Excitation Energy Division Research Articles

Multinucleon transfer in theXe136+Pb208reaction

The dynamic mechanics in the multinucleon transfer reaction $^{136}\mathrm{Xe}+^{208}\mathrm{Pb}$ at an incident energy of ${E}_{\mathrm{c}.\mathrm{m}.}=450$ MeV is investigated by using the improved quantum molecular dynamics model (ImQMD). The lifetime of the neck directly influences the nucleon exchange and energy dissipation between the projectile and the target. The total-kinetic-energy--mass distributions and excitation energy division of primary binary fragments and the mass distributions of primary fragments at different impact parameters are calculated. The thermal equilibrium between two reaction partners has been observed at the lifetime of a neck larger than $480\phantom{\rule{4.pt}{0ex}}\mathrm{fm}/c$. By using the statistical decay code gemini to describe the de-excitation process of the primary fragments, the isotope production cross sections from Pt to At are compared with the prediction by the dinuclear system and GRAZING model. The calculations indicate that the GRAZING model is suitable for estimating the isotope production cross sections only for $\mathrm{\ensuremath{\Delta}}Z=\ensuremath{-}1$ to +2; the DNS + gemini calculations underestimate the cross sections in the neutron-rich and neutron-deficient regions; and the ImQMD + gemini calculations give reasonable predictions of the isotope production cross sections for $\mathrm{\ensuremath{\Delta}}Z=\ensuremath{-}3$ to 0.

Thermodynamics of nuclei in thermal contact

The thermodynamic properties of a dinuclear system are studied with the methods of statistical mechanics. A schematic model calculation shows that the excitation-energy transfer proceeds in energy steps of considerable amount which are subject to large fluctuations. As a consequence, thermal averaging is strong enough to assure the application of thermodynamical methods for describing the energy exchange between the two nuclei in contact. In particular, thermal averaging justifies the definition of a nuclear temperature. The division of excitation energy in thermal equilibrium is derived for several analytical descriptions of the level density.

Excitation energy division in 51V+197Au collisions 75 MeV above the barrier

We have previously determined the excitation energy division in 51V+197Au collisions at a bombarding energy corresponding exactly to the Bass-model barrier, E cm=B, and at E cm=B+25 MeV. At the barrier, the average excitation energies as a function of Z showed an extreme acceptor-donor asymmetry ("sawtooth" phenomenon) suggesting that the excitation energy division depends on how the neck nucleons are shared at scission. At the 25 MeV higher bombarding energy, a rapid change toward equipartition of the excitation energy was observed. We report here on the determination of the excitation energy division in the same system, using the same experimental technique, at E cm=B+75 MeV. At this bombarding energy, the average excitation energies indicate that a temperature equilibrium has been reached for most partitions except for the - on the average - incompletely damped events near the entrance channel charges, where some remnant of the "sawtooth" phenomenon is still visible.

Excitation energy division in 51V + 197Au collisions at and near the barrier

Mass- and charge-yield distributions for 19 < Z < 84 were determined radiochemically for the binary collision products of 51V + 197Au collisions at a bombarding energy corresponding exactly to the Bass-model barrier, E cm = B, and at E cm = B + 25 MeV. The average excitation energies as a function of Z are determined by comparing the centroids of the experimental, secondary mass distributions for given values of Z with the calculated primary centroids from minimization of the potential energy of the di-nuclear system, i.e. from the missing masses. At the barrier, in striking contrast to a thermal equilibrium, we find an extreme donor-acceptor asymmetry in the excitation-energy division reminiscent of the “sawtooth” phenomenon in low-energy nuclear fission. Here, the excitation energy sharing is apparently dominated by shape fluctuations at scission. At the slightly higher bombarding energy, E cm = B + 25 MeV, we observe a rapid change toward equipartition of the excitation energy indicating that, here, the excitation energy division due to shape fluctuations is already covered up by the dissipative exchange of nucleons. Also, the balance of integral cross sections for fusion fission, deep-inelastic scattering, and quasi fission is investigated and is shown to contain important information about the dynamical evolution of the 51V + 197Au system after having passed the entrance channel barrier.

Excitation energy division in heavy-ion reactions.

The excitation energy of the primary products from the reaction $^{56}\mathrm{Fe}$ on $^{165}\mathrm{Ho}$ at 672 MeV was determined by the kinematic coincidence technique. The fraction of the total excitation energy of the system stored in the projectilelike fragment was found to decrease with increasing energy loss. However, thermal equilibrium is not reached, even at the highest energy damping. A small correlation between excitation energy partition and reaction exit channel was observed. Monte Carlo simulations of the present experiment confirmed that some of this correlation is due to the finite resolution of the measured parameters. \textcopyright{} 1996 The American Physical Society.

