Upper limits on parity-violating terms in nuclear electromagnetic operators and nuclear structure effects

Parity-violating (PV) nuclear experiments offer a suitable tool for investigating parity conservation in the electromagnetic interaction of nucleons. It has been estimated that PV electromagnetic vertices induced by weak interactions are very small. Thus, any sizeable PV nucleon electromagnetic couplings indicate the presence of a genuine PV electromagnetic interaction. The upper limit to such a PV electromagnetic interaction can be estimated from the existing experimental value for the circular polarization of γ-rays emitted in the radiative capture of a thermal neutron (n+p → d+γ). This estimate is consistent with the value for the circular γ-polarization measured in deformed nuclei (¹⁸¹Ta, ¹⁷⁷Hf). Useful additional information can be extracted from the measurement of the γ-asymmetry in the n+p → d+γ reaction and from the measurements of circular polarization in the decays of selected spherical nuclei (²⁰³Tl, ¹¹¹In, ⁶³Cu). Extensive theoretical analysis taking into account nuclear structure effects shows that the decay of ⁶³Cu is particularly suitable for this purpose.     

On the nuclear interaction potential in the reactions with heavy ions

The interaction potential of heavy ions⁴He,⁶Li,¹²C and¹⁶O is constructed in the folding model. The density distribution of nuclear matter for these nuclei is calculated in the framework of the hyperspherical function method. For the calculation of the folding potentials we have employed the Skyrme nucleon-nucleon forces. The influence of several effects on the results of calculations is studied: the role of the three-body forces of the nucleon-nucleon interaction, dependence of the folding potential on the mass numbers of the colliding nuclei and the possibility of observing the monopole resonance in the ion inelastic scattering. Using our folding potential as a real part of the optical potential we have calculated the differential cross section of elastic scattering of⁶Li from¹²C at laboratory energy of lithium ionsT _L =90.0 MeV. Reasonable agreement with experiment is obtained.     

Phenomenology of nuclear shadowing in deep-inelastic scattering

We investigate shadowing effects in deep-inelastic scattering from nuclei at small valuesx < 0.1 of the Bjorken variable. Unifying aspects of generalized vector meson dominance and color transparency we first develop a model for deep-inelastic scattering from free nucleons at smallx. In application to nuclear targets we find that the coherent interaction of quark-antiquark fluctuations with nucleons in a nucleus leads to the observed shadowing atx < 0.1. We compare our results with most of the recent data for a large variety of nuclei and examine in particular the Q² dependence of the shadowing effect. While the coherent interaction of low mass vector mesons causes a major part of the shadowing observed in the Q² range of current experiments, the coherent scattering of continuum quark-antiquark pairs is also important and guarantees a very weak overall Q² dependence of the effect. We also discuss shadowing in deuterium and its implications for the quark flavor structure of nucleons. Finally we comment on shadowing effects in high-energy photon-nucleus reactions with real photons.     

The role of Fermi motion and quark exchange as a nuclear medium effect in the nuclear structure function of 3He, 3H nuclei and their EMC effects

To extract the nuclear structure function of light nuclei like ³He and ³H, the convolution of proton and neutron structure functions in the conventional approach could not explain the modification of nuclear structure functions of these nuclei because of some nuclear medium effects. There are some models that have some success to explain the modification of nuclear structure functions in limited x range. So in this investigation the quark exchange model is considered to extract the nuclear medium effect of this phenomenon (quark exchange effect) for nuclear structure functions of ³He and ³H nuclei. The Fermi motion part of the nuclear structure functions for these nuclei is extracted by taking the GRV’s neutron and proton structure functions in the conventional approach. So the nuclear structure functions and the EMC ratios of ³He and ³H nuclei are calculated by considering both Fermi motion and the nuclear medium effect of quark exchange. Finally the extracted results are compared with available data.     

Nuclear spin dependent atomic parity violation,nuclear anapole moments and the hadronic axial neutral current

