Fission and quasifission in U-induced reactions

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

Fusion and quasi-fission in the formation of heavy elements

“…The time scales of certain quasi-fission reactions have been determined from the rotation angle of the system before breakup [2,12]. This method can provide an almost modelindependent estimate of time scales.…”

Systematic study of quasifission characteristics and timescales in heavy element formation reactions

“…Quasifission mass-angle distributions (MAD) first measured at GSI in the 1980s [2,5] showed that quasifission timescales could often be shorter than the rotation time of ∼10 −20 s. However, subsequently only a few measurements [6,7] were made until recent years, when an extensive series of experiments (using the Australian National University Heavy Ion Accelerator Facility and CUBE spectrometer) were carried out [3,[8][9][10][11][12][13][14][15][16]. The kinematic coincidence technique used in the measurements [2,3,17] provides direct information on the mass-ratio of the fragments at scission; thus, the data are represented in terms of mass ratio M R , rather than pre-or According to the characteristics of the MAD (minimum mass yield at symmetry, mass-angle correlation with peak yield at symmetry, and no significant mass-angle correlation), they are assigned as type MAD1, MAD2 and MAD3 respectively [3].…”

Fusion probability and survivability in estimates of heaviest nuclei production

“…CN formation, in contrast, is characterized by equilibration of all degrees of freedom, and hence completes loss of identity of the entrance channel. Intermediate between DIC and CN fission, QF has full energy dissipation but incomplete drift toward the energetically favoured mass-symmetric configuration [5,7].…”

“…Intensive studies of the mechanisms of complete fusion in fission experiments [4,6,7] and in experiments on ER production [9][10][11][12] in reactions leading to heavy and very heavy composite systems, make it evident that the growing QF effect leads to a rapid decrease in the fusion probability and consequently to the drop of production cross sections for the heaviest elements as their atomic number is increased. Such dependence is clearly seen in the case of 'cold' fusion reactions [1].…”

“…Complete fusion reactions have been successfully used in synthesis of the heaviest elements at the border of nuclear stability in experiments performed during the last 20 years [1,2]. Such experiments are extremely challenging as the formation of heavy/superheavy ER is heavily suppressed not only by the CN-fission, but also by a non-equilibrium process called quasi-fission (QF) [3][4][5][6][7]. Following the capture of a projectile and target nuclei, a system may re-separate prematurely, not forming a true CN.…”