Projectile fission in 58Ni-induced reactions at 15.3 MeV/u

The present study was intended as a search for sequential fission of projectilelike fragments for reactions induced by intermediate-mass heavy ions. The objective was to determine the fission probability as a function of excitation energy or energy loss in the first step of the reaction. The experimental configuration was optimized for the detection of three-body sequential fission events over the full range of possible energy losses. The detector arrangement included a large-area ionization chamber which had two independent upper and lower halves, each capable of measuring the energy, E, and the energy loss, ΔE, of reaction products as well as their (x,y) position. A second detector was operated in coincidence on the other side of the beam axis. The inclusive charge distributions measured with the large-area ionization chamber for reactions of 58Ni + 58Ni at 15.3 MeV/u were found to be dominated by products with Z < 28. Very little yield was observed for nuclei with charges greater than that of the projectile. It has been shown that most of the shift toward lighter products is consistent with the effects of equilibrium evaporation. At the largest calculated energy losses, the inclusive charge distributions were found to be slightly asymmetric due to an increasing component of fragments with half or less of the projectile charge. These light fragments were observed to have much broader angular distributions than the corresponding heavier fragments of the same energy loss. These observations from the inclusive measurements already suggest the occurrence of sequential fission of the projectilelike fragment. More conclusive evidence is obtained from the coincidence measurements