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Overview of the fragmentation mechanism
José Benlliure Universidad of Santiago de Compostela Workshop on Nuclear Physics for Galactic Cosmic Rays Grenoble, December
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Layout Basic concepts on fragmentation reactions
Experimental investigation Production of residual nuclei Future perspectives Conclusions José Benlliure, NP4GCR Grenoble, December 2012
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Basic concepts Heavy-ion induced reactions at relativistic energies where a residue from the projectile/target survives the collision. kinetic energies above the Fermi energy up to few GeVs per nucleon peripheral or mid-peripheral collisions participant-spectator picture José Benlliure, NP4GCR Grenoble, December 2012
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Basic concepts Heavy-ion induced reactions at relativistic energies where a residue from the projectile/target survives the collision. kinetic energies above the Fermi energy up to few GeVs per nucleon peripheral or mid-peripheral collisions participant-spectator picture Production of nuclei far from stability and applications J. Pochodzalla et al., PRL 75, 1040 (1995) Equation of state of nuclear matter 1.8 A GeV 40Ar Ag H.L. Bradt and B. Peters, PR 80, 1950 (1950) Cosmic rays composition 1950 1960 1970 1980 1990 2000 2010 2020 cosmic rays Bevalac GSI RIBF FAIR FRIB NSCL / GANIL / RIKEN José Benlliure, NP4GCR Grenoble, December 2012
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Basic concepts A two-step reaction mechanism
1. Formation of the spectator pre-fragment Microscopic degrees of freedom (nucleons and mesons) are more relevant than the nuclear mean field Radial nuclear densities and nucleon-nucleon cross sections Nuclei are moving faster than their constituents, nucleons (adiabatic approximation) only nucleons in the overlapping region (participant) will contribute to the collision non-overlapping regions (spectators) will be almost undisturbed impact parameter defines these regions 2. De-excitation of the spectator pre-fragment Spectator pre-fragments gain excitation energy and some angular momentum according to the impact parameter Statistical de-excitation emitting neutrons, protons, clusters and gammas assuming thermal equilibrium is achieved José Benlliure, NP4GCR Grenoble, December 2012
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Basic concepts Spallation reactions
This is a particular case of fragmentation where one of the colliding nuclei is an hadron Spectator and participant overlap Collision governed by the propagation of a nucleon-nucleon interaction cascade Fast nucleons emitted during the nucleon nucleon cascade (pre-equilibrium) define the size and excitation energy of the pre-fragment Meson production becomes relevant The de-excitation stage is similar statistical equilibrium and particle emission excitation energy gained during the collision is strongly correlated with the initial kinetic energy José Benlliure, NP4GCR Grenoble, December 2012
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Experimental investigation
Experimental approaches Only light particles leave the target Residual nuclei can only be investigated using b-delayed gamma spectroscopy only isobaric distributions are provided no kinematic information Neutron production has been investigated for heavy nuclei with complex detection setups Light nuclei have been measured (Z identification and sometimes A) in inclusive measurements with partial angular acceptance X. Ledoux et al. PRL (1999) José Benlliure, NP4GCR K. Barna et al. NIMA (2003) Grenoble, December 2012
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Experimental investigation
Experimental approaches Heavy residual nuclei and light nuclei can be identified in coincidence Kinematic information can be obtained The detection technology defines the accuracy and completeness of the measurements Z identification of residual nuclei Z and A identification of residual nuclei and kinematics Z identification in coincidence of heavy and light fragments 56Fe+C 1 A GeV 56Fe+p 1 A GeV 56Fe+p 1 A GeV E. Le Gentil et al. PRL (2008) W. Webber et al. PRC (1988) C. Villagrasa et al. PRC (2007) José Benlliure, NP4GCR Grenoble, December 2012
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Experimental investigation
Inverse kinematics GSI More than 1000 fission fragments identified in the reaction 238U(1 A GeV)+d José Benlliure, NP4GCR Grenoble, December 2012
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Experimental investigation
Fragment Separator: zero-degree high-resolving power spectrometer Br ~ 10 – 15 Tm F2-F4: 56Fe + p X A/A ~ B/~ ToF ~ 70 ps L ~ 36 m José Benlliure, NP4GCR Grenoble, December 2012
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More than 6900 cross sections
Production of residual nuclei More than 6900 cross sections T. Enqvist, P. Napolitani, M.V. Ricciardi GSI L. Audouin, I. Mustapha, J. Taieb IPNO E. Casarejos, J, Pereira U. Santiago B. Fernández, W. Wlazlo, C. Villagrasa Saclay José Benlliure, NP4GCR José Benlliure Grenoble, December 2012 Ejercicio 1
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Production of residual nuclei
