A.I. Vdovin

FRAGMENTATION OF GIANT MULTIPOLE RESONANCES OVER TWO-PHONON STATES IN SPHERICAL NUCLEI

Abstract A method for the calculation of the strength function photoexcitation of giant multipole resonances in spherical nuclei is presented. The method is developed in the framework of the model based on the quasiparticle-phonon interaction. The one-phonon states are calculated in the random phase approximation. The fragmentation of one-phonon states over two-phonon ones is calculated. The strength function b(Eλ, η) and the positions of giant isovector dipole and isoscalar quadrupole resonances in 90Zr, 120Sn and 124Te are found. The calculations of the transition strengths and positions of these resonances are in reasonable agreement with experiment. The strength function b(E2, η) is calculated for the isovector quadrupole resonance in 90Zr. The description of the lowenerev octupole resonances in 90Zr and 120Sn is in agreement with experiment.

The description of the fragmentation of one-quasiparticle states in spherical nuclei

Abstract The fragmentation of hole states in spherical nuclei is calculated within the quasiparticlephonon nuclear model. The model equations and the numerical method for their solution are given. The influence of the “quasiparticle plus two phonon” components of the wave function and of phonons of different multipolarity on the fragmentation of one-quasiparticle states is investigated. The necessity of taking into account the “quasiparticle plus two phonon” components and of calculating with a large phonon basis is shown. The calculated values of the centroid E x , width Γ ↓ and spectroscopic factor S j , are in satisfactory agreement with the experimental data for the fragmentation of the neutron states 1 d 3 2 and 1f 7 2 in 57 Ni, 1g 9 2 in Sn and 123 Te, and of the proton state 1g 9 2 in 143 Pm, etc. The calculated strength concentration of the lg 9 2 state decreases in the peak when passing from 115 Sn to 119 Sn. However, this decrease is less than that observed experimentally.

Magnetic quadrupole resonance in spherical nuclei

Abstract The distribution of the M2 strength in spherical nuclei is studied within the quasiparticle-phonon nuclear model. It is shown that the interaction of the one- and two-phonon states affects strongly this distribution at the excitation energies E x > 15 MeV. In all the nuclei the strength of the M2 transitions is concentrated in the excitation energy region of 6–12 MeV. At these energies the calculated total value of B (M2)↑ is in good agreement with the experimental data in 90 Zr and 208 Pb. The calculations show that a group of states observed in 58 Ni at an energy of about 7 MeV in the (e, e′) experiments is a part of the M2 resonance.

Gamow-Teller strength distributions at finite temperatures and electron capture in stellar environments

We propose a new method to calculate stellar weak-interaction rates. It is based on the thermofield dynamics formalism and allows calculation of the weak-interaction response of nuclei at finite temperatures. The thermal evolution of the GT{sub +} distributions is presented for the sample nuclei {sup 54,56}Fe and {sup 76,78,80}Ge. For Ge we also calculate the strength distribution of first-forbidden transitions. We show that thermal effects shift the GT{sub +} centroid to lower excitation energies and make possible negative- and low-energy transitions. In our model we demonstrate that the unblocking effect for GT{sub +} transitions in neutron-rich nuclei is sensitive to increasing temperature. The results are used to calculate electron capture rates and are compared to those obtained from the shell model.

SEMI-MICROSCOPIC CALCULATION OF THE LEVEL DENSITY IN SPHERICAL NUCLEI

The level density at the neutron binding energy for 90 spherical nuclei in the interval 50 < A < 205 is calculated by a method of direct counting of the number of states taking into account collective vibrational excitations. The results of calculations are in satisfactory agreement with the experimental data. The difference in the level density of doubly even and odd-A nuclei is correctly described. The effect of nuclear vibrations on the level density is studied, and it is shown that the account of them leads to an increase in the density by a factor of 1.5–10 and to a decrease in the density fluctuations. It is also studied how the level density depends on excitation energy. With increasing excitation energy, our results come nearer the corresponding values obtained by the statistical model. It is found that the density fluctuations decrease with increasing excitation energy but remain still strong at the neutron binding energy for nuclei with A = 50–70 and for nuclei around closed shells. The density ρ(Iπ) is studied as a function of spin and parity. It is shown that at the neutron binding energy the ratio ρ(I+)ρ(I−) is different from unity for the majority of nuclei. This difference is especially striking for 57Fe and 58Fe nuclei.

Semi-microscopic calculation of the neutron strength functions of spherical nuclei

The fragmentation of single-particle states in odd-A spherical nuclei is studied and the s- and p-wave neutron strength functions are calculated in the framework of the model based on the quasiparticle-phonon interaction. The calculations have no free parameters. The theoretical values of the s- and p-wave strength functions for the compound nuclei 117-123Sn, 125-131Te are approximately equal to S0=(0.1-0.2)*10-4, S1=(2-4)*10-4. They are in satisfactory agreement with experiment.

On the M1 and M2 strengths in 140Ce

Abstract The available experimental and theoretical data on M1 and M2 states in 140 Ce are analyzed. Comparison of the experimental form factors of inelastic scattering of electrons at 165° with DWBA calculations within the quasiparticle-phonon model shows that the experiment does not contradict the existence of noticeable M1 transitions in 140 Ce.

Transversal E1-Resonance in Spherical Nuclei

where ~b‘), K’) and K

Test of the modified BCS model at finite temperature

“), K:”) are the isoscalar and isovector The spin-flip El-states in medium and heavy spherical nuclei are investigated within the q;iasiparticlephonon nuclear model. The RPA-calculations predict the existence of a collective 1-state formed by the isovector spin-dipole force. Its excitation energy is about 20MeV. This state is intensively excited by the inelastic electron scattering at momentum transferred q = 0.5-0.7 fm-’, at the scattering angles 0 > 60’ the main contribution to its excitation comes from the transversal form factor. Therefore, it can be recognized as the transversal Elresonance. The interaction with two-phonon states causes a very strong spreading of the resonance, thus making its experimental observation hardly probable.

Reply to 'Comment on 'Test of the modified BCS model at finite temperature''

A recently suggested modified BCS (MBCS) model has been studied at finite temperatures. We show that this approach does not allow the existence of the normal (nonsuperfluid) phase at any finite temperature (FT). Other MBCS predictions, such as a negative pairing gap, pairing induced by heating in closed-shell nuclei, and superfluid to super-superfluid phase transition are discussed also. The MBCS model is tested by comparing it with exact solutions for the picket fence model. Severe violation of the internal symmetry of the problem is detected. The MBCS equations are found to be inconsistent. The limit of the MBCS applicability has been determined to be far below the superfluidchar21{}normal phase transition of the conventional FT-BCS, where the model performs worse than the FT-BCS.