Investigating fast neutron interaction with chromium nuclei

A new technique based on a Frisch grided ionization chamber and digital signal processing unit is used for low-background measurements of a cross section of the (n, α) reaction proceeding on a solid target mounted on a cathode. Measurements of the (n, α) reaction cross section for the ⁵⁰Cr and ⁵²Cr isotopes are carry out. It is shown that the data for ⁵⁰Cr differ from the ENDFB-VII evaluation by a factor of 20. The excitation function reveals a structure missing from our assessment. The ⁵²Cr(n, α) reaction cross section is measured for neutrons with energies of less than 14 MeV for the first time.     

An extended phonon projection model for even-even nuclei

An extension of the projection model of Lipas et al. is presented. This extended phonon projection model (EPM) includes diagonalization in a projected-state basis and the extension of the laboratory hamiltonian to include anharmonic terms. The oriented system is used in generating the model space. A comparison of the EPM predictions with those of the six-parameter 1BA-1 model is made for ^148–152Sm.     

Deformed even-even nuclei in a quark-meson coupling model with tensor coupling

Neutron total cross sections of ¹⁹⁷Au and ^natTa have been measured at the nELBE photoneutron source in the energy range 0.1–10MeV with a statistical uncertainty of up to 2% and a total systematic uncertainty of 1%. This facility is optimized for the fast neutron energy range and combines an excellent time structure of the neutron pulses (electron bunch width 5ps) with a short flight path of 7m. Because of the low instantaneous neutron flux transmission measurements of neutron total cross sections are possible, that exhibit very different beam and background conditions than found at other neutron sources.     

Low-energy pion scattering from nuclei and the α-particle model

The elastic scattering of pions on carbon and oxygen for T_π = 50–80 MeV is calculated in the Glauber theory using a non-interacting α-particle model wave function for nuclei. The results are in agreement with experimental data.     

Spin dependence of the shape of even-even nuclei within the Davydov-Chaban model

Within a model of a nonaxial even-even nucleus soft in β vibrations, the shape parameters of the ¹⁵⁴Gd, ^156,158,160Dy, ^164,168Er, ¹⁶⁸Yb, ¹⁷⁶Hf, and ¹⁸⁰W nuclei are calculated in the quadrupole approximation as functions of the excited-state spin.     

The effective neutron—proton interaction in rare-earth nuclei

The available experimental information concerning the low-lying rotational-band energies in rare-earth nuclei with odd neutron and proton numbers is analysed within the framework of the unified model. We extract 69 empirical values for 51 two-particle matrix elements of the effective neutron—proton interaction in 16 nuclei. This information deals with 43 two-quasi-particle configurations. Strength and range parameters for various types of effective interaction are determined from least-squares fits of calculated matrix elements to the empirical values. We find a significant improvement in the fit when spin-polarized and long-range components are included in the effective force, indicating the importance of core-polarization effects. We investigate the sensitivity of the calculation to the choice of experimental information, to the single-particle model, and to the radial shape of the force. Some predictions are made concerning as yet unconfirmed or unobserved configurations.     

Real and complex boson expansions in even-even deformed nuclei

Analysis of real and complex boson expansions of the Kishimoto-Tamura type is performed in a deformed basis in order to allow a further study of the anharmonicities of vibrations in deformed nuclei. It is shown that complex solutions cannot be found in the cases where no real one exits.     

An optical model analysis of neutron scattering

Neutron elastic scattering data in the range A > 16, 7 < E < 30 MeV are analyzed using a standard local optical model potential. The obtained parameters are compared with similar optical model parameters derived from proton elastic scattering. Empirical values for the Coulomb correction term, isospin dependence and energy dependence are obtained. The results are compared with theoretical calculations.     

Anomalies in orientation interaction of slow neutrons with nuclei and the problem of neutron molecule existence

Based on the Dirac equation, the features of long-range electromagnetic orientational interaction of slow neutrons with even-even and even-odd nuclei are considered. This interaction is controlled by a narrow potential barrier arranged beyond the nucleus. The barrier height is U _tot = 20–40 eV and depends on Z, A, and the nucleus magnetic moment μ_nucl. The barrier formation is associated with the ponderomotive nonlinear interaction of the anomalous neutron moment with the nucleus electric field. The barrier transparency for thermal neutrons is D(E) ≈ 0.8–0.95. For cold neutrons, the barrier transparency and their reaction cross sections with nuclei sharply decrease and, at E → 0, their cross sections tend toward zero. It was shown that the combined effect of the magnetic dipole-dipole and ponderomotive interaction of the neutron and even-odd nucleus results in the formation of removed symmetrically positioned potential wells for neutrons beyond the nucleus. The presence of these wells results in the possible existence of short-lived or virtual nucleus-neutron molecules and the “neutron halo” effect beyond the nucleus.     

Simple phenomenological model of neutron-resonance densities in 60<A<250 even-even compound nuclei

A simple relationship between the observed density of s-neutron resonances and the energy of the first 2⁺ excited level of even-even compound nuclei is established. The corresponding phenomenological parameters are determined from an analysis of data on stable initial nuclei and are compared with available experimental data on β-unstable nuclei.     

Study of the neutron halo structure in interaction of 6He with nuclei of photoemulsion

To investigate the structure of the halo of Borromean nuclei, we have studied quasi-free scattering of the proton by constituents of halo-nuclei ⁶He. The experimental study is performed using the ⁶He beam of the Flerov Laboratory of Nuclear Reactions (JINR) at an energy of about 10?MeV/u and the technique of nuclear photoemulsions. Searching for events of quasi-free scattering and their processing is performed using the PAVICOM-setup at the Lebedev Physical Institute.     

S-wave nucleon-antinucleon interaction in a model with confined channel

A model of the low energyN¯N interaction based on an idea of a quark channel acting as a doorway state is proposed. The salient features of the experimental data are well reproduced.     

Quantum phase transitions and backbending in even-even N ∼ 90 nuclei

We extend the classical Landau theory for rotating nuclei and show that the backbending in ¹⁶²Yb, which comes about as a result of the two-quasiparticle alignment, is identified with the second-order phase transition. We found that the backbending in ¹⁵⁶Dy, caused by the instability of γ vibrations in the rotating frame, corresponds to the first-order phase transition. We suggest an empirical rule to determine the type of the phase transition in rotating nuclei undergoing backbending. The text was submitted by the authors in English.     

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.