Angular momentum misalignment in deep inelastic processes and angular distribution of sequentially emitted particles and gamma rays

The angular momentum misalignment for fragments produced in deep inelastic scattering is discussed in terms of the thermal excitation of angular-momentum-bearing modes in the intermediate complex. Analytical expressions for the in- and out-of-plane angular distributions are obtained for sequentially emitted particles and fission fragments. The angular momentum dependence of the ratio between particle and neutron decay widths is explicitly treated and found to be quite important. Similarly angular distributions are obtained both for dipole and quadrupole gamma decay. The theoretical results are compared with experimental angular distributions of sequential fission fragments, sequential alphas and gamma rays, and a good agreement is found.     

Asymmetric mass distribution from the fusion-fission reaction40Ar+243Am

The mass distributions and total c.m. kinetic energies of fission fragments formed in the reaction⁴⁰Ar+²⁴³Am at bombarding energies of 214, 222, 240 and 300 MeV have been measured using the angular correlation method. Angular distributions and anisotropy for 222 and 300 MeV have also been obtained. A symmetric mass distribution corresponding to the decay of a highly excited compound nucleus was obtained at 300 MeV bombarding energy. However, with decreasing bombarding energy the fission fragment mass distribution becomes asymmetric, the most probable heavy fragment mass being about 200–210 amu.     

Decisive role of wriggling vibrations in the formation of angular and spin distributions of products originating from binary and ternary fission of oriented nuclei

It is shown that the multiplicities and angular and energy distributions of neutrons and photons evaporated from thermalized fragments originating from the spontaneous and low-energy induced fission of nuclei, the relative yields of ground and isomeric states of final fragments, and the features of delayed neutrons emitted upon the beta decay of the above fragments can successfully be described by employing nonequilibrium distributions of spins and relative orbital angular momenta of fission fragments formed in the vicinity of the scission point for the fissile nucleus being studied. It is also shown that these distributions, which are characterized by large mean values of the spins and orbital angular momenta directed orthogonally to the symmetry axis of the fissioning nucleus are successfully constructed upon simultaneously taking into account zero-mode transverse wriggling and bending vibrations of a fissile compound nucleus in the vicinity of its scission point, the wriggling vibrations being dominant. It is confirmed that the zero-mode wriggling vibrations considered immediately above are directly involved in the formation of the angular distributions of fragments originating from the spontaneous and low-energy fission of nuclei. This makes it possible to describe successfully such distributions for photofission fragments.     

Decay and fission of oriented nuclei

The angular distributions of fragments originating from the binary decay of oriented spherical and deformed nuclei are investigated with allowance for correct transformation properties of wave functions under time inversion. It is shown that, as in the case of protonic decay, the adiabatic approximation for collective rotational degrees of freedom of the systems under investigation is inapplicable in describing the angular distributions of fragments of the deep-subbarrier alpha and cluster decays of nuclei. It is demonstrated that this approximation is justified in describing spontaneous and induced low-energy nuclear fission. The dependence of partial fission widths on the orientation of intrinsic axes, spins, and projections of spins and relative orbital angular momenta of fission fragments is analyzed by using the formalism of the unified theory of nuclear reactions and the theory of open Fermi systems. It is shown that the adiabatic approximation leads to the coherent interference between the wave functions for the relative motion of fragments, whereby the universal angular distributions of fission fragments of oriented nuclei is formed. Deviations from the A. Bohr formula are investigated for these distributions.     

Angular momentum transfer and spin dealignment mechanisms in damped nuclear reactions Ar+Bi and Ni+Pb

The angular momentum transferred to fragment spins has been studied in the damped nuclear reactions Ar+Bi at 255 MeV and 295 MeV and Ni+Pb at 435 MeV from measurement of the angular distribution of the fission fragments of the heavy-recoil nucleus in coincidence with the projectile-like fragment. The heavy-fragment spin is strongly aligned along the normal to the reaction plane and the rigid-rotation limit of the dinuclear system is attained. The dealignment mechanisms produce spin components mainly located in a plane approximately perpendicular to the heavy-recoil lab direction. They are well described by a dynamical model based on the nucleon exchange between the two ions during the collision. The spin-component fluctuations reach high values. In the heavy-recoil direction, these fluctuations are increasing with the total kinetic energy loss and the charge transfer from the projectile to the target. The spin values extracted from both the angular distributions and the fission probabilities are seen to be compatible.     

Ternary fission of nuclei in the adiabatic approximation

On the basis of a generalization of integral formulas for nuclear-decay widths to the three-body case, the spontaneous and the low-energy induced ternary fission of nuclei are investigated by using the adiabatic approximation. The properties of energy distributions, of partial fission widths, and of the angular distributions of fission fragments are analyzed for the case of ternary fission. Conditions are found under which the angular distributions of two heavy fragments originating from ternary fission are similar to the analogous distributions of fragments originating from binary fission. The features of angular distributions are investigated, along with the parities and angular momenta of the third (light) ternary-fission fragment.     

