Nuclear Isospin Research Articles

Probing the nuclear matter isospin asymmetry by nucleon-induced reactions at Fermi energies

Despite the fact that valuable experimental measures are still lacking, available nuclear data on nucleon-induced reactions open new opportunities to address either reaction mechanisms or nuclear interaction characteristics. In this work single and double differential cross sections of emitted particles are analyzed and compared with the experiment. It will be evidenced that these cross sections follow a precise hierarchy. The preequilibrium components of the spectra are built up by the dynamics of the reaction as well as by the properties of the nuclear interaction.

Neutrino beams as a probe of the nuclear isospin and spin–isospin excitations

We explore the possibility of performing nuclear structure studies using low-energy neutrino beams. In particular, low-energy beta-beams and conventional sources (muon decay-at-rest) are considered. We present results on the total charged-current as well as flux-averaged cross sections associated to electron (anti-)neutrino scattering on oxygen, iron, molybdenum and lead, as typical examples. It is shown that by using neutrinos from low-energy beta-beams, information on forbidden states, in particular the spin-dipole, could be extracted.

Instabilities in nuclear matter and finite nuclei

Spinodal instability in nuclear matter and finite nuclei is investigated. This instability occurs in the low-density region of the phase diagram. The thermodynamical and dynamical analysis is based on Landau theory of Fermi liquids. It is shown that asymmetric nuclear matter can be characterized by a unique spinodal region, defined by the instability against isoscalar-like fluctuation, as in symmetric nuclear matter. Everywhere in this density region the system is stable against isovector-like fluctuations related to the species separation tendency. Nevertheless, this instability in asymmetric nuclear matter induces isospin distillation leading to a more symmetric liquid phase and a more neutron-rich gas phase.

Splitting of the isovector giant dipole resonance in neutron-rich spherical nuclei

The well-known splitting of the isovector giant dipole resonance is traditionally explained as a phenomenon of the nuclear isospin asymmetry (isospin splitting model) or the nuclear deformation. We suggest a new mechanism of the splitting of the giant multipole resonances in spherical neutron-rich nuclei resulting from the interplay of the isovector and isoscalar sounds with different velocities. Our approach is based on the collisional Landau kinetic theory and can be used for description of the splitting phenomena for both the isoscalar and the isovector modes in a wide region of nuclear masses $A~40\text{\ensuremath{-}}240$. For the isovector dipole modes, the evaluated values of the splitting energy, the relative strength of the main and satellite resonance peaks, and the contribution to the energy-weighted sum rule are in agreement with experimental data.

Bulk and single-particle properties of hyperonic matter at finite temperature

Bulk and single-particle properties of hot hyperonic matter are studied within the Brueckner-Hartree-Fock approximation extended to finite temperature. The bare interaction in the nucleon sector is the Argonne V18 potential supplemented with an effective three-body force to reproduce the saturating properties of nuclear matter. The modern Nijmegen NSC97e potential is employed for the hyperon-nucleon and hyperon-hyperon interactions. The effect of temperature on the in-medium effective interaction is found to be, in general, very small and the single-particle potentials differ by at most 25% for temperatures in the range from 0 to 60 MeV. The bulk properties of infinite matter of baryons, either nuclear isospin symmetric or a \ensuremath{\beta}-stable composition that includes a nonzero fraction of hyperons, are obtained. It is found that the presence of hyperons can modify the thermodynamical properties of the system in a non-negligible way.

Quasiparticle random phase approximation based on the relativistic Hartree-Bogoliubov model. II. Nuclear spin and isospin excitations

The proton-neutron relativistic quasiparticle random-phase approximation (PN-RQRPA) is formulated in the canonical single-nucleon basis of the relativistic Hartree-Bogoliubov model, for an effective Lagrangian characterized by density-dependent meson-nucleon couplings. The model includes both the $T=1$ and $T=0$ pairing channels. Pair configurations formed from the fully or partially occupied states of positive energy in the Fermi sea, and the empty negative-energy states from the Dirac sea, are included in PN-RQRPA configuration space. The model is applied to the analysis of charge-exchange modes: isobaric analog resonances and Gamow-Teller resonances.

