Maximum excitation energy in fusion-evaporation reactions from light particle multiplicity measurements in32S+58Ni at 25 MeV/A

By using two 4 detector systems for charged particles and neutrons, we measured the evaporated light particles emitted in coincidence with evaporation residues (ER) produced in the reaction³²S+⁵⁸Ni atE(³²S) 820 MeV. From the analysis of the light particle multiplicities as a function of the ER velocity, we extracted the maximum excitation energy reached in fusion evaporation reactions for the studied system.Supported by the European Community Programme Human Capital and Mobility     

Mechanism of enhanced yield of light particles in compound nucleus formation: Diffusion description

In the formation of a compound nucleus the evolution of a dinuclear system is considered. The enhanced yield of light particles for some reactions is explained by the dynamic reasons. The role of quantum and thermal fluctuations is discussed. The results of the previous paper are confirmed.Authors wish to thank Prof. V.V. Volkov for his active attention and comments.     

Complete and incomplete fusion in12C+93Nb and16O+89Y reactions

Excitation functions for the evaporation residues for the reactions¹²C+⁹³Nb and¹⁶O+⁸⁹Y in the projectile energy range of 4 to 6.5 MeV/amu have been measured using off-line gamma spectrometry. The excitation functions for neutron(xn), proton(pxn) and one alpha(xn) emission channels are practically similar for both the reactions. However the products formed by two alpha(2xn) emission show much higher cross sections in the¹²C+⁹³Nb than the¹⁶O+⁸⁹Y system. This has been explained in terms of the incomplete fusion process involving transfer of an alpha particle from the projectile to the target in the former case.Authors thank Shri D.C. Ephraim for making the rolled metal foils and the operation crew of PELLETRON facility for their help in carrying out the irradiations. Authors are grateful to Dr. P.R. Natarajan, Head Radiochemistry Division for his keen interest in this work.     

The effect of angular-momentum dissipation on fluctuations of excitation functions in heavy-ion collisions

We discuss here the effect of dissipation of relative angular momentum on fluctuations of excitation functions in dissipative heavy-ion collisions. Dissipation and fluctuation of relative angular momentum modify and smooth the time-angle localization of the rotating dinuclear system. The secondary maxima in the energy correlation function of the cross-section shift to smaller values of the energy difference, the shift depending on the relaxation time and the diffusion coefficient for angular-momentum dissipation. The results are illustrated for the collision²⁸Si(E _lab=130 MeV)+⁴⁸Ti.Partly supported by the Alexander von Humboldt Foundation     

Light charged particle decay in the reaction7Li+16O

Angular distributions and excitation functions for the emission of a large number of proton, deuteron, triton, and-particle groups in⁷Li+¹⁶O reactions have been measured in the vicinity of the Coulomb barrier. Within the framework of the statistical reaction model, two approaches are presented that can reproduce the only weakly anisotropic shape of the angular distributions and the absolute cross section for those groups of ejectiles where contributions from direct reaction modes are small. When a standard Woods-Saxon potential deduced from elastic scattering is used, the entrance channel angular momentum distribution must be limited to values below critical angular momental _cr which are smaller than the grazing angular momental _gr if fusion is to be described. A global proximity potential with a parameter set that has been adjusted to reproduce the fusion reactions of a variety of p- and sd-shell nuclei yields very similar results when applied to⁷Li+¹⁶O. The proximity potential effectively introduces a similar angular-momentum limitation. This analysis proves the existence of a fusion cross section limitation and the importance of strong direct reaction modes (transfer and possibly inelastic processes) in⁷Li+¹⁶O reactions at energies close to and even below the Coulomb barrier. Another aspect of⁷Li+¹⁶O is addressed briefly. The resonance-like structure observed in the heavy-ion radiative capture reaction⁷Li(¹⁶O, ₀₊₁)²³Na atE _x (²³Na)=25.4 MeV is not observed in the particle decay channels investigated in the present work.The authors would like to acknowledge the help received from B. Bellenberg, B. Dechant, H. Hemmert, T. Krischak, E. Kuhlmann, H. Putsch, and C. Scholz during the experiments.     

A complete ridge-line potential for complex fragment emission

Cross sections were measured for fragments (4<><27) from=" the=" 5.0,6.2,6.9,8.0,10.2=" and=" 12.7=">⁶³Cu+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.This work was supported by the Director, Office of the Energy Research, Office of High Energy and Nuclear Physics, Division of Nuclear Physics, of the U.S. Department of Energy under Contract No. DE-AC03SF00098.     

Partition of excitation energy between reaction products in heavy ion collisions

In the single-particle approach a partition of the excitation energy between the reaction products in deep inelastic collisions of heavy ions are investigated. The role of the particle-hole excitations and the nucleon exchange is considered. The ratio of the projectile excitation energy to the total excitation energy for the reactions²³⁸U(1468 MeV)+¹²⁴Sn,²³⁸U(1398 MeV)+¹¹⁰Pd,⁵⁶Fe(505MeV)+¹⁶⁵Ho,⁷⁴Ge (629 MeV)+¹⁶⁵Ho and⁶⁸Ni(880 MeV)+¹⁹⁷Au is calculated. The results of calculations are in good agreement with the experimental data.We are grateful to Dr. N.V. Antonenko for valuable discussions. This work was supported partly by the Russian Minister for Education and Research under the Grant N2-61-13-28.     

