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PubMed2005

Hohlraum-driven ignitionlike double-shell implosions on the omega laser facility.

High-convergence ignitionlike double-shell implosion experiments have been performed on the Omega laser facility [T. R. Boehly, Opt. Commun. 133, 495 (1997)] using cylindrical gold hohlraums with 40 drive beams. Repeatable, dominant primary (2.45 MeV) neutron production from the mix-susceptible compressional phase of a double-shell implosion, using fall-line design optimization and exacting fabrication standards, is experimentally inferred from time-resolved core x-ray imaging. Effective control of fuel-pusher mix during final compression is essential for achieving noncryogenic ignition with double-shell targets on the National Ignition Facility [Paisner, Laser Focus World 30, 75 (1994)].

Journal Article↗

PubMed2005

B(E2) upward arrow measurements for radioactive neutron-rich ge isotopes: reaching the N=50 closed shell.

The B(E2;0(+)(1)-->2(+)(1)) values for the radioactive neutron-rich germanium isotopes (78,80)Ge and the closed neutron shell nucleus 82Ge were measured at the HRIBF using Coulomb excitation in inverse kinematics. These data allow a study of the systematic trend between the subshell closures at N=40 and 50. The B(E2) behavior approaching N=50 is similar to the trend observed for heavier isotopic chains. A comparison of the experimental results with a shell model calculation demonstrates persistence of the N=50 shell gap and a strong sensitivity of the B(E2) values to the effective interaction.

Journal Article↗

PubMed2005

Four-electron shell structures and an interacting two-electron system in carbon-nanotube quantum dots.

Low-temperature transport measurements have been carried out on single-wall carbon-nanotube quantum dots in a weakly coupled regime in magnetic fields. Four-electron shell filling was observed, and the magnetic field evolution of each Coulomb peak was investigated. Excitation spectroscopy measurements have revealed Zeeman splitting of single particle states for one electron in the shell, and demonstrated singlet and triplet states with direct observation of the exchange splitting at zero-magnetic field for two electrons in the shell, the simplest example of Hund's rule.

Journal Article↗

PubMed2006

Direct observation of vortex shells and magic numbers in mesoscopic superconducting disks.

We have studied vortex configurations in mesoscopic superconducting disks using the Bitter decoration technique. For a broad range of vorticities L the circular geometry is found to lead to the formation of concentric shells of vortices. From images obtained on disks of different sizes in a range of magnetic fields we traced the evolution of vortex states and identified stable and metastable configurations of interacting vortices. Furthermore, the analysis of shell filling with increasing L allowed us to identify magic numbers corresponding to the appearance of consecutive new shells.

Journal Article↗

PubMed2006

Implications of deformation and shape coexistence for the nuclear shell model.

The successes of the nuclear shell model in explaining the stability properties of magic nuclei are challenged by the observation of rotational bands for which the sequential filling of single-particle energy levels of the spherical shell model are not respected. This Letter proposes criteria for identifying the shell-model configurations appropriate for describing such bands of states.

Journal Article↗

PubMed2006

Failure of viral shells.

We report a combined theoretical and experimental study of the structural failure of viral shells under mechanical stress. We find that discontinuities in the force-indentation curve associated with failure should appear when the so-called Föppl-von Kármán (FvK) number exceeds a critical value. A nanoindentation study of a viral shell subject to a soft-mode instability, where the stiffness of the shell decreases with increasing pH, confirms the predicted onset of failure as a function of the FvK number.

Biomechanical Phenomena↗

PubMed1995

Water-dependent domain motion and flexibility in ribonuclease A and the invariant features in its hydration shell. An X-ray study of two low-humidity crystal forms of the enzyme.

The crystal structures of 88 and 79% relative humidity forms of ribonuclease A, resulting from water-mediated transformations, have been refined employing the restrained least-squares method using X-ray data collected on an area detector to R = 0.173 for 15 326 observed reflections in the 10-1.5 A resolution shell and R = 0.176 for 8534 observed reflections in the 10-1.8 A shell, respectively. The comparison of these structures with those of the native, the phosphate-bound and the sulfate-bound forms demonstrates that the mobility of the ribonuclease A molecule involves hinge-bending movement of the two domains and local flexibility within them, particularly at the termini of regular secondary structures and in loops. The comparison also leads to the identification of 31 invariant water molecules in the hydration shell of the enzyme, many of which are involved in holding different parts of the molecule together and in stabilizing local structure. The conformational changes that accompany the partial removal of the surrounding water, particularly those observed in the 79% form, could be similar to those that occur during enzyme action.

