Excited states of deformable even-even atomic nuclei

A method of calculating the collective excited states of even-even atomic nuclei is proposed. A formula depending on three parameters appropriate for calculating the energy ratios of the excited states of deformed and spherical nuclei as well as those in the intermediate region is obtained.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 11, pp. 19–24, November, 1972.     

A calculation of low-lying collective states in even-even nuclei

We present results of a calculation of the low-lying collective quadrupole states in even-even nuclei within the framework of the proton-neutron interacting boson model.     

Theory of the excited states of deformable even-even nuclei

The excited states of nonspherical even-even nuclei are discussed taking the change in nuclear shape during excitation into account.     

On the description of collective negative parity bands in even-even deformed nuclei

The collective bands of negative parity are studied via a microscopic approach where Coriolis forces and octupole correlations are treated on an equal footing. The apparent moment of inertia of these bands are nicely reproduced.     

Excited states of deformable nonaxial odd nuclei

Rotationally single-particle and vibrational excitations of deformable nonaxial odd nuclei are investigated with allowance for the interaction of collective and single-particle states. The ratios of excitation energies, of reduced probabilities of E2 transitions, and of quadrupole moments for deformed nonaxial odd nuclei are calculated up to high-spin states.     

Generators of collective vibrations in heavy nuclei

Solving harmonic equations of motion for a realistic single-particle potential, the generators of various isoscalar collective vibrations up to a multipolarity of 4 are extracted. Transition strengths, transition currents, and energy systematics are studied in comparison with full RPA calculations, fluid dynamical calculations, and experimental data. It is shown that all these states can be well described by simple generators and that the influence of the detailed structure of the residual two-body force is small compared with that of the single-particle level scheme and of geometric constraints.     

Transition probabilities and static moments of excited states in even-even nuclei

A new approach is made to calculate the generalized single-particle density matrixp ^μv in even-even nuclei. It is shown that this quantity is included in the three particle response function, for which we derived a renormalized equation. Taking into account in a consequent way the effective particle-hole interaction we received a formula for the static moment of excited states and the transition probability between such states which is essentially different from the usual RPA theory.     

A shell-model description of 0+ intruder states in even-even nuclei

Starting from the nuclear shell structure in medium-heavy and heavy nuclei, the excitation energy for low-lying 0⁺ intruder states is studied. Taking as a simplified model two particle-two hole (2p-2h) excitations across closed shells, the effects of the pairing and the proton-neutron (monopole and quadrupole component) residual interaction on the unperturbed energies are calculated. Application to major closed-shell (fZ = 50, Z = 82) and to subshell (Z = 40, Z = 64) regions is performed. We especially concentrate on 0⁺ intruder states in the even-even Pb nuclei.     

On collective two-phonon states in deformed nuclei

The centroid energies of the two-phonon states in doubly even deformed nuclei are calculated within the quasiparticle-phonon nuclear model taking into account the Pauli principle in the two-phonon components of the wave functions. It is shown that the collective two-phonon energies are shifted by 1–2 MeV to higher energies due to the Pauli principle. A strong fragmentation of the two-phonon collective states over many nuclear levels occurs at the excitation energies of 3–4 MeV. It is concluded that the two-phonon states cannot exist in deformed nuclei. The analysis of the available experimental data on the two-phonon states shows that the experimental data do not contradict the results of these calculations.     

Isomeric states in heavy nuclei

The one- and two-quasiparticle states in heavy nuclei are treated. The change of one-quasiparticle states in isotone chain seems to be rather smooth. Two-quasiparticle states in nuclei of alpha-decay chain of ²⁷⁰Ds are discussed.     

A method of angular momentum projection for excited states in even-even nuclei

A method of angular momentum projection for the excited states in even-even nuclei is formulated within the framework of the Hartree-Bogoliubov theory. A preliminary application of this method is performed for the K=0^? band in ¹⁶²Er.     

Subthreshold photofission of even-even nuclei

Within quantum-mechanical fission theory, the angular distributions of fragments originating from the subthreshold photofission of the even-even nuclei ²³²Th, ²³⁴U, ²³⁶U, ²³⁸U, ²³⁸Pu, ²⁴⁰Pu, and ²⁴²Pu are analyzed for photon energies below 7 MeV. Special features of various fission channels are assessed under the assumption that the fission barrier has a two-humped shape. It is shown that the maximum value of the relative orbital angular momentum L _m of fission fragments can be found upon taking into account deviations from the predictions of A. Bohr’s formula for the angular distributions of fission fragments. The result is L _m ≈ 30. The existence of an “isomeric shelf” for the angular distributions of fragments from ²³⁶U and ²³⁸U photofission in the low-energy region is confirmed.     

Dynamics of single-particle and collective excitations in heavy nuclei

The propagation of single-particle and small-amplitude collective excitations in a heavy nucleus is considered. We calculate perturbatively corrections to the mean-field approximation induced by the coupling of one-particle and collective motion via the residual particle-hole interaction. Special attention is paid to the energy variation of the quasiparticle effective mass near Fermi energy. We conclude from the calculation that particles and holes excited in low multipolarity giant resonances have average effective masses of the order of 0.8 m rather than m. The mechanism for the decrease is provided by the enforced decoupling of the quasiparticles from surface oscillations due to the high frequency of the giant resonances. We also study the role of surface modes in the decay of giant resonances. Considerable reduction of the damping into 2p-2h states expected from the absorptive part of the optical potential is found. The correlated particle-hole pairs interact with each other by exchanging surface oscillations which adds a destructive interference term to the decay widths of giant resonances. The reduction depends on the multipolarity of the mode and is only large for low angular momenta.     

Excited states in neutron deficient even-even thorium isotopes (218≦A≦222)

The nuclei^{218, 220, 222}Th were investigated by conversion electron andγ-ray spectroscopy after compound nucleus reactions of¹⁴N with²⁰⁹Bi and^{16, 18}O with²⁰⁸Pb. The intenseγ-background from fission was suppressed by spectroscopy of conversion electrons andγ-rays in coincidence with the evaporation residues or theirα-decay. Level schemes were determined for²¹⁸Th up toI ^π=10⁺ and for²²⁰Th and²²²Th up toI ^π=15^?. The observed structure of²¹⁸Th may be explained in the spherical shell model with residual interaction by two neutron excitation. ForN≧130 completely different level schemes are observed showing very intense, collective electric dipole transitions (B(E1)?10^?2 W.u.) with energies of typically 200 keV. Interpretation in terms of stable octupole deformations of the ground state as well as in anα-cluster picture are discussed.     

Binding energies of even-even superheavy nuclei

The structure of the even-even superheavy nuclei with the proton number Z=98–110 is studied using the self-consistent relativistic mean-field theory. The calculated binding energies are in good agreement with the available experimental ones. An upper limit and a lower limit on the binding energies are set by the calculations. This is useful for future calculations of properties of superheavy nuclei and for the experimental synthesis of superheavy nuclei. The energy surface of some relevant superheavy nuclei is also given and it confirms the correctness of the calculations.