Entrance channel effects on preequilibrium emission and incomplete fusion in Promptly Emitted Particle model

In intermediate energy heavy ion collision prompt particles emitted in the early stages of the reaction affect the properties of the incompletely fused composite. We have studied the entrance channel effects on preequilibrium proton emission and various observables, like temperature, residual velocity, and linear momentum transfer of the incompletely fused residue, in the framework of Promptly Emitted Particle (PEP) model. The calculated preequilibrium proton energy spectra for Oxygen and Sulphur induced reactions on various targets have been confronted with the respective experimental data and the agreement between the two has been found to be quite satisfactory. Proton multiplicity has been found to decrease/increase with the increase in target/projectile mass. Residual velocity and linear momentum transfer have been found to have weak dependance on target mass. With the increase in incident energy, the calculation predicts a tendency towards limiting the temperature of the residue for all the target masses. The limiting temperature has been found to decrease with increase in the mass of the residue which is in accordance with the experimental observations.One of the authors (S.D) is thankful to R. Auble and his collaborators at Oak Ridge National Laboratory, for providing the experimental data. The authors would also like to thank Dr. S.K. Basu for his help in running the code PACE2.     

Total neutron multiplicities as a measure of the violence in heavy ion reactions

Energy spectra and multiplicities of neutrons from the reaction system 838 MeV³²S projectiles on¹⁹⁷Au have been measured in coincidence with binary fragmentations. Neutron detection was performed simultaneously in a 4 scintillator sphere and by time-of-flight. The linear momentum transfer (LMT) and the excitation energyE _CN ^* are deduced with the folding angle technique. Neutron multiplicities are compared for consistency and discussed as a measure of LMT andE _CN ^* . The saturation ofM ₄ (E _CN ^* ) beyondE _CN ^* 400 MeV seen for several systems of high fissility (x0.8) is attributed to the spreading of the folding angle distribution and the increasing competition of charged particle evaporation.     

Mass and charge release by the evaporation of particles from compound nuclei around mass 60

Reaction product cross sections have been measured for reactions of⁴He projectile with^54,56Fe,⁵⁸Ni and⁵⁹Co targets in the bombarding energy range 14–27 MeV. The cross-section-weighted average charge and mass removed from the compound system were deduced and compared with results reported for heavy-ion induced reactions. Variation of with E _CN ^* is independent of the initial angular momentum of compound nucleus in theA=60 mass region in contrary to the rare-earth region. This would suggest a strong angular momentum removal from the compound nucleus by evaporation of particles forA60 nuclei. The average evaporated charge depends on both the CN excitation energy E _CN ^* and the compound nucleusN andZ numbers. The and dependence on excitation energy and the projectile-target combination can be understood in terms of the statistical model calculations.We would like to acknowledge the interest and support of this work by Prof. A. Budzanowski. We wish to express our gratitude to Dr. E. Rurarz for his help with preparing the scattering chamber and Faraday cup used in this work. This work was supported by the Polish Government Central Research Program Grant CPBP 01.09 and the Polish Ministry of Education Grant G-MEN-207/90.     

Competition between binary reactions and fusion in heavy-ion collisions at the Coulomb barrier

Mass and charge distributions for binary reaction channels have been measured for the reactions⁸⁶Kr with⁷⁶Ge,¹⁰⁴Ru and¹³⁰Te at the Coulomb barrier using chemical separations and-ray spectroscopy. These systems span the region where dynamical hindrance to complete fusion sets in. The binary reactions can be subdivided into two components associated withi) reflection from the outer potential barrier (quasielastic), andii) reseparation after passing the barrier (complex reactions). The sum of complex-reaction channels and evaporation residues from complete fusion can be reproduced by a barrier passing calculation. The fraction of the barrier passing flux leading to reseparation increases from 26±10% for the lightest system to more than 90% for the heaviest system. The data indicate that fusion hindrance is primarily caused by reseparation shortly after passage of the barrier before Swiatecki's conditional saddlepoint is overcome, resulting in partitions close to the entrance channel configuration. In addition, for the heaviest system, a quasifission component representing somewhat less than 20% of the barrier-passing flux was observed. From the missing masses of fragment pairs we can deduce that the reseparating complex-reaction products have kinetic energies well below the fusion barrier and share the excitation energy in a way similar to the sawtooth-like curve known from low-energy fission. The quasielastic, predominantly one- and two-nucleon transfer channels, have strongly varying cross sections for the three systems despite similar effectiveQ-values. A systematics of one-neutron transfer cross sections at the Coulomb barrier is established and shown to differ considerably from the smooth behaviour observed at energies 20–30% above the barrier. The connection to nuclear polarization phenomena and orbit matching is pointed out.Nuclear reactions:⁷⁶Ge,¹⁰⁴Ru,¹³⁰Te(⁸⁶Kr, X).E=3.22 MeV/u, (3.64) 3.84 MeV/u, 3.96 MeV/u; enriched targets; catcher foil technique, chemical separations,-ray spectroscopy; deduced mass and charge distributions for binary reactions; competition with complete fusion     

