On the interpretation of positron-annihilation data in powders and fine-grained materials

Monte-Carlo simulations of positron diffusion are carried out for powders consisting of spherical and ellipsoidal particles with and without defects. Following Bergersen et al. [1], elastic positron-phonon interaction is considered to be dominant for scattering processes in positron diffusion. The central question is which fraction of the positrons would be able to reach the particle boundaries. Hence, we calculate the Fraction of Positrons reaching particle Surface (FPS). The presence of defects in the particles can drastically reduce FPS depending on the defect concentration and capture rate. We demonstrate that for small-grained materials the grain surface can influence the lifetime signal significantly.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Luminescence centers in porous silicon

Photoluminescence studies on porous silicon show that there are luminescence centers present in the surface states. By taking photoluminescence spectra of porous silicon with respect to temperature, a distinct peak can be observed in the temperature range 100–150 K. Both linear and nonlinear relationships were observed between excitation laser power and the photoluminescence intensity within this temperature range. In addition, there was a tendency for the photoluminescence peak to red shift at low temperature as well as at low excitation power. This is interpreted as indicating that the lower energy transition becomes dominant at low temperature and excitation power. The presence of these luminescence centers can be explained in terms of porous silicon as a mixture of silicon clusters and wires in which quantum confinement along with surface passivation would cause a mixing of andX band structure between the surface states and the bulk. This mixing would allow the formation of luminescence centers.     

Multiple scattering of narrow light beams in aerosols

A multiple scattering propagation model of narrow light beams in aerosol media is described. It is based on a paraxial approximation of the radiative transfer equation in which the flux normal to the incident beam direction is modeled by a diffusion process. The model solutions are the forward- and backscattered intensity profiles for the specified geometry and receiver aperture and field of view. The required inputs are the system parameters, and the aerosol single scattering angular phase function and extinction and scattering coefficients which are allowed to vary along the beam axis. Good agreement is shown with measurements performed in the laboratory over scales ranging from a few tens of mm to a few m, and in the atmosphere over a scale of the order of 1 km. The solutions are valid for optical depths smaller than 10, for phase functions corresponding to average size parameters of order one or greater, and for off-axis positions not exceeding 25% of the reciprocal of the scattering coefficient.     

CW diode pumping and FM mode locking of a Nd: KGW laser

We have demonstrated cw diode end pumping of Nd: KGW, a novel solid-state gain medium, with up to 30% conversion efficiency into near-TEM₀₀ (M² < 1.05) output at = 1.067 µm for a pump level of 2.7 W. The slope efficiency was limited by intracavity reflections to 36%; however, direct comparison to a similar Nd:YAG laser indicates the same intrinsic slope efficiency of 60%. FM mode locking of this laser at 200 MHz has produced 12 ps pulses (compared to 16 ps for Nd: YAG), although an intracavity etalon was required. Considerable reduction in pulse width is possible (the line width limit is 0.5 ps) but different techniques may be necessary. Spatial hole burning was evident in both the 120 GHz free-running spectrum and the etalon-limited mode-locked spectrum.     

Exact plane-symmetric solutions of the nonlinear equations of a spinor field in the theory of gravitation

We obtain exact plane-symmetric solutions of the spinor field equations with a nonlinear term that is an arbitrary functions of the invariant and with the self-gravitational field taken into account. Conditions are formulated for which the initial system of Einstein's equation and the spinor field equations with a power-law nonlinearity have regular solutions with localized (negative) spinor field energy density: so-called soliton-like solutions. Exact solutions of the spinor field equations are also obtained in flat space—time in this case and it is shown that the initial system of equations does not have soliton-like solutions. Hence the self-gravitational field plays a crucial (regularizing) in soliton-like solutions of the nonlinear spinor field equations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 63–68, August, 1995.     

Multiple time scale analysis of two models for the peroxidase-oxidase reaction

A multiple time scale analysis of two four-variable models of the peroxidase-oxidase reaction, the DOP, and the Olsen model, is carried out. It is shown that autonomous limit cycle oscillations are exhibited by the fast subsets of these two models, but only in certain regions of parameter space, confirming the prior suggestion that the slow variable (NADH) is not essential for oscillatory behavior. However, it is found that the slow variable is essential for oscillatory behavior over other ranges of parameter values, and is always essential for complex oscillatory and chaotic behavior. This latter conclusion is based on a study involving driving the fast subset with a sinusoidally varying (NADH). This study suggests the level of coupling between fast and slow variables of an autonomous system necessary to cause the chaos observed in the DOP model. Further study of the driven system allows for the identification of a natural period of the nonoscillatory but bistable fast subsystem and a set of rules for applying a parametric driving in such a way as to generate a more complete Farey sequence from a truncated Farey sequence. These conclusions are used to compare the very similar DOP and Olsen models, which, nevertheless, exhibit quite different Farey sequences and routes to chaos. (c) 1995 American Institute of Physics.     

Reaction studies of carbon clusters

Reaction studies of carbon clustersC _n in the rangen=8–37, produced by laser vaporisation in a supersonic nozzle, have been investigated using time-of-flight mass spectrometry. Clear differences in reaction products formed on hydrogenation are detected which show that even and odd clusters behave quite differently and furthermore thatat least two different types of even cluster appear to exist. The reactivity patterns for clusters C _n withn=16, 18 and 22 are in a different class from those withn=20, 24, 26 ..., a behaviour consistent with the existence of closed cage fullerene structures for even clusters with 20 or more carbon atoms (other thann=22).     

Determination of alpha contaminated soil profiles using Nuclear Track Detectors

Nuclear Track Detectors (NTD's) are a useful option forin situ measurements of the distribution of alpha contamination as a function of soil depth. The contamination profile of alpha emitting elements, e.g. Pu, Am and U, can be determined by detecting their alpha emission at varying depths. This paper discusses a stake type device, containing strips of CR-39 (allyl diglycol carbonate) that can be inserted into the soil up to ten centimeters or more, depending on the firmness of the soil. The CR-39 is exposed directly to the contaminated soil for a few hours. The stake is then withdrawn from the soil, the plastic detectors recovered and the alpha tracks developed by chemical etching with KOH. The distribution of tracks can be used to determine the alpha contamination depth profile as well as for detecting hot spots. It has a sensitivity of less than a pCi/g of soil.Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract W-7405-ENG-48.