AGATAAGATAAdvanced Gamma Tracking ArrayAdvanced Gamma Tracking Array

Determination of position-dependent pulse Determination of position-dependent pulse shapes of Ge detectorsshapes of Ge detectors

N.GoelN.Goel,,T. Engert, J. Gerl, C.Domingo,T. Engert, J. Gerl, C.Domingo,

I. Kojouharov, H. Schaffner,S. TashenovI. Kojouharov, H. Schaffner,S. Tashenov

Outline of the talkOutline of the talk

1. Introduction Fundamental parameters of -detector arrays

1. Total photopeak efficiency 2. Energy resolution -> - spectroscopy with relativistic beams

2. AGATA 1. Tracking

2. Pulse shape analysis

3. Scanner at GSI 1. Principle

2. Experimental position dependent pulse shapes

4. Limitation and improvements in first prototype

1. Tests with LYSO crystal scintillator

2. New scanning approach

5. Conclusions

IntroductionIntroductionPhotopeak efficiency-high multiplicity Photopeak efficiency-high multiplicity -ray -ray cascadescascades

BGOshields

Ge

Suppress the Compton scattered events

30% of total solid angle covered by Ge

Adjacent Gecrystals operated in ADD BACK mode

For high multiplicity M, wrong summing of energies takes place

Discrimination between scattered events and individual hits possiblewith TRACKING

COMPOSITE DETECTORS SEGMENTED DETECTORSSHIELDED DETECTORS

IntroductionIntroductionEnergy resolution: Energy resolution: spectroscopy with spectroscopy with relativistic beamsrelativistic beams

SegmentedDetectors

Reduced solid angle with segmentation/ ~ sin( Φ)•

∆Φ

Intrinsic resolution 2.5keV@ 1.3MeVDoppler Effect@(v/c) ~ 0.5

First interaction point hasposition resolution proportional to the size of segment

Analysis of segment pulse shapesgives position resolution even smaller than the size of segment

Φ

∆Φ

∆Φ

∆Φ

Φ

Outline of the talkOutline of the talk

1. Introduction Fundamental parameters of -detector arrays

1. Total photopeak efficiency 2. Energy resolution -Spectroscopy with relativistic beams

2. AGATA 1. Gamma ray tracking2. Pulse shape analysis

AGATAAGATAAAdvanced Gamma Tracking dvanced Gamma Tracking ArrayArray

4π gamma tracking array for nuclear physics experiments at European accelerators providing radioactive and highly intense stable beams

61.08mm

52.0

9m

m

60o

90 m

m

10o

0

612

24

18 713

19

311 25

814

20

2632

2

159

333

27

21 1016

22

28

344

1117

23

355 2930

10o

AGATAAGATAGamma ray trackingGamma ray tracking

Recognize the individual 3D interaction points

Reconstruct the track of photon using Compton scattering formula

Full energy events distinguished from scattered events => improved photopeak efficiencyDetermining the incoming direction with a very good position resolution( 2-3mm)

Doppler shift in energyfor v/c ~ (0.2 -0.5)

E' = E ( 1- (v/c) cos )

Φ

AGATAAGATAPulse Shape AnalysisPulse Shape Analysis

time(ns)time(ns)

Radial dependence of pulse shape

For each event, 37x100 samples of

preamplifier signals

Digital electronics to record the segment

signals

am

plit

ud

e(a

dc

un

its)

am

plit

ud

e(a

dc

un

its)

Improvement in Doppler Improvement in Doppler correction with segmentation correction with segmentation and PSAand PSAIN-Beam test at Cologne, GermanyIN-Beam test at Cologne, Germany

1. The center of the detector.

2. The center of the firing segment.

3. Position given by pulse shape analysis.

48Ti(d,p)49Ti 100MeV

Det = 32 keV1382 keV Seg = 11 kevDet = 32kevPSA = 5.5 keVSeg = 11 keVDet = 32keV

Energy resolution

F. Recchia , ACTA PHYSICA POLONICA B Vol.38(2007)

∆Φ

Energy(keV)

Cou

nts

SCANNERSCANNERPSA requires a system to scan the detectorPSA requires a system to scan the detector

For interaction at each 3D point inside a segmented detector, there corresponds a unique pulse shape.

