Breakup effects of weakly bound nuclei on the fusion reactions C.J. Lin, H.Q. Zhang, F. Yang, Z.H. Liu, X.K. Wu, P. Zhou, C.L. Zhang, G.L. Zhang, G.P. An, H.M. Jia, and X.X. Xu ( China Institute of Atomic Energy )

§ Dynamics of both fusion and elastic scattering are influenced by coupling to direct-reaction channels (including breakup if the projectile and/or target nuclei are weakly bounded); § These couplings generate a distribution of potential barriers; § Barrier distributions can derived from the excitation functions of fusion and elastic (or quasielastic) scattering.

Barrier distributions extracted from fusion excitation functions N. Rowley et al., Phys. Lett. B254, 25(1991)

48 Ca+ 90,96 Zr fusion ◆ The experiment was carried out at the XTU Tandem accelerator of Legnaro National Laboratory, Legnaro Italy; ◆ The evaporation residues (ER) were separated from the background of beam and beam-like particles by means of an electrostatic deflector ;

The energy ranges are : for 48 Ca+ 90 Zr for 48 Ca+ 96 Zr in 0.5 MeV energy steps.

Barrier distribution deduced from quasielastic scatering ※ is the transmission coefficient for angular momentum ; ※ the reflection coefficient of.

※ is the quasielastic differential cross section at back angles. H. Timmer et al., Nucl. Phys. A584, 190(1995)

Barrier distributions from elastic scattering N. Rowley et al., Phys. Lett. B373, 23 (1995)

Barrier distributions for tightly-bound projectiles

The results of tightly bound projectile can be taken as a reference for probing the break- up effects of weakly bound- projectiles on fusion

Barrier distributions for 16 O+ 152 Sm quasielastic scattering ※ The experiment was carried out with 16 O beam from the HI-13 tandem accelerator at CIAE, Beijing; ※ Three gas-ionization chambers set at 156 0,160 0,164 0 were used to measure scattering particles.

32 S+ 90,96 Zr experiment ※ The excitation functions of quasielastic scattering were measured at back angles. ※ The barrier distributions for these two systems are compared. ※ The influence of neutron transfer on the barrier of 32 S+ 96 Zr are observed. This system have positive Q-values for 1n, 2n, and 4n neutron transfer.

Structures are very similar at high energies.

Comparison of the barrier height distributions obtained from excitation functions of fusion and elastic scattering for more systems. (N. Rowley et al., Phys. Lett. B 373(1996)23)

Conclusion 1: Except some details, the barrier distributions derived from the data of fusion and elastic/quasielastic scattering are almost the same.

Barrier distributions for weakly bound projectiles The experiments were carried out at the HI-13 tandem accelerator at CIAE, Beijing;

6,7 Li are weakly bound stable light nuclei.

6 Li+ 208 Pb fusion The compound nucleus 214 At formed following complete fusion de-excites dominantly through 1n, 2n, 3n, 4n evaporation resulting residue At. These Nuclei are particle emitters. Their half-life time and energies are known.

On-line measurements

Off-line measurements

Elastic and breakup experiment The experiments were carried out at the HI-13 tandem accelerator at CIAE, Beijing with 6,7 Li beams bombarding 208 Pb target. The detector system arrangement is similar to 16 O+ 152 Sm experiment.

E=33MeV

Conclusion 2: For the weakly bound projectiles, the Barrier distributions extracted from the data of (complete) fusion and scattering are different, but nearly the same if the contributions of breakup are added into the differential cross sections of quasielastic scattering.

Possible Reason Barrier distribution is a good probe for the breakup effects on fusion and elastic scattering.

Complete fusion of weakly bound nuclei at above barrier energies Fusion cross section of tightly-bound nuclei can be satisfactory described with the semi-classical approach formula at above barrier energies:

The tightly bound system 16 O+ 208 Pb can be taken as a reference for study of breakup effects.

Complete fusion and total fusion ※ For weakly bound-projectile, the total fusion cross section is the sum of the complete fusion and partial fusion cross sections,. ※ The fusion of weakly bound nuclei is suppressed at the above barrier energies.

Conclusion 3: ◆ The complete fusion of weakly bound projectile with heavy target is suppressed, whereas in most cases breakup has very little effects on the total fusion at the above barrier energies; ◆ This result indicates that partial fusion of weakly bound nuclei most likely takes place near the absorption region.

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