Monte Carlo Simulations for Modern gammatracking

Transcription

1 Monte Carlo Simulations for Modern gammatracking Arrays E.Farnea INFN Sezione di Padova, Italy

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

3 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...

4 European γ-ray detection systems EUROGAM EUROBALL III TESSA ESS30 GASP EUROBALL IV

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

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

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

8 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

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

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

11 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

12 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

13 Data Analysis Pulse shape generation 0.55 r [cm] A B C D E F G [cm] 27 H D C B A z ϕ 0 0 E F G H rel. amplitude t [ns] t [ns] H G F E D C B A rel. amplitude Event generation Detector response Packing and smearing of simulated data γ-ray tracking Electronics Response Function Pulse Shape Analysis to decompose recorded waves

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

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

16 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 the shapes of the detectors. Ball with 180 hexagons. Space for encapsulation and canning obtained cutting the crystals. In the example 3 crystals form a triple cluster Add encapsulation and part of the cryostats for realistic MC simulations

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

18 Geodesic Tiling of Sphere using hexagons and 12 pentagons

19 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)

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

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

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

23 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

24 Effect of the recoil velocity β=20% The comparison between spectra 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 (%) δ s (cm) σ dir (degrees) β (%) Uncertainty on the recoil direction (degrees)

25 The First Step: The AGATA Demonstrator Objective of the final R&D phase Main issue is Doppler correction capability coupling to beam and recoil tracking devices 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 Cost ~ 7 M

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

27 Effect of the recoil velocity Agata Geant4 code (EF) + PRISMA simulation (A.Latina) AGATA Demonstrator + PRISMA 90 Zr recoils with E~350 MeV (with 10% dispersion) assumed. β from reconstructed trajectory length and TOF. Direction from start detector.

28 Performance Photopeak efficiency P/T Ratio ~14cm: Possible target-detector distance for the Demonstrator on PRISMA 1 MeV photons, point source at rest. Tracking is performed.

29 Effect of the recoil velocity Peak FWHM Photopeak efficiency Typical values for reaction products at PRISMA 1 MeV photons, M γ = 1. Tracking is performed.

30 AGATA vs. Conventional arrays AGATA 1π 1 GASP Conf. II 45 HPGe detectors (15 triple clusters) 40 HPGe detectors with anti Compton

31 Realistic Simulations γ Fold 1 γ Fold 3 AGATA 1π array GASP Conf.II 28 Si + 28 Si@125 MeV. Particle detection with EUCLIDES. Kinematical recalibration. E.Farnea, E.Farnea, F.Recchia F.Recchia

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33 The code: physics 1. Possibility to choose set of Geant4 interactions for photons (standard treatment or low-energy treatment) 2. Compton profile optionally considered 3. Linear polarization of the photons optionally considered Starting considerations: in in principle, linear polarization of of photons is is included into into the the Geant4 standard libraries. A non- nonstandard approach is is used, defining a polarization vector specifying the the direction of of the the electric field vector. Does this this produce the the correct results?

34 Unpolarized Compton scattering The angular distribution of the scattered photon is a function of the photon energy and of the scattering angle: W E E r (,θ ) = sin θ W () θ ( γ sin θ ) 0 + = = E 0 m c E 0 E E 0 2 E E ( 1 cosθ ) 0 E E 1 2 α 1 kev 255 kev 511 kev 1022 kev 2044 kev

35 Polarized Compton scattering In this case the angular distibution depends also on the direction of the polarization (in the pictures the direction e 0 is along the x axis). In case of a fully polarized photon beam: r E 1 E 1 E α 2 2 W ( θ, ϕ ) = ( ) 0 2sin θ cos γ 2sin θ cos 2 E + ϕ = 0 E 0 E ϕ 0 2 r r sinθ cos ϕ = cosδ = e d kev 255 kev 511 kev 1022 kev 2044 kev

36 The formalism A convenient formalism to treat polarization (linear and circular) is that of the Stokes parameters and of the scattering matrix developed by Fano et al. Order zero zero approximation: define polarization through the the Stokes parameters and and convert internally to to the the native Geant4 formalism. Check with with simple ideal cases that that the the results are are consistent with with theory.

37 Our test bench Our test bench was an ideal 8-elements polarimeter (plus a central scatterer and an additional external scatterer to study double scattering) The red detector lies in the scattering plane

38 Asymmetry The asymmetry ratio is used to benchmark symmetric polarimeters: A ( θ, ϕ ) = W θ, W θ, ( ϕ ) W ( θ, ϕ + 90 ) ( ϕ ) + W ( θ, ϕ + 90 ) Using a beam of known polarization one can determine the polarization sensitivity: A A 1 A ( 90,90 ) th γ Q th = = for fully polarized beams 1 The experimental asymmetry (to be compared to the theoretical value) should be corrected by the polarization sensitivity: A exp = A Q

39 Non-symmetrical polarimeter, or polarization non-orthogonal to the scattering plane Data are fitted with the following expression: I( ϕ ) 2 ( γ sin θ ( 1 + P cos2 ( ψ ) )) = I ϕ 0 From P and ψ the maximum asymmetry is found, A max = A(θ,ψ), which should be compared to the theoretical value: γ Psin sin 2 A max ( θ, P ) = 2 θ θ E 0 = kev θ = 90 χ 2 = kev at (30,60 ) modified Standard PolarizedComptonScattering I 0 = 504.8(3) P = 0.76(1) ψ = (4) A max = 0.54(2)

40 Check with G4LowEnergy interaction 511 kev at 90 (E 1 = kev) Roughly a factor 2 difference! Given the symmetry, opposite detectors can be summed. Individual analysis can put in evidence anomalies. Stokes Angle A exp Q= A exp /A th ψ exp (deg) A exp = A exp /Q ψ th (deg) [1 1 0] Cal (7) 0.702(1) ψ = angle where the minimum of the angular distribution lies A = (I max -I min )/(I max +I min ) A th [1 0 0] (90,90) (26) (4) [1 1 0] (90,90) (20) (3) [1 1 0] (90,30) (30) 81(4) 0.22(4)

41 Problems with G4 interactions Careful inspection of the code show that the low energy interaction sets provided with the Geant4 package treats polarization in a conceptually wrong way, resulting in a factor 2 attenuation of the anisotropy The standard interaction set treats polarization properly from the conceptual point of view, but the implementation fails Both of them were rewritten in a more satisfactory way (D.Bazzacco).

42 General comparison LowE Std Stokes newstd newlowe [100] [110] [100] [110] [110] 30-60

43 Summary The performance of AGATA (and GRETA) under a wide range of conditions has been evaluated in a realistic way using a specially written, Geant4-based C++ code The treatment of linear polarization provided by Geant4 has been revised in order to obtain results compatible with the theoretical expectations