1. Performance test of STS demonstrators Anton Lymanets 15 th CBM collaboration meeting, April 12 th, 2010

2. 2 Outline Demonstrators tested so far. n-XYTER energy calibration. ADC response. Pedestal position and effective amplitudes of  -peaks. Pedestal profile dependence on current consumption. Crosstalk studies.

3. 3 The demonstrators “D boards” FEB rev. B, C, (D) Q boards Detectors (CBM01B2, CBM02B2s) FEB rev. B: Every second channel bondable. Still good for lab tests for timing studies or ADC response (without clustering). FEB rev. C: All channels are usable But thermal stability becomes an issue. Detector-FEB cable: Turns out to work if shielded properly. Detectors of CBM01 and CBM02 type “behave” similarly (bad), poor charge collection at n-sides. FEB 4nx: Cooling plates improve thermal stability Problems with surviving potential of the chips on board. Beam time : vastly different count rates in different stations caused by the beam. Conclusion: depletion conditions should be controlled carefully.

4. 4 Energy calibration is important for beam time data analysis and estimates of signal-to-noise performance, charge collection efficiency, etc. Cons: capacitance is small => strongly depends on stray capacitance. C VV QQ  Q=C∙  V Create known voltage step over known capacitance Use x-rays with Si strip detector Pros: well defined energy. Range of 59 keV electrons is ~ 15 μm => full  energy is absorbed.   

5. 5 Energy calibration with 241 Am Using 300 μm pitch detector => no significant charge sharing Energy gain = 110.6 e - /ADC cnt + one can obtain pedestal energy (not necessarily zero) Noise 460 e - @ 6 pF

6. 6 Calibration line Energy calibration is obtained, but extrapolated pedestal amplitude is ~3 kElectrons. Possible reasons: non-linearity, bias due to peak detector.

7. 7 Controlling detector depletion X-ray characterization: count rate vs. bias Current-voltage measurement 241 Am x-rays measured on p-side Method works for p-strips before type inversion or n-strips after type inversion Q6

8. 8 ADC response to the MIPs Landau peak with maximum corresponding to ~16.5 ke - Expectation in ~300 μm Si: 23 ke - Landau + Gaussian component

9. 9 Pedestal position crosses zero at ~1.4 V  Dynamic range is reduced! Indirect measurement: VbiasS is measured in test channel, not in ch.46 Measured in FEB B03, ch. #46 Pedestal position

10. 10 Peak1: 7.2 ke - Peak2: 16.3 ke - Amplitude = peak position - pedestal Hitting the lower rail Hitting the upper rail Peak amplitudes

11. 11 Peak detect & hold in out Peak detect & hold circuit “remembers” the maximum amplitude and keeps it until it is transferred to analog FIFO. Offset in peak detector output may cause pedestal ≠ 0 The role of peak detector Observed pedestal/peak shifts are not reproducible in device simulations

12. 12 n-XYTER chip Input pads Output pads Power lines current Channel 127 Channel 0 Test channel

13. 13 Pedestal profile over channels Pedestal “sag” is observed with maximum in channel #64  To be addressed in the upcoming engineering run done in Heidelberg Univ. (H. K. Soltveit)

14. 14 Crosstalk problem – looking into the test channel Look with scope into test channel and fire pulses in its neighbor ch. 0 analog part digital part

15. 15 Default chip settings, test pulses in 32 channels Questions: digital or analog pickup? dependence on channel number? local effect?

16. 16 Part of the channels masked, pulses in 16 channels. Channels 0..63 masked Channels 64..127 masked Crosstalk is not related to activity in neighboring channel, but to number of active channels => Non-local effect

17. 17 Does the effect have analog nature? cal = 128 Change test pulse height (cal setting) cal = 256 Very small effect of amplitude seen => Mostly digital effect

18. 18 Crosstalk problem – using laser pulses XT - signal transmitted in one channel creates undesired effect in another channel. Crosstalk in detector vs. crosstalk in read-out chip. n-XYTER review meeting: December 11th, 2009. The way to go: create signals using laser in isolated channels, look for the response in neighboring channels. ? ? ? ? Advantage Advantage: Study crosstalk in the chip avoiding crosstalk in the detector. detector read-out chip

19. 19 Channel hit occupancy Channel number Counts Big laser spot - equal number of hits in each channel. Hit count rate corresponds to pulse rate.

20. 20 ADC distribution Raw spectrum Baseline subtracted. Line shape corresponds to intensity distribution in the laser spot. Using automatic baseline correction

21. 21 Method: look at signal peak mean and RMS with noise present High noise Low noise With low noise in the channel of interest the observed effects are caused by increased occupancy in other channels

22. 22 Signal mean and RMS vs. n-XYTER occupancy Channel 69 Signal peak width increases vs. total chip occupancy. Signal amplitude drops down linearly with increasing total count rate.

23. 23 Conclusions I Energy calibration has been done. Energy gain = 110 e - /ADC count. Pedestal “amplitude” depends on VbiasS voltage. Conditions for pedestal zero “amplitude” have been determined, but then the dynamic range is reduced.

24. 24 N-XYTER Gain depends on VbiasS voltage. “Sag” in pedestal profile depends on chip power consumption => to be addressed in n-XYTER engineering run. Crosstalk in the chip has digital nature and depends on overall chip activity. Conclusions II

25. 25 Conclusions III – Demonstrator Systems Problems with charge collection in CBM01 and CBM02 – need to control sensor bias. Front-end boards: no operational 4nx- boards and few 1nx-bords are left (this poses a threat to the upcoming beam time).