Journal Article↗

PubMed1996

Analysis of EXAFS Data from Mixed-Shell Systems.

A new method for analysis of EXAFS data from coordination shells containing two types of atoms distributed at two well defined distances is proposed. The method, which in effect isolates the individual contributions of the two subshells, can be viewed as a refinement of conventional techniques such as beat analysis and multi-shell least-squares fitting. No external information on the structure of any of the contributing subshells is required beyond the usual assumption of small or ;Gaussian' disorder. As much as fivefold reduction in the confidence limits of the coordination numbers in comparison with unrestricted multi-shell fits is demonstrated. The range of applicability and limitations of the method are discussed in detail.

Journal Article↗

PubMed2001

Shell waves and acoustic scattering from ultrasound contrast agents.

Ultrasound contrast agents are encapsulated microbubbles, filled either with air or a higher weight molecular gas, ranging in size from 1 to 10 microns in diameter. The agents are modeled as air-filled spherical elastic shells of variable thickness and material properties. The scattered acoustic field is computed from a modal series solution, and reflectivity and angular scattering are then determined from the computed fields for agents of various properties. We show that contrast agents also support shell resonance responses in addition to the monopole response, which has been the focus of previous contrast agent studies. Lamb waves appear to be the source of these additional responses. A shell or curvature Lamb wave generates dipole peaks in the 1- to 40-MHz range for 2.5 to 3.5 microns radius agents with elastic properties approximating those of albumin protein. The inclusion of damping affects the lower frequency dipole peaks but is less important for responses occurring above approximately 30 MHz. Moreover, these responses hold untapped potential for clinical ultrasound applications such as tissue perfusion studies and high frequency contrast agent imaging.

Biomedical Engineering↗

PubMed2002

An overview of the Salmonella enteritidis risk assessment for shell eggs and egg products.

This article summarizes a quantitative microbial risk assessment designed to characterize the public health impact of consumption of shell eggs and egg products contaminated with Salmonella Enteritidis (SE). This risk assessment's objectives were to: (1) establish the baseline risk of foodborne illness from SE, (2) identify and evaluate potential risk mitigation strategies, and (3) identify data gaps related to future research efforts. The risk assessment model has five modules. The Egg Production module estimates the number of eggs produced that are SE-contaminated. Shell Egg Processing, Egg Products Processing, and Preparation & Consumption modules estimate the increase or decrease in the numbers of SE organisms in eggs or egg products as they pass through storage, transportation, processing, and preparation. A Public Health Outcomes module then calculates the incidence of illnesses and four clinical outcomes, as well as the cases of reactive arthritis associated with SE infection following consumption. The baseline model estimates an average production of 2.3 million SE-contaminated shell eggs/year of the estimated 69 billion produced annually and predicts an average of 661,633, human illnesses per year from consumption of these eggs. The model estimates approximately 94% of these cases recover without medical care, 5% visit a physician, an additional 0.5% are hospitalized, and 0.05% result in death. The contribution of SE from commercially pasteurized egg products was estimated to be negligible. Five mitigation scenarios were selected for comparison of their individual and combined effects on the number of human illnesses. Results suggest that mitigation in only one segment of the farm-to-table continuum will be less effective than several applied in different segments. Key data gaps and areas for future research include the epidemiology of SE on farms, the bacteriology of SE in eggs, human behavior in food handling and preparation, and human responses to SE exposure.

Animals↗

PubMed1993

Introduction of a fluorescence-microscopic technique for the detection of eggs, egg shells, and mites in scabies.

A technique using the fluorescence microscope can prove helpful in the laboratory diagnosis of scabies. Specimens from fifteen patients with scabies were used in this study. All of the specimens were embedded with glycerine instead of potassium hydroxide (KOH) solution. The specimens were examined at 0 min, 30 min, 1 hr, 6 hrs, 24 hrs, and one week after mounting under light and fluorescence microscopes. Specimens embedded with non-fluorescent glycerine were not clear immediately after mounting but became so after about 1 hr. Eggs and egg shells were easily counted in the specimens under the fluorescence microscope but were very hard to identify under the light microscope. Mites were absent in half of the specimens; only eggs and egg shells were present in those specimens found by the fluorescence microscope. The above findings suggest that the detection of egg shells by the use of fluorescence microscope may be helpful for the diagnosis of scabies, in particular with mite negative specimens. Slides prepared with non-fluorescent glycerine were more stable and could be preserved for a long time. However, this method is time-consuming and requires expensive equipment.