Left-right asymmetries in atomic transitions, depending upon the nuclear spin, could be a source of information on the neutral hadronic axial current. We show that the relevant electroweak parameter can be extracted from experiment by measuring hyperfine component ratios which do not involve the knowledge of the atomic wave function. In the standard electroweak model, the parity violating electron-nucleus interaction associated with the hadronic axial neutral current is accidently suppressed and, as a consequence, dominated by the electron interaction with the nuclear anapole moment, which describes the effect of the parity violating nuclear forces on the nucleus electromagnetic current. One of our objectives was to identify the various physical mechanisms which determine the size of the nuclear anapole moments. As an important step, we have established a simple relation between the anapole moment and the nuclear spin magnetization. From this relation it follows that the computation of the anapole moment can be reduced to that of one-body operators. The basic tool is a unitary transformationW which eliminates the one-body parity violating potential from the nuclear hamiltonian. It generalizes, to more realistic situations, a procedure used by F.C. Michel in the case of constant nuclear density. The fact that the transformationW does not commute with the residual spin-dependent nucleon-nucleon interaction can be accounted for — within some approximation — by a renormalization of the effective coupling constants which appear in the one-body reduction of the two-body parity violating nucleon-nucleon interaction induced by meson exchange. A particular attention was paid to nuclear correlation effects. They are treated semi-empirically in the independent pair approximation. The nuclear anapole moments of⁸⁵Rb,¹³³Cs, and²⁰⁹Bi have been evaluated for three sets of parity violating meson-nucleon coupling constants, taking into account configuration mixing effects in a semi-empirical way. We suggest a possible strategy to disentangle the axial neutral current from the anapole moment contribution. It requires experiments, accurate to few tenths of a percent, performed on several heavy nuclei. The results should be collected in a two-dimensional plot involving a suitably chosen set of variables (X, Y). In an ideal situation — small theoretical uncertainties —the points corresponding to various nuclei should fall on a straight line which crosses the lineX=0 at a point the ordinate of which is the sought for axial coupling constant.     

Finite nuclei to nuclear matter: a leptodermous approach

The liquid drop model (LDM) expansions of energy and incompressibility of finite nuclei are studied in an analytical model using Skyrme-like effective interactions to examine, whether such expansions provide an unambiguous way to go from finite nuclei to nuclear matter, and thereby can yield the saturation properties of the latter, from nuclear masses. We show that the energy expansion is not unique in the sense that, its coefficients do not necessarily correspond to the ground state of nuclear matter and hence, the mass formulas based on it are not equipped to yield saturation properties. The defect is attributed to its use of liquid drop without any reference to particles as its basis, which is classical in nature. It does not possess an essential property of an interacting many-fermion system namely, the single particle property, in particular the Fermi state. It is shown that, the defect is repaired in the infinite nuclear matter model by the use of generalized Hugenholtz–Van Hove theorem of many-body theory. So this model uses infinite nuclear matter with well defined quantum mechanical attributes for its basis. The resulting expansion has the coefficients which are at the ground state of nuclear matter. Thus a well defined path from finite nuclei to nuclear matter is found out. Then using this model, the saturation density 0.1620 fm⁻³ and binding energy per nucleon of nuclear matter 16.108 MeV are determined from the masses of all known nuclei. The corresponding radius constant r₀ equal to 1.138 fm thus determined, agrees quite well with that obtained from electron scattering data, leading to the resolution of the so-called ‘r₀-paradox’. Finally a well defined and stable value of 288±20 MeV for the incompressibility of nuclear matter K_∞ is extracted from the same set of masses and a nuclear equation of state is thus obtained.     

Quasimolecular nuclear optical potentials

A theory is presented to calculate potentials for the elastic nuclear heavy-ion scattering in a phenomenological way. The density properties of finite nuclei are derived with a schematic ansatz for the interaction energy between nuclear matter. The same interaction energy is applied to the calculation of the real part of the heavy-ion potential, which is of the quasimolecular type. The imaginary part is connected with the outflow time of nuclear matter out of compressed regions of overlapping nuclei. The resulting cross section for the elastic O¹⁶-O¹⁶ scattering reproduces the experiment up to 30 MeV quite well. An effective compression modulus of the S³² compound system can be deduced from the scattering experiment. It results to be about 200 MeV.     

Photoproduction of neutral pions to discrete nuclear states

We study the photoproduction of neutral pions on nuclei to discrete final states at intermediate energies. Both photoproduction and distortion of the π⁰ are dominated by Δ-excitation, which requires a careful treatment of the Δ-nucleus dynamics. We use the Δ-hole approach, which has been used to describe a variety of pion- and photon-induced nuclear reactions. It is shown that π⁰ photoproduction is very sensitive to medium effects of Δ-propagation and can therefore be used to investigate the Δ-nucleus interaction. The validity of the distorted-wave impulse approximation for this reaction is examined. Results for low-lying excitations in ¹²C are compared with the available data.     

