Volume 28, Issue 6, November 1997
28(1997); http://dx.doi.org/10.1134/1.953054View Description Hide Description
The various experimental methods used in studying spontaneous and low-energy nuclear fission in order to determine the fragment kinetic energy, mass, charge, excitation energy, and spin are reviewed. Special attention is paid to the description of a new approach to the study of fission based on the spectrometry of multiple prompt fission γ rays. Some new data on the spontaneous fission of obtained using this approach are presented.
28(1997); http://dx.doi.org/10.1134/1.953055View Description Hide Description
The many models adopted to study the properties of the low-lying magnetic dipole excitations known as the scissors mode observed in most deformed nuclei are reviewed. Attention is focused first on the geometrical two-rotor model (TRM), whose predictions gave the motivation for seeking such a mode. The consistency of these predictions with the most meaningful collective properties of the mode is emphasized. More sophisticated descriptions carried out within different boson models are then reviewed. Their strict connection with the TRM is proved. An even closer link is shown to exist between the TRM and the schematic random-phase approximation (RPA). From the phenomenological and schematic models, confined to the description of the collective features of the M1 transitions, the analysis moves to the fully microscopic approaches, the only ones capable of accounting for the global properties of the mode as well as for the fragmentation of its M1 strength. Shell-model approaches, widely adopted for light and medium-light nuclei, are discussed. A more detailed analysis is devoted to the RPA, the most widely adopted microscopic scheme, especially in heavy nuclei. The path leading from the early incomplete and too approximate approaches yielding contradictory results to the most recent and refined studies converging to similar conclusions is sketched. The quasiparticle–phonon model (QPNM) as a way of improving the RPA description of the M1 spectrum by including the coupling to two-phonon RPA states is finally illustrated, and the related results are discussed. The study of the M1 spectra observed recently in deformed odd-mass nuclei carried out in a QPNM context completes the review.
28(1997); http://dx.doi.org/10.1134/1.953056View Description Hide Description
The results of recent experimental and theoretical studies of low-energy nucleon–nucleon and nucleon–deuteron collisions are studied and compared with the older data. Special attention is devoted to two problems: that of the extrapolation of the phase shifts, amplitudes, and cross sections of such collisions to experimentally inaccessible energies, and that of separating the contributions of nuclear and electromagnetic interactions in the parameters of low-energy elastic scattering. Various classical and recent approaches to the solution of these problems are analyzed. A method is developed for constructing low-energy expansions which permits information on the nuclear interaction to be extracted from the measured phase shifts in systems of two and three nucleons or nuclei. The role of electromagnetic corrections to the Coulomb interaction in such scattering reactions can also be studied using this method.
28(1997); http://dx.doi.org/10.1134/1.953057View Description Hide Description
Photodisintegration and radiative capture in the and channels are studied on the basis of the corresponding two-cluster potential models. The astrophysical factors at low energies are calculated. It is shown that various electromagneticcharacteristics of light nuclei, including the Coulomb form factors at momenta of up to 4 can be studied using a single set of intercluster interactions containing forbidden states and consistent with the phase shifts of the cluster low-energy elastic scattering. The calculated inelastic form factors have a second maximum at 3–4 although it is lower than the experimental one. The orbital states are classified for the lightest cluster systems, and the phase shifts and intercluster potentials are separated on the basis of Young orbital schemes.