Nuclear fission phenomenon—At a glance

This article gives an overview of the physics of the fission phenomenon. It provides a brief introduction to the various aspects of the fission process such as liquid drop model fission barriers, different stages of the fission process, fragment kinetic energy and mass distributions, nuclear shell effects on fission barriers, fragment angular distributions and rare fission modes.     

Étude de la dissipation d'énergie dans la fission de 234u à l'aide de la réaction 233U(d,pf)

The energy balance in the fission of ²³⁴U has been investigated on the basis of experimental results from the ²³³U(d, pf) reaction. Taking into account the neutron evaporation we have deduced the total kinetic energy and excitation energy distributions of the primary fragments as functions of the excitation energy of the fissioning nucleus. The neutron evaporation temperatures have been adjusted so as to reproduce the average value and width of the measured kinetic energy distributions for each fragmentation. Excitation energy distributions of the fragments have been deduced. The data are discussed in the framework of the liquid-drop model with shell corrections. Evidence for energy dissipation in the fission of ²³⁴U, involving drastic changes in the scission configuration, is shown for some fragmentation modes.     

Fission of Pb in the He-induced reactions at 0.65–12.7 GeV

The fission processes from the interactions of 0.65, 1.74, 5.1, 8.8 and 12.7 GeV ⁴He with Pb have been analyzed using the polycarbonate track detector Makrofol in the form of a sandwich. The decay channels ending with one, two, or more (three, four) fragments were detected. We analyzed events in which only two heavy fragments (Z>20) are detected, irrespective of the existence of coincident intermediate mass fragments (8Z20). Using the correlation between the common observables which characterize fission events, we identified events originated from thermal (soft) fission and more violent processes. Cross sections, angular distributions and excitation energy have been determined for two fission mechanisms, and their variation as a function of the incident energy has been investigated.     

Quantum molecular dynamics approach to estimate spallation yield from<Emphasis Type="Italic">p</Emphasis> +<Superscript>208</Superscript>Pb reaction at 800 MeV

The spallation yield of neutrons and other mass fragments produced in 800 MeV proton induced reaction on²⁰⁸Pb have been calculated in the framework of quantum molecular dynamics (QMD) model. The energy spectra and angular distribution have been calculated. Also, multiplicity distributions of the emitted neutrons and kinetic energy carried away by them have been estimated and compared with the available experimental data. The agreement is satisfactory. A major contribution to the neutron emission comes from statistical decay of the fragments. For mass and charge distributions of spallation products the QMD process gives rise to target-like and projectile-like fragments only.     

Charge and angular distributions and secondary fission of deep inelastic products from the reaction 197Au + 979 MeV 136Xe 1443 08

Kinetic energy spectra, charge and angular distributions are presented for over 40 elements (Z = 27?68). The kinetic energy spectra show two distinct components: a deep inelastic one and another from secondary fission of the target-like product. The charge distribution widths are comparable to those observed in Kr bombardments, the angular distributions, however, are more extensively side peaked.     

Fission modes in charged-particle induced fission

The population of the three fission modes predicted by Brosa's multi-channel fission model for the uranium region was studied in different fissioning systems. They were produced bombarding²³²Th and²³⁸U targets by light charged particles with energies slightly above the Coulomb barrier. Though the maximum excitation energy of the compound nucleus amounted to about 22 MeV, the influences of various spherical and deformed nuclear shells on the mass and total kinetic energy distributions of fission fragments are still pronounced. The larger variances of the total kinetic energy distributions compared to those of thermal neutron induced fission were explained by temperature dependent fluctuations of the amount and velocity of alteration of the scission point elongation of the fissioning system. From the ratio of these variances the portion of the potential energy dissipated among intrinsic degrees of freedom before scission was deduced for the different fission channels. It was found that the excitation remaining after pre-scission neutron emission is mainly transferred into intrinsic heat and less into pre-scission kinetic energy.     

Correlation between neutrons and fragments in 252Cf (sf)

The gridded ion chamber developed at CBNM provides a powerful tool for measurements of fission fragment angular, kinetic energy and mass distributions with an angular efficiency close to 4π. In the present experiment it is used together with a neutron time-of-flight detector to measure the correlation between neutron emission, fragment angle, mass and energy in the spontaneous fission of ²⁵²Cf.     

