Abnormal nuclear matter and pion condensation

The possibility of nuclear matter undergoing a combined phase transition into abnormal matter and a pion condensate is investigated. Various lagrangians for the meson (σ and π_i) fields, based on the σ-models, are used in mean-field approximation, and the entire system (mesons + nucleons) is treated fully relativistically. Equilibrium conditions of nuclear matter are obtained with NN repulsion, parametrized by the excluded volume approximation. It turns out that the formation of abnormal matter depends crucially on the choice of the σ-model lagrangian and considerably less on the additional pion condensate.     

A simplified approach to three-body correlations in nuclear matter through a non-zero particle spectrum

Self-consistent nuclear-matter calculations are presented which take into account Dahlblom's results for the contribution to the binding energy due to three-body correlations. We propose a justified parametrization of the single-particle potential for particle states, the energy contribution of which cancels approximately the energy from certain three-body correlations. Indications are given of how to fix this particle-state potential for a given two-body interaction. Two nucleon-nucleon potentials are used: the Reid soft-core potential and a fully momentum-dependent one-boson-exchange potential similar to the form proposed by Ingber and Potenza. The mechanism of the increase in the total wound due to three-body correlations is investigated and reasons are given why this does not prevent the saturation densities from moving to higher values. Due to three-body correlations and with self-consistency on the hole spectrum, the increase in nuclear-matter binding energy is 0.60 MeV/A for the Reid soft-core interaction and 0.68 Mev/A for the OBEP. The saturation momentum is shifted from 1.42 fm^?1 to 1.44 fm^?1 for the Reid potential and from 1.58 fm^?1 to 1.62 fm^?1 for the OBEP.     

On the self-consistency requirement in the low density expansion of the optical potential in nuclear matter

We critically discuss the choice of the auxiliary potential U which is introduced in the low density expansion of the mass operator M(k, w). This choice is related to the analytical properties of M(k, w) in the complex w-plane and we take due account of momentum conservation in the intermediate states appearing in the diagrams associated with M(k, w). We also provide a computation of the one-hole line, rearrangement and renormalization contributions to the optical potential, of the hole state spectral function and of the momentum distribution in nuclear matter. We use a real auxiliary potential which is self-consistent up to the order considered here, i.e. which takes into account the rearrangement and the renormalization corrections. Rearrangement is treated rigorously. The dependence of the obtained results on the choice of the nucleon-nucleon interaction, namely the Hamman-Ho Kim one in our calculation, is discussed.     

Non-linear density dependence of the Δ-self-energy in nuclear matter and its implications in pionic atoms

The $\text{Δ(}\text{3}\text{2}\text{,}\text{3}\text{2}$) self-energy in a nuclear medium is shown to be highly non-linear in density, when proper care is taken of the virtual meson propagation in the medium and retardation effects. As a consequence the p-wave absorptive potential for pionic atoms diverges appreciably from the standard ρ² form. A fit to the existing data on pionic atoms is carried out with the new functional of the density and turns out to be as good as those with the ρ² functional. The successful fits with such different density functionals are due to a very narrow range of nuclear effective densities felt by the pion in the observed pionic atoms. The influence of these effects in related problems is discussed along with the suggestion to widen the range of nuclear densities felt by the pions by looking at other nuclear phenomena.     

Functional approach to nuclear field theory: A schematic model with pairing and particle-hole forces

Path integral techniques in collective variables are applied to a schematic model with monopole pairing and particle-hole forces. The single-particle and collective excitation modes of the system for various kinds of phase transitions are discussed. We formulate a modified perturbation theory (loop expansion) from which, finally, nuclear field theory (NFT) is obtained. The NFT Lagrangian is strictly derived. The graphical rules of the NFT expansion come out automatically.     

Total nuclear photoabsorption cross sections in the region 150 < A < 190

The curves of the total gamma-absorption cross sections (σ_tot) in the E1 giant resonance energy range for the nuclei ¹⁵⁴Sm, ¹⁵⁶Gd, ¹⁶⁵Ho, ¹⁶⁸Er, ¹⁷⁴Yb, ¹⁷⁸Hf, ¹⁸⁰Hf, ¹⁸¹Ta, ¹⁸²W, ¹⁸⁴W, ¹⁸⁶W and ¹⁹⁷Au have been measured using the absorption method. Parameters of the Lorentz curves fitting the measured cross sections σ_tot are given. Quadrupole moments (Q₀) and nuclear deformation parameters (β) were obtained.For deformed nuclei in the ～ 155 < A < ～ 180 region a violation of the correlation between giant resonance widths (Γ) and nuclear deformation parameters was found. Γ₁ and Γ₂, the widths of the resonances corresponding to vibrations of nucleons along and across the nuclear deformation axis, were observed to decrease with the increase of A which could be accounted for by the presence of an N = 108 subshell.     

