AGATA: Status AGATA: Status and Perspectives and Perspectives - PowerPoint PPT Presentation

AGATA: Status AGATA: Status and Perspectives and Perspectives E.Farnea INFN Sezione di Padova, Italy on behalf of the AGATA Collaboration

Outline Outline • Basic concepts: pulse shape analysis and gamma-ray tracking • Gamma-ray tracking arrays: AGATA and GRETA (in strict alphabetical order) • Status of AGATA

European γ -ray detection systems European γ -ray detection systems EUROBALL III EUROGAM TESSA ESS30 GASP EUROBALL IV 1980 1986 1992 1996

Composite & encapsulated detectors Composite & encapsulated detectors CLOVER EB-CLUSTER

Why do we need AGATA? Why do we need AGATA? Our goal is to extract new valuable information on the nuclear structure through the γ -rays emitted following nuclear reactions Problems: complex spectra! Many lines lie close in energy and the “interesting” channels are typically the weak ones ...

Challenges in Nuclear Structure Shell structure in nuclei • Structure of doubly magic nuclei Shape coexistence • Changes in the (effective) interactions Transfermium nuclei Proton drip line and N=Z nuclei • Spectroscopy beyond the drip line • Proton-neutron pairing Nuclear shapes • Isospin symmetry • Exotic shapes and isomers • Coexistence and transitions 100 Sn Neutron rich heavy nuclei (N/Z → 2) 48 Ni • Large neutron skins (r ν -r π → 1fm) • New coherent excitation modes 132+x Sn • Shell quenching 78 Ni Nuclei at the neutron drip line (Z → 25) • Very large proton-neutron asymmetries • Resonant excitation modes • Neutron Decay

Why do we need AGATA? Why do we need AGATA? FAIR • Low intensity SPIRAL2 • High background SPES • Large Doppler broadening REX-ISOLDE • High counting rates MAFF EURISOL • High γ -ray multiplicities HI-Stable High efficiency Harsh conditions! High sensitivity Need instrumentation with High throughput Ancillary detectors Conventional arrays will not suffice!

Efficiency vs. Resolution Efficiency vs. Resolution With a source at rest, the intrinsic resolution of the detector can be reached; efficiency decreases with the increasing detector-source distance. With a moving source, due to the Doppler effect, also the effective energy resolution depends on the detector-source distance Small d Large Ω High ε Poor FWHM Large d Small Ω Low ε Good FWHM

Compton scattering Compton scattering The cross section for Compton scattering in germanium implies quite a large continuous background in the resulting spectra P/T~30% P/T~50% Concept of anti-Compton shield to reduce such background and increase the P/T ratio

From conventional Ge to γ -ray tracking From conventional Ge to γ -ray tracking Compton Shielded Ge Efficiency is lost due to the solid angle covered by the ε ph ~ 10% shield; poor energy resolution at high recoil N det ~ 100 velocity because of the θ ~ 8º Ω ~40% large opening angle Ge Sphere Using only conventional Ge detectors, too many ε ph detectors ~ 50% are needed to avoid N det ~ 1000 summing effects and keep θ ~ 3º the resolution to good values Ge Tracking Array The proposed solution: ε ph ~ 50% Use the detectors in a N det ~ 100 non-conventional way! θ ~ 1º Ω ~80% AGATA and GRETA

Ingredients of Gamma Tracking 4 Identified 1 interaction points Highly segmented Reconstruction of tracks (x,y,z,E,t) i HPGe detectors evaluating permutations of interaction points Pulse Shape Analysis to decompose · recorded waves e 3 3 e 1 · 3 θ 1 E γ E γ 2 1 2 0 θ 2 E γ 1 e 2 2 Digital electronics to record and Reconstructed process segment gamma-rays signals

Arrays of segmented Ge detectors (for Doppler correction) EXOGAM segmented clovers with 4x4 fold segmentation MINIBALL triple-clusters with 6 and 12 fold segmentation Segmented Germanium Array (SeGA) with 32-fold segmentation

Pulse Shape Calculations and Pulse Shape Calculations and Analysis by a Genetic Algorithm Analysis by a Genetic Algorithm ρ ρ ( ) ρ Weighting field measured Weighting field 0 = − ⋅ ⋅ i q E v E signals rel. amplitude method: -0.2 method: e/h e/h W drift -0.4 -0.6 • • A ••••••• 0.55 -0.8 r [ cm ] • signals B 1.0 • C 1.45 -1 • reconstructed D 1.9 0 50 100 150 200 250 • E 2.35 t ns from base • F 2.8 G 3.25 • H 3.7 • GA 1.13 GA ∗ 0.94 • 0.63 z • 0.31 [ cm ] 0.0 ϕ 15˚ 22.5˚ 27˚ Sets of ⊕ 7.5˚ 0˚ interaction points net charge signals transient signals (E; x,y,z) i Base system 0 E F of signals 0.2 G -0.25 A H „fittest“ set measured or calculated 0 -0.5 B C D G rel. amplitude E rel. amplitude H F -0.75 -0.2 • • • • D C B A -1 0 Reconstructed set 0.2 -0.25 of interaction points 0 -0.5 (E; x,y,z) i -0.75 -0.2 • • • • Th. Kröll, NIM A 463 (2001) 227 -1 100 200 300 100 200 300 100 200 300 100 200 300 t [ ns ] t [ ns ]

