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Sub-Barrier enhancement of Fusion Probability in 6 He ?

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### Sub-Barrier enhancement of Fusion Probability in 6He ?

Study of 6He reactions on 238U target

core

Least bound

orbital states

r

z1

z2

r

Strong force overcomes

Coloumb repulsion

(Fusion barrier surmounted)

Competing Factors

Halo Nuclei and Fusion ProbabilityEnhanced:

Strong force : Short ranged within nucleus

Degree of overlap of nuclear matter couples strongly to fusion probability

Seen in heavy deformed nuclei (16O on 148Sm)

Halo nucleus thus can provide longer ranged attractive force

Other enhancements shown in other nuclei through other channels of neutron transfer

Dasso et al. Phys. Rev. C 50, R12 (1994).

Suppressed:

Results in weakly bound neutron states Esep~.971MeV for 2n states in 6He

(~8MeV for 1n stable nuclei 4He)

Break-up occuring before fusion barrier

Incomplete fusion; other fragments escaping

Hussein et al., Phys. Rev. C 46, 377 (1996).

Previous experiments:

6He + 238U Trotta et al. PRL, 84 2342Enhancement

11Be + 209Bi C. Signorini et al., E. Phys. 2, 227 No effect

6He + 209Bi P. DeYoung et al., Phys. Rev. C. 58 3442Enhancement

Experimental Signatures

Fission is experimental signature for fusion 6He on 238U

Other mechanisms can also cause fission

inelastic scatter, nucleon transfer, or breakup

due to weak binding, breakup s large

Complete

Incomplete

R. Raabe et al., Nature 431, 823

Experiment able to differentiate between true fusion and fission

induced by breakup/transfer channels!

Effect of transfer mechanism found to be larger than breakup

for 6He+65Cu reactions (A. Navin, Phys Rev C 70, 044601)

Significant, must be disentangled

Reactions

R. Raabe et al., Nature 431, 823

6He beam:15MeV to 29.5MeV onto UF4 target

Complete fusion events:

6He + 238U : complete fusion ~20 MeV

fission barrier ~5.4 MeV

Events other than complete fusion:

3rd projectile like particle

Must be charged (if result of transfer is alpha

then Q = -6 MeV and fission not possible)

Fission fragments detected as charged particles

Silicon detectors detect charged particles

2 coincidence and 3 coincidence events measured

Clear enhancement in fission signal for halo nucleus

Reaction Analysis

- Significant number of 3 coincidence interactions seen for 6He; none for 4He
- Reactions were of direct nature
- Angular distributions peaked at grazing angle
- Energy of fragment too high for inelastic scatter
- 1 n transfer COM distribution not consistent with observations (isotropic breakup of 5He leads to spread in energy of alpha)
- Distorted Wave Born Approximation calculations for 2-n transfer to excited states in 240U

Ranges based on Q-dependence

and energy required for fission

for different processes

Only multiplicity 3 events below barrier

Overwhelmingly due to 2n transfer

Cannot rule out influence of other direct processes

No fission induced

No sub-barrier enhancement!

Enhancement of fusion?
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