K.X. Jing

Time-scale and Branching Ratios in Sequential Multifragmentation

Abstract Experimental intermediate-mass-fragment multiplicity distributions are shown to be binomial at all excitation energies. From these distributions a single binary event probability can be extracted that has the thermal dependence p = exp [ -B T ] . Thus, it is inferred that multifragmentation is a sequence of thermal binary events. The increase of p with excitation energy implies a corresponding contraction of the time-scale and explains recently observed fragment-fragment Coulomb correlations.

Sources of complex fragment emission in lanthanum-induced reactions at E/A = 14.7 and 18.0 MeV

Abstract Complex fragments with 4 ⩽ Z ≲ 50 have been detected in the reactions of 139 La + 12 C and 27 Al at E / A = 14.7 and 18.0 MeV. From the measured angular distributions, the cross sections for the isotropic, target-like and projectile-like components were extracted. The roles of deep-inelastic, fast-fission, and incomplete fusion processes, and the statistical, compound nucleus emission of complex fragments are discussed.

Transition state rates and mass asymmetric fission barriers of compound nuclei 90,94,98Mo

Abstract Excitation functions were measured for complex fragments with atomic number Z = 5–25 emitted from the compound nuclei 90,94,98Mo produced in the reactions 78,82,86 Kr + 12 C . Mass-asymmetric fission barriers were extracted by fitting the excitation functions with a transition state formalism. The extracted barriers are several MeV higher on average than the calculations of the Rotating Finite-Range Model and substantially lower than predicted by the Rotating Liquid Drop Model. The symmetric fission barriers measured support the hypothesis of a congruence term that doubles for the fission of strongly indented saddle-point shapes. The excitation functions were analyzed to search for atomic number Z- and energy E-dependent deviations from transition-state-method predictions. All of the measured excitation functions can be scaled onto a single universal straight line according to the transition-state predictions. No Z- and/or E-dependent effects that could be attributed to transient effects are visible.

Equilibrium and Non-equilibrium Complex Fragment Emission in 50-100 MeV/u 139La + 12C Reactions

Abstract Complex fragment emission ( Z > 2) has been studied in the reactions of 50, 80, and 100 MeV/u 139 La + 12 C. Charge, angle, and energy distributions were measured inclusively and in coincidence with other complex fragments, and were used to extract source rapidities, velocity distributions, and cross sections. The binary signature of the coincidence events and the sharpness of the velocity distributions illustrate the primarily 2-body nature of these reactions. Calculations based on statistical compound nucleus decay have been compared with the experimental data. The emission velocities, angular distributions, and absolute cross sections of fragments of 20 ⩽ Z ⩽ 35 at 50 MeV/u, 19 ⩽ Z ⩽ 28 at 80 MeV/u, and 17 ⩽ Z ⩽ 21 at 100 MeV/u are consistent with the binary decay of compound nuclei formed in incomplete fusion reactions in which the 139 La projectile picks up about one-half of the 12 C target. At 80 and 100 MeV/u, statistical model calculations are also able to reproduce the isotropic portion of the cross section for lighter and heavier fragments. However, a significant fraction of the total cross section for these fragments is due to non-equilibrium emission. Although the emission process is still mainly binary, and the relative velocity between the fragments is determined by their mutual Coulomb repulsion, the anisotropic angular distributions and the magnitudes of the absolute yields are incompatible with standard compound-nucleus statistical decay.

Mass asymmetric fission barriers for 75Br

Fragments with atomic numbers covering nearly the entire range of the mass-asymmetry coordinate (4 < Z < 27) were observed from the 5.0, 6.2, 6.9, 8.0, 10.2 and 12.7 MeV/A 63Cu + 12C reactions. Energy spectra and angular distributions show the presence of projectile-like and target-like components along with an isotropic component. The isotropic component appears as a Coulomb ring in the invariant cross-section plots indicating the presence of a binary compound nucleus decay which is confirmed by the coincidence data. Excitation functions were constructed for each Z value and a nearly complete set of mass-asymmetric barriers has been extracted for 75Br. There is excellent agreement between the experimentally determined barriers and the finite-range model predictions.

Scaling laws in 3He induced nuclear fission.

Nuclear Science Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720(February 8, 2008)Fission excitation functions of compound nuclei in a massregion where shell effects are expected to be very strong areshown to scale exactly according to the transition state pre-diction once these shell effects are accounted for. The factthat no deviations from the transition state method havebeen observed within the experimentally investigated exci-tation energy regime allows one to assign an upper limit forthe transient time of 10

Excitation functions and mass asymmetric fission barriers for compound nuclei 70,76Se

Abstract Excitation functions were measured for complex fragments with atomic number Z=5−20 emitted from the compound nuclei 70,76 Se produced in the reactions 58,64 Ni + 12 C. Mass asymmetric fission barriers were extracted by fitting the excitation functions with a transition state formalism. The extracted barriers were compared with those calculated from macroscopic nuclear models. The measured barriers for symmetric fission seem to support the hypothesis of a shape-dependent congruence energy, which doubles for fission of strongly indented saddle-point shapes. All of the measured excitation functions can be scaled onto a single straight line according to the transition state prediction.

Particle structure function and subbarrier fusion in hot nuclei

The study of particle evaporation spectra can provide information about shape polarization phenomena induced by the nascent particle on the residual nucleus, and about optical modulations felt by the particle as it is preformed inside the nucleus. These aspects can be studied as a function temperature. Preliminary experimental evidence about these features has been obtained.

Low-Z impurities of carbon foils

Abstract The level of purity of a target foil is very important in low cross section nuclear reaction experiments. Carbon foils prepared by two different techniques were analyzed for their content of light impurities such as nitrogen, oxygen, and sodium. These data indicate that the technique used to manufacture a foil is the largest source of contamination. Foils prepared by vacuum evaporation showed sizable levels of light impurities, whereas foils prepared by chemical vapor deposition were substantially purer.

A complete ridge-line potential for complex fragment emission

Cross sections were measured for fragments (4<Z<27) from the 5.0,6.2,6.9,8.0,10.2 and 12.7 MeV/N63Cu+12C reactions. Excitation functions were constructed for each Z value, and a nearly complete set of mass-asymmetric barriers has been obtained for75Br. There is excellent agreement between the experimentally determined barriers and the finite-range model calculations, while there is strong disagreement with the liquid-drop model calculations.