Comment on "Energy partition in near-barrier strongly damped collisions 58Ni+208Pb"

Chatterjee et al. [Phys. Rev. C 44, R2249 (1991)] report that the sharing of dissipated energy in the $^{58}\mathrm{Ni}$ + $^{208}\mathrm{Pb}$ reaction at E/A=6.5 MeV is not influenced appreciably by the direction and magnitude of net charge (and mass) transfer. We point out that neither this conclusion nor one arguing for a correlation between mass flow and excitation energy division is supported by the data and analysis presented in their communication.

Energy partition in near-barrier strongly damped collisions 58Ni+208Pb.

Neutron spectra from the inverse-kinematics reaction $^{58}\mathrm{Ni}$${+}^{208}$Pb at ${\mathit{E}}_{\mathrm{lab}}$/A=6.65 MeV have been measured in coincidence with nickel-like fragments. Neutron emission patterns for net pickup and stripping channels have been analyzed in terms of sequential evaporation from fully accelerated projectile-like and target-like fragments. Like at higher energies, these patterns suggest an absence of appreciable correlations between net mass transfer and excitation energy division for strongly damped collisions, at the present near-barrier energy. The overall multiplicities, as well as energy spectra and angular distributions of neutrons, are well reproduced by simulation calculations, assuming an energy division always in favor of the heavy fragment.

Correlations between energy and mass partition in the damped reaction 165Ho+74Ge at Elab=8.5 MeV/nucleon.

Data from a kinematical coincidence experiment on the damped reaction $^{165}\mathrm{Ho}$${+}^{74}$Ge at 8.5 MeV/nucleon have been reanalyzed. Although the new analysis confirms the presence of some correlations between the excitation-energy division and the mass asymmetry, the magnitude of these correlations is found to be significantly smaller than that previously reported. Proton-neutron symmetry of the mechanism of heat generation through nucleon exchange is revealed and a possible acceptor-donor asymmetry of this mechanism is discussed.

Heat partition in damped reactions.

The effect of driving force on the division of excitation energy between the asymmetric colliding partners in deep inelastic heavy-ion collisions is studied in the nucleon-exchange model. For this purpose an event-by-event analysis using Monte Carlo simulation technique is performed. It is seen that the fraction of the total excitation energy carried by the projectile-like fragment is sensitive to the mass drift. The model is also applied to analyze the correlation between fragment mass and excitation energy for a given total energy loss and significant correlation is found. The reactions induced by $^{56}\mathrm{Fe}$ at 9 MeV/nucleon and $^{74}\mathrm{Ge}$ at 8.5 MeV/nucleon on $^{165}\mathrm{Ho}$ are considered in our analysis and the calculated correlations are in good agreement with the results obtained from the kinematic coincidence experiments.

Aspects of kinematical coincidence measurements of excitation energy division in damped reactions

It is shown that the finite resolution inherent in the kinematical coincidence method leads to systematic errors in the deduced (primary) physical quantities if the latter are calculated based on mass and linear momentum conservation equations alone. As an example, application of this method for measuring excitation energy of the fragments from damped reactions is reviewed. In such a case, finite resolution effects generate significant instrumental, or “background” correlations between the physical quantities reconstructed in a straightforward fashion, hence, if not accounted for, they may lead to the qualitative misinterpretation of the data. Experimental measures are discussed which appear necessary in order to ensure proper accuracy of the finite resolution corrections. An alternative method of data analysis is presented which is much less susceptible to the finite resolution effects discussed.

Additional evidence for fusion-fission in S32+24Mg reactions: Division of excitation energy and spin in the fission fragments

Additional evidence for fusion-fission in S32+24Mg reactions: Division of excitation energy and spin in the fission fragments

Excitation energy equilibration in damped 139La+40Ar collisions at 15 MeV per nucleon.

Energy and angular distributions of neutrons emitted in coincidence with projectile-like fragments from the damped reaction $^{139}\mathrm{La}$${+}^{40}$Ar at a bombarding energy of 600 MeV have been measured. Components corresponding to sequential emission from two fully accelerated reaction fragments as well as preequilibrium neutron emission have been separated as a function of energy loss and fragment atomic number. The deduced division of total excitation energy between the reaction partners is very well described by the one-body nucleon-exchange model for all energy losses. The data provide no evidence for correlations between the excitation energy division and the measured projectile-like fragment atomic number.

