Fusion of 16,18O + 58Ni at energies near the Coulomb barrier Huiming Jia Nuclear Reaction Group, China Institute of Atomic Energy

Outline ♠ Introduction ♠ Some available result for O + Ni ♠ Experiment and result ♠ Systematic analysis ♠ Conclusion

Introduction Basic aspect: Fusion reaction between heavy-ions involves complex mass rearrangement and therefore complex reaction dynamics. …offer an opportunity for examining and improving the theories for the reaction dynamics. Some relevance: …the production mechanism of the solar energy. …the production of nuclei in massive pre-supernova stars via tunneling processes. …the current successful way for synthesizing the new elements and especially the superheavy elements (SHE).

Introduction: theoretical description… A single barrier penetration… Barrier Distribution Quantum Classical Coupled-channels (CC) effects -> Multi-dimensional barrier penetration. Neck-formation model: NPA 472, 571 (1987); 500, 195 (1989); PLB 201, 22 (1988). CCFULL: K. Hagino et al., Comput. Phys. Commun.123, 143 (1999). Semiclassical approach - Pollarolo: Phys. Rev. C 62, 054611 (2000). Semiclassical approach - Zagrebaev: Phys. Rev. C 67, 061601(R) (2003). Microscopic DC-TDHF: R. Keser et al., Phys. Rev. C 85, 044606 (2012). ……

Introduction The CC effect due to inelastic excitation states has been described well, but the positive Q-value neutron transfer (PQNT) effect is still not clear. …first experimental discovery: Beckerman et al., Phys. Rev. Lett. 45, 1472 (1980). …first theoretical explanation: Broglia et al, Phys. Rev. C 27, R2433 (1983); Phys. Lett. B 133, 34 (1983). ♠…the rudiment of the multi- dimensional barrier penetration model. ♠…the origin of the idea that the increased kinetic energy, in the intermediate-channel after n-transfer, enhances the fusion cross section.

Introduction Experimental inconsistence: …positive Q-value n-pickup… A.M. Stefanini et al., Phys. Rev. C 52, R1727 (1995). Experimental inconsistence: …positive Q-value n-pickup… But, most systems for the corresponding n- stripping show no such an effect: 17O + 117Sn (Q-1n = +5.18 MeV), 18O + 74Ge,112Sn, 36S + 58Ni… …only one system shows enhancement due to the positive Q-value n-stripping system: 18O + 58Ni, Q-2n = +8.20 MeV. Some experimental study of the correlation of the fusion enhancement with the transfer reactions. P. Jacobs et al., Phys. Lett. B 175, 271 (1986) Theoretical difficulty: ♠ The complexity of the transfer reaction: orbital populations, optimal Q-window… ♠ Coupling: form factor, coupling strength…

Some available result for O + Ni D. Pereira et al., PRC 73, 014601 (2006) A.M. Borges et al., Phys. Rev. C 46, 2360 (1992). Q-2n = +8.20 MeV …bigger error. …not extend to lower energy region. Explanation: 2n stripping leads to the population of the 2n-pairing vibrational state of 60Ni.

Some available result for O + Ni R.F. Simões et al., Phys. Lett. B527, 187 (2002). ‘qe’: quasi-elastic scat., including the direct processes of elastic, inelastic and transfer… @ θlab. = 161° …a sign of a larger quasi- elastic flux deviated to fusion. …a complex barrier distribution.

Some available result for O + Ni PACE4: 31.21, 31.59MeV V.I. Zagrebaev, Phys. Rev. C 67, 061601(R) (2003). Experimental data from A.M. Borges et al., Phys. Rev. C 46, 2360 (1992).

H.Q. Zhang et al., Chin. Phys. C 34, 1628 (2010). Experiment and result ♠ Electrostatic deflector ♠ HI-13 tandem accelerator at CIAE ♠ Separation: electrical rigidity η = E/q; Optimal high voltage; ♠ Identification: TOF-E (MCP + QSD); ♠ Angular distribution; ♠ Transmission efficiency. H.Q. Zhang et al., Chin. Phys. C 34, 1628 (2010).

Experiment and result ♠ Beam: 16,18O, 2-20 pnA, 31.8 – 60 MeV, energy step 2.4/1.2/0.6 MeV. ♠ Target: 58Ni, 99.80% isotope enrichment. ♠ Beam monitors: 4 Si detectors at θlab. = 17°.

Experiment and result CCFULL: K. Hagino et al., Comput. Phys. Commun.123, 143 (1999). Akyüz -Winther Potential (energy-independent bare)

Experiment and result …modest fusion enhancement effect! …the possible difference between DFus and DQEL?

Systematic analysis The Q-value for multi-neutron pickup from g.s. to g.s. CCFULL …bare Akyüz-Winther energy-independent potential + inelastic coupled-channels effect.

Systematic analysis Residual enhancement (RE) (except inelastic coupling effect) Benchmark: RE ~ 1 for the reference systems without expected PQNT effect. C.L. Jiang et al., Phys. Rev. C 82, 041601(R) (2010).

Systematic analysis …a positive-going correlation of RE with the Q-values.

Systematic analysis …a negative-going correlation of RE with the Q-values. ？the inelastic effect of 94Zr. ？the effect of the pairing-vibration state 02+ after 2n transfer

Conclusion The fusion excitation function of 16,18O + 58Ni at near-barrier energy region was re-measured with high precision. Only modest fusion enhancement effect for 18O + 58Ni compared with the available data and coupled-channels calculations, although of the high Q-2n value. Systematic analysis based on CCFULL code may provide a further reliable proof for the occurrence of the PQNT effect. The systematic analysis gives a visualized sub-barrier fusion enhancement excluding the inelastic coupling effect. This shows a complex correlation of the enhancement effect with the neutron- transfer channels.

Thanks for your attention!