1. AGATA Advanced Gamma Tracking Array
Determination of position-dependent pulse shapes of Ge detectors
N.Goel,T. Engert, J. Gerl, C.Domingo,
I. Kojouharov, H. Schaffner,S. Tashenov

2. Outline of the talk 1. Tests with LYSO crystal scintillator
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

3. Introduction Photopeak efficiency-high multiplicity -ray cascades
SHIELDED DETECTORS
COMPOSITE DETECTORS
SEGMENTED DETECTORS Ge
BGO
shields
Adjacent Ge
crystals operated
in ADD BACK mode
For high multiplicity M,
wrong summing of energies takes place
Discrimination between scattered events and individual hits possible
with TRACKING
Suppress the Compton scattered events
30% of total solid angle covered by Ge

4. Introduction Energy resolution:  spectroscopy with relativistic beams
Intrinsic resolution 1.3MeV
Doppler ~ 0.5
Reduced solid angle with
segmentation
/ ~ sin( Φ)• ∆Φ ∆Φ ∆Φ Φ Φ
First interaction point has
position resolution proportional
to the size of segment
Analysis of segment pulse shapes
gives position resolution even
smaller than the size of segment
Segmented
Detectors ∆Φ

5. Outline 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 tracking
2. Pulse shape analysis

6. AGATA Advanced Gamma Tracking Array
4π gamma tracking array for nuclear physics experiments at European accelerators providing radioactive and highly intense stable beams
61.08mm
90 mm
52.09mm
60o
10o
10o 34 33 35 31 32 4 3 27 28 5 29 30 24 2 26 1 25 18 9 10 22 11 23 6 7 19 8 21 20 13 15 16 17 12 14

7. AGATA Gamma 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 efficiency
Determining the incoming direction with a very good position resolution( 2-3mm)
Doppler shift in energy
for v/c ~ ( )
E' = E ( 1- (v/c) cos ) Φ

8. AGATA Pulse Shape Analysis
For each event,
37x100 samples of
preamplifier signals
Digital electronics
to record the segment
signals
Radial dependence of pulse shape
amplitude(adc units)
amplitude(adc units)
time(ns)
time(ns)

9. F. Recchia , ACTA PHYSICA POLONICA B Vol.38(2007)
Improvement in Doppler correction with segmentation and PSA IN-Beam test at Cologne, Germany
48Ti(d,p)49Ti 100MeV ∆Φ
Energy resolution
1382 keV
Det = 32 keV
Seg = 11 kev
Det = 32kev
PSA = 5.5 keV
Seg = 11 keV
Det = 32keV
1. The center of the detector.
2. The center of the firing segment.
3. Position given by pulse shape
analysis.
Counts
Energy(keV)
F. Recchia , ACTA PHYSICA POLONICA B Vol.38(2007)

10. SCANNER PSA 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 shapes
for all possible interaction
position inside the
detector
An efficient system to
scan the
detector

11. Outline of the Talk 1. Tests with LYSO crystal scintillator
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

12. SCANNER PRINCIPLE Determination of incoming direction of photon
Ge detector
to be scanned
22Na → 2γ, 511 keV
256x scintillator
fibres
4x 64 channel PMTs

13. SCANNER PRINCIPLE Determination of incoming direction of photon
Y(arb units)
X(arb units) b c
X = (b+c)/(a+b+c+d) Y = (a+b)/(a+b+c+d)
-> A matrix is reconstructed which gives map of (X,Y).

14. SCANNER PRINCIPLE Determination of Z coordinate of interaction position
6 ring slits
42 BGOs
22Na → 2γ, 511 keV
256x scintillator
fibres
4x 64 channel PMTs

15. SCANNER PRINCIPLE Energy spectra under triple coincidence and Agata energy gate
True Event
False Event
90o scattering
511keV
511keV
false triggering
of BGO
Counts
Energy(keV)
Even under triple coincidence events its possible to have false events which can be filtered by applying gate on AGATA energy.