Sequential fission angular distributions from mass-asymmetric heavy-ion reactions

The angular distributions of sequential fission fragments have been measured for the reactions of ⁴⁰Ar with ¹⁹⁷Au and ²³⁸U as a function of reaction Q-value and charge transfer. These angular, distributions are used to study the angular momentum and alignment of the deep-inelastic products which undergo fission. All of the fission fragment angular distributions are strongly focused into the plane defined by the beam and the projectile-like fragment velocity vectors. The in-plane angular distributions from reactions with uranium are isotropic for small energy losses and become anisotropic as the energy loss increases. For large negative Q-values, the in-plane anisotropy increases as the deep-inelastic products become more symmetric. The variation of the in-plane anisotropy with mass asymmetry for the two systems in this work was compared to a compilation of previous work and a consistent pattern was found. These alignment data are compared to equilibrium statistical calculations.     

Induced nuclear fission accompanied by pion emission

Basic features of the nuclear-fission process induced by protons of incident energy in the range 150<E _p<600 MeV and accompanied by pion emission are predicted on the basis of the cascade-evaporation-fission model. Specific calculations are performed for the total cross section; and the angular and double-differential distributions of pions; excitation-energy,mass number, and charge-number distributions of compound nuclei; and the mass-energy distributions of fission fragments. Various lines of possible experimental investigations into this fission channel are discussed, including searches for the pionic channel of nuclear decay induced by protons of energy close to the meson-production threshold, advancements to the energy region E _p<100 MeV in order to study of new mechanisms of pion production in nuclear fission, and an extension of investigations to the case of nuclear fission accompanied by kaon emission.     

Heavy ion reactions around the Coulomb barrier

The angular distributions of fission fragments for the 32S+184W reaction near Coulomb barrier energies are measured. The ex perimental fission excitation function is obtained. The measured fission cross sections are decomposed into fusion-fission, quasi-fission and fast fission contributions by the dinuclear system (DNS) model. The hindrance to completing fusion both at small and large collision energies is explained. The fusion excitation functions of 32S+90,96Zr in an energy range from above to below the ...     

Angular distributions,relative orbital angular momenta,and spins of fragments originating from the binary fission of polarized nuclei

The angular distributions of fragments originating from the binary fission of odd and odd-odd nuclei capable of undergoing spontaneous fission that are polarized by a strong magnetic field at ultralow temperatures and from the low-energy photofission of even-even nuclei that is induced by dipole and quadrupole photons are investigated. It is shown that the deviations of these angular distributions from those that are obtained on the basis of the A. Bohr formula make it possible to estimate the maximum relative orbital angular momentum of fission fragments, this estimate providing important information about the relative orientation of the fragment spins. The angular distributions of fragments originating from subthreshold fission are analyzed for the case of the ²³⁸U nucleus. A comparison of the resulting angular distributions with their experimental counterparts leads to the conclusion that the maximum relative orbital angular momentum of binary-fission fragments exceeds 20, the fragment spins having predominantly a parallel orientation. The possibility is considered for performing an experiment aimed at measuring the angular distributions of fragments of the spontaneous fission of polarized nuclei in order to determine both the spins of such nuclei and the maximum values of the relative orbital angular momenta of fission fragments.     

Measurement of the fission moment of inertia of medium mass nuclides

Measurements have been made, by means of mica nuclear track detectors, of the distributions in tracklength and the angular distributions with respect to the beam direction for the fission fragments from the systems Ag, Te and Au plus 80-MeV alpha particles. Analysis of the tracklength distributions indicated that, in each case, full momentum transfer from the incident projectile characterized those interactions that led to fission. Angular-momentum dependent statistical model calculations for the decay of the respective compound nuclei then provided information on fission-evaporation competition in the de-excitation processes, and in particular the distribution in nuclear temperature and angular momentum at which the fission events took place. This information was then employed in the analysis of the measured fission fragment angular distributions, and to extractK ₀ ² values. From these, the moments of inertia of the fission saddle point shapes were calculated, and the results are in good agreement with theoretical estimates.     

Investigation of the reaction <Superscript>208</Superscript>Pb(<Superscript>18</Superscript>O, <Emphasis Type="Italic">f</Emphasis>): Folding angular distributions of fission fragments and gamma-ray multiplicity

Correlations between folding angular distributions of fission fragments and the gamma-ray multiplicity are studied for ¹⁸O + ²⁰⁸Pb interactions at energies of the beam of ¹⁸O ions in the range E _lab = 78–198.5 MeV. The probabilities are determined for complete-and incomplete-fusion processes inevitably followed by the fission of nuclei formed in these processes. It is found that the probability of incomplete fusion followed by fission increases with increasing energy of bombarding ions. It is shown that, for the incomplete-fusion process, folding angular distributions of fission fragments have a two-component structure. The width of folding angular distributions (FWHM) for complete fusion grows linearly with increasing energy of ¹⁸O ions. The multiplicity of gamma rays from fission fragments as a function of the linear-momentum transfer behaves differently for different energies of projectile ions. This circumstance is explained here by the distinction between the average angular momenta of participant nuclei in the fusion and fission channels, which is due to the difference in the probabilities of fission in the cases where different numbers of nucleons are captured by the target nucleus.