Dependence of nuclear deorientation on nuclear spin for recoils in vacuum

Time-integral measurements have been made of the vacuum deorientation of the 2⁺ and 4⁺ states in ¹⁵⁰Sm and the 6⁺ and 8⁺ states in ¹⁵⁶Gd Coulomb-excited by 133 MeV ³⁵Cl ions. The ¹⁵⁰Sm results were deduced from γ-ray angular distributions measured in coincidence with backscattered ³⁵Cl ions. In the ¹⁵⁶Gd measurements, a fixed counter array and a sandwich target were used to measure directly the differences between the γ-ray angular distributions from nuclei recoiling in vacuum and the unperturbed γ-ray angular distributions from nuclei stopped in a thick ¹⁵⁶Gd target. The measured deorientation time constants in ps are τ₂(2⁺) = 27±4; τ₂(4⁺) = 14±4; τ₂(6⁺) = 24±3; τ₂(8⁺) = 23⁺¹⁰_?6; τ₄(2⁺) = 12±2; τ₄(4⁺) = 5±3; τ₄(6⁺) = 7.6±2.5. The 8⁺ data were analyzed assuming τ₂/τ₄ = $\text{10}\text{3}$ as predicted by the Abragam-Pound theory for $\text{I ≧ 4}$ essentially independently of the degree of quadrupole admixture. The other results are consistent with this except for the 2⁺ level which shows some quadrupole effect. The present results, which show strong deorientation of high spin levels, are in contrast to earlier work on neutron deficient Er isotopes. In light of our findings we suggest that the failure to observe deorientation in the high spin states in Er is primarily due to anomalously low g-factors associated with the backbending observed in these nuclei.     

Fusion probability of symmetric heavy,nuclear systems determined from evaporation-residue cross sections

Cross sections were measured for the formation of evaporation residues in ⁴⁸Ca-, ⁸⁶Kr- and ¹²⁴ Sn-induced reactions with Yb, Sb and Zr isotopes. For the nearly symmetric systems, the energy dependence of the fusion probability in central collisions was determined. The fusion probability at the expected fusion barrier as calculated from a one-dimensional heavy-ion potential was found to be reduced by several orders of magnitude. The fusion cross sections increase gradually, and only at energies appreciably above the expected fusion barrier the major part of the cross section of central collisions leads to fusion. The observed hindrance of the fusion process is compared with model predictions.     

The systematics of nuclear single-particle states (II). the region A < 35

The energies of single-particle states in nuclei with A < 35 are obtained as eigenvalues of a local Saxon-Woods potential with depth depending linearly on A and on the nuclear symmetry parameter.     

Determination of nuclear spectroscopic factors from data on differential cross sections for elastic neutron scattering on light nuclei

The values of the coupling constants (spectroscopic factors) dpn, ¹²C¹¹Cn and ¹⁴N¹³Nn have been derived by means of the extrapolation of differential cross section data to the corresponding poles. Some general questions concerning the application of this method to light nuclei are considered.     

Inclusive (p, π) reactions at 201 and 180 MeV

Inclusive proton-induced charged pion production was studied on ¹²C, ⁸⁹Y and ^natPb at 201 and 180 MeV. A QQD spectrometer was used for forward angles and a plastic scintillator range spectrometer for backward angles. The angular dependence, variation with the pion energy, and total cross sections are deduced from doubly differential cross section measurements.     

A study of the Li(π, pp)He reaction within the three-body problem

The differential cross section for the ⁶Li(π⁺, pp)⁴He reaction for symmetrical coplanar kinematics has been calculated at the incident pion energy E_π = 70 MeV within the three-body problem + 2N taking into account the Pauli exclusion principle in each N subsystem. Three-body distortions in the final state have been taken into account in the eikonal approximation. The treatment is based on the superposition of the single-particle and two-particle mechanisms of absorption leading to the rescattering of the π- and ρ-mesons through the Δ-isobar. The analytical character of the adopted wave function for the ⁶Li nucleus (the multidimensional gaussian basis) makes it possible to ascertain clearly that the reaction amplitude of the type under study is insensitive to short-range NN correlations both in the initial and final states. The calculated momentum distribution of recoil -particles is in good agreement with the experimental data available, but the experimental accuracy should be improved in order to see the contribution of a specific three-body part of the ⁶Li wave function orthogonal to the ad channel. Some problems to be solved in future are discussed.     