Present understanding The first stage of the reaction defines the pre-fragment properties Mass, linked to the excitation energy gained ~ 27 MeV * DA1 Isospin, hypergeometrical distribution around the projectile A/Z Angular momentum, also linked to DA1 The de-excitation phase determines the nature of the final fragments The excitation energy and mass of the pre-fragment are the most relevant parameters In some cases A/Z and J also influence the final nature of the fragments hot pre-fragments cold final-residues José Benlliure, NP4GCR Grenoble, December 2012
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Production of residual nuclei
cold final-residues Mass of the final fragments The mass of the final fragments mostly depends on the excitation energy per nucleon of the pre-fragment The emission of a nucleon costs around 10 MeV, then DA2 ~ 27DA1/10 Nucleus-nucleus collisions reach the “limiting fragmentation” regime In spallation reactions this regime is only reached at large kinetic energies Excitation energy 1 A GeV 1 A GeV José Benlliure, NP4GCR Grenoble, December 2012
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Production of residual nuclei
Neutron excess of the final fragments (Sn ~ Sp) (Sn ~ Sp+Ec) The neutron excess of the final fragments is governed by the competition between proton and neutron evaporation Proton evaporation costs more energy (binding+Coulomb) than neutron evaporation (binding) The equilibrium is reached on the left of the beta-stability line This line is called “evaporation corridor” 124Xe+Pb 1 A GeV 136Xe+Pb 1 A GeV evaporation corridor José Benlliure, NP4GCR Grenoble, December 2012
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Production of residual nuclei
Detailed description of final fragments The microscopic nature of the process should be considered The “evaporation corridor” is not so universal as previously thought 136Xe+Pb 1 A GeV 124Xe+Pb 1 A GeV José Benlliure, NP4GCR Grenoble, December 2012
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Production of residual nuclei
Detailed description of final fragments The microscopic nature of the process should be considered The “evaporation corridor” is not so universal as previously though Charge exchange reactions (virtual pion exchange or excitation of the D resonance) may also introduce some differences * 136Xe(1000 A MeV)+Ti 136Xe(200 A MeV)+Ti 136Xe(500 A MeV)+Ti José Benlliure, NP4GCR Grenoble, December 2012
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Production of residual nuclei
Detailed description of final fragments The microscopic nature of the process should be considered The “evaporation corridor” is not so universal as previously though Charge exchange reactions (virtual pion exchange or excitation of the D resonance) may also introduce some differences Nuclear structure effects should also be considered as nucleon pairing N-Z=constant ● N=Z ■ N=Z+2 ▲N=Z+4 N=Z+6 ● N=Z+1 ■ N=Z+3 ▲ N=Z+5 José Benlliure, NP4GCR Grenoble, December 2012
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Production of residual nuclei
Kinematics of the final fragments Kinematic properties of residual fragments contribute to better understand the reaction mechanism For light nuclei, the kinematics made it possible to disentangle different de-excitation mechanisms 1 A GeV 136Xe+Pb José Benlliure, NP4GCR Grenoble, December 2012
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Future perspectives New data
Fragmentation and spallation reactions have been largely investigated because of their interest for the production of nuclei far from stability or the radiological characterization of spallation neutron sources Fragmentation and spallation of lighter systems have strong interest not only because of its implications in GCR propagation but also in hadron-therapy. Moreover, those nuclei present other de-excitation mechanisms that are of interest for investigating the dynamics of nuclear matter at high temperature, Fermi break-up and multi-fragmentation. The investigation of these de-excitation mechanisms characterized by the simultaneous emission of several José Benlliure, NP4GCR Grenoble, December 2012
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Future perspectives Experimental progress SPALADIN FIRST,R3B
Accurate measurements of heavy residual nuclei (A,Z) Charge identification of heavy residual nuclei SPALADIN ANDES Simultaneous detection of heavy and light fragments but only Z identification FIRST,R3B Future complete kinematic experiments with full identification of all fragments including neutrons and gammas. José Benlliure, NP4GCR Grenoble, December 2012
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Conclusions Fragmentation and spallation reactions have been largely investigated because these reaction mechanisms are an optimal tool for investigating the dynamics of nuclear matter at high temperature but also because they use for production of nuclei far from stability or neutrons in spallation neutron source. Today we have a good qualitative knowledge on the production of residual fragments in these reactions, in particular for heavy colliding nuclei. Experimental information on light systems is rather incomplete. An accurate description of the residual nuclei formation in these reactions, in particular for light systems, requires not only new data but also improve models for specific de-excitation mechanisms (Fermi break-up and multi-fragmentation) Present technologies could allow for complete kinematic experiments José Benlliure, EURISOL Town Meeting Lisbon, October 2012
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