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…)