16. SCANNER Experimentally determined pulse shapes6 7 9 10 11 12 8 13 14 15 16 17 18 19 20 21 22 23
time(ns)
amplitude(ADC units) 14 15 20 21

17. SCANNER Radial position information from net charge collecting segment
Pulse rises slowly for interaction occuring close to central or outer electrode.
Pulse for interaction near central electrode is convex while concave for the interaction near outer electrode.
Pulse rises faster for interaction occurring in the middle of the segment.
amplitude(ADC units) 1
time(ns)
amplitude(ADC units) 1 2 3 1 2 3
time(ns)

18. SCANNER Radial position information from polarity image charge
amplitude(ADC units)
time(ns)
time(ns)
+ve -> Interaction close to outer electrode
-ve-> Interaction close to the centre electode

19. SCANNER Azimuthal position information from amplitude of image charge
Pulse shapes
from neighbouring segment
amplitude(ADC units)
time(ns)
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 took
place.

20. SCANNER Summary 2 types of signals have to be analysed
Transient charge signal induced in
neighbouring segment
net charge signal
from hit segment
Azimuthal
position
Radial
position
Radial position is given by rise time
and shape of pulse

21. Outline of the Talk 1. Tests with LYSO crystal scintillator
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

22. Limitation in the first scanner prototype
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.
1 ct/hr
2. Low count rate problem
The true coincidence rate for
back segments is about 1
count/hour.
BGO
90mm
40ct/hr
10mm

23. Replacement of fibres with LYSO/BGO
Scintillator plate
Position sensitive
PMT

24. Tests with LYSO crystal scintillator
-> Position resolution of LYSO plate
->Wrapping the upper surface of cyrstal for better light collection

25. Tests with LYSO crystal scintillator POSITION RESOLUTION
3 mm thick
6cm
7.6 cm
6cm
Position sensitive PMT
SETUP FOR MEASUREMENT OF POSITION RESOLUTION
LYSO
Reference detector
0.3 mm

26. Tests with LYSO crystal scintillator Position Resolution
Energy gate on LYSO
and reference detector
to filter false events
511 keV
511 keV
counts
counts
Pulse height(channel number)
Pulse height(channel number)
width
2.3mm
counts Y X
Y(arb units)

27. Tests with LYSO crystal scintillator Position Resolution
surface of LYSO
0.3mm
Calculated position of source
moved in steps of 1mm
Measured position of source
reference
detector
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.

28. Tests with LYSO crystal scintillator 2
Tests with LYSO crystal scintillator 2. Wrapping material for better light collection
TEFLON
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.
Peak pos.=
ESR
Peak pos.=
Amplitude

29. Improvements possible:
Circular and bigger scintillator crystals(LYSO/BGO)
Edge effects
32 anode signals of HAMAMATSU R2486
Position accuracy
0.1 mm precision X-Y table
Efficient and reliable measurements

30. NEW SCANNING APPROACH
Ge-Detector
Side scanning detector
X-Y detector

31. NEW SCANNING APPROACH
Recording the pulse shapes for two positions (a) and (b).
Comparing the pulse shapes from two data sets
Signal of one set will be identical to signal of other set at the crossing points.
Position sensitive
Detector
Na-22 source
90o a)
Na-22
Rotated by 900 b)

32. 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.

33. 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.

34. DANKE SCHOEN

35. POSITION RESOLUTION HAMAMATSU 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 using
X =(Xa/Xa+Xb) Y=(Ya/Ya+Yb)
->A matrix is reconstructed which gives map of (X,Y).

36. SIMULATED RESULTS OF PERFORMANCE
CONFIG.
NO.OF DETECTORS
AMT. OF GERMANIUM(Kg)
Eph[P/T]%
My=1
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]

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

38. Liverpool Scanning System:34
BGO's
Cs-137
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