Damped Reactions Research Articles

Bombarding energy dependence of theSm144+Kr84reaction

Results are presented for the damped reaction /sup 144/Sm+/sup 84/Kr at laboratory energies of 470, 595, and 720 MeV, corresponding to 1.2, 1.6, and 1.9 times the Coulomb barrier energy, respectively. Correlations between angle, kinetic-energy loss, and charge of the projectile-like fragments are discussed. A probability for orbiting is observed at all bombarding energies, with fragment kinetic energies indicative of large exit-channel deformations. This study shows that energy dissipation (up to roughly-equal100 MeV) and charge exchange are correlated in a manner which appears to be independent of initial relative ion velocity. The results are compared with a classical dynamical transport model.

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

Alpha-particle emission in coincidence with damped reaction fragments is studied for the 165Ho + 56Fe (Elab = 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 Mα depends strongly on the fragment excitation energy and atomic number, with an average multiplicity 〈Mα〉 = 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.

Deep inelastic reactions and fusion

During the last decade considerable progress has been made in our understanding of the mechanisms of reactions between complex nuclei. In Session 3, some aspects of this progress as well as some of the outstanding problems are reviewed for damped or deep inelastic reactions and fusion.

Oasis: The superhilac on-line isotope separator

An on-line isotope separator at the Berkeley SuperHILAC is described. It is based on a gas-cooled target coupled with a surface ion source or with a newly developed space-charge compensated electron-bombardment source. Compound nucleus and damped reactions can be used for isotope production. The analyzing magnet is a 180° sector field with a field index of 0.5. Several operating features described include beam intensity measurements and computer control, and the detection systems. Some preliminary off-line and on-line results are reported.

The analysis of simultaneous mass and charge data from damped heavy-ion reactions

Procedures are described to identify simultaneously the mass and charge of projectile-like fragments produced in damped heavy-ion reactions and derived from data recorded with a ΔE− E time-of-flight telescope. Corrections for the effects of particle emission from the primary reaction products are also discussed. The average properties of the resultant charge, neutron and mass distributions can be described by six parameters obtained from a two-dimensional Gaussian fit. These parameters, which are a function of kinetic energy damping, summarize the properties of nucleon exchange in the damped process. After the effects of experimental resolution (in two dimensions, charge and mass) and the effects of binning of the data have been taken into account, most physically relevant quantities of interest and their uncertainties can be calculated with the formulas presented.

Dynamics of Damped Nuclear Reactions

The nucleon-exchange model for damped nuclear reactions is briefly reviewed in the framework of macroscopic transport theory. Recent developments are discussed and some future directions indicated.

Mechanisms of heavy ion dissipative collisions: TheBi209+Kr84reaction atElab=712MeV

Projectilelike reaction products from the $^{209}\mathrm{Bi}$+$^{84}\mathrm{Kr}$ reaction were studied at a laboratory bombarding energy of 712 MeV. The fragments were identified by atomic number, and their kinetic energies and angular distributions were measured with solid-state $\ensuremath{\Delta}E\ensuremath{-}E$ telescopes. The angular distribution is found to be peaked at an angle slightly smaller than the grazing angle and the $Z$ distribution is peaked near $Z=36$. Correlations of the experimental variables show that the angular and $Z$ distributions broaden drastically with kinetic energy dissipation in damped reactions. Various correlations between experimental observables, including the variance of the $Z$ distributions as a function of the reaction $Q$ value and the relationship between kinetic energy and reaction angle, are interpreted in terms of a classical dynamical model using a proximity one-body transport formalism.NUCLEAR REACTIONS $^{209}\mathrm{Bi}$($^{84}\mathrm{Kr}$,HI); ${E}_{\mathrm{lab}}=712$ MeV; measured $\ensuremath{\sigma}(\ensuremath{\theta}, E, Z)$; damped reaction; extracted moments of element distributions and centroids of angular distributions as a function of $Q$ value; comparison with dynamical calculations employing proximity one-body transport model.

Distinctive features of heavy ton dissipative collisions

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.

Transport of angular momentum in damped nuclear reactions

The transport of angular momentum in damped nuclear reactions is investigated in a model considering the stochastic exchange of individual nucleons between the two reactants. Starting from a general transport description of the dynamical evolution of the macroscopic variables, the equations of motion for the first and second moments of the distribution function are derived. The evolution of the angular-momentum carrying variables is discussed in detail and illustrated by calculations for cases of experimental interest.

Fragment spin correlations in damped nuclear reactions

The correlation between the spins of damped nuclear reaction products are studied theoretically. The dynamical exchange of nucleons preferentially excites positive-signature dinuclear rotational modes and produces significant fragment spin correlations, in contrast with the results of a purely statistical model. Illustrative calculations are made for the angular correlations between fission fragments following the reaction 1768 MeV 208Pb + 238U.

Correlated mass and charge transport induced by statistical nucleon exchange in damped nuclear reactions

Experimental relations between mass, charge and energy loss of fragments from damped reactions are analyzed in terms of an independent-particle transport model. A quantitative description of data is achieved including the energy-loss dependence of isobaric and isotopic fragment distributions.

The significance of rolling friction and nuclear potentials in classical trajectory calculations

The influence of various conservative nuclear potentials (proximity, modified proximity, and Woods-Saxon potential) on the properties of damped reactions and heavy-ion induced fusion have been studied for a light and a heavy system by means of trajectory calculations. The equations of motion are numerically solved taking into account friction against rolling, whose strength is treated as free parameter to demonstrate its importance for the dynamics of the reaction. It is found that the amount of dissipated angular momentum as well as the fusion cross section sensitively depends on this friction term, which should not be neglected, in particular for asymmetric systems.

Effect of Pauli Blocking on Exchange and Dissipation Mechanisms Operating in Heavy-Ion Reactions

Systematic properties of dissipation and exchange mechanisms associated with damped nuclear reactions are obtained from available data, yielding results that cannot be consistently described in a classical approach. However, correlations between energy loss and the variances of the fragment $A$ and $Z$ distributions are understood on the basis of an exchanged-induced dissipation mechanism, if account is taken of the Pauli exclusion principle.

Structure in the energy spectra of the fragments produced in damped heavy-ion reactions

Structure in the energy spectra of the fragments produced in damped heavy-ion reactions

Measurement of the Energy Division versus Mass in Highly Damped Reactions

Nuclear charge, mass, and kinetic energies were measured for fragments from the reactions of $^{86}\mathrm{Kr}$+ $^{124}\mathrm{Sn}$ at 440- and 720-MeV bombarding energies. From these data it was deduced that the internal excitation energy of the strongly damped products divides proportionally with the mass.

Interaction times for damped heavy-ion collisions

Angular-momentum dependent interaction times for damped heavy-ion reactions are deduced from experimental angular distributions. The angular momentum l is related to the experimental kinetic-energy loss assuming the energy loss to increase monotonically with decreasing l and the experimental cross section to be given by the sharp cutoff model. Although the analysis depends on the moment of inertia of the intermediate double-nucleus system, rather short interaction times are obtained ranging from 10/sup -22/ to several times 10/sup -21/ sec. Assuming this time scale, nucleon diffusion coefficients are deduced from experimental fragment charge distributions.

On the time scale of 40Ca+ 40Ca strongly damped reactions

The time dependence of 40Ca+ 40Ca deep inelastic collisions is studied by means of the fragment change and mass distributions. Two time components are unfolded. The “fast” one exhibits all features of “quasi-fission”. The characteristics of the “slow” component cannot be distinguished from compound nucleus fission.