If a neutrino has both Dirac and Majorana masses (m,μ), as some recent models predict, ν⇆ν¯ oscillations within a single generation may cause a violation of the nucleosynthesis bound on the number of light neutrino species. Taking into account the effects of finite temperature and density, we find that μm must be less than 3×${10}^{\mathrm{-}5}$ ${\mathrm{eV}}^2$, which is too small to explain atmospheric neutrino observations. The bound can be softened to 0.05 ${\mathrm{eV}}^2$ if the right-handed components couple to massless bosons (Majorons). We introduce a simple but quantitatively accurate formalism for tracking the approach of the sterile state to thermal equilibrium.

Exclusive measurements of neutrons and charged products have been performed using a combination of 4π neutron and 4π charged-particle detectors. The maximum observed energy dissipation corresponds to only approximately one-half of the available kinetic energy. For any degree of dissipation, the velocity distributions of charged particles are characteristic of sequential emission following binary collisions. The data imply that central collisions also lead to bimodal emission patterns or that they are not sufficiently well isolated by the requirement of high particle multiplicities.

The energy spectrum and the angular dependence relative to the fission direction of photons in the energy region between 2 and 40 MeV have been measured for the spontaneous fission of ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$. A large anisotropy was found in the energy region 8 to 12 MeV implying that photons in this region are emitted from a nuclear system which is highly elongated along the fission axis.

A sodium atomic beam is deflected by two optical standing-wave fields which excite near resonance Raman transitions. Observed deflections are consistent with theoretical predictions of a long-range dc component of the force on a three-level atom in the Λ configuration. The low laser intensities (and small detunings) used, combined with observed evidence of damping in the three-level Λ system, may ultimately lead to the design of high-density, all optical traps wherein the atoms would be mostly in the dark state.

We report the first experimental results on the state selective cross-section ratio ${\mathrm{\sigma }}^{+\mathrm{*}}$/${\mathrm{\sigma }}^{\mathrm{*}}$ of helium for electron and proton impact. Our results show consistently higher yields for electrons than for protons at higher velocities. A comparison is made between the cross-section ratios ${\mathrm{\sigma }}^{2+}$/${\mathrm{\sigma }}^{+}$ of helium for electron and proton impact.

A turbulent homogeneous perfect gas with no forcing is computed using the piecewise-parabolic method on a uniform grid of ${256}^3$ zones. The initial rms Mach number is one. The main results are (i) persistence of a quasisupersonic phase with strong density contrasts over several shock formation times; (ii) late appearance of a post-supersonic self-similarly decaying regime in which the total velocity spectrum varies with wave number as ${\mathit{k}}^{\mathrm{-}1}$; and (iii) both shock curvature and velocity gradients are important mechanisms of vorticity production in the first half of the supersonic phase.

An experimental study of the parametric instability of a liquid-vapor interface close to the critical point is presented. A cell containing carbon dioxide at the critical density is vertically vibrated sinusoidally; the acceleration amplitude ${\mathit{a}}_0$ and the instabiilty wavelength ${\lambda }_0$ at the instabiilty onset are measured with respect to the excitation frequency and the critical temperature difference ${\mathit{T}}_{\mathit{c}}$-T. These measurements provide a simple way to determine the surface tension and the density difference between the two phases. When ${\mathit{T}}_{\mathit{c}}$-T diminishes ${\lambda }_0$ decreases and reaches a finite value whereas ${\mathit{a}}_0$ diverges. The shape of the wave pattern appears to depend on ${\mathit{T}}_{\mathit{c}}$-T.

The slow upward migration, due to buoyancy, of a nonwetting fluid thorugh a saturated, three-dimensional porous medium has been studied experimentally, and by computer simulations. The height h of the invading fluid structure is proportional to the mass M of the invading fluid. The linear mass density S=M/h is found to scale with the dimensionless Bond number Bo (ratio of bouyancy and capillary forces) as S=A‖Bo${\mathrm{\Vert }}^{\mathrm{-}\mathrm{\alpha }}$, with α=0.72±0.06, consistent with theory and simulations. A noisy behavior in h(M) results from dynamical processes such as snapoffs and bursts.

A two-dimensional periodic modulation of an ambient magnetic field generates a quasiperiodic perturbation for the Alfvén wave. A simple quasiperiodic wave equation has been derived, which resembles the Schrödinger equation for a valence electron in a one-dimensional quasicrystal. Let σ be the tangent of the angle between the direction of the unperturbed magnetic field and the periodicity of the perturbation. If σ is an irrational number, the perturbed wave potential is quasiperiodic. By a large perturbation, the Alfvén continuum is changed into point spectra for almost all σ.

A class of exact one-dimensional solutions for modulated light pulses coupled to electron plasma waves in a relativistic cold plasma is investigated. The solutions are in the form of isolated envelope solitons and the nonlinear relationship between their group velocity, amplitude, and frequency are discussed. Numerical results are presented for intense pulses propagating close to the velocity of light; such pulses are of great interest from the point of view of particle and photon accelerators.

The internal dynamics of high molar mass polystyrene in semidilute solution have been examined using pulsed gradient spin echo NMR. We find evidence for reputationlike motion characterized by mean-square displacement φ(t)∼${\mathit{t}}^{1/4}$, ${\mathit{t}}^{1/2}$ and t regimes. We observe clear transitions between φ(t)∼${\mathit{t}}^{1/2}$ and φ(t)∼t regimes and the date are broadly consistent with the quantitative predictions of Doi-Edwards theory. In an experiment akin to dynamic neutron scattering, we obtain a structure factor from which some information about entanglement tube dimensions can be deduced.

