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Sub-Barrier enhancement of Fusion Probability in 6 He ?

Sub-Barrier enhancement of Fusion Probability in 6 He ?. Study of 6 He reactions on 238 U target. r. core. Least bound orbital states. r. z 1. z 2. r. Strong force overcomes Coloumb repulsion (Fusion barrier surmounted). Competing Factors. Halo Nuclei and Fusion Probability.

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Sub-Barrier enhancement of Fusion Probability in 6 He ?

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  1. Sub-Barrier enhancement of Fusion Probability in 6He ? Study of 6He reactions on 238U target

  2. r core Least bound orbital states r z1 z2 r Strong force overcomes Coloumb repulsion (Fusion barrier surmounted) Competing Factors Halo Nuclei and Fusion Probability Enhanced: 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

  3. 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

  4. 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

  5. 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

  6. 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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