N.I. Pyatov

Collective 1+ states in doubly even deformed nuclei

Abstract The properties of collective I π = 1 + ( K = 0 and 1) states in doubly even deformed nuclei are investigated in the framework of the Tamm-Dankoff (TDA) and random-phase (RPA) approximations. The Hamiltonian of the model includes the pairing and spin-spin residual interactions. The strength of the latter is found from calculations of magnetic moments in odd-mass nuclei. The calculations of the B ( M 1, 0 + → 1 + ) values, dipole sum rules and strength functions for M1 transitions show that in rare-earth nuclei the spin-spin correlations concentrate the main strength of M1 transitions in the high-energy region (≈ 10 MeV) where the coherent 1 + states may appear (magnetic dipole resonance).

Excitation of 0+ states in two-particle transfer reactions

Abstract The spectroscopic factors for the two-particle transitions to the ground and excited 0 + states are studied in the model with pairing, quadrupole and spin-quadrupole forces. A qulaitative explanation of the observed cross-section ratios in the actinide nuclei is obtained.

Microscopic DWIA analysis of the (p, n) reactions

Abstract The charge-exchange nuclear excitations for the isobars 48 Ca → 48 Sc , 90 Zr → 90 Nb and 208 Pb → 208 Bi are considered in the framework of the theory of finite Fermi systems by exactly taking into account the particle-hole continuum. The distorted-wave impulse approximation (DWIA) is employed to calculate the angular distributions for the isobaric analogue states (IAS), Gamow-Teller resonances (GTR), dipole ( L = 1, S = 0), spin-dipole ( L = 1, S = 1), and so on, up to L = 7 excitations in the (p, n) reactions at bombarding proton energies E p between 100 and 200 MeV. The results obtained are compared with experimental data and the conclusions concerning both the effective nucleon-nucleon interaction in the chargeexchange channel and the local quasiparticle charge e q [ στ ] with respect to the στ fields are discussed. It occurred that just below the GTR at small angles there is essentially no background from transitions with L > 0. It is shown that the theory describes fairly well the experimental data with the Landau-Migdal spin-isospin strength parameter g ′ = 1.1 ( G ′ = 330 MeV · fm 3 ) and e q [ στ ] = 0.8. The latter characterizes the quenching of the low-energy spin-isospin-flip transitions, i.e. the renormalization of the axial-vector vertices in nuclear matter.

Non-adiabatic calculation of positive parity states in 161Er and 163Er

Abstract A calculation of positive parity states in 161 Er and 163 Er, within the non-adiabatic rotational model, has been performed, taking the Coriolis coupling of seven Saxon-Woods potential orbitals into account. Two parameters, the rotational parameter h z.shtsls; 2 2φ and the effective pairing force strength G eff were used in the calculation. It is shown that a satisfactory fit of the calculation to the experimental energy levels with spin values up to 29 2 can be achieved only if a strongly reduced pairing force and rotational parameter values close to those for neighbouring even nuclei are used.

0+ and 1+ unlike particle-hole states in deformed odd-odd nuclei and ß-strength functions

Abstract The properties of collective 0 + and 1 + states built of unlike particle-hole excitations in deformed odd-odd nuclei have been investigated in a microscopic approach. Their s-strength functions are calculated and compared with other predictions.

SPIN POLARIZATION EFFECTS IN ODD-MASS DEFORMED NUCLEI.

Abstract Within the framework of the Tamm-Dancoff method the spin polarization effects are interpreted as caused by the “scattering” of the odd nucleon on the 1 + ( K =0 orl) excitations of the even core. On this assumption the values of the effective factors g s z and g s + are connected with the “scattering” process on the 1 + 0 and 1 + 1 excitations respectively. It is pointed out that it is necessary to take the latter process into account in calculating the decoupling parameter for the K= 1 2 spin up states. The numerical calculation is performed for the 1 2 − [510] Nilsson state.

N-forbidden beta transitions in odd-mass deformed nuclei

Abstract The effect of the ΔN =2 mixing of states from different principal harmonic oscillator shells on β-transitions between odd-mass deformed nuclei is considered using Woods-Saxon single-particle wave functions. The value of log ( ft ) s.p. = 6.2 − 6.4 is obtained for all the N -forbidden transitions. Taking into account pairing correlations brings into satisfactory agreement the calculated and experimental log ft .

A study of the 90Zr(6Li, 6He)90Nb reaction at E6Li = 93 MeV

Abstract The energy spectra of 6 He nuclei from the 90 Zr( 6 Li, 6 He) reaction at E 6 Li = 93 MeV have been measured over the angular range of 7 ° ⩽ θ lab ⩽ 20 °. The theoretical DWBA analysis of these spectra and angular distributions was performed employing transition densities of bound and resonance states obtained on the basis of the theory of finite Fermi systems. It is shown that up to excitation energies of E x ≈ 10 MeV in 90 Nb a direct charge-exchange mechanism dominates in this reaction, while at higher E x the contribution made by multistep processes increases significantly. It is also shown that the quasi-elastic cross section is determined by a sum of partial contributions of spin-isospin-flip transitions with multipolarities L =0 through L =6.

Spin-Isospin Excitations in Nuclei and (p,n) Reactions

The review is given of theoretical studies of spin-isospin excitations of nuclei in (p,n) reactions at intermediate energies. The microscopic approach used combines the distorted-wave impulse approximation (DWIA) for the reaction mechanism with the nuclear structure description within the theory of finite Fermi-systems (TFFS). The model was applied to the calculation of angular distributions for bound and resonance states, inclusive energy spectra of neutrons and spin observables for some closed shell nuclei.

Microscopic Description of (p, n) Reactions at Intermediate Energies

Recently,(p, n) reactions at intermediate proton energies (Ep ≳ 100 MeV) have turned out to be a fruitful tool for studying charge-exchange excitations of nuclei, in particular, in the spin-isospin channel. Thus, the Gamow — Teller resonance (GTR) was discovered and the quenching of the integral strength of spinisospin transitions with a small momentum transfer in the low-energy range of the excitation spectrum (0 ≤ Ex ≤ 30 MeV) was established[1]. To obtain a quantitative estimate of the quenching factor in the continuous spectrum region, it was necessary to develop a microscopic approach for the description of observed inclusive neutron spectra and separation of contributions of various multipolarities. First estimates of a background below the GTR were made in papers[2] without taking into account the effective interactions of guasiparticles. Further development of this model and detailed calculations for 90Zr are given in ref.[3]. Analogous calculations by the Hartree-Fock method with the Skyrme interactions were carried out in ref.[4]. We have developed a microscopic model[5] for the description of angular distributions for individual excitations and inclusive spectra of neutrons, which differs from the above-mentioned approaches, mainly, by the method of structure calculations.