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GEANT4 simulations for a new generation of -ray detector : AGATA
(Advanced Gamma Tracking Array)
J. Roccaz, K. Hauschild, A. Korichi, A. Lopez-Martens, S. Mohammadi, S. Siem
Nuclear Structure Group, CSNSM Orsay, CNRS-IN2P3, France
Geant 4 Workshop, CERN (Geneva) November 11th-15th, 2002
Outline
I. Introduction to superdeformation
II. An example of a current array : EUROBALL
III. Limits of actual generation
IV. The new generation : AGATA
Description of the project
Work to be performed with GEANT4
V. Summary
Superdeformation (SD)
Definition :
• a state of the nucleus
• shape of a rugby ball : long axis/short axis = 2
How can we explain the existence of SD nuclei? :
Experimental signature :
A rotational spectrum
Regular spacing of -rays
E40keV (~50keV) A~190 (~150)
(elongation)
Nucleus
quantal object
Appearance of a second minimum which stabilizes the nucleus in a SD state
Production & deexcitation of SD nuclei
Fusion-evaporation :
• production of compound nucleus at high angular momentum (l >30ћ) and E* ~ 50MeV
• deexcitation via particle emission (n, p, which removes a lot of energy (~ 10MeV/n)
• emission of rays when no more particles can be emitted
average emission : 30
SD nuclei are rare events : 1% of the reaction channel!!
fusion-evaporation , mb for 36S+160Gd at 159MeV
SD transitions = 1% = 1mb
high sensitive detection device is needed : EUROBALL
EUROBALL @ Vivitron (Strasbourg)
Photopeak efficiency : 6.5% Resolution (Ge) : 2.1 keV Peak/Total (related to Signal/Noise) : 40%
at 1.3MeV M=30
beam
239 Ge detectors cooled by liquid N, surround the target
Another material is used : BGO (calorimeter, anti-Compton shields surrounding each Ge)
Anti-Compton shields : Why?
60Co source
Totally absorbed = good event
Scattered = background
Signal from Ge + signal from BGO=> rejected event
BGO Ge
E
Composite detectors
Goal : to reduce -ray broadness due to Doppler effect
Limits of the current generationSearch for hyperdeformation (long axis/small axis=3) few nbarns (expected)!!
With EB we can study weak SD structures : = 10µb
Limits of current Ge arrays with AC shields
EUROBALL :
~40%, =6.5%, P/T=40% for M=30
Need to replace shields by active material (Ge) to increase solid angle
AGATA :
~80%, =25%, P/T=50% for M=30
We are interested in the individual energies in cascade, so we need tracking
To do tracking : Crystal segmentation and digital electronics are necessary
Interaction in segment 2:
PSA on 2 : height of signal energy
rise time radius
precision ~1cm
PSA on 2+1+3 precision ~ few mm
Development of digital electronics
Segmentation and Pulse Shape Analysis (PSA)
Miniball segmented crystal
(E,x,y,z) for each interaction point
Principle of Tracking
E=e1+e2
Smallest ² = path followed by the photon
1 : (e1,x1,y1,z1)
2 : (e2,x2,y2,z2)
( Measured)
source
0
1
20
1
2
a b
e’1
e’2
a) b)
a or b
Eee ii
i
'2
i=1, 2
= 1.33 MeV
M = 30
goodbad
detected ~ 23 photopeaks16 reconstructed ~ 14 photopeaks
30 + 4 interactions/ (1MeV)= 1031 possible combinations previous method cannot be used
algorithms of reconstruction must be developed
Example : Cluster algorithmto group points into candidate events (clusters)
Application to an ideal Ge shell
•Partly escaped•Partly absorbed
•First hit
World map representation
Principle of tracking for AGATA
Reconstruction efficiency : 60 % (14/23)
Total Photopeak Efficiency : 47% (14/30)
2 examples of possible geometry :•Planar geometry
72 cryostats : a stack of 4 planar detectors
Each detector
is segmented into 16
segments
~4800 electronic channels
R = 16 cm
entrée faisceausortie faisceau
Geometry of AGATA
•Geometry with 190 coaxial Ge detectors
~7000 electronic channels
each Ge is segmented
into 36 segments
Simulations in order to compare , P/T… between the different geometries and also between GEANT3 and GEANT4
optimization of geometry
Creation of a database containing interaction points
test and develop the algorithms of tracking and the PSA
Work to be performed with GEANT4
Work started using GEANT3.
We will use GEANT4 because of its lower energy threshold
1 keV < interactions < 10keV are not lost
Summary
Current arrays better knowledge of the structure of the atomic nucleus (e.g. superdeformation)
We need a more powerful array : AGATA hyperdeformation
A lot of work still has to be done : •Simulations with GEANT
•Pulse Shape Analysis
•Tracking algorithms
•Prototype detectors testing ...
First experiment with AGATA in 2007-2008
(if all goes well…)