Increased asymmetries in 2-deoxyglucose uptake in the brain of freely moving congenitally acallosal mice

To investigate the role of the corpus callosum in the expression of functional brain asymmetries, we compared left and right uptake of [14C]2-deoxyglucose in 43 brain regions measured in 10 C57B1/6 mice with a normal corpus callosum and in 12 congenitally acallosal mice, after 45 min of free activity in a novel, large open-field arena. The metabolic patterns across the brain appeared to be similar in the two groups of mice, as well as the average direction of asymmetry in tracer incorporation, which was higher at right in most of the brain regions for both acallosals and controls. However, the direction of the metabolic asymmetries of any given region was not consistent across individual animals. The largest asymmetries were found in the central auditory nuclei in both groups of mice, with extreme values in some acallosals. Significantly larger asymmetries were found in acallosal mice for the brain and the cortex as a whole, as well as for the lateral geniculate and pretectal nuclei, the olfactory tubercles, and retrosplenial, infrarhinal and perirhinal cortices. The metabolic asymmetries of the thalamic sensory nuclei were correlated with the asymmetries of the corresponding sensory cortical fields in the acallosal, but not in control mice. On the other hand, asymmetries of the cortical regions were largely intercorrelated in control mice, resulting in a general activation of one hemisphere over the other, while in acallosals they were more independent, resulting in a "patchy" pattern of cortical asymmetries. These results suggest that callosal agenesis, combined with the occurrence of ipsilateral Probst bundles, leads to a loss of co-ordination in the activation of different sensory and motor areas. The impaired co-ordination might then be distributed through cortico-subcortical loops, resulting in larger asymmetries throughout the brain. Thus, a normal corpus callosum appears to balance and synchronize metabolic brain activity, perhaps by smoothing the effects of asymmetrically activated ascending systems.     

Master equation and Fokker-Planck methods for void nucleation and growth in irradiation swelling

A complete theory of void swelling in irradiated metals requires the treatment of defect cluster nucleation events, as well as subsequent growth of stable clusters. One difficulty is that small-voids evolve rapidly and reversibly, whereas the secular evolution of the overall system is extremely slow. Thus, rate theory models for the void size distribution entail a set of stiff, coupled equations. A combined Master equation and Fokker-Planck numerical approach is introduced to address this problem and permit large time-steps at late times. Calculations are stable in practice, easily converged, and computationally efficient to large doses over a wide range in temperatures. The results are encouraging compared to experiment and earlier, related calculations.     

On the coefficient for bulk recombination of vacancies and interstitials

Bulk recombination in the rate theory of void swelling and irradiation creep is a significant process at low temperatures. The rate of recombination is proportional to the recombination radius which is evaluated in the present paper. The computed radius is in good agreement with measured values for dilute copper alloys at cryogenic temperatures. The theoretical results can therefore be extended to the temperature range for void swelling, and it is found that the recombination radius is about two times the lattice parameter.     

Crystallographic anisotropy of growth and etch rates of CVD diamond

The investigation of orientation dependent crystal growth and etch processes can provide deep insights into the underlying mechanisms and thus helps to validate theoretical models. Here, we report on homoepitaxial diamond growth and oxygen etch experiments on polished, polycrystalline CVD diamond wafers by use of electron backscatter diffraction (EBSD) and white-light interferometry (WLI). Atomic force microscopy (AFM) was applied to provide additional atomic scale surface morphology information. The main advantage of using polycrystalline diamond substrates with almost random grain orientation is that it allows determining the orientation dependent growth (etch) rate for different orientations within one experiment. Specifically, we studied the effect of methane concentration on the diamond growth rate, using a microwave plasma CVD process. At 1% methane concentration a maximum of the growth rate near <100> and a minimum near <111> is detected. Increasing the methane concentration up to 5% shifts the maximum towards <110> while the minimum stays at <111>. Etch rate measurements in a microwave powered oxygen plasma reveal a pronounced maximum at <111>. We also made a first attempt to interpret our experimental data in terms of local micro-faceting of high-indexed planes.     

The role of dispersed barriers in the pulsed irradiation creep of magnetic fusion reactor materials

The effects of irradiation pulsing on the climb-glide creep, and the role of the glide barrier height are investigated. Only tokamak-type pulsing is considered. We develop a new formulation for the creep strain increment per pulse for large barriers, for which more than one pulse is needed for glide to occur. This formulation is applied to typical tokamak-type conditions, including the UWMAK-I and INTOR designs. It is concluded that no significant enhancement over steady irradiation occurs for τ_v ? burn-time. However, in long burn-time Tokamaks with τ_v?on-time and off-time, it is found that the pulsed creep enhancement can be significant. For example, for a duty factor of 0.9 the enhancement is about 3 for small barriers using a dose-equivalent average damage rate when comparing pulsed and steady irradiation. The maximum enhancements are diminished to about 2 when equal instantaneous damage rates are used.     

Reversible expansion of gallium-stabilized δ-plutonium

It is shown that the transient expansion of plutonium-gallium alloys observed both in the lattice parameter as well as in the dimension of a sample held at ambient temperature can be explained by assuming incipient precipitation of Pu₃Ga. However, this ordered ζ′-phase is also subject to radiation-induced disordering. As a result, the gallium-stabilized δ-phase, being metastable at ambient temperature, is driven towards thermodynamic equilibrium by radiation-enhanced diffusion of gallium and at the same time reverted back to its metastable state by radiation-induced disordering. A steady state is reached in which only a modest fraction of the gallium present is arranged in ordered ζ′-phase regions.     

A high speed,precision voltage buffer in a submicron CMOS process

A new buffer architecture was introduced by Comer and Comer (1998, International Journal of Electronics, 84, 345). This buffer uses an active feedback network based on a transconductance amplifier. An implementation of the new buffer was done in a CMOS process. The buffer was intended for the output stage of a 10-bit video digital-to-analogue converter. The circuit was fabricated on the American Microsystems 0.6 μm process. Design specifications called for a gain accuracy of 0.1%, an offset voltage shift of no more than 1mV over a commonmode input range of 50% of supply voltage and a bandwidth of 500MHz. The actual circuit showed a gain error of less than 0.1%, a common-mode offset variation of less than 2mV, and a bandwidth of 450MHz.     

Controlled incorporation of mid-to-high Z transition metals in CVD diamond

We report on a general method to fabricate transition metal related defects in diamond. Controlled incorporation of Mo and W in synthetic CVD diamond was achieved by adding volatile metal precursors to the diamond chemical vapor deposition (CVD) growth process. Effects of deposition temperature, grain structure and precursor exposure on the incorporation efficiency were systematically studied, and doping levels of up to 0.25 at.% have been achieved. The metal atoms are uniformly distributed throughout the diamond grains without any indication of inclusion formation. These results are discussed in context of the kinetically controlled growth process of CVD diamond.