Measurement of Evaporation Residue and Fission Cross Sections of the Reaction ³⁰Si + ²³⁸U at Subbarrier Energies

Abstract

Effects of the prolate deformation of ²³⁸U on fusion were studied in the reaction ³⁰Si + ²³⁸U at bombarding energies close to the Coulomb barrier. The fission (capture) cross sections were measured at the JAEA tandem accelerator to see the enhancement of the cross sections in the subbarrier energy due to the lower Coulomb barrier in the collisions of projectile at the polar sides of ²³⁸U. In order to obtain the direct evidence for complete fusion, evaporation residue cross sections were measured at UNILAC of GSI. At the subbarrier energy of E_c.m. = 133. 0 MeV, where only polar collisions to ²³⁸U occur, we measured three spontaneously fissioning nuclei which we assigned to the isotope ²⁶⁴Sg. The obtained cross section (10⁺¹⁰_-6) pb was smaller than the calculation that the system captured at the polar collisions results in complete fusion, suggesting the competition between fusion and quasifission. At the above barrier energy of E_c.m. = 144.0 MeV, where collisions on the equatorial side of ²³⁸U start to contribute, we observed α-decay chains starting from ²⁶³Sg and the cross section was determined to be (67⁺⁶⁷_-37) pb. The data was reproduced by the calculation that the system captured from the equatorial side of ²³⁸U results in complete fusion .