Competitive Reaction Mechanisms in Exotic Nuclear Reactions

1
Competitive Reaction Mechanisms in Exotic Nuclear
Reactions
Yoritaka Iwata1, Naoyuki Itagaki2, Takaharu
Otsuka1,2,3
1Department of Physics, University of Tokyo.
2CNS, University of Tokyo.
3RIKEN.
Thanks to
J. A Maruhn (University of Frankfurt)
K. Yabana T. Nakatsukasa (Univ. Tsukuba)
C. Simenel (Saclay)
2
Reaction mechanism with exotic nuclei
i) Different effect of Pauli Principle
Based on quantum numbers (equivalence
difference )
ii) Spin excitation mechanism
For spin difference
iii) Isospin dependent motion
For isospin difference
The competition between these i), ii), iii)
3
Reaction mechanisms I

• Spin dependence of the force ( spin-orbit
  force)
• causes spin-excitation

On the reaction plane
4
Reaction mechanisms II

• Isospin dependence of the force causes
  isospin-excitation

or
or
n
p
p
n
Stronger attraction
Stronger attraction
Neutron-rich
Proton-rich
probe
Effective interaction
or
Nucleon
Target
(as a probe)
Target nucleus
5
Validity of isospin dependent motion
The isovector GDR is experimentally observed in
a-scatterings
Exp.
Isovector expectation value lt Isospin
dependent motion of each nucleon
4He 40Ca ? 44Ti
TDHF calc.
Non-trivial oscillation exists !
Contact time 225fm/c
6
The competition Specific dynamics
Isospin p or n
Pauli Principle
Spin or -
? Independent motion
Spin excitation
Isospin excitation
For spin difference
For isospin difference
time
time
Neutron
Proton
Spin-up
Spin-down
space
space
Trajectory
Trajectory
7
Alpha-particle in collision
initial alpha-particle
Dynamically fluctuating alpha-particle

• For the initial a-particle, the center-of-mass
  positions and the center-of-mass velocities are
  taken to be exactly the same.

Interaction between the target

• For a bounded a-particle, internal interactions
  (blue arrows in the figure) are equilibrated.

(External interaction)

• For an isolated a-particle (before the
  collision), external interactions (red arrows in
  the figure) are turned off.

Domain limit of attraction due to the nuclear
force from the target nuclei
8
We would like to see the competition of these
fundamental dynamics
Dynamically fluctuating a-particle
casting an a-particle for Ca-isotopes
a-particle
Nice probe for the competition
9
4He 40Ca ? 44Ti
TDHF3d
b 0
total
neutron
proton
Spin excitation is relatively dominant
10
Time hindrance of spin-excitation in exotic
reactions
4He ACa ? A4Ti
b 0
High mountain
Hindrance of spin excitation
Hindrance of spin excitation
A 40
Low land
Small mountain
A 34
A 46
The spin excitation is mainly observed in the
collision with total isospin symmetric case
i.e. A40, Z20 (in this case).
11
Theoretical framework for competitive reaction
mechanism
Iwata-Itagaki et. al.
l amplitude of excitation
linear
nonlinear
12
Non-central collision exhibiting the competition
TDHF3d
b 2.5
total
alpha 40Ca
neutron
proton
TDHF Center-of-mass trajectory (200 fm/c to 1125
fm/c)
On the reaction plane
Dineutron, diproton-like cluster
Deuteron-like cluster
gt
neutron v.s. proton
spin-up v.s. spin-down
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Fusion reaction with heavy nuclei by TDHF3d
Exotic reaction mechanism
Now in progress
i) Pauli Principle
ii) Spin excitation mechanism
iii) Isospin dependent motion
iv) Coulomb force
For fusion reactions with heavy nuclei, it should
be prominent !
The competition between these i), ii), iii), iv)
Advice from Prof. Dr. K. Yabana Prof. Dr. T.
Nakatsukasa (Univ. Tsukuba)
The attention are especially paid to the Coulomb
int.
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Fusion reactions with heavy nuclei
A1
A2
Head-on collision
Z1
Z2
Ri r0 A 1/3
(spherical nuclei)
R1
Fusion process
RC R1 R2
Ref.)
1 Swiatecki, 82.
2 Wada and Abe
3 Washiyama, Yilmaz, Ayik and Takigawa (Quantum
diffusion theory)
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Barrier top fusion
Box 96fm80fm80fm
16O 16O
157.5 fm/c
112.5 fm/c
292.5 fm/c
202.5 fm/c
247.5 fm/c
Keep oscillating
Contact (1st Barrier)
472.5 fm/c
512.5 fm/c
382.5 fm/c
337.5 fm/c
427.5 fm/c
Keep oscillating
Fusion occurs
Total density plot
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Barrier top fusion
132Sn 132Sn
Box 96fm80fm80fm
Repulsion
67.5 fm/c/c
157.5 fm/c/c
247.5 fm/c/c
Contact (1st Barrier)
The 2nd internal Barrier (Extra-push)
337.5 fm/c/c
427.5 fm/c/c

• Central peak density distribution can never
  achieved in this case.

Total density plot
Fusion never occurs
Hindrance by two steps T.Abe, T.Wada et al.
17
100Sn 100Sn
132Sn 132Sn
Barrier-top fusion (TDHF3D)
80Zr 80Zr
Boundary of the hindrance
56Ni 56Ni
40Ca 40Ca
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Summary Note

• Two principal reaction dynamics in heavy-ion
  collisions

• Spin excitation originated from the spin-orbit
  force

gtgtgt dynamical LS-splitting due to time-odd part
J.A. Maruhn, P.-G.Reinhard et al., PRC74 (2006)

• Iso-spin-excitation originatedfrom isospin
  dependence of the force.

gtgtgt It is also the origin for isovector-GDR.

• There is a competition between these two
  reactions including the Pauli effect.

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Summary
Reaction mechanism in the frontier of heavy nuclei
i) Pauli Principle
ii) Spin excitation mechanism
iii) Isospin dependent motion
iv) Coulomb force
For fusion reactions with heavy nuclei, it should
be prominent !
The competition between these i), ii), iii), iv)