In-beam test of PSA: MARS detector In-beam test of PSA: MARS detector Coulex. of 56 Fe at 240 MeV on 208 Pb, v = 0.08c MC limit Corrected using points FWHM assuming determined with a 4.5 keV 5 mm FWHM Genetic Algorithm position resolution: Corrected using 4.2 keV FWHM center of segments 6.3 keV � 24 detectors with ∆θ ≈ 9° Corrected using FWHM center of crystal recoil 16.5 keV � one detector with ∆θ ≈ 22° − 1 β cos( θ ) CM = Lab E E γ γ − 2 1 β E γ (keV) Position resolution � 5 mm FWHM Similar result from an experiment done with the GRETA detector

An alternative approach: Grid search An alternative approach: Grid search • Search the best χ 2 for pulse shapes in the reference base r φ • The pulse shapes associated to one point in the reference base are choosen as sample z • The χ 2 of the sample is calculated for all the points in the reference base • The results obtained for the in-beam experiment are quite similar to those obtained with a genetic algorithm r 2 r 1 φ 2 Genetic algorithm Best pulse shapes search Raw φ 1 z 2 z 1 F = 1 16 keV F = 2 e 1 e 2 F = 3 R.Venturelli, Munich PSA meeting, September 2004 Other approaches (neural networks, wavelets, etc.) are currently attempted within the collaboration

γ -ray tracking γ -ray tracking Photons do not deposit their energy in a continuous track, rather they lose it in discrete steps A high multiplicity event E γ =1.33MeV, M γ =30 One should identify the sequence of interaction points belonging to each individual photon Tough problem! Especially in case of high-multiplicity events

Interaction of photons in germanium Interaction of photons in germanium Mean free path determines size of detectors: λ ( 10 keV) ~ 55 µ m λ (100 keV) ~ 0.3 cm λ (200 keV) ~ 1.1 cm λ (500 keV) ~ 2.3 cm λ ( 1 MeV) ~ 3.3 cm λ ( 2 MeV) ~ 4.5 cm λ ( 5 MeV) ~ 5.9 cm λ (10 MeV) ~ 5.9 cm

Tracking algorithms Tracking algorithms Basic ingredient: Compton scattering formula − − N 1 N 1 ∑ ∑ E = E = E e E e γ γ i ' i = = + i n i n 1 E ⋅ E 01 12 γ = ⇒ = P P cos θ E γ ' E E ( ) ⋅ 01 12 γ + − P 1 1 cos θ 2 m c 0 2  −  2 E P χ = χ χ − E E N 1 ≈ 2 2 ∑   ⇒ γ ' γ '   n n σ   = n 1

Reconstruction of multi-gamma events Reconstruction of multi-gamma events Analysis of all partitions of measured points is not feasible : Huge computational problem (~10 23 partitions for 30 points) Figure of merit is ambiguous � the total figure of merit of the “true” partition not necessarily the minimum 1 – Cluster (forward) tracking 2 – Backtracking 3 – Other approaches (fuzzy tracking, etc.)

Forward tracking (G.Schmid, 1999; mgt implementation by D.Bazzacco, Padova) 1. Create cluster pool => for each cluster, E γ 0 = ∑ cluster depositions 2. Test the 3 mechanisms 1. do the interaction points satisfy the Compton scattering rules ? 2   2 − χ E Pos − N 1 E ≈ ∑   γ ' ⋅ W γ '   n E   = n 1 γ n 2. does the interaction satisfy photoelectric conditions (e 1 ,depth,distance to other points) ? 3. do the interaction points correspond to a pair production event ? E 1st = E γ – 2 m e c 2 3. Select clusters based on χ 2

Backtracking (J. Van der Marel et al., 1999) Photoelectric 83% energy deposition 87% is approximately independent of incident energy and is peaked around 100-250 keV => interaction points within a given deposited energy interval (e min < e i < e max ) will be considered as the last interaction of a fully absorbed gamma

Backtracking B. Cederwall, L. Milechina, A. Lopez-Martens 1. Create photoelectric interaction pool: e min < e i < e max 2. Find closest interaction j to photoelectric interaction i prob. for photoelectric interaction > P phot,min distance between interaction points < limit E inc = e i + e j , E sc = e i 3. Find incident direction from incident + scattered energies cos θ = 1 – m e c 2 (1/E sc –1/E inc ) 4. Find previous interaction k or source along direction cos θ (energy) - cos θ (position) < limit E inc = e i +e j +e k prob. for Compton interaction > P comp,min E sc = e i +e j distance between interaction points < limit the last points of the sequence are low energy and close to each other � bad position resolution and easily packed together