A data base containing pulse shapesfor all possible interaction

position inside the detector

An efficient system to scan thedetector

Outline of the TalkOutline of the Talk

1. Introduction Fundamental parameters of -detector arrays

1. Total photopeak efficiency 2. Energy resolution - Spectroscopy with relativistic beams

2. AGATA 1. Tracking

2. Pulse shape analysis

3. Scanner at GSI 1. Principle

2. Experimental position dependent pulse shapes

4. Limitation in first prototype

1. Tests with LYSO crystal scintillator

2. New Scanning approach

5. Conclusions

SCANNER PRINCIPLESCANNER PRINCIPLEDetermination of incoming direction of photonDetermination of incoming direction of photon

22Na → 2γ, 511 keV

256x scintillatorfibres

4x 64 channel PMTs

Ge detectorto be scanned

SCANNER PRINCIPLESCANNER PRINCIPLEDetermination of incoming direction of photonDetermination of incoming direction of photon

X = (b+c)/(a+b+c+d) Y = (a+b)/(a+b+c+d)-> A matrix is reconstructed which gives map of (X,Y).

X(arb units)

Y(a

rb u

nit

s)

a

b

d

c

SCANNER PRINCIPLESCANNER PRINCIPLEDetermination of Z coordinate of interaction positionDetermination of Z coordinate of interaction position

22Na → 2γ, 511 keV

256x scintillatorfibres

6 ring slits

42 BGOs

4x 64 channel PMTs

False Event

SCANNER PRINCIPLESCANNER PRINCIPLEEnergy spectra under triple coincidence and Agata Energy spectra under triple coincidence and Agata energy gateenergy gate

Even under triple coincidence events its possible to have false events which can be filtered by applying gate on AGATA energy.

511keV511keV

false triggering of BGO

90o scattering

True Event

Energy(keV)

Counts

6 7 9 10 11

12

8

13 14 15 16 17

18 19 20 21 22 23

time(ns)

am

plit

ude(A

DC

unit

s)

14

SCANNERSCANNER Experimentally determined pulse shapes Experimentally determined pulse shapes

15

20 21

SCANNERSCANNERRadial position informationRadial position information from net charge from net charge collecting segmentcollecting segment

1. Pulse rises slowly for interaction occuring close to central or outer electrode.

2. Pulse for interaction near central electrode is convex while concave for the interaction near outer electrode.

3. Pulse rises faster for interaction occurring in the middle of the segment.

1

23

1

1 2

3

time(ns)am

plit

ude(A

DC

unit

s)

time(ns)

am

plit

ude(A

DC

unit

s)

SCANNERSCANNERRadial position information from polarity image Radial position information from polarity image chargecharge

+ve -> Interaction close to outer electrode -ve-> Interaction close to the centre electode

time(ns) time(ns)am

plit

ude(A

DC

unit

s)

SCANNERSCANNERAzimuthal position information from amplitude of Azimuthal position information from amplitude of image chargeimage charge

Segment fired

The height of pulses from the segments close to the segment which is fired for a given event is strongly related to the point

at which the interaction tookplace.

time(ns)

am

plit

ude(A

DC

unit

s)

Pulse shapesfrom neighbouring segment

SCANNERSCANNERSummarySummary

2 types of signalshave to be analysed

net charge signal from hit segment

Transient charge signal induced in neighbouring segment

Radial position is given by rise time and shape of pulse

Azimuthal position

Radial position

Outline of the TalkOutline of the Talk

1. Introduction Fundamental parameters of -detector arrays

1. Total photopeak efficiency 2. Energy resolution - Spectroscopy with relativistic beams

2. AGATA 1. Tracking

2. Pulse shape analysis

3. Scanner at GSI 1. Principle

2. Experimental position dependent pulse shapes

4. Limitation and improvements in first prototype

1. Tests with LYSO crystal scintillator

2. New Scanning approach

5. Conclusions

Limitation in the first scanner Limitation in the first scanner prototypeprototype

1. Efficiency of fibres???Most of the Compton electrons leave the active volume of fibres .Due to length of Fibre(8cm) light collection suffers.Much lower count rate contrary to expectation.

2. Low count rate problemThe true coincidence rate for back segments is about 1 count/hour.

BGO

90mm

10mm

1 ct/hr

40ct/hr

Replacement of fibres with Replacement of fibres with LYSO/BGOLYSO/BGO

Position sensitive PMT

Scintillator plate

Tests with LYSO crystal Tests with LYSO crystal scintillatorscintillator

-> Position resolution of LYSO plate

->Wrapping the upper surface of cyrstal for better light collection

Tests with LYSO crystal scintillatorTests with LYSO crystal scintillator POSITION RESOLUTIONPOSITION RESOLUTION

6cm

6cm

3 mm thick

Position sensitive PMT

0.3 mm

LYSOReference detector

SETUP FOR MEASUREMENT OF POSITION RESOLUTION

7.6 cm

Energy gate on LYSOand reference detector

to filter false events

Tests with LYSO crystal scintillatorTests with LYSO crystal scintillator Position ResolutionPosition Resolution

