Two-phonon giant resonances

The status of double-phonon giant resonances in nuclei is briefly reviewed. We summarize the presently available experimental data and discuss related theoretical efforts. Emphasis is paid to the electromagnetic excitation in heavy ion collisions at near relativistic energies, a process which yields the largest cross sections for two-phonon giant dipole resonances.     

Photonuclear reactions of actinides in the giant dipole resonance region

Photonuclear reactions at energies covering the giant dipole resonance (GDR) region are analyzed with an approach based on nuclear photoabsorption followed by the process of competition between light-particle evaporation and fission for the excited nucleus. The photoabsorption cross-section at energies covering the GDR region is contributed by both the Lorentz-type GDR cross-section and the quasi-deuteron cross-section. The evaporation-fission process of the compound nucleus is simulated in a Monte Carlo framework. Photofission reaction cross-sections are analyzed in a systematic manner in the energy range of ∼ 10-20 MeV for the actinides ²³²Th , ²³⁸U and ²³⁷Np . Photonuclear cross-sections for the medium-mass nuclei ⁶³Cu and ⁶⁴Zn , for which there are no fission events, are also presented. The study reproduces satisfactorily the available experimental data of photofission cross-sections at GDR energy region and the increasing trend of nuclear fissility with the fissility parameter Z ²/A for the actinides.     

Photonuclear physics when a multiterawatt laser pulse interacts with solid targets

When a laser pulse of intensity 10(19) W cm(-2) interacts with solid targets, electrons of energies of some tens of MeV are produced. In a tantalum target, the electrons generate an intense highly directional gamma-ray beam that can be used to carry out photonuclear reactions. The isotopes 11C, 38K, (62,64)Cu, 63Zn, 106Ag, 140Pr, and 180Ta have been produced by (gamma,n) reactions using the VULCAN laser beam. In addition, laser-induced nuclear fission in 238U has been demonstrated, a process which was theoretically predicted at such laser intensities more than ten years ago. The ratio of the 11C and the 62Cu beta(+) activities yields shot-by-shot temperatures of the suprathermal electrons at laser intensities of approximately 10(19) W cm(-2).     

Nuclear giant resonances and linear response

We search for nonlinear effects in nuclear giant resonances (GRs), in particular the isovector dipole and the isoscalar quadrupole modes. To that end, we employ a spectral analysis of time-dependent Hartree-Fock (TDHF) dynamics using Skyrme forces. Based on TDHF calculations over a wide range of excitation amplitudes, we explore the collectivity and degree of harmonic motion in these modes. Both GR modes turn out to be highly harmonic in heavy nuclei from A = 100 on. There is no trace of a transition to irregular motion and multiple resonances are predicted. Slight anharmonicities are seen for light nuclei, particularly for ¹⁶O. These are mainly caused by the spin-orbit splitting.     

Landau zeroth-sound and nuclear giant resonances

Analytical solution for the zeroth-sound in the infinite matter is used as a basis for constructing the distribution functions in finite-size nuclei. Simple characteristic equation is obtained which determines frequencies of isoscalar nuclear giant resonances due to the zeroth-sound modes coupled to the surface distortions.     

Photonuclear reactions: Modern status of the data

Photonuclear reaction data play an important role in basic and applied research. Radiation shielding design, radiation transport analysis, activation analysis, astrophysical nucleosynthesis, safeguards and inspection technologies, human body radiotherapy absorbed dose calculations, beam monitoring in heavy-ion dissociation research at ultrarelativistic energies, etc., could be mentioned. However, there exist quite evident systematic discrepancies in both shapes and magnitudes between photonuclear cross sections measured in various laboratories. These discrepancies noticeably reduce the accuracy and reliability of data. A systematic overview of various types of data contained in the international database is given. The modern status of the data is discussed. The reasons for significant discrepancies between various photonuclear data are analyzed and methods to reduce them are suggested.     

Nuclear viscosity and widths of giant resonances

We write down a set of coupled hydrodynamic equations of the Navier-Stokes type which describe the motion of two compressible, viscous nuclear fluids. The solutions of these equations give rise to giant resonances of both isoscalar and isovector type. The viscosity terms in the equations are responsible for the damping of these resonances. Within this framework we obtain expressions for the width of the resonances as a function of the mass number A, and relations between the widths and the excitation energies for various multipolarities (J = 0⁺, 1^?, 2⁺, 3^?, 4⁺), and isospins (T = 0,1). The A dependence of the calculated widths exhibit the experimental trends of the giant dipole and isoscalar quadrupole widths. Also, as a result of the calculation we obtain estimates of the values of viscosity coefficients in nuclei.     

Double giant resonances in nuclei

A brief review of the results of microscopic calculations aimed at describing the characteristics of double giant dipole resonances (DGDR) is presented. Special attention is paid to a systematic microscopic study of the anharmonic properties of DGDRs for nuclei with mass numbers 40≤A≤208. It is found that the corrections of the energy centroid of a DGDR from its harmonic limit are negative, have a value on the order of a few hundred keV, and follow an A ^?1 dependence.     

