integrated beam intensity Markus Keil, CERN

16. Fluence Expected for pA Run in 2004 • 2004: High intensity protonn run (~10 weeks) • Fluence of 1013neq/cm2/week expected • Up to now everything working fine, but spare planes ready to be exchanged Markus Keil, CERN

17. A Really Damaged Plane (September 2004) • The „worst“ plane at a bias voltage of 30 V • Online plot from this year‘s proton run • Large number of pixels type inverted • But: still working fine at high enough depletion voltage (250 V) Markus Keil, CERN

18. Summary • A silicon pixel telescope has been operated in a harsh radiation environment. • Fluences of several 1013 neq/cm2 have been accumulated in 2003, and similar doses in 2004. • A simple occupancy measurement can make the effect of type inversion visible. • The change of depletion voltage and reverse-bias current are similar to the expectations. • Up to now the telescope is working fine, although the initially p-in-n type sensors are partially type inverted. Markus Keil, CERN