The (p, t) and (p, τ) reactions on O at 39.8 MeV

Differential cross sections have been measured at forward angles for (p, t) and (p, τ) transitions from ¹⁷O to the ⁻ ground states and lowest-energy ⁻ states in the ¹⁵O and ¹⁵N mirror nuclei. The data are compared with DWBA calculations using simple single-particle and single-hole wave functions. When the (p, t) and (p, τ) transitions are considered separately, the calculated and experimental ratios of the ⁻ integrated cross sections to the ⁻ integrated cross sections agree to within 30 %; however, the ratios of (p, τ) cross sections to the mirror state (p, t) cross sections are calculated to be about twice as large as actually measured. This experimentally observed reduction of the (p, τ) cross section relative to the (p, t) cross section can possibly be attributed to interference between the S = 0 and S = 1 components of the (p, τ) transitions.     

Angular momentum transfer and spin dealignment mechanisms in damped nuclear reactions Ar+Bi and Ni+Pb

The angular momentum transferred to fragment spins has been studied in the damped nuclear reactions Ar+Bi at 255 MeV and 295 MeV and Ni+Pb at 435 MeV from measurement of the angular distribution of the fission fragments of the heavy-recoil nucleus in coincidence with the projectile-like fragment. The heavy-fragment spin is strongly aligned along the normal to the reaction plane and the rigid-rotation limit of the dinuclear system is attained. The dealignment mechanisms produce spin components mainly located in a plane approximately perpendicular to the heavy-recoil lab direction. They are well described by a dynamical model based on the nucleon exchange between the two ions during the collision. The spin-component fluctuations reach high values. In the heavy-recoil direction, these fluctuations are increasing with the total kinetic energy loss and the charge transfer from the projectile to the target. The spin values extracted from both the angular distributions and the fission probabilities are seen to be compatible.     

Formation of covalent nuclear molecules in the 13C + 12C system

Angular distribution's of the elastic scattering of ¹³C on ¹²C and the inelastic scattering leading to the lowest-lying $\text{1}\text{2}{}^{\mathrm{+}}$ and $\text{5}\text{2}{}^{\mathrm{+}}$ states in ¹³C have been measured at energies close to the Coulomb barrier. Analyses carried out in the framework of a complete coupled-reaction-channel theory show that extremely polarized single-particle molecular orbits (hybridization) are formed during the scattering process which give rise to a multiple-step interaction of the valence nucleon, i.e. the formation of a covalent nuclear molecule.     

Modifications of single-particle properties in nuclear matter induced by three-body forces

Within the self-consistent Green’s function formalism, we study the effects of three-body forces on the in-medium spectral function, self-energy and effective mass of the nuclear matter constituents, analyzing the density and momentum dependence.     

Study of 39K levels via the 39K(α, α') reaction

The ³⁹K(α, α') reaction has been studied at E_α = 31 MeV. A number of hole-core coupled states were identified by their strong excitations with angular momentum transfers of L = 3 or 5. The L = 3 transitions showed fairly good agreement with strengths predicted from holecore coupled wave functions. However, agreement between experiment and theory for the states excited by L = 5 excitations $\text{(}\text{11}\text{2}{}^{\mathrm{?}}\text{and}\text{13}\text{2}{}^{\mathrm{?}}\text{levels}\text{)}$ requires accounting for the blocking effects in the wave functions for these levels.     

Nuclear charge radii from X-ray transitions in muonic atoms of carbon,nitrogen and oxygen

Energies of muonic X-rays of the K-series of carbon, nitrogen and oxygen have been measured with an accuracy of about 15 eV. Root mean square radii of the nuclear charge distributions were deduced. The results 2.49±0.05 fm for carbon, 2.55±0.03 fm for nitrogen and 2.71 ±0.02 fm for oxygen are in good agreement at comparable accuracy with recent electron scattering data.     

Charge dependence of nuclear forces in ΔJ = 0, ΔT = ±1 beta decays

By assuming the validity of the conserved vector current theory, the Fermi nuclear matrix elements M_F for the beta decays of ²⁰F, ⁴¹Ar, ⁴⁴Sc, ⁵²Mn and ⁵⁶Co which violate the ΔT = 0 selection rule are calculated theoretically in the jj-coupling, shell model. In these calculations, M_F is obtained by considering a short-range, charge-dependent, internucleon potential in addition to the usual Coulomb potential. Experimental values of M_F have been obtained by measuring the asymmetry parameter A appearing in the β-γ circular polarization correlation. Comparison between the theoretical and experimental values yields the result that the deviation of nuclear forces from exact charge independence is about a few per cent, while the deviation from charge symmetry is consistent with a value an order of magnitude smaller. If the conserved vector current theory is not valid, additional contributions to the Fermi nuclear matrix elements can arise from mesonic exchange currents. It is noted that such effects could be as large as those due to charge-dependent forces and would therefore cloud the above interpretation.