Positron lifetime as a function of grain size

Published data on positron annihilation lifetime in copper as a function of grain size have been analyzed to show that there is a linear relationship between the internal grain boundary surface area, per unit volume,S _v, and the positron lifetime, τ. The analysis indicates that grain boundaries are important in the trapping of positrons. It is suggested that the slope of the resulting straight line,dS _v/dτ, can be used to determine the annihilation rate of the grain boundaries.     

The nuclear size dependence of large momentum transfer scattering and quark interaction dynamics

A simple dynamical picture for hadron interactions is used to discuss the A dependence of large p_⊥ scattering and dilepton production. Observations in large p_⊥ experiments are explained. Important corrections to dilepton production cross sections are found, leading to acceptable agreement between colored-quark theory and experiment.     

Structure of nuclear multipole resonances as a function of momentum transfer

The momentum-transfer dependence of transverse and longitudinal isovector form factors is studied for the electroexcitation of sd-shell self-conjugate nuclei. The effect of the interference between the orbital and spin components of the intranuclear current on the behavior of transverse form factors is traced. The results of the particle-core version of the shell model are compared with experimental data on E1 and M6 multipole resonances.     

Brillouin scattering in ice and deuterated ice as a function of temperature

The elastic constants c₁₁, c₃₃, c₄₄ for ordinary ice and c₁₁ for D₂O ice between 12 and 250°K are deduced from light scattering Brillouin measurements for sound waves of wavevector q = 2.35 × 10⁵cm^?1. A range of temperature is found between 70 and 130°K where the elastic constants display an abnormal behaviour.     

Performance of data link modules as a function of multimode fiber size

Commercial data link modules from three manufacturers were measured for transmitter launch power and receiver sensitivity when used with the five common graded-index multimode fiber types: 50 μm core/NA_mk, = 0.20; 50 μm/NA_m = 0.23; 62.5/0.275; 85/0.26: and 100/0.29. Depending on the model, transmitters launched from 2.8 to 4.4 dB more power into the 100 pm fiber than the 62.5 μm fiber, whereas receiver sensitivities decreased by up to 3.5 dB. These variations in launch power and receiver sensitivity complicate the selection of an ideal fiber and emphasize the need for specifying data link performance with the particular fiber of interest. In general, the system gain of typical transmitter/receiver pairs is less favorable to large diameter fibers than previously believed.     

Performance of data link modules as a function of multimode fiber size

Commercial data link modules from three manufacturers were measured for transmitter launch power and receiver sensitivity when used with the five common graded-index multimode fiber types: 50 μm core/NA_mk, = 0.20; 50 μm/NA_m = 0.23; 62.5/0.275; 85/0.26: and 100/0.29. Depending on the model, transmitters launched from 2.8 to 4.4 dB more power into the 100 pm fiber than the 62.5 μm fiber, whereas receiver sensitivities decreased by up to 3.5 dB. These variations in launch power and receiver sensitivity complicate the selection of an ideal fiber and emphasize the need for specifying data link performance with the particular fiber of interest. In general, the system gain of typical transmitter/receiver pairs is less favorable to large diameter fibers than previously believed.     

Multiple nuclear Bragg scattering

Multiple nuclear resonant scattering from an ⁵⁷FeBO₃ single crystal has been observed in the three-beam case. A change of the lifetime and a modulation of the quantum beat were observed in our study. The high brilliance of synchrotron radiation and the high perfection of the synthetic ⁵⁷FeBO₃ single crystal enabled us to observe such an effect. This revised version was published online in August 2006 with corrections to the Cover Date.     

Hadron-nucleus interactions and the leading particle effect in a dual-parton model

We describe hadron-nucleus interactions at smallP _T in the framework of a dual-parton model which is a generalization of the two-chain dual-partonmodel of hadron-hadron interactions introduced recently in the literature. The model (of a multiple scattering type) is formulated in the framework of the 1/N expansion in dual theories. Regge singularities are used to determine the (joint) momentum distribution functions of the various sets of constituents (valence and sea quarks) which yield hadron jets in successive orders of the 1/N expansion. These jets result from a standard color separation mechanism, inspired by QCD. We study the inclusive spectra of various types of secondaries for various projectiles and targets. Particular emphasis is laid on theA-dependence in the projectile fragmentation region. A comparison with available experimental data is presented.     

