An APD Readout for EL DetectorsThorsten Lux

IFAE, Barcelona, Spain

Introduction

• R&D started 1-1.5 years ago• Funded mainly by Consolider CUP• Motivation: alternative to SiPMs in the NEXT

detector• APDs known to give similar energy resolution as

PMTs in EL detectors (1 sensor chambers)• What can one achieve with Multi-APD readouts?

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Previous Measurements

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• Large set of studies performed at University of Coimbra

• Excellent energy resolution of < 5% at 22 keV achieved (4-6 bar)

• Only one APD (16 mm diameter)

• Similar performance under identical conditions and similar coverage for APDs and PMTs

• What one could achieve with a multi-APD readout?

Setup

• Kapton coated Copper as cathode• 15 mm drift• 7 mm EL gap• Completely built at IFAE

hot getter

~20 cm

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APDs

• Modified standard APDs from Hamamatsu• Deliverable in large quantities• Size 5x5, 10x10 mm2

• High sensitivity directly to 175 nm• Measurements: 70+/-16 % ( in collaboration

with Coimbra and presented at EXRS conference)

• Chamber was equipped with 5 APDs• Size 5x5 mm2, pitch 10 mm• All APDs were connected to the same

HV channel• Each APD was read out independently

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Electronics

• One APD measurements could be performed with commercial electronics

• Not feasible for more APDs• Low cost readout (preamp/shaper) was designed and built at IFAE• Capable to process signals of several s• Followed by a 8 channel digitizer from CAEN (V1724)• DAQ system based on Midas

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Data Taking

• Triggering only on central APD• 1000 signal events, 10 pedestal events• Converted to root format and analyzed with scripts• Scans of large set of parameters (APD bias voltage, EL field, drift

field) for several pressures• One long run for each pressure

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Common Noise

• Pedestals highly correlated (70%)• Coming from ground and bias voltage

common lines.

Pedestals are Gaussians

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Intercalibration

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0

1

2

3

4

• Plotting charge in the center versus the asymmetry in the side APDs allows intercalibration

• If charge in APD 0 maximal, there should be no asymmentry

• Additional intercalibration from the spectra• Central APD: Minimum when varying E=E0+Eout

• Depends on APD voltage (currently calibration of 400 V applied to all data)

(E1-E3)/(E1+E3)

E0

Position Determination

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• Center-of-Gravity method• APD plane seem to be shifted in one direction/collimator

opening deformed?• We will use in the future a collimator with displaced hole to

test this further• Monte Carlo predicts resolutions better than 1 mm

Y (mm)

X (mm)

Energy Resolution Method

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• Various methods to measure the spectra were tested• Best results with:

• Determining the position of the minimum in the central APD• Integrating +/-200 time bins from this position for all APDs• Dividing by number of time bins• Substracting the pedestal for each APD• Correct for the left-right (up-down) asymmetry

ΣE

(E1-E3)/(E1+E3)

Energy Resolution

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• Position +/- 5mm in respect of the center• Fitting 2 Gaussians (width and position fixed for 25 keV peak wrt 22 keV)• Additional peak at 8 keV -> x-ray fluoroscence of copper• Best result until now: 9.7% FWHM@22 keV (not optimal settings)• Ratio of the two Gaussians in agreement with literature• MC predicts 7% from pure ionization and photon fluctuations

9.7% FWHM=> 0.9%@2.5 MeV

• 1.65 bar• APD: 410 V• EL: 4 kV/cm/bar• Drift: 200 V/cm/bar• Calibration for 400 V

Threshold

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• Applying the same procedure to pedestal (except the minimum/maximum requirment)• Low noise when adding all APDs• Threshold of 2-3 keV seems to be possible• Still improvable -> common noise problem, temperature reduction, etc.

2 Extreme Cases

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• Energy resolution only for central APD and only for outer APDs• No calibration or corrections• Position: +/- 5 mm from the center• Central APD: 17.5 % FWHM• Sum of outer APDs: 12.4% FWHM (conversion happens within the empty space)

17.5% 12.4%

Spatial Energy Distribution

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Conversion in the EL

region

Cd peaks

Cu peak

Cosmics

Pedestals

Drift Field Scan

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50 100 150 200 250 300 350 400 450 500 55020.00

21.00

22.00

23.00

24.00

Gain

50 100 150 200 250 300 350 400 450 500 5500

5

10

15

20

Resolution

0 50 100 150 200 250 300 35014

14.5

15

15.5

16

16.5

0 50 100 150 200 250 300 350.00

5.00

10.00

15.00

20.00

25.00

1.6 bar

1.6 bar

1. bar 1. bar

V/cm/bar V/cm/bar

• Preliminary results• APD bias 405 V• EL Field 3.5 kV/cm/bar

EL Field Scan

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• Preliminary results• APD bias 405 V• Drift Field 200 V/cm/bar• Pressure 1.6 bar

V/cm/bar V/cm/bar

2000 2500 3000 3500 4000 4500.0keV

5.0keV

10.0keV

15.0keV

20.0keV

25.0keV

30.0keV

Gain

2000 2500 3000 3500 4000 4500.00%

5.00%

10.00%

15.00%

20.00%

25.00%

Resolution

APD Bias Voltage Scan

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325 335 345 355 365 375 385 395 405 4150

5

10

15

20

25

30

Gain

APD Voltage [V]

Peak

Pos

ition

325 335 345 355 365 375 385 395 405 415 4250

5

10

15

20

25

30

35

Energy Resolution

APD Bias Voltage [V]

Ener

gy R

esol

ution

[%]

• Preliminary results• Drift Field 200 V/cm/bar• EL Field 3.5 kV/cm/bar• Pressure 1.6 bar

Tracking Chamber

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• In collaboration with CIEMAT, Madrid• Setup for about 30 APDs and 2 PMTs• Active volume: diameter 20 cm, length 12 cm• Aim: evaluating the tracking capabilities + effect of longer drift distances• Design finished and production started

Summary and Outlook

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• An EL setup with 5 APDs was succesfully designed and operated• Suitable for EL applications in which moderate position information and

good energy resolutions are required • Good performance of the Hamamatsu APDs observed• For the case of a DB experiment based on xenon <=1% energy resolution

at 2.5 MeV seems to be achievable• Room for improvements: reduction of noise, larger APDs, higher pressure,

better calibration, temperature corrections, etc.• A chamber with 30 APDs and 2 PMTs is currently constructed to study

tracking and longer drift distances• Tests with Argon planned in the future

Space for Improvements

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• Very new results• Calibration for 410 V• Same run as before

8.8 % FWHM@ 22 keV=> 0.8 %@ 2.5 MeV