Effects Of Nuclear Orientation Research Articles

Dynamical approach to heavy-ion induced fission using actinide target nuclei at energies around the Coulomb barrier

In order to describe heavy-ion fusion reactions around the Coulomb barrier with an actinide target nucleus, we propose a model which combines the coupled-channels approach and a fluctuation-dissipation model for dynamical calculations. This model takes into account couplings to the collective states of the interacting nuclei in the penetration of the Coulomb barrier and the subsequent dynamical evolution of a nuclear shape from the contact configuration. In the fluctuation-dissipation model with a Langevin equation, the effect of nuclear orientation at the initial impact on the prolately deformed target nucleus is considered. Fusion-fission, quasifission, and deep quasifission are separated as different Langevin trajectories on the potential energy surface. Using this model, we analyze the experimental data for the mass distribution of fission fragments (MDFF) in the reactions of ${}^{34,36}$S + ${}^{238}$U and ${}^{30}$Si + ${}^{238}$U at several incident energies around the Coulomb barrier. We find that the time scale in the quasifission as well as the deformation of fission fragments at the scission point are different between the ${}^{30}$Si + ${}^{238}$U and ${}^{36}$S + ${}^{238}$U systems, causing different mass asymmetries of the quasifission.

Dynamical approach to fusion-fission process in superheavy mass region

In order to describe heavy-ion fusion reactions around the Coulomb barrier with an actinide target nucleus, we propose a model which combines the coupled-channels approach and a fluctuation-dissipation model for dynamical calculations. This model takes into account couplings to the collective states of the interacting nuclei in the penetration of the Coulomb barrier and the subsequent dynamical evolution of a nuclear shape from the contact configuration. In the fluctuation-dissipation model with a Langevin equation, the effect of nuclear orientation at the initial impact on the prolately deformed target nucleus is considered. Fusion-fission, quasifission and deep quasifission are separated as different Langevin trajectories on the potential energy surface. Using this model, we analyze the experimental data for the mass distribution of fission fragments (MDFF) in the reaction of 36 S+ 238 U at several incident energies around the Coulomb barrier.

Effects of nuclear orientation on fusion and fission process for reactions using 238U target nucleus

Fragment mass distributions for fission after full momentum transfer were measured in the reactions of 30Si,34,36S,31P,40Ar + 238U at bombarding energies around the Coulomb barrier. Mass distributions change significantly as a function of incident beam energy. The asymmetric fission probability increases at sub-barrier energy. The phenomenon is interpreted as an enhanced quasifission probability owing to orientation effects on fusion and/or quasifission. The evaporation residue (ER) cross sections were measured in the reactions of 30Si +238U and 34S+ 238U to obtain information on the fusion probability. In the latter reaction, significant suppression of fusion was implied. This suggests that quasifission has large fraction in the mass-symmetric fission events. The results are supported by a model calculation based on a dynamical calculation using Langevin equation, in which the mass distribution for fusion-fission and quasifission fragments are separately determined.

Effects of nuclear orientation on fusion and fission in the reaction using238U target nucleus

Fission fragment mass distributions in the reaction of 30 Si+ 238 U were measured around the Coulomb barrier. At the above-barrier energies, the mass distribution s howed a Gaussian shape. At the sub- barrier energies, triple-humped distribution was observed, which consists of symmetric fission and asymmetric fission peaked at AL/AH≈ 90/178. The asymmetric fission should be attributed to quasifission from the re sults of the measured evaporation residue (ER) cross-sections for 30 Si+ 238 U. The cross-section for 263 Sg at the above- barrier energy agree with the statistical model calculation which assumes th at the measured fission cross-section originates from fusion-fission, whereas the one for 264 Sg measured at the sub-barrier energy is smaller than the calculation, which suggests the presence of quasifission.

Total Neutron Cross Section of OrientedHo165from 2 to 135 MeV

The effect of nuclear orientation on the total neutron cross section of the highly deformed $^{165}\mathrm{Ho}$ nucleus has been measured from 2 to 135 MeV. The data are successfully fitted by adiabatic coupled-channel calculations. The origin of this effect can also be understood by a semiempirical model which makes use of both the black-nucleus model and the nuclear Ramsauer effect.

Transmission of 8 and 15 MeV neutrons through oriented 165Ho

The effect of nuclear orientation on the neutron total cross section of 165Ho has been measured at neutron energies at 8 and 15 MeV. The result is in reasonable agreement with the theoretical prediction of a coupled channel calculation.

Effects of nuclear orientation and electron polarization in electro-excitation of nuclei

The differential cross section for electro-excitation of nuclei and the polarization of the scattered beam are presented for an oriented target and/or polarized incident electrons. The electron states are represented by plane waves. Transitions of arbitrary multipolarity are considered, and both the Coulomb and the transverse parts of the interaction are taken into account. Experiments with aligned targets can be used to determine the reduced nuclear matrix elements contributing to a mixed (E L, M L±1) transition. The change in the electron polarization vector by inelastic scattering from an unoriented target is discussed in some detail for the cases of pure Coulomb and pure transverse scattering.

Cryogenics and Nuclear Physics

Static and dynamic methods for orienting nuclei at low temperatures are reviewed. The effects of nuclear orientation on alpha -, beta -, and gamma - emission angular distributions are outlined. Attention is devoted to beta decay and parity nonconservation. The achievement of nuclear cooling'' by a method analogous to that used in the isentropic demagnetization of paramagnetic salts is described. (T.F.H.)