705 research outputs found
Mechanical behavior of polycrystalline ceramics: Brittle fracture of SiC-Si3N4 materials
The first study area involved magnesium oxide and the role of anion impurities, while the second area was directed toward slow crack growth in silicon nitride-silicon carbide ceramics. The oxide program involved development of fabrication techniques for anion doped materials and evaluation of the role of these anions in the hot pressing response, grain boundary diffusion of nickel doped material, grain boundary microhardness, and grain growth
Small eta-N scattering lengths favour eta-d and eta-alpha states
Unstable states of the eta meson and the 3He nucleus predicted using the time
delay method were found to be in agreement with a recent claim of eta-mesic 3He
states made by the TAPS collaboration. Here, we extend this method to a
speculative study of the unstable states occurring in the eta-d and eta-4He
elastic scattering. The T-matrix for eta-4He scattering is evaluated within the
Finite Rank Approximation (FRA) of few body equations. For the evaluation of
time delay in the eta-d case, we use a parameterization of an existing Faddeev
calculation and compare the results with those obtained from FRA. With an eta-N
scattering length, fm, we find an eta-d unstable
bound state around -16 MeV, within the Faddeev calculation. A similar state
within the FRA is found for a low value of , namely, fm. The existence of an eta-4He unstable bound state close to
threshold is hinted by fm, but is ruled out by
large scattering lengths.Comment: 21 pages, LaTex, 7 Figure
Novel insights into protein structure and dynamics utilizing the red edge excitation shift approach
A shift in the wavelength of maximum fluorescence emission toward higher wavelengths, caused by a corresponding shift in the excitation wavelength toward the red edge of the absorption band, is termed the red edge excitation shift (REES). This effect is mostly observed with polar fluorophores in motionally restricted media such as viscous solutions or condensed phases where the dipolar relaxation time for the solvent shell around a fluorophore is comparable to or longer than its fluorescence lifetime. REES arises from slow rates of solvent relaxation (reorientation) around an excited state fluorophore which depends on the motional restriction imposed on the solvent molecules in the immediate vicinity of the fluorophore. Utilizing this approach, it becomes possible to probe the mobility parameters of the environment itself (which is represented by the relaxing solvent molecules) using the fluorophore merely as a reporter group. Further, since the ubiquitous solvent for biological systems is water, the information obtained in such cases will come from the otherwise 'optically silent' water molecules. This makes REES extremely useful since hydration plays a crucial modulatory role in the formation and maintenance of organized molecular assemblies such as folded proteins in aqueous solutions and biological membranes. The application of REES as a powerful tool to monitor the organization and dynamics of a variety of soluble, cytoskeletal, and membrane-bound proteins is discusse
Mechanical behavior of polycrystalline ceramics: Brittle fracture of Si C - Si3N4 materials
The results are described of the final stage of the research involving the role of anions in the behavior of magnesium oxide, as well as the continued efforts of the fracture behavior of silicon nitride materials. These efforts, particularly the first, are further sub-divided in subsections describing individual types of behavior of materials
Bose-Einstein Condensate Driven by a Kicked Rotor in a Finite Box
We study the effect of different heating rates of a dilute Bose gas confined
in a quasi-1D finite, leaky box. An optical kicked-rotor is used to transfer
energy to the atoms while two repulsive optical beams are used to confine the
atoms. The average energy of the atoms is localized after a large number of
kicks and the system reaches a nonequilibrium steady state. A numerical
simulation of the experimental data suggests that the localization is due to
energetic atoms leaking over the barrier. Our data also indicates a correlation
between collisions and the destruction of the Bose-Einstein condensate
fraction.Comment: 7 pages, 8 figure
L-MM Auger electron emission from chlorinated organic molecules under proton impact: angular distribution and total cross section measurement
We have measured absolute total cross section for LMM Auger electron emission
of Cl in chlorinated methane and benzene chloride in collision with H+ ion.
Projectile energy dependence of the total yield as well as the angular
distribution has been studied. Incident proton energy has been varied from 125
keV to 275 keV in steps of 50 keV. C KLL Auger yield have been compared with
previous studies and found to be in agreement within the effect of chemical
species It has been found that the LMM Auger yield of Cl is much more
significantly affected by molecular environment than the C KLL
Critical view of WKB decay widths
A detailed comparison of the expressions for the decay widths obtained within
the semiclassical WKB approximation using different approaches to the tunneling
problem is performed. The differences between the available improved formulae
for tunneling near the top and the bottom of the barrier are investigated.
Though the simple WKB method gives the right order of magnitude of the decay
widths, a small number of parameters are often fitted. The need to perform the
fitting procedure remaining consistently within the WKB framework is emphasized
in the context of the fission model based calculations. Calculations for the
decay widths of some recently found super heavy nuclei using microscopic
alpha-nucleus potentials are presented to demonstrate the importance of a
consistent WKB calculation. The half-lives are found to be sensitive to the
density dependence of the nucleon-nucleon interaction and the implementation of
the Bohr-Sommerfeld quantization condition inherent in the WKB approach.Comment: 18 pages, Late
Hidden evidence of non-exponential nuclear decay
The framework to describe natural phenomena at their basics being quantum
mechanics, there exist a large number of common global phenomena occurring in
different branches of natural sciences. One such global phenomenon is
spontaneous quantum decay. However, its long time behaviour is experimentally
poorly known. Here we show, that by combining two genuine quantum mechanical
results, it is possible to infer on this large time behaviour, directly from
data. Specifically, we find evidence for non-exponential behaviour of alpha
decay of 8Be at large times from experiments.Comment: 12 pages LaTex, 3 figure
Fragmentation dynamics of diatomic molecules under proton impact: Kinetic energy release spectra of CO^{q+} and NO^{q+} (q = 2, 3) molecular ions
We report on the fragmentation dynamics of triply charged, diatomic,
molecular ions of NO and CO. Dissociative fragmentation after multiple
ionization of NO and CO is studied under the impact of 200 keV proton beam
using recoil-ion momentum spectrometer. Kinetic Energy Release distributions
(KERDs) for various fragmentation channels were obtained. We have also
calculated the potential energy curves (PECs) for ground and several excited
states of NO^{3+} and CO^{3+} molecular ion. The obtained KERDs are discussed
in the background of the calculated PECs as well as the simple Coulomb
excitation model. Coulomb breakup of the unstable precursor molecular ion shows
a clear preference for the N^{2+} + O^+ (and C^{2+} + O^+) fragmentation
channel.Comment: 8 pages, 6 figures, 3 table
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