Search

Effective energy-loss functions were derived for oxygen-adsorbed amorphous silicon surfaces from a reflection electron energy-loss spectroscopy analysis based on the extended Landau theory. This study has revealed that the intensity of the surface-plasmon-loss peak for a clean surface decreases and its peak position shifts towards the lower-energy losses as oxygen exposure proceeds (⩽1000 L). To understand the above behavior of the surface-plasmon-loss peak, the distribution of the energy losses was calculated using the hydrodynamic model. The decrease and shift of the surface-plasmon-loss peak has been described with considerable success by assuming that the quasifree static electron density in the vicinity of the silicon surface decreases as oxygen adsorption proceeds owing to oxygen’s high electron affinity.

We present a nonlinear modulation scheme based on coupling between optically excited surface plasmons in a thin gold film and the heating of the film by a pulsed direct current, and also for the reverse process. The reflected surface-plasmon excitation beam is shown to undergo a spatial deformation as a function of a pulsed current through the foil. Similarly, the current through the thin film is modulated by the action of periodic excitation of surface plasmons using infrared photons.

Chemical sensing using optical diffraction from an array of microcantilevers is demonstrated. Properly fashioned arrays of micromachined silicon-nitride cantilevers containing embedded deformable diffraction gratings are functionalized with chemically selective coatings. Adsorption of specific molecules on the cantilever leads to bending, which changes the diffraction pattern of a laser beam reflecting off the array. Quantitative chemical information can be obtained by monitoring the displacement of diffraction peaks as a function of analyte exposure.

Two sets of physical interaction cross sections for detailed electron track-structure calculations in liquid water and water vapor have been used to investigate the possible magnitude of phase-dependent differences in the primary yields of ionizations and their fluctuations produced during complete slowing down of electrons in the energy range from 10 eV to 10 keV. For fast electrons the calculated values of the mean energy absorbed per ion pair are 25.8 eV/ip for the liquid as compared to 30.0 eV/ip for the vapor; both results are consistent with experimental data. A similar phase effect is found in the ionization yields from each molecular subshell, since essentially the same partitioning of the total ionization cross section has been used in the calculations for the liquid and the vapor. The relative fluctuations of the ionization yields as described by the Fano factor are 0.15 for the liquid and 0.25 for the vapor; the Fano factors for each single molecular orbital are typically between 0.7 and 0.9 in both phases. The distributions in the total number of ionizations produced by the complete slowing down of electrons with initial energies above about 100 eV are Gaussian within about two standard deviations around the mean. The significance of the differences in the yields of ionizations and excitations calculated for both phases is discussed. In the absence of adequate experimental data for the basic processes, the differences are interpreted in terms of the respective assumptions made for single scattering processes.

The problem considered is that of measuring the fast neutron tissue dose. A special proportional counter has been designed whose response to collimated neutrons is such that the number of counts produced is approximately proportional to dose, when the neutron energy is between 0.2 Mev and 10 Mev. The response has been tested experimentally with monoergic neutrons of various energies between 365 kev and 5.2 Mev and at 14 Mev and is in good agreement with the dose curve.

The shape of a proportional counter pulse due to an extended track of ionization is determined, before and after passing through a linear pulse amplifier, for various values of track extension and positive ion collection time. It is found that the maximum height of the amplifier output pulse is essentially independent of the length and orientation of the track in the counter, provided the time of collection of the outermost electrons is not longer than the amplifier time constant.

The formal solutions of problems involving transient heat conduction in infinite internally bounded cylindrical solids may be obtained by the Laplace transform method. Asymptotic series representing the solutions for large values of time are given in terms of functions related to the derivatives of the reciprocal gamma function. The results are applied to the case of the internally bounded infinite cylindrical medium with, (a) the boundary held at constant temperature; (b) with constant heat flow over the boundary; and (c) with the ``radiation'' boundary condition. A problem in the flow of gas through a porous medium is considered in detail.

In this paper a simple derivation is given of the distribution of photons emitted when an energetic charged particle passes through a thin layer of dielectric material. This derivation emphasizes the individual microscopic events which occur during such a process. The distribution function is shown to agree with that obtainable from the transition radiation theory of Frank and Ginzburg and from the plasmon decay theory of Ferrell for the case of a thin foil. The special case of a double foil bounded by vacuum is considered in detail, and a general formula is found for the photon distribution function. An approximation analogous to that of Born's in quantum mechanics is developed to treat the case of a general composite foil whose thickness is small compared with λ but which may be large compared with vλ/c, where λ is the wavelength of the light emitted and v is the speed of the charged particle.

The coupled electron‐hole cascade in an electron gas is described by a space‐ and angle‐independent transport equation. Analytic cross sections employed for the various processes are appropriate to a high density electron gas and for low to moderate excitation energies. The solution of the integral transport equation may be expressed in analytic form. It is found that the cascade process results in the accumulation of extremely large numbers of electrons and holes close to the Fermi surface.