Pion scattering and nuclear dynamics

A phenomenological optical-model analysis of pion elastic scattering and single- and doublecharge-exchange scattering to isobaric-analog states is reviewed. Interpretation of the opticalmodel parameters is briefly discussed, and several applications and extensions are considered. The applications include the study of various nuclear properties, including neutron deformation and surface-fluctuation contributions to the density. One promising extension for the near future would be to develop a microscopic approach based on powerful momentum-space methods brought to existence over the last decade. In this, the lowest-order optical potential as well as specific higher-order pieces would be worked out in terms of microscopic pion-nucleon and delta-nucleon interactions that can be determined within modern meson-theoretical frameworks. A second extension, of a more phenomenological nature, would use coupled-channel methods and shell-model wave functions to study dynamical nuclear correlations in pion double charge exchange.Invited talk to the symposium 'Mesons and Light Nuclei IV, Bechyn, Czechoslovakia, September 5–10, 1988.The author would like to express his appreciation to Dr. Mach and the organizers for their the hospitality at the Conference.     

General properties of nuclear resonant scattering

The process of nuclear resonant scattering resonant scattering is considered on the basis of an optical model. The coherent properties coherent properties of the radiation and scattering mechanism are described. The complementary pictures of γ-ray resonant scattering in energy and time domains are presented. Special attention is paid to scattering of a γ quantum by an ensemble of nuclei. The central concept of the theory of nuclear resonant scattering, the nuclear exciton, nuclear exciton as a delocalized nuclear excitation, is described in detail. It is shown that both temporal and spatial aspects of coherence play a crucial role in the evolution of the nuclear exciton. A large place is given to the analysis of resonant scattering of synchrotron radiation by nuclear ensembles.

     

Phonation thresholds as a function of laryngeal size in a two-mass model of the vocal folds

This letter analyzes the oscillation onset-offset conditions of the vocal folds as a function of laryngeal size. A version of the two-mass model of the vocal folds is used, coupled to a two-tube approximation of the vocal tract in configuration for the vowel /a/. The standard male configurations of the laryngeal and vocal tract models are used as reference, and their dimensions are scaled using a single factor. Simulations of the vocal fold oscillation and oral output are produced for varying values of the scaling factor. The results show that the oscillation threshold conditions become more restricted for smaller laryngeal sizes, such as those appropriate for females and children.     

Core polarization contribution to the nuclear matter response function in the quasi-elastic scattering region

The lowest order core polarization contribution to the nuclear response function, which is seen in the quasi-elastic electron scattering, is calculated in nuclear matter, using a finite range effective interaction with a tensor component. The relationship of our calculation with a sum rule approach is discussed. At this stage the fit with available experimental data is found not to be improved.     

Perturbed nuclear scattering of synchrotron radiation

Mössbauer spectroscopy in the time domain with SR as a source may become a powerful tool in investigations of time-dependent hyperfine interactions caused by external perturbations. This is firstly due to the easy possibility to synchronize the excitation of the nuclei with an external perturbation and secondly due to the insensitivity of Mössbauer time spectra to mechanical vibrations which are generated in samples by these perturbations. As a particular case of perturbed nuclear scattering, the coherent scattering under conditions of 90°- and 180°-switching of the magnetic hyperfine field direction is considered. The results of the first experimental studies are discussed.     

Interference between nuclear resonance and nuclear Thomson scattering of photons

The elastic differential scattering cross section from ⁵⁵Mn has been measured at θ = 140° with E_γ = 5.5–11.4 MeV photons. Evidence for destructive interference between nuclear resonance (NR) scattering and nuclear Thomson (NT) scattering has been obtained.     

Monte Carlo studies of two measures of polymer chain size as a function of temperature

Monte Carlo simulations of single polymer chains with both excluded volume and nearest-neighbor interaction energies are discussed. Two measures of chain size are obtained in the simulation, the radius of gyration of the polymer chain and the inverse radius of the polymer chain. Both of these are reported as a function of temperature, or interaction energy, and chain length,N. The possibility of estimating the fractal dimensions of these measures from the Monte Carlo data is discussed in the context of two different interpolation functions for the temperature dependence of the fractal dimensions. The approach to the fractal dimension as a function of chain length,N, is studied. It is suggested that the approach to fractal dimension of the measures of chain size of polymers is slow, perhaps a fractional power itself.     

Hadron formation in deep-inelastic positron scattering in a nuclear environment

The influence of the nuclear medium on the production of charged hadrons in semi-inclusive deep-inelastic scattering has been studied by the HERMES experiment at DESY using a 27.5 GeV positron beam. The differential multiplicity of charged hadrons and identified charged pions from nitrogen relative to that from deuterium has been measured as a function of the virtual photon energy and the fraction z of this energy transferred to the hadron. There are observed substantial reductions of the multiplicity ratio at low and at high z, both of which are well described by a gluon-bremsstrahlung model of hadronization. A significant difference of the -dependence of is found between positive and negative hadrons. This is interpreted in terms of a difference between the formation times of protons and pions, using a phenomenological model to describe the - and z-dependence of . Received: 20 February 2001 / Revised version: 3 May 2001 / Published online: 8 June 2001