Nonlinear physics and modern technology

Typical systems considered in nonlinear physics are complex dynamics systems in which matter and energy fluxes are accompanied by and controlled by information fluxes. In this paper, we compare two types of complex partially controllable systems: on the one hand, a modern physics laboratory in which processes are investigated in solids (dielectrics, semiconductors, metals, and structures) exposed to high-power pulsed fluxes of photons and massive particles; and on the other hand, an industrial shop using modern technology for applying anticorrosion and protective electrically insulating and chemically resistant coatings on steel pipes for thermal pipelines. The systems to be compared (the physics laboratory and the shop) have much in common from the standpoint of nonlinear physics. This allows us to investigate and optimize the structure of modern production processes and the corresponding production equipment (production lines) by methods of nonlinear physics, and also to show how important it is for students (who will be production equipment designers and process engineers) to spend considerable time doing practical work in a modern experimental physics laboratory. Tomsk Polytechnical University, Tomskénergo OAO [Open Joint-Stock Company]. Institute of Power Electronics, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 104–115, November, 1997.     

Isotopic dependence of giant multipole resonances

A procedure is presented which allows the application of linear response theory and the random phase approximation to an open shell. The procedure is applied to Ca isotopes. The general features of giant multipole resonances are found to vary smoothly with the mass. The resonances exhibit more structure in the open 1f_{$\text{7}\text{2}$} shell nuclei. While the energy-weighted dipole sum is practically constant in all isotopes, the isoscalar quadrupole and octupole energy weighted sums increase continuously by ~ 30 % from ⁴⁰Ca to ⁴⁸Ca.     

High temperature giant dipole and isoscalar resonances

We present a systematic study of the Giant Dipole Resonance (GDR) at high temperatures (T≧4 MeV) in the framework of a semiclassical approximation that uses them ₁ andm ₃ RPA sum rules to estimate the GDR mean energy. We focus on the evolution withT of the collective nature of the GDR and of theL=0, 2, 3 and 4 isoscalar resonances. We find that the GDR remains particularly collective at highT, suggesting that it might be possible to observe it experimentally even at temperatures close to the maximum one a nucleus can sustain.     

Collisional damping of giant monopole and quadrupole resonances

Collisional damping widths of giant monopole and quadrupole excitations for ¹²⁰Sn and ²⁰⁸Pb at zero and finite temperatures are calculated within Thomas-Fermi approximation by employing the microscopic in-medium cross-sections of Li and Machleidt and the phenomenological Skyrme and Gogny forces, and are compared with each other. The results for the collisional widths of giant monopole and quadrupole vibrations at zero temperature as a function of the mass number show that the collisional damping of giant monopole vibrations accounts for about 30 - 40% of the observed widths at zero temperature, while for giant quadrupole vibrations it accounts for only 20 - 30% of the observed widths at zero temperature. Received: 9 January 2001 / Accepted: 29 March 2001     

Collective potentials and dipole and quadrupole giant resonances

The dynamic collective model is extended into the energy region immediately above the giant dipole resonances, i.e. into an energy region between 20 and 28 MeV. The total Hamiltonian is constructed and the dynamical problem is solved by diagonalizing the Hamiltonian in the basis of a five-dimensional harmonic oscillator. In schematical studies the splitting of giant quadrupole resonances is shown. For some elements the potential energy surfaces (PES) are constructed within the collective model developed by Gneuss et al. and the quadrupole resonances have been calculated in the framework of the dynamic collective model. In the last part the agreement with experimental data is shown.     

将近代物理内容加入"大平台"普通物理实验

介绍了将近代物理内容加入“大平台”普通物理实验的具体情况.     

Coupling of giant resonances via residual interactions

The coupling between E2 and M1 modes in deformed ¹⁵⁴Sm is investigated within the novel averaging RPA approach with the factorized residual interaction. The calculations show that the E2 giant resonance is not noticeably affected by the coupling. At the same time, the M1 response demonstrates a new structure (high-energy branch of the scissors mode) at 24–25 MeV.     

Coulomb excitation of double giant dipole resonances

We implement the Brink–Axel hypothesis for the excitation of the double giant dipole resonance (DGDR): the background states which couple to the one-phonon giant dipole resonance are themselves capable of dipole absorption. These states (and the ones which couple to the two-phonon resonance) are described in terms of the gaussian orthogonal ensemble of random matrices. We use second-order time-dependent perturbation theory and calculate analytically the ensemble-averaged cross section for excitation of the DGDR. Numerical calculations illuminate the mechanism and the dependence of the cross section on the various parameters of the theory, and are specifically performed for the reaction ²⁰⁸Pb + ²⁰⁸Pb at a projectile energy of 640 MeV/nucleon. We show that the contribution of the background states to the excitation of the DGDR is significant. We find that the width of the DGDR, the energy-integrated cross section and the ratio of this quantity over the energy-integrated cross section for the single giant dipole resonance, all agree with experiment within experimental errors. We compare our approach with that of Carlson et al. who have used a similar physical picture.     

Pionic excitation of giant resonances in Y

Inelastic scattering of π⁺ and π⁻ exhibits strongly excited giant resonance structures. Besides the giant quadrupole resonance centered around 14 MeV, structures are observed at 16.8 MeV which can be fitted with a mixing of L=0 and L=1, and around 25 MeV (E_x = 110A^−1/3).     

Semiclassical description of isoscalar giant multipole resonances

The scaling approximation in a semiclassical theory of nuclear collective motions based on the Vlasov equation is applied to the study of isoscalar giant resonances. Analytic forms are obtained for the frequencies of any multipolarity, expressed just in terms of local density distributions, using realistic nuclear effective forces. The importance of non local interactions and diffuse surfaces is clearly shown. The limits of the scaling picture in describing high multipolarity resonances are finally discussed.