Isospin effects in nuclear multifragmentation

We develop an improved Statistical Multifragmentation Model that provides the capability to calculate calorimetric and isotopic observables with precision. With this new model we examine the influence of nuclear isospin on the fragment elemental and isotopic distributions. We show that the proposed improvements on the model are essential for studying isospin effects in nuclear multifragmentation. In particular, these calculations show that accurate comparisons to experimental data require that the nuclear masses, free energies and secondary decay must be handled with higher precision than many current models accord.

New magnetic dipole phenomena in atomic nuclei

In recent years a number of nuclear phenomena was discovered that are accompanied by strong magnetic dipole (MI) transitions. Measurements of MZ matrix elements enabled nuclear physicists to deduce new structure information in a model-independent way, since the electromagnetic interaction is well understood. Due to the dominantly isovector character of the electromagnetic MI transition operator (see Appendix A), information on MI matrix elements helps to clarify various aspects of the nuclear isospin degree of freedom (see Appendix B). This is of particular importance as a prerequisite for making full use of newly established or planned radioactive ion beam (RIB) facilities probing nuclei at extreme values of isospin.

Isospin dependent Pauli blocking and nucleon mean free path in isospin-asymmetric nuclear matter

The isospin, density, and energy dependence of the nucleon mean free path in isospin-asymmetric nuclear matter is obtained through geometrical considerations of isospin dependent Pauli blocking and Fermi motion. The nuclear medium isospin effects on nucleon mean free path are studied systematically. It is found that the isospin-dependent nuclear medium effects are important for mean free path of a nucleon in isospin-asymmetric nuclear matter. Meanwhile, the importance of isospin-dependent Pauli blocking in isospin-asymmetric nuclear matter is also discussed.

A nuclear isospin potential from inversion of scattering data

Inverse scattering theory has been used to specify central and isospin dependent potentials from analyses of the complementary sets of differential cross sections from the 334.4 MeV 3He 14C and the 72 MeV 14C 3H reactions. A numerical inversion (conventional optical model search process) and a global inversion (fully quantal Lipperheide-Fiedeldey scheme) method has been used to to specify those interactions. The results of the two procedures are dramatically different.

Nuclear surface, mean field and isospin effects in Boltzmann master equation theory of pre-equilibrium reactions

Pre-equilibrium emission of nucleons and light charged particles is investigated in central collisions leading to the complete fusion of two heavy ions, within the Boltzmann master equation approach. Realistic nucleon momentum density distributions for both projectile and target nuclei are used and mean field effects are schematically introduced in the early stage of the reaction process. The isospin dependence of the calculation parameters is also discussed. Theoretical predictions of the spectra of the particles emitted both in symmetric and asymmetric interactions at incident energies of a few ten MeV/nucleon are in satisfactory agreement with the experimental findings.

Neutrons, Nuclei and Matter: An Exploration of the Physics of Slow Neutrons

A selection of topics: The neutron as an elementary particle: The nuclear atom The constituents of nuclei Discovery of the neutron Neutron capture Mechanical properties of the neutron Neutron sources Neutron detection and measurement. Neutron scattering and two-particle interactions: Fundamental interactions of the neutron Two-particle collisions Scattering in a central potential Scattering parameters Scattering length and effective range Discrete states of the scattering potential The neutron-proton system. Neutrons and nuclei: Nuclear stability and binding energy Nuclear sizes Nuclear forces Nuclear isospin pions. Neutrons and nuclear structures: Models of the nucleus The nuclear shell model Simple shell-model tests The extended shell-model Collective motion collective motion and unitary symmetry Neutron reactions and nuclear structure. Neutrons and nuclear energy: Energetics of nuclear transformation and decay Nuclear fission Neutron transport and diffusion Neutron moderation The nuclear fission chain reactor Controlled nuclear fusion. Foundations of neutron optics: Wave-particle duality The eikonal approximation for neutrons Neutron propagation in condensed media Phase relations and superposition of states. Wave phenomena with neutrons: The Huygens-Fresnel principle for neutrons Neutron diffraction Neutron interference Neutron polarization Neutron spin resonances. The neutron and condensed states of matter: Applied neutron optics Neutron scattering in condensed systems The Van Hove theory of space-time correlations Neutron scattering in magnetic media Neutron scattering instrumentation. The neutron as a composite system: Strong interactions and flavour SU(3) symmetry Electromagnetic interactions Weak interactions. Fundamental processes and the role of the neutron: Parity violation in the nucleus The Standard Model of quarks and leptons Quark models of the nucleon Neutron physics beyond the Standard Model The neutron in astrophysics and cosmology.

