J. R. Birkelund

Heavy-ion fusion: Comparison of experimental data with classical trajectory models

Abstract Currently available data on fusion excitation functions for heavy-ion induced reactions over a wide mass range are compared to results calculated with a classical dynamical model based on the proximity nuclear potential of Blocki et al., the Coulomb potential of Bondorf et al., and one-body nuclear friction in the proximity formalism of Randrup. With these conservative and dissipative forces and the radial parameters of Myers, overall good agreement is obtained between the theoretical excitation functions and most of the available data. Extensive calculations have been performed to test the sensitivity of the calculated fusion cross-sections to a number of parameters, including the radial dependence of the Coulomb and nuclear potentials, the radial and tangential friction form factors as well as the projectile and target radii. The theoretical excitation functions for the lighter heavy-ion systems are rather insensitive to changes in either the conservative or dissipative forces. The calculations show that tangential friction sufficient to produce the rolling condition is necessary to explain the magnitude of the fusion cross-sections at high energies, which are also sensitive to the magnitude of the radial friction component. This is in contrast to the fusion cross-sections at low energies which are determined by the nuclear potential at larger separations, and to a lesser extent by tangential friction. The low energy fusion data are most sensitive to the nuclear radii. The calculations reveal the importance of more experimental measurements of fusion cross-sections at high energies, especially for heavy systems where the magnitudes of the fusion cross-sections are the most sensitive to the assumed forces. However, even for these cases the effects of the conservative and dissipative forces are similar and difficult to separate. These studies indicate, however, that it is possible to construct a conservative potential that will give calculated fusion excitation functions which are in good agreement with all experimental data over the entire mass range. The maximum fusion cross-sections as defined here exceed considerably the liquid-drop limiting value for heavy systems.

Reaction parameters for heavy-ion collisions

Abstract These tables present reaction parameters for all combinations of 27 projectile and 16 target nuclei in a laboratory bombarding energy range of 1–50 MeV/u. The reaction parameters are derived from the Fresnel model of heavy-ion scattering, the droplet model, and the rotating liquid-drop model, or from systematics of experimental data.

Neutron emission in the reactionHo165+Fe56atElab=8.5MeV/u

Neutron emission and ..cap alpha..-particle emission in the /sup 165/Ho + /sup 56/Fe reaction have been measured for the damped-collision and fusion-fission components of this reaction at a bombarding energy of 8.5 MeV/u. The c.m. neutron energy spectra for damped collisions indicate equal temperatures for the light and heavy fragments, a result consistent with the equilibration of the excitation energy during the interaction time. Pre-equilibrium neutron emission is found to contribute less than 5%. An out-of-plane neutron anisotropy A/sub 2/ = 0.2 +- 0.1 is found. The measured ratios of the neutron multiplicities for the light and heavy fragments suggest a rapid approach of the fragment N/Z ratios during the collision to that corresponding to minimum potential energy of the composite system.

A two-dimensional position sensitive ΔE-E counter for energetic light charged particles

Abstract A large area position sensitive ΔE - E detector for energetic light charged particles is described. A single wire gas proportional counter is used as the ΔE detector, with stop detectors consisting of three 19 mm thick NE 102 scintillators coupled to RCA 8053 photomultipliers. The total active area of the ΔE - E detector is 43 cm 2 . Two dimensional position readout is obtained from the proportional counter by a combination of charge division and electron drift time measurements. The detector is suitable for α-particles with energies between 6.5 and 180 MeV. Position resolution is 1 mm in both dimensions, ΔE resolution is 10% and the energy resolution of the scintillators is 3%. The detector is compact enough to fit in a 30″ scattering chamber, where solid angles ≳200 msr may be attained. An application of this detector in studies of charged particle emission in a heavy ion induced reaction is discussed.

Alpha-particle emission in the strongly damped reaction of 165Ho + 56Fe at 8.3 MeV/nucleon

Abstract Alpha-particle emission in coincidence with damped reaction fragments is studied for the 165 Ho + 56 Fe (E lab = 465 MeV) reaction. Statistical emission from the two fully accelerated strongly damped reaction fragments accounts for the dominant sources of the α particles. The α-particle multiplicity M α depends strongly on the fragment excitation energy and atomic number, with an average multiplicity 〈 M α 〉 = 0.2 for Fe-like fragments from damped reaction events with − Q > 100 MeV. Temperatures derived from neutron and α-particle spectra are in good agreement.

Distinctive features of heavy ton dissipative collisions

Abstract Examples of experimental angular, charge (mass) and energy distributions of reaction products from damped heavy-ion reactions are presented. Correlations between these observables, as well as with the associated light particles emitted in the deexcitation stage, are illustrated and shown to give important insights into the reaction mechanism of dissipative collisions. Comparisons of experimental results with dynamical transport calculations based on one-body nucleon exchange give strong evidence that stochastic exchange of independent nucleons account for the dominant part of the dissipative and fluctuative phenomena observed in damped reactions.

Neutron emission in the reaction /sup 165/Ho + /sup 56/Fe at E/sub lab/ = 8. 5 MeV/u

Neutron emission and ..cap alpha..-particle emission in the /sup 165/Ho + /sup 56/Fe reaction have been measured for the damped-collision and fusion-fission components of this reaction at a bombarding energy of 8.5 MeV/u. The c.m. neutron energy spectra for damped collisions indicate equal temperatures for the light and heavy fragments, a result consistent with the equilibration of the excitation energy during the interaction time. Pre-equilibrium neutron emission is found to contribute less than 5%. An out-of-plane neutron anisotropy A/sub 2/ = 0.2 +- 0.1 is found. The measured ratios of the neutron multiplicities for the light and heavy fragments suggest a rapid approach of the fragment N/Z ratios during the collision to that corresponding to minimum potential energy of the composite system.

Neutron emission in the reaction Ho-165 + Fe-56 at Elab=8.5 MeV/u

Neutron emission and ..cap alpha..-particle emission in the /sup 165/Ho + /sup 56/Fe reaction have been measured for the damped-collision and fusion-fission components of this reaction at a bombarding energy of 8.5 MeV/u. The c.m. neutron energy spectra for damped collisions indicate equal temperatures for the light and heavy fragments, a result consistent with the equilibration of the excitation energy during the interaction time. Pre-equilibrium neutron emission is found to contribute less than 5%. An out-of-plane neutron anisotropy A/sub 2/ = 0.2 +- 0.1 is found. The measured ratios of the neutron multiplicities for the light and heavy fragments suggest a rapid approach of the fragment N/Z ratios during the collision to that corresponding to minimum potential energy of the composite system.