Monte Carlo Simulations for Modern gamma- tracking Arrays E.Farnea - PowerPoint PPT Presentation

Monte Carlo Simulations for Modern gamma- tracking Arrays E.Farnea INFN Sezione di Padova, Italy

Outline • From conventional to gamma-ray tracking arrays • Results from Monte Carlo simulations for AGATA • Polarization studies with Geant4

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 ...

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

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? 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!

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

AGATA • High efficiency and P/T ratio. • Good position resolution on the individual γ interactions in order to perform a good Doppler correction . • Capability to stand a high counting rate. Pulse shape analysis + γ -ray tracking

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 E γ 1 θ 2 e 2 2 Digital electronics to record and Reconstructed process segment gamma-rays signals

Benefits of the γ -ray tracking v/c = 20 % scarce Detector Doppler correction photon direction Definition of the capability Segment Pulse shape analysis + good tracking γ Energy (keV)

Why Monte Carlo Simulations? • Careful optimization of the geometry of the array • Evaluation of the expected performance of the array in a consistent way • Production of controlled datasets to develop and train the required algorithms

The Monte Carlo code for AGATA • Based on Geant4 C++ classes • Event generation suited for in-beam experiments • gamma-ray tracking is not included directly in the code (complicated process in itself!) • “Raw” data produced by the Geant4 program are processed with a tracking code (in this work, mgt) and analyzed with other programs

Detector response Pulse shape generation Event generation • • A 0.55 r [ cm ] • • B 1.0 • C • 1.45 • D 1.9 • E • 2.35 • • F 2.8 • • G 3.25 • H • 3.7 • 1.13 0.94 ∗ • 0.63 z 0.31 • [ cm ] 0.0 ϕ 15˚ 22.5˚ 27˚ 0˚ 7.5˚ 0 E F 0.2 G -0.25 A H 0 -0.5 B C D G E rel. amplitude rel. amplitude H F -0.75 -0.2 • • • • D C B A -1 0 0.2 -0.25 0 -0.5 -0.75 -0.2 • • • • -1 100 200 300 100 200 300 100 200 300 100 200 300 Data Analysis t [ ns ] t [ ns ] Electronics Response Function Packing and smearing of simulated data γ -ray tracking Pulse Shape Analysis to decompose recorded waves

Class structure of the program Agata *Agata *Agata Agata Agata Agata SteppingAction RunAction EventAction PhysicsList VisManager Agata *Agata GeneratorOmega CSpec1D Agata Agata Analysis GeneratorAction SteppingOmega CSpec2D *Agata Emitted *Agata *Agata InternalEmission ExternalEmission *Agata Agata Detector *Agata Emitter *Agata *Agata Shell Detector InternalEmitter ExternalEmitter Construction *Agata Detector Simple * Possibility to *Agata *Agata *Agata change parameters DetectorAncillary DetectorArray SensitiveDetector via a messenger class CConvex Agata Messenger classes are not shown! Messenger classes are not shown! Polyhedron HitDetector

Building a Geodesic Ball (1) Building a Geodesic Ball (1) Start with a On its faces, draw a regular platonic solid pattern of triangles grouped Project the faces on e.g. an icosahedron as hexagons and pentagons. the enclosing sphere; E.g. with 110 hexagons and flatten the hexagons. (always) 12 pentagons

Building a Geodesic Ball (2) Building a Geodesic Ball (2) Al capsules 0.4 mm spacing 0.8 mm thick Al canning 2 mm spacing 2 mm thick A radial projection of the spherical tiling generates Space for encapsulation and the shapes of the detectors. canning obtained cutting the Ball with 180 hexagons . Add encapsulation and crystals. In the example 3 part of the cryostats for crystals form a triple cluster realistic MC simulations

Building a Geodesic Ball (3) Building a Geodesic Ball (3)

Geodesic Tiling of Sphere using 60–240 hexagons and 12 pentagons 60 80 110 120 150 200 240 180