Restoration of isospin symmetry in highly excited compound nuclei.

We have deduced compound nuclear isospin mixing in $^{28}\mathrm{Si}^{\mathrm{*}}$ and $^{26}\mathrm{Al}^{\mathrm{*}}$ at excitation energies between 33 and 65 MeV. The \ensuremath{\gamma}-ray yield from the decay of giant dipole resonances built on excited states is found to be suppressed, implying that isospin mixing is small. By comparison with data from the literature at lower excitation energy, we conclude that isospin is a better symmetry in these compound nuclei at high excitation energy.

THE DETERMINATION OF Vud AND A TEST OF THE UNITARITY OF THE QUARK MIXING MATRIX

The significant violation of unitarity implied by recently published results for the elements Vud, Vus and Vub of the first row of the quark mixing matrix is considered. A possible way out of the difficulty is described, which gives a different value of |Vud|. It involves close scrutiny of the calculated values of the nuclear isospin breaking correction δc to the ft values for superallowed β decays.

Strength of intermediate-range forces coupling to isospin

An experimental search for new forces coupling to nuclear isospin with a range of \ensuremath{\ge}3 m was conducted using a torsion balance driven in resonance by a set of masses configured to generate a nearly pure isospin source field. The strength of any such coupling \ensuremath{\xi} in units of gravity per atomic mass unit is found to be bounded by -2.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$\ensuremath{\le}\ensuremath{\xi}\ensuremath{\le}+2.7\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$, where the positive sign represents an attractive force between like isospin charges.

Electroproduction of charged pions from light nuclei

Electroproduction of charged pi mesons from light nuclei to discrete final states is studied in a distorted wave impulse approximation. The Blomqvist and Laget pion photoproduction operator is extended to the case of electroproduction by including virtual photons. Gauge invariance is included in the theory by making the Feynman diagrams for the process individually gauge invariant. Conservation of the electroproduction current and isolation of the electron's kinematical aspects are done by introducing modified virtual photon polarization vectors. Cross section calculations for single nucleon electroproduction performed with the real photoproduction parameters of Blomqvist and Laget and standard forms of electromagnetic form factors agree with data. The single nucleon virtual photoproduction operator is used in a distorted wave impulse approximation to describe electroproduction from a nuclear target leading to a discrete final nuclear state. Nuclear transition densities which enter into elastic electron scattering and electroproduction processes are compared. It is found that because of the change of nuclear isospin, electroproduction provides information on a larger number of transition densities than inelastic electron scattering for a given target nucleus. Methods of integrating over the electron's kinematics are presented for single arm experiments, that is, where only the produced pion is detected. Single arm electroproduction cross sections are calculated for 10B and 12C targets. For the 10B case, good agreement with data is found. For the 12C electroproduction cross sections, no conclusions can be reached because the quality of present data is poor. It is found that in these experiments, contributions to the cross section from longitudinal virtual photon components are negligible because of small values of virtual photon invariant mass.

Search for an intermediate-range composition-dependent force.

We have conducted an experiment to detect a composition-dependent force with range \ensuremath{\lambda} between 10 m and 1 km, and find a statistically significant effect. If interpreted as arising from a new force, this result and other recent measurementes would be consistent in strength only if the coupling were predominantly to nuclear isospin.

