Ch. Stoyanov

Damping of high-lying single-particle modes in heavy nuclei

Abstract The recent experimental and theoretical results on the damping of high-lying single-particle modes in heavy nuclei are reviewed. In one-nucleon transfer reactions these states manifest themselves as broad “resonance”-like structures superimposed on a large continuum. The advantages and the limitations of the transfer reaction approach will be presented using the results from neutron and proton pick-up and stripping reactions. The problem raised by the subtraction of the underlying background, the assumptions made to describe the reaction process and the method used to extract the strength distributions are presented. The existing empirical systematics is summarized for nuclei ranging from 90Zr to 208Pb. The theoretical approaches used to explain the damping of the high-lying single-particle modes are based on the coupling between collective and single-particle degrees of freedom. In a first step the bare single-particle mode is spread over several doorway collective states due to the interaction with surface vibrations. In a second step the doorway states spread their strengths over many other degrees of freedom. These two steps of the damping mechanism are discussed in detail within the framework of the quasiparticle-phonon nuclear model. A large-scale comparison between the measured and calculated average energies, spreading widths and spectroscopic strengths of the high-lying single-particle (hole) states in heavy nuclei is presented. The systematic features of the damping (energy, angular momentum and isotopic dependence) are discussed. Recent advances of the experimental approaches, such as the γ-decay of the high-lying states or the use of heavy-ion transfer reactions at intermediate energies, are outlined. The detailed study of the damping mechanism of high-lying single-particle modes reveals new features and leads us to a new field in nuclear structure: “the spectroscopy of inner and outer subshells”.

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.

Boson forbidden low-energy E1-transitions in spherical nuclei

Low-energy E1-transitions in spherical nuclei forbidden in the ideal boson picture are considered. For that the internal fermion structure of nuclear excitations is taken into account. Several examples of such transitions calculated within the Quasiparticle Phonon Model are considered and the role of dipole core polarization is discussed. It is shown that transition probabilities of an order of 10 3 W.u. observed experimentally are well described by this model. c 1998 Elsevier Science B.V.

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.

Quasiparticle random phase approximation with finite rank approximation for Skyrme interactions

A finite rank separable approximation for the particle-hole RPA calculations with Skyrme interactions is extended to take into account the pairing. As an illustration of the method energies and transition probabilities for the quadrupole and octupole excitations in some O, Ar, Sn and Pb isotopes are calculated. The values obtained within our approach are very close to those that were calculated within QRPA with the full Skyrme interaction. They are in reasonable agreement with experimental data.

Nuclear properties in the lead region within the quasiparticle-phonon nuclear model

Abstract The properties of the low-lying states and giant resonances in 208Pb are calculated within the quasiparticle-phonon nuclear model, the model parameters being fixed. The fragmentation of the deep hole states 1 h 11 2 , 1 g 7 2 and 1 g 9 2 in 207Pb and 1 g 7 2 , 1 g 9 2 , 2 p 1 2 and 2 p 3 2 in 203, 205, 207Tl is calculated, and qualitative agreement with the available experimental data is obtained. The s-, p- and d-wave neutron strength functions are calculated for 206Pb + n and 207Pb + n resonances. Their energy dependence is correctly described up to 0.9 MeV above the neutron binding energy Bn. The reduced E1 transition probabilities and the total photoabsorption cross sections σγt in 206, 208Pb are calculated. It is shown that substructures observed in the energy dependence σγt are caused by the fragmentation of one-phonon states. A fine structure of the isoscalar quadrupole resonance is explained, and the description of the LEOR and HEOR in 208Pb is obtained, which is in agreement with the experimental data.

Two-phonon J = 1 states in even-mass Te isotopes with A = 122–130☆

Abstract Excited states of the nuclei 122,126,130 Te were populated via the (γ, γ′) reaction at endpoint energies of the bremsstrahlung between 4.5 and 5.5 MeV. Gamma rays were detected with a EUROBALL CLUSTER detector and a single HPGe detector. In all investigated nuclei two or three prominent dipole transitions were identified at E γ ≈ 3 MeV. The corresponding low-lying J = 1 states are interpreted as two-phonon excitations. Quasiparticle-phonon-model calculations predict at about 3 MeV one 1 − state arising from the coupling of the first quadrupole and the first octupole phonon, and one 1 + state arising from the coupling of the first and the second quadrupole phonon, where the latter has isovector character. Such an excitation mode can be considered as an analogue of the scissors mode in vibrational nuclei. The calculated transition strengths are compatible with experimental ones within a factor of about 1.5.

The influence of the giant dipole resonance on radiative strength functions in spherical nuclei

Abstract The El strength functions for spherical doubly even nuclei are calculated in a wide energy interval including the giant dipole resonance (GDR). A good description of the GDR widths in 124 Te, 140 Ce and 142 Ce is obtained. The calculated values of the radiative strength functions near the neutron binding energy B n are in good agreement with the experimental data. The influence of GDR on the radiative strength functions and on the total photoabsorption cross sections is studied. It is shown that in single-closed-shell nuclei the El strength functions near B n are slightly influenced by the GDR. The values of strength functions are determined by the fragmentation of one-phonon states lying near B n For nuclei far from the closed shells, the influence of GDR on the radiative strength functions increases, and, for example, in 136 Ba, 144 Nd and 146 Nd it becomes essential. The dipole photoabsorption σ γ ,t cross sections as functions of the excitation energy are calculated. The existence of substructures but not their energy location is reliably predicted by the theory. The available experimental data confirm the existence of substructures in the energy dependence of σ γ ,t It is shown that the Lorentzian extrapolation of the GDR tail proved to be rough for the description of σ γ t near B n .

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.