No abstract

No abstract

Triple-differential cross sections have been measured as a function of product mass, total kinetic energy, and center-of-mass scattering angle in reactions induced by "'U on ' 0, Mg, "Al, "S, "Cl, Ca, Ca, and ""Zn targets at several bombarding energies between 4.6 and 7.5 MeV /nucleon. The analysis focuses on binary processes in which the product masses are substantially different from the target-projectile masses. These include the complete fusion followed by fission as well as quasifission processes in which large mass transfers occur on a short time scale. The relative contributions of these two components are estimated from the mass-angle correlations and analyzed within the extra and extra-extra push concepts. The time scale for mass transfer in quasifission reactions is derived from turning angles of the intermediate complex, and it is found that the mass drift toward symmetry shows the characteristics of an overdamped motion with a universal time constant independent of scattering system and bombarding energy. This is consistent with the one-body nuclear dissipation mechanism being responsible for the damping in the mass asymmetry degree of freedom. Also the average total kinetic energy of reaction products in quasifission is independent of temperature, supporting the one-body dissipation hypothesis.The elimination of background arising from, e.g. , target impurities and ternary processes, does, however, in practice require the determination of additional parameters. This is especially important in the present experiment where the contribution from the various target constituents of composite targets, such as ZnS and LiCl, can be separated on the basis of these additional parameters.The experimental arrangement consists of four large area (20&(30 cm ) position sensitive avalanche detectors, ' two of which are positioned side by side around the beam axis at a distance of -60 cm from the target, the remaining two being situated at larger angles on opposite sides of the beam axis at a distance of -35 cm. The detector arrangement is shown in Fig. 1. Both binary and ternary coincidences occurring in the four detectors within a resolving time of -SOO -700 ps are recorded. This arrangement allows for the detection of coincident binary products over an angular range of 6' -70 in the laboratory corresponding to 0, =18 -162 . The system is efficient for products spanning the entire mass range from the target to the projectile, and the complete range of fragment kinetic energies.An identical detector arrangement and method of analysis were employed in earlier experiments as described in more detail in Ref. 13. The c.m. angular resolution and the mass resolution were b, HFwHM=2 (FWHM denotes full width at half maximum) and 6 A = 5 u, respectively.In addition, three 7.5 em&7. 5 cm diam NaI detectors were placed at backward angles for the detection of y rays

Multi-fragment decays of 129 Xe, 197 Au, and 238 U projectiles in collisions with Be, C, Al, Cu, In, Au, and U targets at energies between E/A = 400 MeV and 1000 MeV have been studied with the ALADIN forward-spectrometer at SIS. By adding an array of 84 SiCsI(Tl) telescopes the solid-angle coverage of the setup was extended to θ lab = 16 • . This permitted the complete detection of fragments from the projectile-spectator source.The dominant feature of the systematic set of data is the Z bound universality that is obeyed by the fragment multiplicities and correlations. These observables are invariant with respect to the entrance channel if plotted as a function of Z bound , where Z bound is the sum of the atomic numbers Z i of all projectile fragments with Z i ≥ 2. No significant dependence on the bombarding energy nor on the target mass is observed. The dependence of the fragment multiplicity on the projectile mass follows a linear scaling law.The reasons for and the limits of the observed universality of spectator fragmentation are explored within the realm of the available data and with model studies. It is found that the universal properties should persist up to much higher bombarding energies than explored in this work and that they are consistent with universal features exhibited by the intranuclear cascade and statistical multifragmentation models.

No abstract