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GEANT4 simulations for a new generation of g -ray detector : AGATA

GEANT4 simulations for a new generation of g -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.

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GEANT4 simulations for a new generation of g -ray detector : AGATA

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  1. GEANT4 simulations for a new generation of g-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

  2. Outline • Introduction to superdeformation • An example of a current array : EUROBALL • Limits of actual generation • The new generation : AGATA  Description of the project  Work to be performed with GEANT4 V. Summary

  3. 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 DEg ~ 40keV (~50keV) A~190 (~150) Nucleus quantal object b(elongation) Appearance of a second minimum which stabilizes the nucleus in a SD state

  4. 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, a) which removes a lot of energy (~ 10MeV/n) • emission of g rays when no more particles can be emitted •  average emission : 30 g • SD nuclei are rare events : 1% of the reaction channel!! • fusion-evaporation , s=100mb for 36S+160Gd at 159MeV • SD transitions = 1% = 1mb • high sensitive detection device is needed : EUROBALL

  5. EUROBALL @ Vivitron (Strasbourg) beam 239 Ge detectors cooled by liquid N, surround the target Another material is used : BGO (calorimeter, anti-Compton shields surrounding each Ge) Photopeak efficiency : 6.5% Resolution (Ge) : 2.1 keV Peak/Total (related to Signal/Noise) : 40% at 1.3MeV Mg=30

  6. Anti-Compton shields : Why? Totally absorbed  = good event Scattered  = background BGO Ge Signal from Ge + signal from BGO=> rejected event 60Co source

  7. Composite detectors Goal : to reduce -ray broadness due to Doppler effect Dq   DE 

  8. Limits of the current generation Search for hyperdeformation (long axis/small axis=3)  s=few nbarns (expected)!! With EB we can study weak SD structures :  = 10µb Limits of current Ge arrays with AC shields EUROBALL: ~40%, e=6.5%, P/T=40% for Mg=30 Need to replace shields by active material (Ge) to increase solid angle • AGATA : • ~80%, e=25%, P/T=50% for Mg=30 • We are interested in the individual energies in cascade, so we need tracking • To do tracking : Crystal segmentation and digital electronics are necessary

  9. Segmentation and Pulse Shape Analysis (PSA) • 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 Miniball segmented crystal  (E,x,y,z) for each interaction point

  10. 1 1 0 0 2 2 Principle of Tracking b) a) a b source 1 : (e1,x1,y1,z1) 2 : (e2,x2,y2,z2) ( Measured) e’1 E=e1+e2 e’2 a or b i=1, 2 Smallest ² = path followed by the photon

  11. Eg = 1.33 MeV Mg = 30 bad good Principle of tracking for AGATA • Partly escaped • Partly absorbed • First hit 30 + 4 interactions/ (1MeV)= 1031 possible combinations  previous method cannot be used algorithms of reconstruction must be developed Example : Cluster algorithm to group points into candidate events (clusters) Application to an ideal Ge shell 27 g detected ~ 23 photopeaks 16 reconstructed ~ 14 photopeaks World map representation Reconstruction efficiency : 60 % (14/23) Total Photopeak Efficiency : 47% (14/30)

  12. Geometry of AGATA 2 examples of possible geometry : • Planar geometry • 72 cryostats : a stack of4 planar detectors • Each detector • is segmented into 16 • segments •  ~4800 electronic channels • Geometry with 190 coaxial Ge detectors each Ge is segmented into 36 segments  ~7000 electronic channels

  13. 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 • 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

  14. 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…)

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