Prediction of hyperdeformed nuclear states at very high spins

Calculations based on the generalized cranking-Strutinsky method with the deformed Woods-Saxon potential predict the existence of extremely elongated hyperdeformed nuclear shapes with the axis ratios significantly exceeding2:1. The strongest effect is expected to take place for nuclei around ¹⁶⁶Er, ¹⁶⁸Yb, and ¹⁷⁰Hf (Z = 68, 70, 72; N = 98) for spins as high as the fission limit down to I ∼ 10–20. The chances for observing those states in nature are discussed in detail. Systematic occurrences of the superdeformed and hyperdeformed states also in lighter (A ∼ 70, and A ∼ 100) nuclei are suggested as a consequence of the approximate pseudo-oscillator (or pseudo-SU(3) symmetries of the realistic nuclear mean field.     

Thermostatic properties of symmetric nuclear matter

The thermostatic properties of symmetric nuclear matter are calculated by extension of a recent Thomas-Fermi approach to ground-state nuclei by Myers and Swiatecki [1]. We have computed the free energy per nucleon f(T, n) in Landau's quasiparticle approximation and have derived from it the relevant thermostatic properties. In view of its application to finite excited nuclei, the degenerate limit of nuclear matter is discussed in particular. As an interesting result we find at higher temperatures van-der-Waals-like isotherms in the p-n plane. Below the critical temperature T_c = 17.3 MeV two phases of nuclear matter, liquid and vapour, are defined by these. Comparing these results with the reduced phase transition data of ³He, ⁴He, and “inert gases,” we find that nuclear matter is similar to the He-isotopes, but differs considerably from the inert gases.     

Allowed β-transitions,weak magnetism and nuclear structure in light nuclei

The general formalism for allowed β-transitions is derived in the framework of the elementary particle as well as in the impulse approximation treatment. Explicit expressions for the shape factor, the asymmetry and anisotropy of electrons emitted from oriented nuclei, the β-γ (CP) correlation and the β-γ angular correlation are given. Emphasis is imputed to the question how model independently these quantities can be evaluated. This problem has been studied in detail for the β-transitions in the nuclei ¹²B¹²N, ²⁰F²⁰Na, ²²Na and ²⁴Na²⁴Al. The computations of the nuclear matrix elements are carried out for the A = 12 system in intermediate coupling and for the sd-shell nuclei in the SU(6)-SU(3) coupling scheme using different residual interactions. Comparison to experiments is made. All results can be explained with ordinary CVC predictions without invoking second class currents.     

Electron spin-echo envelope modulation at S-band. 2: Hyperfine and weak nuclear quadrupole interaction effects forI>1/2

The analysis of the nuclear modulation often observed in the electron spin-echo signals gives information on the structural arrangement around the paramagnetic center and on the interactions undergone by the spin system. Most applications to date have been carried out at X-band (≈9 GHz). However, to run experiments at different operating frequencies yields more accurate results and increases the number of experimental situations that can be investigated. Here we analyze the two- and three-pulse modulation patterns simulated at S-band (≈3 GHz) where the modulation is due to two different nuclei of experimental interest at different electron-nucleus distances, and comparison is done with the corresponding X-band patterns.²H (I=1) and⁷Li (I=3/2) nuclei are chosen due to their low quadrupole moment, so that comparison can be done between the effects due to weak nuclear quadrupole interaction at the two bands. It is shown that at short distances the complicated patterns due to hyperfine couplings are successfully interpreted by Fourier transform in the frequency domain, and that the use of S-band is really of some advantage with respect to X-band in detecting the presence of a weak nuclear quadrupole interaction.     

Heavy element nuclear chemistry at JAERI

The present status of heavy element nuclear chemistry research at JAERI (Japan Atomic Energy Research Institute) is reviewed. Production of the transactinide nuclei ²⁶¹Rf and ²⁶²Db via the reactions of ²⁴⁸Cm(¹⁸O,5n) and ²⁴⁸Cm(¹⁹F, 5n), respectively, at the JAERI tandem accelerator is reported. Study of the aqueous chemistry of Rf is being carried out with a newly developed rapid ion-exchange separation apparatus. Anion-exchange behavior of Rf in acidic solution is briefly discussed. Recent experimental results on decay studies of neutron-deficient actinide nuclei using the gas-jet coupled JAERI-ISOL are given. We also discuss characteristics of nuclear deformation properties at scission in symmetric and asymmetric fission of actinides. Prospects for studies in the near future are briefly considered.     