The critical behavior of the polymer blend d-PB/PS was investigated by small-angle neutron scattering experiments. 3D Ising behavior was clearly observed with the critical exponents γ=1.26±0.01, ν=0.59±0.01, and η=0.047±0.004. The crossover to mean-field behavior occurs at ${\mathit{T}}^{\mathrm{*}}$=${\mathit{T}}_{\mathit{c}}$+5.4 K. This is compared with the results of other experiments and the Landau-Ginzburg criterion. The Q dependence of the structure factor S(Q) follows the Ornstein-Zernike form in both regimes.

Coexistence curves of polystyrene PS(${\mathit{M}}_{\mathit{w}}$=3.7×${10}^4$ g/mole, ${\mathit{M}}_{\mathit{w}}$/${\mathit{M}}_{\mathit{n}}$<1.06) and poly(2-chlorostyrene) P2CIS(${\mathit{M}}_{\mathit{w}}$=2.33×${10}^5$ g/mole, ${\mathit{M}}_{\mathit{w}}$/${\mathit{M}}_{\mathit{n}}$≃1.19) in the presence of 22.6 wt % of di-n-butylphthalate (DBP) and of PS (${\mathit{M}}_{\mathit{w}}$=3.7×${10}^4$ g/mole, ${\mathit{M}}_{\mathit{w}}$/${\mathit{M}}_{\mathit{n}}$<1.06)/ P2CIS(${\mathit{M}}_{\mathit{w}}$=11.2×${10}^4$ g/mole, ${\mathit{M}}_{\mathit{w}}$/${\mathit{M}}_{\mathit{n}}$≃1.09) were determined by using a combination of a novel centrifugal method and synchrotron small-angle x-ray scattering. The coexistence curves could be fitted to yield a β value of ∼0.33 for the blend with DBP and a β value of 0.5 for the blend without DBP.

We present possible equilibrium phase diagrams on the temperature-concentration plane for binary interpenetrating polymer networks (IPN) in which network junctions are formed by reversible cross-linking. It is shown that, in most parts of the phase plane, the formed networks of both species are mutually exclusive so that the system demixes into small domains of heterophases, but that there is a small region just above the top of the miscibility gap in which a molecularly mixed IPN can stay in a stable phase. The condition for the existence of a stable IPN is examined. Competition between network formation and two-phase demixing is studied.

The spreading profiles of small (picoliter) droplets of polydimethylsiloxane on silver surfaces have been monitored in the thickness range below one monolayer. The profile shapes are distinctly non-Gaussian and can be approximated rather well by spherical caps. The experimental shapes and spreading dynamics are compared with Monte Carlo simulations of spreading polymer liquids. Excellent agreement is found.

Measurements of the muon spin relaxation of muonium show that the ${\mathrm{Mu}}^{\mathrm{I}}$ center in CuCl is an example of metastability in which isolated muonium tunnels locally around the tetrahedral interstitial site with four Cu nearest neighbors, with the stable form (${\mathrm{Mu}}^{\mathrm{II}}$) quasistationary at the same interstitial. For T<30 K the inverse correlation time associated with the damped tunneling motion increases with decreasing temperature according to a power law ${\mathit{T}}^{\mathrm{-}2.7\left(1\right)\mathrm{}}$, which is close to that observed previously for long-range quantum diffusion of muonium in insulators.

The atomic geometry of the CuCl(110)-(1×1) surface is determined by dynamical analysis of low-energy electron-diffraction intensities. This surface undergoes a relaxation characterized by a ∼30° Cu-Cl surface bond rotation, a 0.15 Å contraction of the top-to-second layer distance, and a 0.4 Å horizontal displacement of Cl relative to Cu. The relaxation is consistent with the ‘‘universal’’ structure deduced from the analysis of cleavage surface of tetrahedrally coordinated III-V and II-VI compounds, thereby revealing that this featuare of the structure does not depend significntly on the ionicity of the compound.

A quantitative structural study of Na and Rb adsorption on Al(111) using normal incidence standing x-ray wave-field triangulation shows that Rb occupies atop adsorption sites and both systems show a constant adsorbate-substrate bond length over the alkali coverage range 0.12–0.33 monolayer. These results indicate that there is directional local bonding in this simple alkali s-p band metal adsorption system, and show that there is no ionic-covalent bonding transition in this coverage range for either system.

We have irradiated fused silica with monochromatic x rays from a synchrotron radiation source, and have shown for the first time that the efficiency of producing E’ spins increases nonlinearly with photon energy for silica containing a large concentration of preexisting defects, and almost linearly for silica containing very few preexisting defects. It is concluded that E’ defects are created or activated with efficiencies proportional to approximately the first or second powers, respectively, of the initial energy of the energetic electrons produced by x-ray absorption.

The OH stretch mode of hydrogen defect ensembles in cubic and tetragonal ${\mathrm{SrTiO}}_3$ and ${\mathrm{KTaO}}_3$:Li has been investigated with polarized Raman scattering. Introducing the Raman behavior type analysis to tetragonal crystals allows the determination of the local symmetry and proves for ${\mathrm{SrTiO}}_3$ that protons vibrate along the O-O bonds on the edges of the oxygen octahedron. Moreover, the different local symmetries of OH dipoles in antiferrodistortive ${\mathrm{SrTiO}}_3$ and ${\mathrm{KTaO}}_3$:Li with polar off-center ${\mathrm{Li}}_{\mathrm{K}}$ are reflected experimentally in the behavior type of the Raman scattering intensities.