Study of the two-nucleon mechanism of pion absorption in nuclei

The two-nucleon mechanism of pion absorption by nuclei is investigated in the energy region of the πN P 33 Δ-resonance. The basic absorption process is governed by a πNN … NΔ … NN transition matrix, derived from the phenomenological Hamiltonian of M. Betz and T.-S. H. Lee ( Phys. Rev. C, 23 (1981) , 375), which was constructed to describe NN scattering phase-shifts up to 1 GeV. The model allows a realistic description of pion absorption on a pair of bound nucleons with quantum numbers and relative radial wave functions different from those of the physical deuteron. The deuteron-like 3 S 1 ( T = 0) pairs are shown to play a privileged role, in accordance with the assumption underlying the conventional quasideuteron model. We then enbed the two-nucleon mechanism into complex nuclei, using the impulse approximation. The many-body effects on the two-body absorption mechanism are analyzed in detail by using the Faddeev wavefunction for 3He and harmonic oscillator shell-model wavefunction for 1 p-shell target nuclei in our calculations. All nonlocal effects owing to nucleon Fermi motion and NΔ off-shell propagation are treated rigorously. The main features of ( π +, p) reactions on 3He, 4He, and 12C are predicted correctly, when large pion distortion effects are taken into account by using the isobar-hole model with all of the parameters pradetermined from earlier studies of pion nucleus scattering. The predicted cross sections at the two-body absorption peaks overestimate the data by a factor ranging from 1.2 ∼ 2 for the ( π +, p) reactions, and as much as a factor of ∼ 4 for the coincidence ( π +, pp) on 12C. Our results indicate that a consistent description of all inclusive data can be obtained only when we further assume that the outgoing nucleons must be strongly rescattered by nuclear medium in 12C and heavier nuclei. When the calculated absorption cross sections are integrated over entire kinematic regions at each pion energy, we find that the calculated values are only about 1 3 to 1 2 of the total absorption cross section extracted from earlier measurements. By combining our results and calculations by K. Masutani and K. Yazaki ( Nucl. Phys. A, 407 (1983) , 309), it is concluded that a large part of total absorption cross section could originate from the inelastic absorption process: ( π, π′ N) nucleon knockout followed by pion absorption. We are, however, unable to calculate microscopically this inelastic absorption cross section unambiguously, and hence the existence of other absorption mechanisms and possible contributions to proton spectra from targer fragmentation cannot be excluded from our analysis. These possibilities become even more likely when the weaker ( π −, p) cross section is found to be undertestimated by the theory by a very large factor. The origin of this problem is identified to be the suppression, by nuclear geometry and isospin selection rules, of the two-body process πNN … NΔ … NN initiated by π −. We discuss the necessary experimental information for future improvements of the theory.

Antinucleon as a probe of nuclear spin and isospin excitations

Using two models for the antinucleon-nucleon ($\overline{\mathrm{N}}\mathrm{N}$) interaction, we obtain complex, energy dependent $\overline{\mathrm{N}}\mathrm{N}$ transition operators $t$ appropriate for ($\overline{\mathrm{N}}, {\overline{\mathrm{N}}}^{\ensuremath{'}}$) inelastic scattering studies on complex nuclei. It is shown that the spin-isospin dependence of the $\overline{\mathrm{N}}\mathrm{N}$ annihilation potential plays an important role in determining the dominant spin-isospin modes in the nuclear response. In particular, for the most realistic of our $\overline{\mathrm{N}}\mathrm{N}$ models, a large spin dependent component ${t}_{\ensuremath{\sigma}}$ is obtained, leading to the strong excitation of isoscalar spin-flip states; this term is suppressed in the corresponding NN $t$ matrix. The central spin and isospin independent term, ${t}_{0}^{c}$, is large for $\overline{\mathrm{N}}\mathrm{N}$, at all relevant momentum transfers $q$. At high $q$ the isoscalar spin-orbit (${t}_{0}^{\mathrm{LS}}$) and isovector tensor (${t}_{\ensuremath{\tau}}^{T}$) components are important. Cross section and analyzing power predictions for $\overline{\mathrm{N}}$ inelastic scattering as well as corrections due to energy and density dependence are discussed.

Isospin dependence of second-order pion-nucleus optical potential

A universal form for the dependence of the second-order pion-nuclear optical potential on the nuclear and pionic isospin and on the densities $\ensuremath{\rho}$ and $\ensuremath{\Delta}\ensuremath{\rho}$ is derived, where $\ensuremath{\rho}$ is the total density and $\ensuremath{\Delta}\ensuremath{\rho}$ is the valence neutron density. The result is characterized by five complex parameters for each partial wave channel contributing to the optical potential. Our result applies to scattering in the vicinity of the (3,3) resonance, and the parameters may be most cleanly determined phenomenologically by applying the theory to elastic, and single- and double-charge-exchange data for medium to heavy weight $J=0$, spherical nuclei. The relationship between the parameters and a microscopic cluster decomposition of the optical potential is explored. The parameters are calculated theoretically for selected second-order processes to obtian a first orientation to the magnitude of the terms. The results show, in particular, that large contributions to double charge exchange arise from nonanalog intermediate nuclear states and that these contributions have a characteristic isospin dependence which is different from that found in the simple models previously studied.