Nuclear Reactions II

1
Nuclear Reactions - II
A. Nucleon-Nucleus Reactions A.1
Spallation A.2 Induced Fission B.
Nucleus-Nucleus Reactions B.1
Fragmentation B.2 Multifragmentation B.3
Vaporization
2
Nuclear Reactions - II Intermediate and
Relativistic Energies

• Spallation
• Induced Fission
• Fragmentation
• Multifragmentation
• Vaporization

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A-1 Spallation - 0 Introduction
Generalities Light particle emissions
Residues The two stage of the spallation
process The Intra-Nuclear Cascade
Production of pions Evaporation Comparison
with experimental results Main physical
aspects Applications
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A.1 Spallation 1 Generalities
Definition (Encyclopedia Britannica) high-energy
nuclear reaction in which a target nucleus struck
by an incident (bombarding) particle of energy
greater than about 50 million electron volts
(MeV) ejects numerous lighter particles and
becomes a product nucleus correspondingly lighter
than the original nucleus. The light ejected
particles may be neutrons, protons, or various
composite particles equivalent
projectile (p, n, p, ...)
target
Drawing from the INC model
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A.1 Spallation 2 Generalities
1947 E.O. Lawrence discovers that the number of
emitted nucleons, especially neutrons, may be
quite large depending upon the conditions of the
spallation reaction. 90s new interest for
spallation reactions ? they are of pivotal
importance for the development of powerful
neutron sources for various purposes hybrid
systems, be devoted to energy production or to
incineration of nuclear wastes so-called
multi-purpose spallation sources devoted to
irradiation studies, material structure analysis,
future tritium production units Other
interest the production of isotopes ?
spectroscopy ? reaction
mechanisms ? astrophysics Main
aspects energy spectrum and angular
distribution of emitted light particles
production rate of residues
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A.1 Spallation 3 Light particle emission
Neutron spectra
Conclusion the spallation reaction is a TWO STEP
process!
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A.1 Spallation 4 Residues
They are observed in the very late stage of the
process after the cascade, the evaporation, and
the possible b-decay of very-short-lived emitters.
fission products
spallation residues
very light fragments produced by fast ejection
(cascade) evaporation of the remnant
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A.1 Spallation 6 Residues
208Pb p at 1 AGeV
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A.1 Spallation 7 The two stages of the
spallation process

• Individual nucleon-nucleon scattering

The incident proton loses part of its energy
cascade ( 1022 s 30 fm/c)
projectile (p, n, p, ...)
target
2. Evaporation of the residue
( 1020 - 10-16 s)
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A.1 Spallation 8 The Intra-Nuclear Cascade
1. J. Cugnon, Phys. Rev. C 22(1980)1885 2.
H.W.Bertini Phys.Rev.188(1969)1711 3. Y. Yariv
and Z. Fraenkel, Phys. Rev. C 20(1979)2227
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A.1 Spallation 9 The Intra-Nuclear Cascade
Liège
Bertini/Isabel
D
D
p
p
in the nucleus
in the nucleus
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A.1 Spallation 10 Production of Pions
The pions are the decay products of the D
resonances. They are mesons, exchange particles
of the strong interaction between nucleons. The D
resonances and pions are coming from inelastic
nucleon-nucleon reactions at beam energies above
few hundred of AMeV (Mp 140 MeV).
Pion cycle ( D ? Np ? D )
NN ? DN hard D production D ? Np D
decay DN ? NN D absorption Np ? D soft
D production
4 sorts of Ds
D ? 1 (pp) D ? 2/3 (pp0) 1/3 (np) D0
? 2/3 (np0) 1/3 (pp-) D- ? 1 (np-)
C. Hartnack et al., Nucl. Phys. A 495(1989)303
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A.1 Spallation 11 Evaporation
These models are used to simulate the second step
of the reaction, i.e. the decay of the excited
residue. Main features - the life-time of
the excited residue is much longer than its
formation time T(residue) several hundreds of
fm/c T(formation) 30 fm/c - the
individual properties of the quantum states have
a negligible effect at high excitation
energy, due to the small distance between the
energy levels, in particular in heavy
nuclei. All the states are equiprobable so the
deexcitation of the nucleus may be treated in a
statistical way. In other words, a statistical
model considers the probabilities of the
different deexcitation possibilities with
comparable weights, which correspond to
individual processes of similar time length.
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A.1 Spallation 12 Evaporation
Most of the current evaporation code describe the
residue deexcitation according to the Weisskopf
theory which is based on the energy conservation
and the assumption of the micro-reversibility of
the process. This assumption is verified for the
light particle emissions (n, p, d, t, 3He, 4He),
but not for the emission of heavier nuclei or for
fission.
Note the average total neutron multiplicity is
about 4 times the multiplicity of the neutrons
coming from the cascade stage.
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A.1 Spallation 13 Comparison with experimental
results
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A.1 Spallation 14 Comparison with experimental
results
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A.1 Spallation 15 Main Physical Aspects

• 1. The available incident energy is
  progressively shared by the incident particle
  itself, the ejectiles, the pions and the target.
• The proton travels trough the target in 10 fm/c
  (1 fm/c 3.10-24 s)
• A large amount of the energy is removed quickly
  ( 20 fm/c) by the emission of a few fast
  nucleons and some pions.
• The nuclear density is depleted when the proton
  enters the target.
• A kind of hole is drilled into the target.
• The density becomes more or less uniform before
  the ejection of the fast particles is over.
• The relative energy transfer is maximum for
  incident energies between 1 and 2 GeV.
• On average, the proton loses energy with a rate
  which is universal.

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A.1 Spallation 16 EURISOL
EURopean Isotope Separation On-Line radioactive
nuclear beam facility ? http//www.ganil.fr/euri
sol/ ? radioactive beams at very high
intensities
Applications proton neutron
drip-lines changes in shell structure
halo structure high-spin physics isospin
effects in nuclear matter superheavy
nuclei nuclear astrophysics tests of
the standard model muons and
antiprotons solid state physics
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A.1 Spallation 17 Spallation sources
European Spallation Source (ESS) EU ?
http//www.neutron-eu.net/en/index.php ???? Spall
ation Neutron Source (SNS) Japan ?
http//jkj.tokai.jaeri.go.jp/
(2007) Spallation Neutron Source (SNS) USA ?
http//www.sns.gov (2006!)

• Applications
• - chemistry
• complex fluids
• crystalline materials
• disordered materials
• engineering
• magnetism and
• superconductivity
• polymers
• - .

Note a proton of 1 GeV on Pb target with a
thickness of 60 cm produces about 25 neutrons!