Investigation of the fusion of heavy nearly symmetric systems

Excitation functions in the vicinity of the Coulomb barrier have been measured for the formation of evaporation residues in¹⁰⁰Mo-induced fusion reactions with^{90, 92, 96}Zr,^{92, 96, 98, 100}Mo,¹⁰⁴Ru and¹¹⁰Pd as well as for the system⁹⁶Zr+⁹⁶Zr. From these data the fusion probability in central collisions was extracted covering a range of 4 orders of magnitude. At the fusion barriers expected from systematics we find that the fusion probability is suppressed by one to three orders of magnitude. It is rising very gradually at higher energies and reaches for the heaviest systems saturation only at energies as high as 30 MeV above the barrier. The observed hindrance of the fusion process increases roughly with the growing Coulomb repulsion between the collision partners, but there is also a distinct influence of their individual nuclear structure. The data are compared to the extra-push model, the surface-friction model and the diabatic fusion model. A parameterisation of the extra-push energy and its fluctuation in terms of a macroscopic quantity like the Coulomb repulsion combined with a microscopic quantity characterizing the nuclear structure is proposed. As a byproduct of this work a new alpha emitter,¹⁹¹Po, could be identified. Its half-life is (15.5 _–2.5 ⁺⁶ ) ms, the alpha energy is (7314±20) keV.     

Production of fast evaporation residues by the reaction20Ne+208Pb at projectile energies of 8.6, 11.4 and 14.9 A.MeV

Velocity distributions and production cross sections of evaporation residues have been measured in the reaction²⁰Ne+²⁰⁸Pb at projectile energies of 8.6, 11.4, 14.9 A.MeV. Essential deviations from statistical model of deexcitation have been observed. Monte Carlo simulations involving emission of non-equilibrium particles have been used in order to reproduce experimental velocity, charge and mass distributions of evaporation residues and to estimate indirectly multiplicities of pre-equilibrium particles. Communicated by V. Metag     

Excitation functions for production of heavy residues in the interaction of12C with181Ta

The excitation functions for production of 21 isotopes and isomers of Au, Pt, Ir, Os and Re in the interaction of¹²C with¹⁸¹Ta from 54 to 98 MeV incident energy have been measured by the activation technique. The analysis of these data allows one to estimate the reaction cross-section and the cross-sections for complete fusion of¹²C and the incomplete fusion of⁸Be and fragments with tantalum.     

Fusion cross-section of the 7Li + 11B reaction

The ⁷Li + ¹¹B reaction has been investigated in the energy range 5.5 MeV < E _lab < 19MeV, by detecting γ-ray resulting from the de-excitation of evaporation residues. Statistical compound-nucleus calculations have been performed in order to extract both the cross-sections of individual exit channels and the fusion cross-section of the system. The total angular momentum that the compoundn ucleus ¹⁸O can support has been deduced and is seen to exhibit saturation for a limiting value of 8.5ħ at the high-energy extreme. The results are discussed in terms of the entrance channel and statistical yrast line limitations.     

Preequilibrium versus thermalized light-charged particle emissions in the40Ar (1100MeV)+24Mg reaction. Dynamical calculations

In an attempt to separate preequilibrium and thermalised emissions of light particles in low impact parameter heavy-ion collisions, the⁴⁰Ar+²⁴Mg reaction has been studied at 27.5 MeV/nucleon. Exclusive measurements have permitted us to examine, in some detail, heavy fragments and charged particles (p, d, t,-particle). The fragments recognized as evaporation residues have been selected and, due to inverse kinematic conditions, the related preequilibrium and statistical emissions of light particles resulting from incomplete fusion reaction appear to be distinguishable to a fair extent. This separation is fully supported by Monte Carlo calculations. Some experimental characteristics of the light particles have been examined and compared to the predictions of dynamical calculations. These calculations, associating a preequilibrium (interpreted as prompt emitted particles) model with a statistical-decay model, follow the evolution of the collision from the point of contact between the projectile and the target to the final evaporation-residue formation on an event by event basis. The predictions of these calculations have been compared to experimental data and satisfactory agreement is achieved for fragment-mass distribution, proton-energy spectra, and proton-angular distribution.     

Influence of pre-fission neutron emission on fission fragment angular distribution for the system209Bi +16O

The data of fission fragment anisotropies measured for the system¹⁶O +²⁰⁹Bi in the centre of mass energy region of 73 to 95 MeV have been compared with the saddle point statistical model calculations. The corrections to the nuclear temperature and the spin distribution arising due to pre-fission neutron emission have been made. While the resultant calculations reproduce very well the data in the near- and sub-barrier energy regions, they deviate from the data at higher energies. This observation is similar to what was already reported for¹⁶O +²⁰⁸Pb system.