The code: geometry 1. Candidate configurations for AGATA which have been investigated have 120 or 180 hexagonal crystals; they have been chosen because of the possibility to form clusters of detectors with few elementary shapes. 2. The solid angle coverage is maximized only using irregular hexagons; with regular hexagons the performance of the array is lower because of the spaces between the crystals. 3. Geodesic tiling polyhedra handled via a specially written C++ class (D.Bazzacco) 4. Relevant geometry parameters read from file (generated with an external program)

GRETA vs. AGATA Ge crystals size: Length 90 mm Diameter 80 mm 120 hexagonal crystals 2 shapes 180 hexagonal crystals 3 shapes 30 quadruple-clusters all equal 60 triple-clusters all equal Inner radius (Ge) 18.5 cm Inner radius (Ge) 23.5 cm Amount of germanium 237 kg Amount of germanium 362 kg Solid angle coverage 81 % Solid angle coverage 82 % 4320 segments 6480 segments Efficiency: 41% (M γ =1) 25% (M γ =30) Efficiency: 43% (M γ =1) 28% (M γ =30) Peak/Total: 57% (M γ =1) 47% (M γ =30) Peak/Total: 58% (M γ =1) 49% (M γ =30)

Expected Performance Response function Absolute efficiency value includes the effects of the tracking algorithms! Values calculated for a source at rest.

Effect of ancillary devices Absolute photopeak Peak-to-total ratio efficiency (tracking included) (response function) Ancillary devices have an impact comparable to Ancillary devices have an impact comparable to the case of conventional arrays the case of conventional arrays (tracking is “robust”!) (tracking is “robust”!)

The code: physics 1. Schematic built-in event generator 2. Possibility to decode “realistic” event structure and sequence from a formatted text file 3. Possibility to couple the code to generic Geant4 event generators

Effect of the recoil velocity The comparison between spectra β =20% obtained knowing or not knowing the event-by-event velocity vector shows that additional information will be essential to fully exploit the concept of tracking β (%) 5 20 50 δ s (cm) 1.5 0.5 0.3 σ dir (degrees) 2 0.6 0.3 ∆ β (%) 2.4 0.7 0.3 Uncertainty on the recoil direction (degrees)

The First Step: The AGATA Demonstrator Objective of the final R&D phase 2003-2008 1 symmetric triple-cluster 5 asymmetric triple-clusters 36-fold segmented crystals 540 segments 555 digital-channels Eff. 3 – 8 % @ M γ = 1 Eff. 2 – 4 % @ M γ = 30 Full ACQ with on line PSA and γ -ray tracking Test Sites: GANIL, GSI, Jyväskylä, Köln, LNL Main issue is Doppler Cost ~ 7 M € correction capability → coupling to beam and recoil tracking devices

AGATA Demonstrator + PRISMA ps, , Y ~ 350 ps X = 1 mm ∆ X = 1 mm t ~ 350 Y = 1 mm ∆ Y = 1 mm Y position ∆ Y = 2 mm Y = 2 mm Y position ∆ t ∆ ∆ AGATA ∆ ∆ X Demonstrator Dipole MCP Ion Chamber Quadrupole MWPPAC 195 MeV 195 MeV 36 36 S + S + 208 208 Pb, Pb, θ lab = 80 = 80 o o θ lab Z=28 Z=28 ∆ Z ~ 60 for Z=20 Z ~ 60 for Z=20 X position X position First installation site ps < 500 ps E/E < 2% ∆ E/E < 2% t < 500 X = 1 mm ∆ X = 1 mm for the Demonstrator: .) a.u.) E (a.u the PRISMA ∆ E ( ∆ t ∆ ∆ ∆ spectrometer at LNL ∆ Z=16 Z=16 Z/ ∆ Z/ E. Fioretto E. Fioretto INFN - LNL INFN - LNL E ( E (a.u a.u.) .)