Mass splittings of nuclear isotopes in chiral solition approach

The differences in the masses of isotopes with atomic numbers between ～10 and ～30 can be described within the chiral soliton model in satisfactory agreement with data. The rescaling of the model is necessary for this purpose—a decrease in the Skyrme constant by ～30%, providing the “nuclear variant” of the model. The asymmetric term in the Weizsäcker-Bethe-Bacher mass formula for nuclei can be obtained as the isospin-dependent quantum correction to the nucleus energy. Some predictions of the binding energies of neutron-rich isotopes are made in this way from, e.g., ¹⁶Be, ¹⁹B to ³¹Ne or ³²Na. The neutron-rich nuclides with high values of isospin are unstable relative to decay owing to strong interactions. The SK4 (Skyrme) variant of the model, as well as the SK6 variant (sixth-order term in the derivatives of the chiral field in the Lagrangian as soliton stabilizer), is considered; the rational-map approximation is used to describe multi-Skyrmions.     

Isospin and density waves in asymmetric nuclear matter

The excitation of small density oscillations (zero sound) and isospin oscillations (isospin sound) in cold asymmetric nuclear matter (in the ground state ?⁰_n> ?⁰_p, ?⁰ = ?⁰_n+?⁰_p = 0.17 nucleons/fm³) is investigated within the framework of the Landau theory of normal Fermi liquids. There is only one undamped mode of excitation, which consists predominantly of isospin oscillations, with some admixture of density oscillations. The phase velocity of this undamped wave depends very weakly on the neutron excess and is close to that of a pure isospin wave (isospin sound) in symmetric nuclear matter of the same density. At the neutron excess corresponding to that existing in heavy nuclei the amplitude of the density oscillations constitutes about 30 % of the amplitude of the neutron excess density oscillations. Calculation with a suitably parametrized charge dependent quasiparticle interaction in asymmetric nuclear matter shows that for (?⁰_n??⁰_p)/?⁰ > 0.63 both zero sound and isospin sound are strongly damped.     

Antiprotons for nuclear structure research

The interaction of antiprotons with nuclei is investigated in a model using the free space $\overline{p} N$ T-matrix and nuclear densities from HFB calculations. Elastic scattering and annihilation reactions are used as probes for nuclear sizes of exotic nuclei. The annihilation cross sections are related to the nuclear root-mean-square radii by a scaling law. A more detailed picture is obtained by detecting in coincidence a pion ejected from the annihilation zone.     

Variational calculations of asymmetric nuclear matter

We report on variational calculations of the energy E(ρ, β) of asymmetric nuclear matter having ? = ?_n + ?_p = 0.05 to 0.35 fm^?3, and β = (?_n ? ?_p/g9 = 0 to 1. The nuclear h used in this work consists of a realistic two-nucleon interaction, called v₁₄, that fits the available nucleon-nucleon scattering data up to 425 MeV, and a phenomenological three nucleon interaction adjusted to reproduce the empirical properties of symmetric nuclear matter. The variational many-body theory of symmetric nuclear matter is extended to treat matter with neutron excess. Numerical and analytic studies of the β-dependence of various contributions to the nuclear matter energy show that at ? < 0.35 fm^?3 the β⁴ terms are very small, and that the interaction energy EI(ρ, β) defined as E(ρ, β) ? T_F(ρ, β), where T_F is the Fermi-gas energy, is well approximated by EI₀(?) + β²EI₂(ρ). The calculated symmetry energy at equilibrium density is 30 MeV and it increases from 15 to 38 MeV as ? increases from 0.05 to 0.35 fm^?3.     

Method of induced activity for studying nuclear reactions on Pb and Sn isotopes

Nuclear reactions at the interaction of particles with heavy targets were studied using the method of induced activity. This method permits an investigation of the mechanism of residual nuclei formation in a wide range of nuclear masses beginning from light nuclei up to the nuclei with masses near to the target mass. The results of investigations of nuclear reactions on separated isotopes of lead (²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb) and tin (¹¹⁸Sn) isotope performed using the method of induced activity are given.     

A shell-model description of 0+ intruder states in even-even nuclei

Starting from the nuclear shell structure in medium-heavy and heavy nuclei, the excitation energy for low-lying 0⁺ intruder states is studied. Taking as a simplified model two particle-two hole (2p-2h) excitations across closed shells, the effects of the pairing and the proton-neutron (monopole and quadrupole component) residual interaction on the unperturbed energies are calculated. Application to major closed-shell (fZ = 50, Z = 82) and to subshell (Z = 40, Z = 64) regions is performed. We especially concentrate on 0⁺ intruder states in the even-even Pb nuclei.