K. Hanold

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.

Azimuthal correlations as a test for centrality in heavy-ion collisions

Abstract Azimuthal correlations of particles emitted in collisions between 197Au target nuclei and projectiles 36Ar (at E A = 35, 50, 80 and 110 MeV ) and 129Xe (at E A = 50 MeV ) have been measured with the MSU Miniball, a 4π phoswich array with a low detection threshold. Various impact parameter filters, based on the charge-particle multiplicity, transverse energy and midrapidity charge are tested by their ability to suppress collective motion as measured by the azimuthal correlations. The usefulness of a directivity cut in selecting central collisions for these systems is evaluated.

Multifragment emission in 36Ar+197Au and 129Xe+197Au collisions. Percolation model

Abstract Relative abundances of intermediate mass fragments and light particles measured for 36 Ar+ 197 Au collisions at E A = 50, 80 and 110 MeV lie within the range of percolation model predictions, but for 129 Xe+ 197 Au collisions at E A = 50 MeV , the percolation model predicts too small admixtures of intermediate mass fragments among the emitted charged particles.

A modular array to detect complex fragments produced in intermediate-energy reverse-kinematics reactions

A segmented silicon-silicon-plastic array was constructed for studying complex fragment production in heavy-ion reactions with incident energies of 35–100 MeV/u. The array was designed: (1) to measure the energy, position and charge of fragments with 1≤Z≤Zproj; (2) to have high efficiency for detecting fragments produced in reverse-kinematics reactions; (3) to detect events with two or more fragments; and (4) to have a flexible configuration. Each array telescope consists of a 300 μm Si detector, a 5 mm Si(Li) detector and a 7.6 cm plastic scintillator. The elements of the telescope are held by interconnecting modular packages which allow the telescopes to be close packed about the beam direction. This modular array has been used in several different experimental configurations and examples of its performance are presented.

Complex fragment production and multifragmentation in 139La-induced reactions at 35, 40, 45 and 55 MeV/u

Abstract Complex fragment emission ( Z >3) has been studied in the reactions of 35, 40, 45 and 55 MeV/u 139 La+X. Charge, angular, and energy distributions were measured inclusively and in coincidence with other complex fragments, and were used to extract source rapidities, velocity distributions, and cross sectins. Multifragment events increase with both bombarding energy and entrance-channel mass asymmetry. The excitation functions for multifragment events rise strongly with excitation energy. These excitation functions are independent of the target-projectile combination and bombarding energy suggesting, the formation of an intermediate nuclear system, whose decay properties depend mainly on its excitation energy and angular momentum.

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.

Multifragment Emission in 36Ar +197Au and 129Xe + 197Au Collisions

Abstract Multifragment disintegrations observed in 36 Ar+ 197 Au and 129 Xe+ 197 Au collisions are compared with theoretical predictions.