Romualdo de Souza

Ternary Fission and Neck Fragmentation

Romualdo de Souza

Ternary Fission

In studying the interplay of Thermodynamics and Dynamical effects, ternary fission is a superior choice over multifragmentation due to low excitation and large deformation.

There are 3 probes of nuclear dynamics in fission:

1) Total kinetic energy of fission fragments

2) Pre-scission emission of neutrons

3) Ternary fission

Ternary fission can provide information on both the magnitude as well as the tensorial properties (one body vs. two body) of nuclear dissipation.

N. Carjan, A. Sierk, and J.R. Nix, Nucl. Phys. A452, 381 (1986)

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Ternary fission can be used as a probe of the scission configuration

nth, ,12C 232Th

Excited Compound Nucleus Possible

Emission of an IMF

neck

Net Force Scission axis

neutrons

Fission fragments

Spontaneous Fission

L (deg.)

E (

MeV

)

Neck (equatorial)emission >97%

Polar Emission 0.3%

252Cf

Theobald et al.

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Cosper et al. [ S.W. Cosper, J. Cerny, and R.C. Gatti, Phys. Rev. 154 1193 (1967).]

• Neck fragments are focused relative to the scission axis.

• Neutron rich isotopes are favored

• Kinetic energy spectra are approximately gaussian.

• Heavier clusters observed: e.g. 10Be

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Experiments on IMF/Cluster Production in Hot Ternary Fission

• 3He + 232Th Elab/A = 90 MeV (IUCF) D.E. Fields et al., PRL 69 3713 (1992)

• 4He + 232Th Elab/A = 50 MeV (IUCF) S.L. Chen et al., PRC 54 R2114 (1996)

• 12C + 232Th Elab/A = 22 MeV (MSU) R. Yanez et al., PRL 82, 3585 (1999)

• 12C + 232Th Elab/A = 16 MeV (ANL)

4He + 232Th E/A=50 MeVE* 140 MeVmax 33

12C + 232Th E/A=22 MeV

E* 230 MeV

max 120 > RLDM 70

Intermediate Mass Fragments (IMF): 3 Z 20

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Focused angular distribution (due to cancellation of Coulomb forces along scission axis)

D.E. Fields et al Phys. Rev. Lett. 69, 3713 (1992)

Detector orthogonal to scission axis!

Other three Detectors are non-orthogonal to scission axis!

Ternary FissionLow kinetic energies (due to emission from an extended system)

3He + 232Th at Elab/A=90 MeV

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Experimental Setup of 4He + 232Th E/A=50 MeV

95.0

BeamTarget

55

PPAC/MWPC

PPAC/MWPC

30 cm

4-pack160.8

143.2

100.0

• 5cm x 5cm 300 m quadrant Si design

• axial IC design

•18-20 torr CF4 in IC

• 3cm CsI(Tl) with PD readout

E Threshold 0.7 MeV/u

• Measurement of IMFs at backward angles eliminates pre-equilibrium component

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Experimental Setup of 4He + 232Th E/A=50 MeV

95.0

BeamTarget

55

PPAC/MWPC

PPAC/MWPC

30 cm

4-pack160.8

143.2

100.0

E Threshold 0.7 MeV/u

• Measurement of IMFs at backward angles eliminates pre-equilibrium component

Experiments on IMF/Cluster Production in Hot Ternary Fission

• 3He + 232Th Elab/A = 90 MeV (IUCF) D.E. Fields et al., PRL 69 3713 (1992)

• 4He + 232Th Elab/A = 50 MeV (IUCF) S.L. Chen et al., PRC 54 R2114 (1996)

• 12C + 232Th Elab/A = 22 MeV (MSU) R. Yanez et al., PRL 82, 3585 (1999)

• 12C + 232Th Elab/A = 16 MeV (ANL)4He + 232Th E/A=50 MeVE* 140 MeVmax 33

12C + 232Th E/A=22 MeVE* 230 MeV

max 120 > RLDM 70

Intermediate Mass Fragments (IMF): 3 Z 20

Ion Chamber-Si-CsI(Tl)/PD telescopes

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• Single wire resolution in X (0.8 )

• Slightly larger than single wire in Y

• Pulse Height separation of FF from alphas

• Time of flight relative to RF (fission mass asymmetry)

Identification of FF by hybrid PPAC/MWPC

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0

200

400

600Be in4-pack

FF2

IMF FF1

FF2

IMF FF1

0

200

400

600C in4-pack

0 20 40 60 800

50

100

150

200 Be inDet. 5

CM Energy Spectra of Be and C12C + 232Th at E

LAB = 264 MeV

0 20 40 60 800

50

100

150

200C inDet. 5

ECM

(MeV)d

N/d

EC

M

Fragments Emitted in Carbon induced reactions show the same features as in the He induced reactions

KE spectra orthogonal to the scission axis are bimodal

Low energy component is focused orthogonal to scission axis

(Near scission/neck emission)

High energy component is “isotropic”

(early stage emission while system is still compact)