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Detector Characterisation Group

A. Boston, D. Cullen, J. Gerl, A. Görgen, S. Gros, K. Hauschild, T. Kröll, J. Ljungvall, N. Saito,

C. Santos, J. Simpson

What is characterisation?

• Specification of AGATA symmetric/asymmetric crystals• Calculation of reference pulse shapes

– Calculation of E-fields– Simulation of interaction points

• Full characterisation of prototype crystals– Detailed source scans

• Define how to characterise detectors– Define reference points

• Standardise scanning set-ups

With contributions from D. Bazzacco, P. Medina and C. Santos

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Why do we need?

• Charge pulse response characteristics for a closed-end coaxial geometry.

• Pulse shape response of each capsule needs to be understood.• Impurity

concentration• Lattice orientation• Surface passivation

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Highly segmented Ge Detectors

16

543

2A B C D

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Segmented Clover detectors

• s

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Automated Scanning Tables

Liverpool System

• Parker linear positioning table• Pacific scientific stepper motors• 0.3mCi 137Cs/0.2mCi 57Co• 1-2mm collimator• Singles/coincidence system

Precise position calibration

GSI / CSNSM Orsay

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Eurisys Mesures 6x6 Segmented Detector

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Charge pulse response

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Detector scan results

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6x6 Risetime Analysis

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Mirror Charge Asymmetry Analysis

rl

rl

QQQQ

A+−

=

• Ql and Qr are magnitudes of mirror charge signal in the left and right neighbour.

• The asymmetry cancels out the radial contribution and yields information regarding the azimuthal position of the main interaction.

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Image charge asymmetry results

1 3

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Energy Calibrated spectra of sector 1

A1 B1 C1 D1

152Eu source, GASP ACQ, high statistics (0.3 MEvs)

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Add-back spectra (all segments)

F1 F2 F3 F4

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F1 F2 F3 F4

0

1

2

3

4

5

6

1 2 3 4 5 6 7Number of Segments

FWH

M (k

eV)

1392

1394

1396

1398

1400

1402

1404

1406

1408

1410

1 2 3 4 5 6 7Number of Segments

Peak

Ene

rgy

(keV

)

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Energy calibration of CC

Using one energy calibrationderived from total CC spectrum

Using 25 energy calibrationsderived from CC spectra incoincidence with segments

If more than one segment fired, CC amplitude is a weighted average of the different calibrations

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Energy correlation between segments

1

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4

32

A B C D

Front

Front-segments: A1-A2

(A1+A2) · (A1-A2)(A1) · (A2)

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Position-dependent (?) energy-losses

344 keV1408 keV

244 keV

122 keV

∆Emax ~ 12%(z-scale is logarithmic)

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90 × 40 mm10º tapering angle

• Prototype symmetric triple cluster• Ordered from EM• Requirement to define segmentation scheme of detector• Database of calculated pulse shapes constructed for

consideration.

Courtesy of A. Görgen and T. Kröll

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Pulse shape database

• Two databases constructed utilising program to generate pulse shapes written by Th. Kröll

• Segmentation schemes considered:– 6 : 10 : 18.5 : 18.5 : 18.5 : 18.5– 10 : 10 : 17.5 : 17.5 : 17.5 : 17.5

• Uniform impurity concentration 1010

• -4kV operating voltage• Centre contact radius 5 mm (16mm depth)• GRETA segmentation scheme (for comparison):

– 10 : 12 : 16 : 18 : 20 : 14– Core depth 9mm

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Calculation of pulse shapesCalculation of pulse shapes

FEM-modelof detector

• Calculation of the signals induced on the contacts using the weighting field method

Calculate weightingfields

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Calculation of pulse shapesCalculation of pulse shapes

1.130.940.63

0.310.0

z[cm]

0˚ 7.5˚ 15˚ 22.5˚27˚

ϕ

A 0.55B 1.0

r [cm]

C 1.45D 1.9

E 2.35F 2.8

G 3.25H 3.7

net charge signals

-0.2

0

0.2H

GF

E

D C B A

-0.2

0

0.2

100 200 300

rel.

am

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100 200 300t [ns]

-1

-0.75

-0.5

-0.25

0

A

BCDEF

GH

-1

-0.75

-0.5

-0.25

0

100 200 300

rel.

am

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100 200 300t [ns]

∗

transient signals

•

•

•

•

•

•

• •

••

•

••••

••

•••••••

• •

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Pulse shape database

• A 1mm grid was used to evaluate the pulse shapes.

• Following cylindrical symmetry arguments interactions only in seg. B → 25,000 grid points were used.

• Net and image charge considered.• 662 keV photons from 137Cs

considered.

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Sensitivity

• Total average sensitivity:

• χ < 1 : Difference in signal is less than noise• χ = 1 : Difference in signal is noise• χ > 1 : Difference in signal is well above noise

2z

2y

2xS χ+χ+χ=

∑ ∑= σ

δ−=χ

i

Tp

0t2

2tt2

x2

))z,y,x(,q)z,y,x(i,q(

Charge collected at time t in segment i for interaction (x,y,z)

Segment that collects net charge and eight direct neighbours

Integration over time in steps of 1ns

Noise factor 1% of total charge

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Sensitivity

• Pulse shapes produced with 1ns precision:– SHOW EXAMPLES

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Sensitivity

• Demonstration of sensitivity: the position sensitivity peaks at the (effective) segment borders

• Regions near the outer surface between segment borders have the poorest sensitivity

total

dz

dy

dx

high

low

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Sensitivity: Conclusions

• Problematic zones:– front corners– back surface

• Very thin front segment reduces effective size of this segment only slightly:

• depth: 6 mm → effective volume 26,000 mm3

• 10 mm → 32,000 mm3

• Second segment can be made larger without significant loss of performance

• Reasonable compromise for segmentation in depth: – 8, 13, 15, 18, 18, 18 mm

• GRETA segmentation scheme (for comparison):– 10 : 12 : 16 : 18 : 20 : 14– Core depth 9mm

http://www-dapnia.cea.fr/Sphn/Deformes/Agata/local/index.shtml

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Design of detectorDesign of detector

• Layout of segments:• Enhance amplitudes of transient

signals• Counting rates per segment• Capacitance of segments• Equalise mean charge collection

times • Folding Algorithm

• Shape of front part of detector• Length of inner hole• We would prefer a more semi-

spherical shape of the front part of the crystal!

(Liverpool/Milano/CSMSM Orsay)

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AGATA capsuleAGATA capsule

• The impact of effective segmentation

0 1 2 3 3.8 0 1 2 3 3.80

10

0 1 2 3 3.80

10

z [cm]

0 1 2 3 3.80

10

tapering angle = 8° r [cm]

geometrical segmentation

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TIGRE simulations

• TIGRE e-drift velocity simulation

Liverpool, Strasbourg, Upsalla collaboration

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TIGRE simulations

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TIGRE simulations

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Conclusions

Status as of today:

• Specification of AGATA symmetric/asymmetric crystals• Calculation of reference pulse shapes

– Calculation of E-fields– Simulation of interaction points

• Full characterisation of prototype crystals– Detailed source scans existing/new detectors

• Define how to characterise detectors– Define reference points

• Standardise scanning set-ups