Low temperature thermal conductivity and specific heat of elastomers

The specific heat C and thermal conductivity κ of polybutadiene are characteristic of all non-crystalline materials at temperatures below ≈ K, reflecting the presence of localized excitations. The changes in C and κ with variation in crosslinking suggest that the relaxation times of the localized excitations may vary with crosslink density. Extension of an elastomer does not reveal a change in density of localized excitations as monitored by measuring κ, possibly because other phonon scattering mechanisms mask the effect. For T ? 10 K the phonon mean-free-path is independent of the microscopic anisotropy of the elastomer.     

Coexistence of competing predators in a coral reef ecosystem

An analysis is made using a four-dimensional mathematical model where there is a constant rate of flow of input nutrient. An organism is introduced in the model which is growing by taking in that nutrient. Two other predators on that organism are also introduced. The predators at the second and third trophic levels belong to the same species, though of different age groups. The predator at the third trophic level exhibits a distinct cannibalistic attitude to the predator of the second trophic level. Our analysis leads to different thresholds in terms of the model parameters acting as conditions under which the species associated with the system cannot thrive even in the absence of competition. Local stability of the system is obtained when one or more of the predators go extinct. Under appropriate circumstances a positive rest point of the system is obtained. Computer simulations have been carried out to illustrate different analytical results.     

Derivation of muon spectrum at high energies from the recent JACEE primary cosmic-ray spectrum using Fermilab data on meson production

Summary The energy spectrum of sea level muons in the range (10²÷10⁴) GeV has been calculated from the latest directly measured JACEE primary spectrum using Fermilab results on pp→π^± X and pp→K^± X inclusive reactions. The conventional pion atmospheric diffusion equation after Bugaevet al. has been used in this analysis to account the flux of muons emerged from the multiple generation of mesons in air. The derived muon spectrum has been compared with the earlier magnetic spectrograph data of Durham and Kiel groups. The latest BAKSAN scintillator telescope data is well in agreement with the calculated integral spectrum originated from the meson decays in the range (1÷10⁴) GeV.     

Thermal instability of a horizontal layer of micropolar fluid with heat source

The thermal instability of a horizontal layer of micropolar fluid which loses heat throughout its volume at a constant rate has been considered. The influence of the various micropolar fluid parameters on the onset of convection have been analysed. It is found that heat source and heat sink have the same destabilising effect in micropolar fluid. It is observed that the horizontal dimension of the cells remains insensitive to the changes in the micropolar fluid parameters and also to the heat source parameterQ except forQ values near zero, where the change is drastic. Further, it is observed that though the vertical component of velocity and the curl of microrotation do not vanish anywhere between the two boundaries forQ=0, they vanish at a point nearer to the lower boundary even for a small change in theQ value.     

Generalized Euler transformation in extracting useful information from divergent (asymptotic) perturbation series and the construction of Padé approximants

Euler transformation for accelerating convergence of a series is considered in the context of handling divergent (asymptotically convergent) perturbation series. A generalized (parametrized) version of this transformation is developed, based on the conjecture of Dalgarno and Stewart, which works better. Viewed from this standpoint, the Padé approximants follow as a special case of the parametrized Euler transformation (PET ), as is the case with the μ transformation procedure of Feenberg in a perturbative context. The PET is shown to serve as a more general method of handling a divergent series and is able to appreciate the construction and convergence behavior of specific sequences of Padé approximants. The role of parametrization in the context of the Z^?1 perturbation theory of atoms is also noted and the workability of the adopted strategy is demonstrated by choosing some specific test cases.     

Molecular inversion in the nπ* states: A theoretical investigation of some model systems in a semiempirical molecular orbital framework

The considerations of Walsh rules are extended to rationalize the loss of planarity in the ^1,3nπ* states of simple carbonyl and thiocarbonyl molecules. The role of Fermi correlation in shaping the differences between conformations in the singlet and the triplet state is emphasized. The role played by the π* orbital is also considered.     

Energy spectra of cosmic pions and nucleons at the top of the atmosphere

The production spectra of cosmic pions and their primaries have been derived from the experimental muon spectrum by using the theories of Barrettet al. and Feynman. The calculated pion production spectrum takes the formI(E _π)d E _π = 0.206E _π ^{? 2.65} d E _πper cm² sec str GeV in the energy range 3<E _π<1500 GeV and the nucleon spectrum follows the relationN(E _p)d E _p = 2.7 × 10⁴ (E _p/GeV)^2.65 d E _pper m² sec str GeV in the range 5 <E _p <3000 Gev.     

A soft-segmentation visualization scheme for magnetic resonance images

Prevalent visualization tools exploit gray value distribution in images through modified histogram equalization and matching technique, referred to as the window width/window level-based method, to improve visibility and enhance diagnostic value. The window width/window level tool is extensively used in magnetic resonance (MR) images to highlight tissue boundaries during image interpretation. However, the identification of different regions and distinct boundaries between them based on gray-level distribution and displayed intensity levels is extremely difficult because of the large dynamic range of tissue intensities inherent in MR images. We propose a soft-segmentation visualization scheme to generate pixel partitions from the histogram of MR image data using a connectionist approach and then generate selective visual depictions of pixel partitions using pseudo color based on an appropriate fuzzy membership function. By applying the display scheme in clinical examples in this study, we could demonstrate additional overlapping regions between distinct tissue types in healthy and diseased areas (in the brain) that could help improve the tissue characterization ability of MR images.     

Stress in polycrystalline GaN films prepared by r.f sputtering

Undoped, Be-doped and Si-doped polycrystalline GaN films were deposited by R.F. sputtering onto fused silica substrates. The films were deposited at various deposition temperatures ranging from 300 K to 623 K and characterized by optical measurements while the microstructural information was obtained from SEM and XRD studies. The compositional study for the GaN film was carried out using SIMS. Residual stresses in these films were evaluated from the band tail of the absorption spectra as well as from direct measurements of hardness by commercially available depth sensing indentometer. It was observed that undoped GaN films had the highest hardness followed by that for Be-doped and Si-doped films. The values of hardness obtained form the above optical measurement tallied quite well with those obtained from direct indentation measurement.