K.-H. Schmidt for the CHARMS collaboration Gesellschaft für Schwerionenforschung (GSI) Darmstadt, Germany Spallation Reactions - Physics and Applications work supported by EU (EURISOL and EUROTRANS)

Spallation reactions – physics and applications - Definition - Applications - Experiments - Physics and models - Conclusion Outline

Definition

What is a spallation reaction ? Violent collision of nucleons (or particles) with heavy nuclei. First studied with cosmic rays. Schopper et al. Naturw. 25 (1937) 557 Collision of a μ+ of 41.2 GeV with an iron nucleus, recorded by the KARMEN detector. Disintegration (spallation) of the nucleus in many pieces. Production of a variety of different particles and fragments.

Applications

Importance of spallation reactions EOS of nuclear matter - Spallation is a way to heat nuclear matter → thermal break-up Astrophysics - Reactions of cosmic rays with interstellar medium → origin of c.r. - Nucleosynthesis in turbulence of Supernova explosions Spallation neutron sources* - Efficient way for producing neutrons ADS* (Accelerator-driven system) - Project for incinerating radioactive waste Secondary-beam facilities* - Production of rare isotopes Radioprotection and medicine

Neutron sources TypeFacility Proton beam Neutron energy Time structure Neutron flux Purpose Fission reactor ILL Grenoble --- cold, thermal, epithermal continuous n/(cm 2 s) mostly solid state Spallation neutron source SINQ Villigen 500 MeV, 1.8 mA continuous n/(cm 2 s) mostly solid state ISIS Rutherford 800 MeV, 200 μA 50 Hz, 400 ns mostly solid state SNS Oak Ridge 1 GeV, 1.4 mA 60 Hz, 695 ns mostly solid state n_TOF CERN 200 GeV/c thermal – several 100 MeV 0.42 Hz, 6 ns nuclear physics Example: Layout of SINQ → (Study of condensed matter.)

ISOL-based secondary-beam facilities FacilityProton beamOutput ISOLDE CERN≤ 1.4 GeVRare isotopes TRIUMF Vancouver200 MeVRare isotopes, neutrons, pions, muons EURISOL project1 GeVRare isotopes

ADS (Accelerator-driven system) Proton accelerator (≈ 1 GeV) Subcritical fission reactor Spallation neutron source Purpose: Incineration of nuclear waste Prototype: Myrrha (Mol, Belgium)

Experiments

Detector systems Normal kinematics (particle on nucleus) Inverse kinematics (nucleus on light target) Neutrons (d2Y/(dE dθ)) (kinematical detectors) Neutrons (total yield) (moderation and capture of neutrons) Light charged particles (d2Y/(dE dθ)) (ΔE -E, e.g. silicon) Heavy residues (a few independent yields, cumulative yields) (irradiation, off-line gamma spectroscopy / accelerator mass spectrometry) Heavy residues (Y(Z,A), dY/dv) (in-flight identification Bρ – ToF – ΔE) Neutrons and light charged particles (advanced installations at

Double-differential neutron spectra SATURNE experiment, S. Leray et al. (2002) Neutrons in forward direction reach up to the energy of the projectiles.

Spectra of light fragments PISA experiment, Jülich, F. Goldenbaum et al. (2003) Almost thermal energy spectrum of light fragments.

Excitation functions of heavy residues Titarenko et al, 2005 Independent and cumulative yields by off-line gamma spectroscopy

GSI facility → inverse kinematics  UNILAC : Up to 20 A MeV  SIS : 50 – 2000 A MeV, up to particles/spill  Beams of all stable nuclides up to 238 U

Fragment Separator (FRS)  max = 15 mrad  p/p =  1.5 % Resolution: -  (  )/   5·  Z   A / A  2.5  ToF   x 1, x 2  B   E  Z Nuclide identification ( 238 U + p, M. V. Ricciardi)

Benefit of inverse kinematics Protons (553 MeV) on lead 208 Pb (500 A MeV) on hydrogen Experiments in inverse kinematics: Complete overview on nuclide production (T 1/2 >100 ns) ; E > several 100 A MeV

Spallation of 238 U – complete overview Data measured at GSI* * Ricciardi et al, Phys. Rev. C 73 (2006) ; Bernas et al., Nucl. Phys. A 765 (2006) 197; Armbruster et al., Phys. Rev. Lett. 93 (2004) ; Taïeb et al., Nucl. Phys. A 724 (2003) 413; Bernas et al., Nucl. Phys. A 725 (2003) More than 1000 different nuclides observed. Features of spallation-evaporation / -fission / -IMF emission

Velocity distributions Typical velocity profiles are characteristic for the reaction mechanism (evaporation, fission and multifragmentation) P. Napolitani, 2007

