Asymmetry of QADC signals in single PbWO4 crystal

Cosenza INFN Group( L. La Rotonda, E. Meoni, A. Policicchio, G.

Susinno,T. Venturelli)

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Summary

GOAL: The goal of this work is to measure the left-right asymmetry of QADC signals from single PbWO4 crystal and then estimate the (average) Cerenkov contribution both for showers at different depths and for mips.This study is related to the paper :“Separation of crystal signals into Cerenkov and scintillation components”.

DATA SETS: we have studied QADC data taken at different angles with: electrons of 50 GeV both without Pb in front to the crystal and with different Pb layers in front to the crystal;Pions of 70 GeV without Pb in front to the crystal.The data are collected at room temperature.

METHOD: We have computed asymmetry versus angle both from the mean of QADC spectra and event by event

OBSERVATION: Since asymmetry depends on beam impact point position, we have triedto study also this dependence for showers in the early stage using electron of 50 GeV atdifferent impact point positions

Pb

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Showers in the early stage

Showers in differentstages

Pions

Showers in the early stageat different impact points

Data sets

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50 GeV electron showers in the early stage

(ADC counts)

(CH 25)

(CH 24)

ADC spectra after pedestal subtraction

(ADC counts)

Applied a cut to suppress the pedestal and eventually the mip background in the beam

0° - 15° - 30° - 60° 0° - 15° - 30° - 60°Channel 25 Channel 25

>0°

=2.2cm=2.5 X0

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The ADC signal increases with the angle, because increases the track length. For positive angles the CH25 signal is larger than CH24 one because CH25 collects a larger Cerenkov component (vice versa for negative ones).

But at fixed positive and negative corresponding angles, the difference between the signal of the 2 CHs is not equal, this is evident also in asymmetry slope.

We have equalized ADC signals of the two channels at 00

ADC CH25ADC CH24 (CH 25)

(CH 24)

AD

C M

ean

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(CH25)ADC(CH24)ADC(CH24)ADC(CH25)ADC

Asymmetrymeanmean

meanmean

ADCmean is the mean of the ADC spectra shown before

The first method (asymmetry from mean of ADC spectra)

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We have computed asymmetry also

event by event at a fixed angle

H25)ADCcount(CH24)ADCcount(C

H24)ADCcount(CH25)ADCcount(CAsymmetry

00 150 300 450

Taken mean value of the asymmetry distribution for each angle

Errors of the plot are RMS of asymmetry distributions

The second method (asymmetry event by event)

Asymm

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The two methods give the same results

Asy comp. evt by evtAsy comp. from ADC spectra

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10mm of Pb

35mm of Pb(ADC counts)

(ADC counts)

(ADC counts)

0°-15°-30°

4mm of Pb

20mm of Pb

50 GeV electron showers at different stages

25mm of Pb

(ADC counts)

(ADC counts) (ADC counts)

ADC spectra after pedestal subtraction and after a cut to suppress the pedestal and eventually the mip background in the beam

Pb

Channel 25

=0.4Pb cm->3.5 Pb cm =0.7 X0-> 6.25 X0

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Asymmetry computed event by event

Asymmetry computed with mean of ADC spectra

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Asymmetry difference between positive and negative angles

From asymmetry results seems that thereis a shift between the “real zero angle” and the “nominal zero angle”

Shift of 2°Shift of 5°

The shift is close to 2° (no data taken at -1° -3°,-4°to perform other checks)

Differences in asymmetry observed for corresponding positive and negative angles

We tried to fix this angle:Equalized the 2 ADC channel data at -2°(-5°)fixed_angle= nominal_angle +2°(fixed_angle=nominal_angle+5°)

Negative anglesPositive angles

Setup without Pb

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Before angle correctionAfter angle correction

Before angle correction After angle correction

First method

Second method

First method

Second method

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Positive angles Negative angles

Cerenkov component estimation

Asymax(30°)=7.0%

Asy(30°)= 7.5%

Asymax(26°)= 7.6%

Error computed from event by event Asymmetry width 5)%(13)(30

SignalCerenkov

3)%(7Asym

Asym)(1

2Asym)(30

SignalCerenkov

stage early the In

10GeV of ele of testbeam previuos of result 6)%(13)(30Signal

Cerenkov

Fit performed with a second-degree polynomial

Asymmetry at 30° (mean between positive & negative)

Cerenkov comp at30° from asymm

Positive & negative angles

Asymax(30°)=7.2%

Asymmetry at 30° (from the global fit)

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70 GeV pions

(ADC counts) (ADC counts)

ADC spectra after pedestal subtraction

Channel 25

(CH 25)

(CH 24)>0°

=2.2cm=2.5 X0

0° - 15° - 30° - 60°-75°

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Computed asymmetry event by event applying different cut of large ADC counts

Before angle correction After angle correction

Cut on 200 countsCut on 300 countsCut on 500 counts

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Taken mean value of the asymmetry distribution for each angle

Errors of the plot are RMS of asymmetry distributions

00 150 300 450

Asymmetry distribution event by event

Asymm

Angle (deg)

Asy

mm

2° 32° 47° 67°

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Positive angles Negative angles

Positive & Negative angles

Cerenkov component estimation

Asymax(38°)= 12.1% Asy(30°)= 11.5%

Asymax(33°)= 11.9% Asy(30°)= 11.8%

Asymax(33°)= 11.7% Asy(30°)= 11.7%

Cerenkov fraction is about 20% at 30°

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Asymmetry versus beam impact potision

Taken a data set with electron of 50 GeV with a beam horizontal scan position from one crystal end to the other at step of 1 cm

CH24CH25

Beam at 8 cmBeam at 3 cm Beam at 0 cmBeam at -3 cmBeam at -6 cm

ADC spectra after pedestal subtraction

Effect due to ADC saturation

(CH 25)

9 0 -9 (cm)

(CH 24)

xRL

We tried in any case to compute Asymmetry event by event

(ADC counts) (ADC counts)

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(CH 25)

9 0 -9 (cm)

(CH 24)

xRL

The asymmetry variation is 0.55%/cm

(CH25)ADC(CH24)ADC(CH24)ADC(CH25)ADC

Asymmetrycountcount

countcount

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Conclusions

We have computed asymmetry with QADC data:

For electron showers in the early stage we obtained a maximumasymmetry of (7± 3)% around 30°corresponding to Cerenkovfraction of (13± 5)%.As expected, the asymmetry as a function of the shower depthdecreases.

For mips (pions) we obtained a maximum asymmetry of 12%corresponding to Cerenkov fraction of 20%

The results are compatible with the ones of the previous testbeam

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We have repeated event by event analysis applying some cuts on the beam position

(CH 25)(CH 24)

x

RL

y

The beam is all contained in x (horizontal coordinate) and is larger than the crystal in y (vertical coordinate)Selected only events that cross crystalaround x_crystal_center ± 0.5 cm(assumed x_crystal_center=x_DREAM_centertrue with a precision of 1 cm) y_crystal_center ± 0.25 cm(y_crystal_center determinated by the beamprofile)

Asymmetry with cut on beam position

Without cut on beam positionWith cut on beam position

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Third method

30-(CH25)(CH24)30(CH25)(CH24)

30-(CH24)(CH25)30(CH24)(CH25)

]ADCADC]ADC[ADC]ADC[ADC]ADC[ADC

Asy[

We have also computed asymmetry using only data at 30° and -30°

ADC(CH) is the mean of ADC spectrum

)%.()( 51430Signal

Cerenkov stage early the In