Performance test of STS demonstrators

Anton Lymanets

15th CBM collaboration meeting, April 12th, 2010

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OutlineOutline

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.

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The demonstratorsThe demonstratorsFEB 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.

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Energy calibrationEnergy calibrationis 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.

CV 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.

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Energy calibration with Energy calibration with 241241AmAmUsing 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

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Calibration lineCalibration line

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

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Controlling detector depletionControlling detector depletion

X-ray characterization:count rate vs. bias Current-voltage measurement

241Am x-rays measured on p-side

Method works for • p-strips before type inversion or • n-strips after type inversion

Q6 Q6

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ADC response to the MIPsADC response to the MIPs

Landau peak with maximum corresponding to ~16.5 ke-

Expectation in ~300 μm Si: 23 ke-

Landau + Gaussian component

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Pedestal position crosses zero at ~1.4 VDynamic range is reduced!Indirect measurement: VbiasS is measured in test channel, not in ch.46

Measured inFEB B03,ch. #46

Pedestal positionPedestal position

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Peak1: 7.2 ke-Peak2: 16.3 ke-

Amplitude = peak position - pedestal

Hitting the lower rail Hitting the upper rail

Peak amplitudesPeak amplitudes

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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 outputmay cause pedestal ≠ 0

The role of peak detectorThe role of peak detector

Observed pedestal/peak shifts are not reproducible Observed pedestal/peak shifts are not reproducible in device simulationsin device simulations

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n-XYTER chipn-XYTER chip

Inpu

t pa

ds

Out

put

pads

Power lines

Power lines

current

current

Channel 127

Channel 0Test channel

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Pedestal profile over channelsPedestal profile over channels

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

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Crosstalk problem – looking into the test Crosstalk problem – looking into the test

channelchannel

Look with scope into test channel and fire pulses in its neighbor ch. 0

analog part digital part

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Default chip settings, test pulses in 32 Default chip settings, test pulses in 32 channelschannels

Questions: digital or analog pickup? dependence on channel number? local effect?

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Part of the channels masked, Part of the channels masked, pulses in 16 channels.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

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Does the effect have analog nature?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

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Crosstalk problem – using laser pulsesCrosstalk 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.

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?

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? AdvantageAdvantage:

Study crosstalk in the chip avoiding crosstalk in the detector.

detectorread-out chip

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Channel hit occupancyChannel hit occupancy

Channel number

Cou

nts

• Big laser spot - equal number of hits in each channel.• Hit count rate corresponds to pulse rate.

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ADC distributionADC distribution

Raw spectrum Baseline subtracted. Line shape corresponds to intensity distribution in the laser spot.

Using automatic baseline correction

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Method: look at signal peak mean Method: look at signal peak mean and RMS with noise presentand RMS with noise present

High noise Low noise

With low noise in the channel of interest the observed effects are With low noise in the channel of interest the observed effects are caused by increased occupancy in other channelscaused by increased occupancy in other channels

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Signal mean and RMS Signal mean and RMS vs. n-XYTER occupancyvs. n-XYTER occupancy

Channel 69Channel 69

Signal peak width increases vs. total chip occupancy.Signal peak width increases vs. total chip occupancy.

Signal amplitude drops down linearly with increasing Signal amplitude drops down linearly with increasing total count rate.total count rate.

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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.

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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

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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).