K. Satou

Effect of nuclear shell structure on fusion reaction

The dependence of the fusion reaction on the nuclear shell structure was investigated for the two reaction systems 82Se + 138Ba and 82Se + 134Ba, where the nucleus 138Ba has a closed neutron shell N=82, while the nucleus 134Ba has a neutron number 78. Evaporation residues for these fusion reactions were measured near the Coulomb barrier region. The measured evaporation residue cross sections for the reaction system 82Se + 138Ba were two orders of magnitude larger than those for the reaction system 82Se + 134Ba in the excitation energy region of 20–30 MeV. The evaporation residue cross sections were compared with those of the other reaction systems that produce the same compound nucleus as the present systems. It was found that the fusion reaction 82Se + 138Ba occurs without hindrance, while that of 82Se + 134Ba is considerably hindered, as commonly observed in the massive reaction system with the charge product ZpZt>1800 of projectile and target. This suggests the importance of the shell closure N=82 in the heavy-ion fusion reaction.

Dependence of Heavy-ion Fusion Reaction on Nuclear Deformation and Nuclear Shell Structure

The dependence of the fusion probability on the orientation of deformed nucleus was investigated for the reactions 60, Ni + Sm and Ge + Nd. Evaporation residues were measured for these reaction systems by the JAERI recoil mass separator in the vicinity of the Coulomb barrier and the fusion probability was extracted as a function of bombarding energy. It was found that the fusion probability depends strongly on the orientation of the nuclear deformation. The fusion probability is considerably reduced when the projectiles collide at the tip of the deformed nuclei. On the other hand, when the projectiles collide at the side of the deformed nuclei, the fusion occurs without hindrance. This phenomenon is understood qualitatively by comparing the distance between the mass centers of two colliding nuclei at touching with the position of the saddle point of the compound nucleus. The dependence of the fusion probability on the nuclear shell closure was also investigated for the reactions Se + Ba, where the nucleus Ba has a closed neutron shell of N = 82 and the nucleus Ba has the neutron number N = 78, four neutrons less than the closed shell. The measured evaporation residue cross section for the reaction Se + Ba was well reproduced by statistical model calculations taking into account a subbarrier fusion enhancement, while the evaporation residue cross section for the reaction Se + Ba was about 100 times smaller than that for the fusion reaction Se + Ba. This suggests that the shell closure plays an important role in the fusion process.

Measurement of Evaporation Residue Cross Sections for the Sub‐barrier 16O + 238U Reaction

Evaporation residue cross sections in the reaction of 16O+238U were measured for the energy range from above‐ to extreme sub‐barrier as direct evidence of complete fusion. The cross sections are reproduced by a statistical‐model calculation, for which partial cross sections are calculated by a coupled‐channel model taking into account the prolate deformation of 238U. It is found that complete fusion occurs in the collision of the projectile with the tips of the 238U target, the same as in the side collision.

Effects of Nuclear Deformation on the Fusion Probability in the Reactions of 76Ge+150Nd and 82Se+natCe

Evaporation residue (ER) cross sections for the 76Ge+150Nd and 82Se+natCe reactions were measured near the Coulomb barrier in order to investigate the effects of nuclear deformation on the fusion process. The first reaction represents fusion involving the prolately deformed target 150Nd, whereas the latter reaction is the fusion with the spherical target natCe. We obtained fusion probabilities from the experimental data with the aid of calculated survival probabilities. The system 82Se+natCe showed fusion hindrance in the form of the extra-extra-push energy of 27±5 MeV, whereas the system 76Ge+150Nd does not show any hindrance at the bombarding energy corresponding to the Coulomb barrier for the collision of 76Ge on the side of 150Nd. Since the side collision is compact in configuration at touching, our results suggest that the reaction starting from the compact touching point results in a higher fusion probability.

Effects of Nuclear Deformation on Fusion Probability in the Reactions of 76Ge+150Nd and 82Se+natCe near the Coulomb Barrier

Evaporation residue (ER) cross sections for 76Ge+150Nd and 82Se+natCe were measured near the Coulomb barrier in order to investigate the effects of nuclear deformation on fusion process. The former reaction represents fusion involving the prolately deformed target l50Nd, whereas the latter reaction is the fusion using spherical target natCe. We obtained fusion probability from the experimental data with the aid of calculated survival probability. The system 82Se+natCe showed fusion hindrance in the form of extra-extra-push energy of 27±5 MeV, whereas the system 76Ge+150Nd does not show any hindrance at the bombarding energy corresponding to the Coulomb barrier of the collision of 76Ge on the side of 150Nd. Since the side collision is compact in configuration at touching, our results indicates that the reaction starting from the compact touching point results in higher fusion probability.