X

Ywidth

2.3mm

Pulse height(channel number) Pulse height(channel number)

cou

nts

cou

nts

Y(arb units)

cou

nts

511 keV 511 keV

Tests with LYSO crystal scintillatorTests with LYSO crystal scintillator Position ResolutionPosition Resolution

74.0 74.5 75.0 75.5 76.0 76.5 77.0 77.5 78.0 78.5 79.0 79.5 80.0

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Y m

easu

red

YReal

Distortion near the edges of plate.Linear central region is approximately 3cm.An accurate position calibration, allows to determine the spatial resolution (in mm). It has an average value of 2.5 mm.

surface of LYSO 0.3mm

reference detector

moved in steps of 1mm

Measured position of sourceCalc

ula

ted p

osi

tion o

f so

urc

e

Tests with LYSO crystal scintillatorTests with LYSO crystal scintillator 2. Wrapping material for better light collection2. Wrapping material for better light collection

•Signal amplitude is higher with Enhanced Specular Reflector(ESR) film as compared to Teflon by a factor of ~ 1.32(30% light collection improvement)

•Higher reflectance increases light collection efficiency.

•Finally the upper surface of LYSO is covered with combination of Teflon and ESR.

Amplitude

TEFLON

ESR

Peak pos.= 346.591

Peak pos.= 457.365

IImprovements possible:mprovements possible:

Circular and bigger scintillator crystals(LYSO/BGO)

Edge effects

32 anode signals of HAMAMATSU R2486

Edge effectsPosition accuracy

0.1 mm precision X-Y table Efficient and reliable measurements

NEW SCANNING APPROACHNEW SCANNING APPROACH

X-Y detector

Ge-DetectorSide scanning detector

Recording the pulse shapes for two positions (a) and (b).Comparing the pulse shapes from two data setsSignal of one set will be identical to signal of other set at the crossing points.

NEW SCANNING APPROACHNEW SCANNING APPROACH

Na-22 source

Na-22

Position sensitive Detector

a)

b)

90o

Rotated by 900

Conclusions:Conclusions:

1. The next generation of arrays for in-beam -ray spectroscopy will be based on the concepts of pulse shape analysis (PSA) and - -ray tracking.

2. Using pulse shape analysis , we can get the position of first interaction point within the hit segment of a multisegmented detector.

3. Experiments have been done already to demonstrate the Doppler correction capability of AGATA detector.

4. GSI implemented a first detector scanner based on principles of PET.

5. Clear capability to obtain the scanning information was demonstrated.

6. Low efficiency of fibre detectors resulted in scanning 10 times slower than originally expected.

Continued....Continued....

7. For this reason , we plan to replace fibres by crystal scintillators.

8. Test done with LYSO coupled to a position sensitive PMT show a position resolution of ~ 2.5 mm and an efficiency of 10% at 511 keV.

9. Pulse shape comparison procedure is proposed for measuring the HPGe detector pulse shapes as a function of the -ray interaction position inside the detector volume.

DANKE SCHOEN

HAMAMATSU R2486HAMAMATSU R2486

Multianode wires crossing each other in X-Y directions.

12 stage coarse mesh dynode structure.

Anode signals taken from 4 corners labelled as Xa,Xb,Ya,Y

->variables (X,Y) are calculated usingX =(Xa/Xa+Xb) Y=(Ya/Ya+Yb)

->A matrix is reconstructed which gives map of (X,Y).

POSITION POSITION RESOLUTIONRESOLUTION

SIMULATED RESULTS OF SIMULATED RESULTS OF PERFORMANCEPERFORMANCE

CONFIG. NO.OF DETECTORS

AMT. OF GERMANIUM(Kg)

Eph[P/T]%My=1

Eph[P/T]%My=30

Ideal 4pi shell

1 233 65[85] 36[60]

AGATA 180 60 clusters 320 38[53] 24[44]

EUROBALL ~120 210 9[56] 6[37]

Matrix ReconstructionMatrix Reconstruction

-> variables (X,Y) are calculated usingX = (b+c)/(a+b+c+d) Y = (a+b)/(a+b+c+d)-> A matrix is reconstructed which gives map of (X,Y).

Liverpool Scanning System:Liverpool Scanning System:

Limitations in the Liverpool Scanning table: -> Uncertainities in the position determined in the X-Y direction due to finite diameter of collimator.-> Time required to scan along 9 radial lines in steps of 1 mm ~ 90 days.-> Count rate in back segments is extremely small, 1ct/ hour

34BGO's

Cs-137