Generation and division of excitation energy in heavy-ion collisions studied by measuring charged-particle survival fractions

Charged-particle survival fractions of primary projectile-like fragments from the 40Ar + 197Au reaction at 450 MeV were measured by using a large array of 32 phoswich detectors operating in coincidence with a detector of projectile-like fragments. Differential survival fractions of the primary pickup and stripping reaction products indicate a dependence of the average excitation energy generated in the primary fragments on the direction of the mass transfer.

Excitation energy division in the quasielastic region from reactions of 12 MeV/nucleon 48Ti with 150Nd.

Internal excitation of projectile-like fragments and target-like fragments was investigated for the system 12 MeV/nucleon $^{48}$Ti${+\mathrm{}}^{150}$Nd, by measuring the discrete \ensuremath{\gamma} rays emitted by both fragments in coincidence with the charge-separated projectile-like fragments. Two quasielastic exit channels of the projectile-like fragments were studied: Z=20 and Z=22. Characteristic \ensuremath{\gamma} rays were used to determine the average masses of products, after separation and neutron evaporation, as a function of kinetic energy loss. Our results show that the mass-to-charge equilibration occurs quickly for the Z=20 exit channel, but is slower for the Z=22 channel. Comparison between the average masses of products with the masses calculated using the statistical model show that the excitation energy is divided approximately equally between the projectile-like and the target-like fragments for the Z=20 exit channel. The excitation energy appears to be divided equally up to \ensuremath{\sim}105 MeV of the kinetic energy loss corresponding to \ensuremath{\sim}36% of kinetic energy damping. This result is consistent with predictions made by the stochastic nucleon-exchange models for a small kinetic energy loss.

Unusual excitation-energy division in quasi-fission reactions between50Ti,54Cr and58Fe with207,208Pb at the barrier

Unusual excitation-energy division in quasi-fission reactions between50Ti,54Cr and58Fe with207,208Pb at the barrier

Excitation energy division in the 9 MeV/nucleon 56Fe+ 165Ho reaction

The centroids and widths of the excitation energy distribution for damped fragments produced in the 56Fe + 165Ho reaction at 9 MeV/nucleon have been determined. There exists a dependence of excitation energy division between the reaction fragments on energy loss and nucleon transfer.

Nonequilibrium population of magnetic substates and excitation-energy division in the decay of an orbiting complex.

$\ensuremath{\gamma}$-ray angular correlations have been measured for the strongly damped reaction $^{12}\mathrm{C}$($^{28}\mathrm{Si}$, $^{12}\mathrm{C}$)$^{28}\mathrm{Si}$ at ${\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.}=180\ifmmode^\circ\else\textdegree\fi{}$ for ${E}_{\mathrm{c}.\mathrm{m}.}=42.7 \mathrm{and} 33.7$ MeV. The $^{12}\mathrm{C}$ 4.44-MeV, ${J}^{\ensuremath{\pi}}={2}^{+}$ state is found to be produced almost exclusively in the $m=0$ magnetic substate with respect to the beam axis. Comparison of singles and coincidence yields shows that the $^{12}\mathrm{C}$ comes out in its ground state most of the time (\ensuremath{\cong} 70%) resulting in the entire excitation energy \ensuremath{\cong} 15-20 MeV appearing in $^{28}\mathrm{Si}$. These results can be reproduced by a constrained phase-space calculation.

Excitation energy division in the first 160 MeV of total kinetic energy loss for the reaction 684 MeV 80Kr+ 174Yb

The fraction of the total excitation energy imparted to the primary target-like fragment produced in the reaction 684 MeV 80Kr+ 174Yb has been determined for one charge asymmetry in the exit channel. This fraction was deduced from the kinetic energy cost of evaporating neutrons from the target-like fragment and is approximately 0.5 for total kinetic energy losses as large as 160 MeV. This result is consistent with the predictions of nucleon exchange models and indicates that for some exit channels thermal equilibrium has not been reached for energy losses corresponding to a sizeable fraction of the maximum kinetic energy relaxation.

The number of neutrons emitted by the individual fragments from 235U fission

The number of neutrons emitted by the individual fragments produced in the thermal neutron fission of 235U has been measured using a large cadmium loaded organic liquid scintillator as a detector. The number of neutrons emitted by the pair of fragments was measured as a function of their mass ratio under conditions of 4π geometry. The semi-empirical mass formula of Weizsäcker was used to determine the amount of excitation energy removed by the evaporated neutrons. It was found that the division of excitation energy between the light and heavy fragments was strongly asymmetric. The results obtained are at variance with the statistical theory of fission given by Fong.