Systematic studies

Collaboration GSI P. Armbruster, A. Bacquias, T. Enqvist, L. Giot, K. Helariutta, V. Henzl, D. Henzlova, B. Jurado, A. Keli ć, P. Nadtochy, R. Pleska č, M. V. Ricciardi, K.-H. Schmidt, C. Schmitt, F. Vives, O. Yordanov IPN-Paris L. Audouin, M. Bernas, B. Mustapha, P. Napolitani, F. Rejmund, C. Stéphan, J. Taïeb, L. Tassan-Got CEA-Saclay A. Boudard, L. Donadille, J.-E. Ducret, B. Fernandez, R. Legran, S. Leray, C. Villagrasa, C. Volant, W. Wlaz ł o University Santiago de Compostela J. Benlliure, E. Casarejos, M. Fernandez, J. Pereira CENBG-Bordeaux S. Czajkowski, M. Pravikoff 14 PhD

(New project at GSI) Neutrons Heavy fragments Exotic beam from Super-FRS Protons Target  -rays Neutrons Protons Tracking detectors:  E, x, y, ToF, B  Neutrons High-resolution spectrometer - Full identification of heavy residues with simultaneous measurement of neutrons, light charged particles and gammas with new R3B magnetic spectrometer.  Aiming for a kinematically complete experiment.

Physics and models

Nucleon-nucleus collision at 1 A GeV Ep 10 MeV137 MeV/c9.03 fm 100 MeV443 MeV/c2.79 fm 1 GeV1692 MeV/c0.73 fm Decisive parameter: de Broglie wavelength of a nucleon: =h/p Compared to nuclear radius (r = 1.16 fm  A 1/3 ) or range of nuclear force (  1 fm) Spallation reaction ≈ collisions of individual nucleons ! No consistent uniform description of the spallation process available.

Modeling of spallation reactions 1. Intranuclear cascade (INCL, ISABEL,...) (quasi-free nucleon- nucleon collisions → high-energy n, p..) 2. Exciton model (sequence of particle- hole excitations → pre-equilibrium emission, included in INCL) Evaporation code (ABLA07,...) (evaporation of particles and fragments, fission) 3. Multifragmentation (expansion and thermal break-up) Specialized codes for different steps of the reaction

Thermal expansion ρ~e S level density S=2√(aE*) Fermi gas a~V level-density parameter grows with volume E* = E 0 * - c·(V-V 0 ) 2 parabolic dependence of nuclear binding on volume or density S~√(V(E 0 *-c · (V-V 0 ) 2 ) Statistical model: The nucleus assumes the configuration which offers maximum number of states. This is also true for the volume.

Multifragmentation Expansion may lead to multifragmentation. (SMM, ABLA07)

The evaporation corridor Decisive influence of evaporation on the nuclide distribution. Residues tend to follow the evaporation corridor (Dufour, Charity).

Fission Fission barrier → Interplay of surface and Coulomb energy.

General features of fission Potential barrier as a function of mass asymmetry. Symmetric fission for heavy systems

Experimental information – low energy K.-H. Schmidt et al., NPA 665 (2000) 221 Experimental survey at GSI by use of secondary beams

Modeling multi-modal fission E* = 60 MeV 20 MeV 10 MeV Many different nuclei with different E* contribute to fission. black: data, red: simulation with ABLA07

Dynamics of fission Fission is a dynamical process, described by the Langevin equation.

Langevin trajectories Fission is hindered by dynamics with respect to evaporation. Fission barrier Ground state

Generalized fission Potential barrier as a function of mass asymmetry. Continuous mass distribution from particle evaporation to symmetric fission (Moretto)

Emission of intermediate-mass fragments Evaporation of IMF (very asymmetric fission) must be considered. (only n,p,α) (n, p, all fragments) Data: 209 Bi + p Yu. E. Titarenko et al., Nucl. Instrum. Methods A 562 (2006) 801

Model Calculation INCL4 + ABLA07

Conclusions - Many fields of application → high interest for good understanding - Two experimental approaches - Direct kinematics - light particles: yields and energy distributions – heavy residues: only long-lived species and cumulative yields - Inverse kinematics – heavy residues: complete overview (≈1000 nuclides / system) - velocity spectra: information on reaction mechanism - new-generation (complete) experiments at - Elaborate codes for the reaction stages (e.g. INCL4 + ABLA07) - INC → (Exciton) → ( Thermal break-up) → Evaporation-fission Spallation reactions

Additional slides

Experimental challenge Short-lived as well as stable nuclei have to be detected.

Excitation functions Titarenko et al, 2005 Independent and cumulative yields - About 100 nuclei/system - Uncertainty 7 – 30 % Additional information: - Miah et al, Nucl. Sc. Tech. Suppl. 2 (2002) Schiekel et al, Nucl. Instr. Meth. B114 (1996) 91 - Adilbish et al, Radiochem. Radioanal. Lett. 45 (1980) Chu et al, Phys. Rev. C 15 (1977) 352

Velocity distributions 238 U (1 AGeV) + 2 H Pereira, PhD thesis For each nucleus: production cross section, velocity and production mechanism FISSION FRAGMENTATION

Experimental progress by inverse kinematics ProjectileTargetEnergy [A GeV] 56 Fe 1 H, 2 H ,124 Xe 1,2 H, Be, Ti, Pb0.2, 0.5, Au 1H1H Pb 1,2 H, Ti0.5, U 1,2 H, Ti, Pb1 Data accuracy: Statistic: below 3% Systematic: % More than 1000 nuclei/system measured Data available at: