Showing papers in "Physical Review C in 1987"
TL;DR: The lifetime of negative muons has been measured in 50 elements plus 8 isotopes, and the total capture rates have been deduced and compared to various calculations.
Abstract: The lifetime of negative muons has been measured in 50 elements plus 8 isotopes. For light elements the accuracy of 2 to 3 ns is a significant improvement over most previous measurements. In heavier elements the accuracy is 1 to 2 ns, which is comparable to, or better than, previous results, with reasonable agreement in most cases. For $^{18}\mathrm{O}$, Sc, Dy, and Er there were no previous data. The total capture rates have been deduced and compared to various calculations.
342 citations
TL;DR: The data suggest that sigma R reaches a minimum located around 300 MeV/nucleon independently of the projectile target combination, and a dependence of s Sigma R on mass asymmetry of the svstem is also demonstrated.
Abstract: Direct measurements of total reaction cross sections (${\ensuremath{\sigma}}_{R}$) have been performed in the energy range of 10--300 MeV/nucleon for heavy ion collisions. A decrease of ${\ensuremath{\sigma}}_{R}$ with increasing energy was observed for a wide range of masses of the colliding systems. The data suggest that ${\ensuremath{\sigma}}_{R}$ reaches a minimum located around 300 MeV/nucleon independently of the projectile target combination. A dependence of ${\ensuremath{\sigma}}_{R}$ on mass asymmetry of the system is also demonstrated. Trends of ${\ensuremath{\sigma}}_{R}$ in this energy range are well reproduced by the predictions of a simple microscopic model based on individual nucleon-nucleon collisions. Our data have been employed in this framework to derive a new semi- empirical parametrization of ${\ensuremath{\sigma}}_{R}$. Most of the experimental results in the intermediate and high energy range have been reproduced by this parametrization using a single energy-dependent parameter.
235 citations
TL;DR: The time scale for mass transfer in quasifission reactions is derived from turning angles of the intermediate complex, and it is found that the mass drift toward symmetry shows the characteristics of an overdamped motion with a universal time constant independent of scattering system and bombarding energy.
Abstract: Triple-differential cross sections have been measured as a function of product mass, total kinetic energy, and center-of-mass scattering angle in reactions induced by $^{238}\mathrm{U}$ on $^{16}\mathrm{O}$, $^{26}\mathrm{Mg}$, $^{27}\mathrm{Al}$, $^{32}\mathrm{S}$, $^{35}\mathrm{Cl}$, $^{40}\mathrm{Ca}$, $^{48}\mathrm{Ca}$, and $^{\mathrm{nat}}\mathrm{Zn}$ targets at several bombarding energies between 4.6 and 7.5 MeV/nucleon. The analysis focuses on binary processes in which the product masses are substantially different from the target-projectile masses. These include the complete fusion followed by fission as well as quasifission processes in which large mass transfers occur on a short time scale. The relative contributions of these two components are estimated from the mass-angle correlations and analyzed within the extra and extra-extra push concepts. The time scale for mass transfer in quasifission reactions is derived from turning angles of the intermediate complex, and it is found that the mass drift toward symmetry shows the characteristics of an overdamped motion with a universal time constant independent of scattering system and bombarding energy. This is consistent with the one-body nuclear dissipation mechanism being responsible for the damping in the mass asymmetry degree of freedom. Also the average total kinetic energy of reaction products in quasifission is independent of temperature, supporting the one-body dissipation hypothesis.
206 citations
TL;DR: The structure of infinite nuclear matter and finite nuclei is studied in the framework of the relativistic Hartree-Fock approximation to obtain a satisfactory description of binding energies and densities for light as well as heavy nuclei.
Abstract: The structure of infinite nuclear matter and finite nuclei is studied in the framework of the relativistic Hartree-Fock approximation. Particular attention is paid to the contribution of isovector mesons (\ensuremath{\pi},\ensuremath{\rho}). A satisfactory description of binding energies and densities can be obtained for light as well as heavy nuclei. The spin-orbit splittings are well reproduced. Connections with nonrelativistic formulations are also discussed.
191 citations
TL;DR: The regions of baryon number, charge, and hypercharge in which various decays are allowed are illustrated graphically, and flow lines and accumulation lines for decaying strangelets are obtained.
Abstract: We develop a mass formula for finite lumps of strange quark matter near flavor equilibrium Our Fermi-gas model includes surface tension and Coulomb energy but not quantum-chromodynamic radiative corrections Assuming strange matter is stable in bulk, we investigate the radioactive decays of finite strangelets (clumps of strange matter with baryon number Aapprox <10/sup 7/) We derive conditions governing instability for all decay modes The regions of baryon number, charge, and hypercharge in which various decays are allowed are illustrated graphically, and flow lines and accumulation lines for decaying strangelets are obtained Finally, we present typical histories of decaying strangelets
150 citations
TL;DR: The characteristics of a stiff equation of state can be obtained with a much softer compressibility in 400 MeV/nucleon heavy ion collisions.
Abstract: We examine the influence of momentum-dependent interactions on the momentum flow in 400 MeV/nucleon heavy ion collisions. Choosing the strength of the momentum dependence to produce an effective mass ${m}^{\mathrm{*}}$=0.7m at the Fermi surface, we find that the characteristics of a stiff equation of state can be obtained with a much softer compressibility.
144 citations
TL;DR: It may be possible to infer the pion dispersion relation in hot and dense nuclear matter by measuring the invariant mass distribution of back-to-back electrons and positrons in the center of mass frame in high energy nucleus-nucleus collisions.
Abstract: The general features of electron-positron emission from high temperature nuclear matter are discussed Estimates are made of the production rates arising from incoherent nucleon-nucleon scattering and from two-pion annihilation It may be possible to infer the pion dispersion relation in hot and dense nuclear matter by measuring the invariant mass distribution of back-to-back electrons and positrons in the center of mass frame in high energy nucleus-nucleus collisions
133 citations
TL;DR: The development shows that current conservation places no restrictions on the use of different electromagnetic form factors for mesons and nucleons, and that phenomenological meson-nucleon form factors can be introduced in a way which is consistent with current conservation.
Abstract: The relation between the nucleon-nucleon interaction and exchange currents needed for current conservation are derived for the Bethe-Salpeter formalism, and for the approach in which the spectator particle is restricted to its mass shell. For both approaches, it is shown how to achieve current conservation for a completely general isospin dependent, energy dependent interaction with arbitrary phenomenological electromagnetic form factors for the nucleon and mesons, and with strong form factors at the meson-nucleon vertices. Contrary to what has often been stated in the literature, the development shows that current conservation places no restrictions on the use of different electromagnetic form factors for mesons and nucleons, and that phenomenological meson-nucleon form factors can be introduced in a way which is consistent with current conservation. The longitudinal part of the exchange current is uniquely determined by current conservation, and for the common case of an interaction that only depends on the invariant momentum transfer variable an explicit expression for this longitudinal exchange current is given. The transverse part, which contains all electromagnetic form factors, is unconstrained by current conservation.
117 citations
TL;DR: The measured peak total photoneutron cross sections are, within an uncertainty of 2--3 %, 174, 252, 340, 502, and 602 mb for /sup nat/Zr, /sup 127/I, / Sup 141/Pr, /Sup 197/Au, and /Sup nat/Pb, respectively.
Abstract: The photoneutron cross sections for /sup nat/Zr, /sup 127/I, /sup 141/Pr, /sup 197/Au, and /sup nat/Pb have been measured near the peak of the giant dipole resonance, with monoenergetic photons, in such a way that the absolute cross sections for these nuclei were intercalibrated, using /sup 141/Pr as a benchmark. The measured peak total photoneutron cross sections are, within an uncertainty of 2--3 %, 174, 252, 340, 502, and 602 mb for /sup nat/Zr, /sup 127/I, /sup 141/Pr, /sup 197/Au, and /sup nat/Pb, respectively. These results resolve outstanding discrepancies among previous measurements.
112 citations
TL;DR: The breakup effect is found to depend little on the energy and target, which is shown by calculating dynamical polarization potentials induced by the breakup process, which explains well the empirical reduction factor of the single-channel double-folding model.
Abstract: For a wide range of incident energy (E/sub lab/ = 40--170 MeV) and target mass number (A = 12--208), projectile breakup effects in elastic scattering of /sup 6/Li have been investigated in the microscopic coupled-channel framework; coupling with /sup 6/Li..--> cap alpha..+d breakup process is taken into account by the method of coupled discretized continuum channels. /sup 6/Li-target interactions are provided by folding of the M3Y effective nucleon-nucleon potential with nucleon densities of colliding nuclei. The calculation reproduces well experimental data of elastic scattering for all the targets and incident energies investigated, without any renormalization of the folding real potential. The breakup effect is found to depend little on the energy and target, which is shown by calculating dynamical polarization potentials induced by the breakup process. The dynamical polarization potential has a repulsive real part with strength of about 40% of the folding potential, the imaginary part being negligible, which explains well the empirical reduction factor of the single-channel double-folding model. The origin of repulsive nature of the breakup effect is discussed.
106 citations
TL;DR: The band condition for relativistic quarks moving in one-dimensional periodic potentials using the transfer matrix method is obtained and an analytic form for the wave function which results when the potential is taken to a delta function limit is given.
Abstract: We obtain the band condition for relativistic quarks moving in one-dimensional periodic potentials using the transfer matrix method. Using a strong electrostatic type of potential in the Dirac equation does not give confining properties, while Lorentz scalar potentials do. We give an analytic form for the wave function which results when the potential is taken to a delta function limit, and discuss the discrepancy between this result and that obtained by ``solving'' the Dirac equation for a delta function potential.
TL;DR: Green's function Monte Carlo calculations of the alpha particle provide the first test of variational methods for systems with spin-dependent interactions, and the results are presented for A = 3 and 4 nuclei with a V6 interaction.
Abstract: The first Green's function Monte Carlo calculations of A=3 and 4 nuclei with spin-dependent interactions are reported. Green's function Monte Carlo methods for calculating the properties of coupled channel quantum systems are described in detail, including both exact and approximate schemes. Results are presented for A=3 and 4 nuclei with a V6 interaction. For the triton, the Green's function Monte Carlo calculations are compared with Faddeev and variational methods. Green's function Monte Carlo calculations of the alpha particle provide the first test of variational methods for systems with spin-dependent interactions. For this interaction, variational methods underestimate the binding energy of the alpha particle by \ensuremath{\approxeq}2 MeV. Other ground state properties of light nuclei have also been determined. Implications of these results for more realistic interactions are discussed, along with the possibility of future extensions of Green's function Monte Carlo methods to treat momentum dependent and three nucleon interactions.
TL;DR: A set of parameters has been derived for a global optical potential from elastic-nucleus scattering with energies higher than 80 MeV from the geometry and energy dependence derived by Put and Paans.
Abstract: A set of parameters has been derived for a global optical potential from elastic \ensuremath{\alpha}-nucleus scattering with energies higher than 80 MeV. The geometry and energy dependence derived by Put and Paans was adopted. The optical model predictions were tested with data from elastic as well as from inelastic scattering.
TL;DR: These findings are consistent with an expanding nuclear system which disintegrates at low temperature and rather low density and within the framework of the quantum statistical model.
Abstract: Two-particle correlations at small relative momenta provide information about the space-time extent and the excitation energy density of the emitting system. Detailed measurements of two-particle correlations were performed for $^{40}\mathrm{induced}$ reactions on $^{197}\mathrm{Au}$ at E/A=60 MeV. In general, more pronounced correlations are measured for particles of higher outgoing energy. From the relative populations of excited states, a mean emission temperature of T\ensuremath{\approxeq}5.5 MeV is extracted. Within the framework of the quantum statistical model, our findings are consistent with an expanding nuclear system which disintegrates at low temperature (T\ensuremath{\approxeq}5.5 MeV) and rather low density (\ensuremath{\rho}\ensuremath{\le}0.1${\ensuremath{\rho}}_{0}$).
TL;DR: At low temperatures, the reaction rate is found to be slightly different from earlier studies, and distortion effects are shown to be important in the spectroscopic properties of C12.
Abstract: The generator-coordinate method is applied to the study of the three-..cap alpha..-particle system. The choice of the basis functions allows us to calculate the ..cap alpha..+ /sup 8/Be phase shifts. We perform a comparison between the full three-cluster approach and two of its approximations. Distortion effects are shown to be important in the spectroscopic properties of /sup 12/C. We present the /sup 8/Be(..cap alpha..,..gamma..) /sup 12/C S factor derived in the three-..cap alpha..-particle model. At low temperatures, the reaction rate is found to be slightly different from earlier studies.
TL;DR: This work calculates the nucleon self-energy and the density-dependent effective NN and N..delta.. cross sections in a nuclear medium and shows a sizable reduction of the pion production cross section and important changes in theelta.. absorption cross sections.
Abstract: Within the relativistic Dirac-Brueckner approach we discuss the properties of highly-energetic nucleons and deltas in dense nuclear matter. The effective NN and N..delta.. interactions are constructed in a fully self-consistent way and reproduce all known properties of nuclear matter. We calculate the nucleon self-energy and the density-dependent effective NN and N..delta.. cross sections in a nuclear medium. Results show a sizable reduction of the ..delta.. (pion) production cross section and important changes in the ..delta.. absorption cross sections.
TL;DR: A fermion dynamical symmetry model that predicts several new dynamical symmetries, one of which has recently been empirically verified, and has limits appropriate to the study of high-spin physics is presented.
Abstract: A fermion dynamical symmetry model is presented. The model has a rich variety of dynamical symmetries, with fully microscopic connections between these dynamical symmetries and the underlying shell structure. In the low angular momentum region, without explicit introduction of bosons, all the dynamical symmetries contained in the phenomenological interacting boson model are recovered. Furthermore, the model predicts several new dynamical symmetries, one of which has recently been empirically verified, and has limits appropriate to the study of high-spin physics.
TL;DR: The particle physics input to the NN charge asymmetric potential due to rho-..omega.. mixing is updated and the calculated changes in scattering lengths chemically bonda/sub nn/chemically bond-chemically Bonda/ sub pp/chemical bond are in good agreement with experiment.
Abstract: The particle physics input to the NN charge asymmetric potential due to rho-..omega.. mixing is updated. The calculated changes in scattering lengths chemically bonda/sub nn/chemically bond-chemically bonda/sub pp/chemically bondapprox.1.3 +- 0.2 fm are in good agreement with experiment (approx.1.4 +- 0.8 fm), as are the contributions to the binding energy difference in the A = 3 mirror nuclei.
TL;DR: The real part of the central neutron-/sup 208/Pb mean field is the sum of a Hartree-Fock component plus a dispersive component, which is assumed to have a Woods-Saxon shape whose depth decreases exponentially with increasing energy and whose radius and diffuseness are independent of energy.
Abstract: The real part of the central neutron${\mathrm{\ensuremath{-}}}^{208}$Pb mean field is the sum of a Hartree-Fock component plus a dispersive component. In keeping with theoretical expectations, the Hartree-Fock field is assumed to have a Woods-Saxon shape whose depth decreases exponentially with increasing energy and whose radius and diffuseness are independent of energy. The dispersive component is determined from the imaginary part of the optical-model potential by making use of the dispersion relation which connects these two quantities. The imaginary part is written as the sum of a volume and a surface-peaked contribution. The dispersion relation then implies that the real dispersive contribution is also the sum of volume and surface-peaked components. The parameters of the complex mean field are determined by fitting the available differential and polarization cross sections in the energy domain [4, 40 MeV] and the total cross sections in the domain [1,120 MeV]; these data are contained in previous published or unpublished reports, but new measurements of the total cross sections are presented from 1 to 25 MeV. Good fits to these cross sections, and also to unpublished total cross sections for energies up to 165 MeV, are obtained despite the fact that the number of adjusted parameters is quite small because of our use of the constraint implied by the dispersion relation.The real part of the mean field is well approximated by a Woods-Saxon shape whose radius decreases with increasing energy between 5 and 25 MeV; its depth is approximately constant from 5 to 15 MeV and then decreases with increasing energy; these findings are in keeping with recent empirical evidence. When the neutron energy decreases below 2.5 MeV, the potential radius decreases; it increases again when the neutron energy decreases below -14.5 MeV. In the domain -20 MeVlEl0 the deduced potential accurately reproduces the experimental single-particle energies as well as the asymptotic values of the single-particle wave functions as measured from sub-Coulomb pickup reactions; it also yields excellent agreement with the spreading width of the deeply bound 1${h}_{11/2}$ hole state. The rms radii, absolute spectroscopic factors, and occupation numbers are calculated for the valence particle and hole states. At the Fermi energy (-6 MeV), the mean field can be identified with the Hartree-Fock potential, for which the present analysis yields a depth of 46.4 MeV, a radius of 1.24${A}^{1/3}$ fm, and a diffuseness of 0.68 fm. In the energy domain 4\ensuremath{\le}E\ensuremath{\le}10 MeV, the already good agreement between the predicted and measured cross sections is further improved if the imaginary part of the mean field is allowed to have its strength depend upon the neutron orbital angular momentum, and its surface diffuseness is allowed to be energy dependent.
TL;DR: These calculations go beyond the simple RIA by resolving ambiguities in the relativistic NN amplitudes and including Pauli blocking corrections.
Abstract: Microscopic relativistic optical potentials have been calculated for closed shell nuclei at energies near 200 MeV. These calculations go beyond the simple RIA by resolving ambiguities in the relativistic NN amplitudes and including Pauli blocking corrections.
TL;DR: Ground-state properties of even-even nuclei in the s-d shell are calculated using relativistic mean- field models of baryon-meson dynamics that include scalar meson self couplings and the implications for quantum hadrodynamics in the mean-field (Hartree) and one-loop approximations are discussed.
Abstract: Ground-state properties of even-even nuclei in the s-d shell are calculated using relativistic mean-field models of baryon-meson dynamics that include scalar meson self couplings. Axial symmetry is assumed. The systematics of intrinsic quadrupole moments are studied for a variety of parameter sets that are fitted to the same nuclear matter saturation properties. The size of the moment is strongly correlated with the nucleon effective mass M/sup */ and the spin-orbit strength, but is only weakly affected by the compressibility or the surface energy. If parameter sets with accurate spin-orbit strengths are used, the trends for s-d nuclei reproduce experimental systematics and are quantitatively similar to those obtained in nonrelativistic Skyrme-Hartree-Fock calculations. The implications for quantum hadrodynamics in the mean-field (Hartree) and one-loop approximations are discussed.
TL;DR: Real-time finite temperature Green's function methods with pair cutoff approximations are applied to the calculation of the equation of state of symmetric nuclear matter and significant differences are found between the pressure-density isotherms at finite temperature given by different Skyrme interactions, although they give quite similar ground state nuclear matter properties.
Abstract: Real-time finite temperature Green's function methods with pair cutoff approximations are applied to the calculation of the equation of state of symmetric nuclear matter. The liquid-gas and the superconducting second-order phase transitions of nuclear matter are studied using, respectively, the normal and abnormal pair cutoff approximations. Several versions of the Skyrme effective interactions are employed. Significant differences are found between the pressure-density isotherms at finite temperature given by different Skyrme interactions, although they give quite similar ground state nuclear matter properties. The critical temperatures ${k}_{B}$${T}_{c}^{(1)}$ for the liquid-gas phase transition given by various Skyrme interactions range from \ensuremath{\sim}15 to \ensuremath{\sim}20 MeV. A strong dependence of ${k}_{B}$${T}_{c}^{(1)}$ on the combination 3${t}_{1}$+5${t}_{2}$, ${t}_{1}$ and ${t}_{2}$ being two parameters of the Skyrme interaction, is observed. For nuclear matter at normal density, nonvanishing energy gap is obtained only for the Skyrme interactions SkI and SkVI. The critical temperatures for the superconducting second-order phase transitions for these two cases are, respectively, 0.5 and 0.345 MeV. Dependence of the energy gap on the nuclear matter density is discussed.
TL;DR: These orbitals and the self-consistently determined mean fields provide a framework for a detailed investigation of the compressibility as well as other relativistic effects.
Abstract: Relativistic Hartree orbitals for nonspherical (even-even) nuclei have been calculated self-consistently from a Lagrangian field theory using the mean field approximation. All parameters in this model are determined from the properties of infinite nuclear matter, so there are no parameters which can be adjusted in the calculation of the orbitals. The energy levels, rms radii, and quadrupole moments are in qualitative agreement with earlier nonrelativistic calculations and with experiment; however, the overall deformations (which are fixed by the self-consistency) are somewhat smaller than those obtained experimentally and from nonrelativistic calculations. The difference may be due to the large compressibility characteristic of relativistic mean field calculations. These orbitals and the self-consistently determined mean fields provide a framework for a detailed investigation of the compressibility as well as other relativistic effects.
TL;DR: In this article, the reduced alpha transfer rates were studied microscopically with a schematic model and the results for ground state to ground state alpha transfer reactions were given, and the authors showed that the reduction in alpha transfer rate can be explained by ground state-to-ground state alpha transfers.
Abstract: The reduced alpha transfer rates are studied microscopically with a schematic model. Results for ground state to ground state alpha transfer reactions are given.
TL;DR: A possible storage-ring experiment is proposed for measuring bound-state ..beta..-decay rates, which will be most easily applied to the decay of /sup 3/H/sup +/. .AE
Abstract: Nuclear ..beta.. decays of highly ionized atoms under laboratory conditions are studied. Theoretical predictions of ..beta..-decay rates are given for a few cases in which bound-state ..beta.. decay produces particularly interesting effects. A possible storage-ring experiment is proposed for measuring bound-state ..beta..-decay rates, which will be most easily applied to the decay of /sup 3/H/sup +/. .AE
TL;DR: The circular polarization of the 1.081 MeV γ rays from 18F has been measured to be (1.7±5.8)×10-4.4), which is significantly smaller than recent theoretical predictions based on the Weinberg-Salam model.
Abstract: The circular polarization of the 1.081 MeV γ rays from 18F has been measured to be (1.7±5.8)×10-4. 18F was produced by a 4.05 MeV 3He+ beam impinging on a recirculating water target with a thin entrance window. The circular polarization was measured with four magnetic transmission polarimeters, each backed by a 150 cm3 n-type Ge detector. A fast data acquisition system limited overall peak losses to 30% at a count rate of 60 kHz per detector. Supplementary experiments were carried out to investigate the sensitivity of the circular polarization measurement to systematic variations of beam position and intensity. These effects were found to be negligible relative to the statistical uncertainty. The experimental result is interpreted in terms of an isovector parity-nonconserving matrix element between the ‖0+,T=1,1.042 MeV〉 and ‖0-,T=0,1.081 MeV〉 states in 18F that is dominated by the weak pion exchange contribution. The present result for the weak pion-nucleon coupling strength is ‖fπ1‖=(0.4-0.4+1.4)×10-7, which is significantly smaller than recent theoretical predictions based on the Weinberg-Salam model.
TL;DR: Inclusive cross sections for the proton and nuclear targets of A = 4, 9, 12, and 16 were measured for 537 and 730 MeV electrons scattered at 37.1 deg.
Abstract: Inclusive cross sections for the proton and nuclear targets of A = 4, 9, 12, and 16 were measured for 537 and 730 MeV electrons scattered at 37.1 deg. Systematic features of the continuum scattering data are compared with other electron scattering data and with photoabsorption measurements. A model calculation based on the isobar-hole formalism is compared with the data in the delta resonance region.
TL;DR: It is shown that evaporation which characterized the liquid phase and expansion which characterizes the gaseous phase have clear experimental signatures and an abrupt change in the behavior of the experimentally inferred lifetime is predicted near the threshold for the liquid-gas phase transition.
Abstract: A formalism is developed for predicting two-particle correlations in terms of the final single-particle phase-space distributions which can be calculated from theoretical models. The ability to determine the phase-space distributions from experiment is discussed. It is shown that evaporation which characterizes the liquid phase and expansion which characterizes the gaseous phase have clear experimental signatures. An abrupt change in the behavior of the experimentally inferred lifetime is predicted near the threshold for the liquid-gas phase transition.
TL;DR: It is shown that the d-wave capture plays an important role in determining the magnitude and shape of S(E) at low energies of astrophysical interest.
Abstract: We calculated the differential cross sections as well as the total cross sections for the /sup 7/Be(p,..gamma..)/sup 8/B at low energies in the radiative direct capture model including the E1, M1, and E2 transitions. The astrophysical S factor at the solar temperature of 20 keV is found to be 0.024 keVX which agrees well with a recent measurement by Filippone et al. We show that the d-wave capture plays an important role in determining the magnitude and shape of S(E) at low energies of astrophysical interest.