Animals↗

PubMed1975

The ultrastructural localization of calcium in the avian shell gland.

Fluctuating concentrations of calcium in the blood supplying the avian shell gland are not reflected at tissue level. Analyses of oviducal mucosa throughout the laying cycle show that the concentration of calcium remains relatively unaltered even at peak activity thereby implying a rapid transfer process. These experiments describe the use of potassium pyroantimonate as a localizing agent. The precipitate produced by injecting the latter via the aorta was identified as calcium pyroantimonate by electron diffraction and the efficiency of the precipitant estimated by neutron activation analysis of eggshells present in the shell gland at the time of operation. Results indicate that calcium is transferred across the surface epithelial cells of the shell gland (tubular and pouch regions). Its localization in the former is taken as an indication of the involvement of this region in the production of the mammillary mantle.

Animals↗

PubMed1994

Isolation and characterization of a carboxysome shell gene from Thiobacillus neapolitanus.

The gene coding for the major carboxysome shell peptide (csoS1) from Thiobacillus neapolitanus has been isolated and sequenced. Oligonucleotide primers for polymerase chain reaction (PCR) amplification of the 5' end of the gene were made possible by amino acid sequencing of the N-terminal residues of the shell peptide. A 41 bp PCR product was used as a probe to isolate the gene. The deduced amino acid composition of the 216 bp gene shows a high degree of hydrophobicity. The gene is located within a series of three repeated regions of DNA and appears to have arisen via gene duplication. The transcript of csoS1 is approximately 400 bases in length. The shell peptide shares significant homology with Synechococcus open reading frames implicated in carboxysome structure/assembly. These open reading frames and csoS1 are related and are probably members of a carboxysome gene family.

Amino Acid Sequence↗

PubMed2000

A closed shell structured eyeball model with application to radial keratotomy.

A closed shell structured eyeball model was developed for predicting the displacements and curvatures in an eyeball due to radial keratotomy. Both the cornea and sclera are modeled as an ellipsoidal cap, and the two caps are connected at the limbus to form a closed shell. The analysis of the number of corneal collagen laminae required for the tissue to be theoretically transversely isotropic was presented. The cornea, as well as the limbus and sclera, is considered as macroscopically homogeneous and isotropic in this study. A procedure to obtain the principal curvature at a point on the exterior surface was established. In the basic formulation, large displacements are contemplated. However, the FORTRAN computer program that was prepared to implement the procedure considers small displacements, and the resulting equations are linear. Although the results from this shell structured eyeball model are fairly good quantitatively, they do show vividly the following qualitative corneal behavior after the operation is performed: The opening of an incision has a V-shape, the radial displacements through the corneal thickness are nearly the same, and the largest in-plane displacement is only one-tenth of the largest radial displacement.

Computer Simulation↗

PubMed1991

On a nonlinear theory for muscle shells: Part I--Theoretical development.

This paper presents a theory for studies of the large-strain behavior of biological shells composed of layers of incompressible, orthotropic tissue, possibly muscle, of arbitrary orientation. The intrinsic equations of the laminated-shell theory, expressed in lines-of-curvature coordinates, account for large membrane [O(1)] and moderately large bending and transverse shear strains [O(0.3)], nonlinear material properties, and transverse normal stress and strain. An expansion is derived for a general two-dimensional strain-energy density function, which includes residual stress and muscle activation through a shifting zero-stress configuration. Strain-displacement relations are given for the special case of axisymmetric deformation of shells of revolution with torsion.

Biomechanical Phenomena↗

PubMed1981

Why sliding seats and short stroke intervals are used for racing shells.

A theory is developed to explain why racing shell crews use sliding seats and shorter-than-necessary stroke intervals, two features that tend to waste energy in a highly competitive, power-limited situation. The theory shows that sliding seats are used because they allow the crew to move the oar blades fast enough during the stroke to transfer all available power to the water. Shorter-than-necessary stroke intervals are used to minimize the sum of the power dissipated in developing thrust, which decreases with the relative length of the stroke interval, and the power dissipated in hull drag due to fluctuations about average velocity, which increases with the relative length of the stroke interval. Using representative dimensions and masses, a typical value of average velocity for a race, and independently estimated values of drag and thrust coefficients, the theory predicts an optimum stroke fraction of 0.400, and optimum stroke rate of 45 per min, and an accountable power of 0.46 hp/man for an eight-oar racing shell, values that agree remarkably well with those used under competitive racing conditions. The theory indicates that using wider-bladed oars might improve performance; the theory also supports a suggestion made to the writer by R. A. Swanson that crossing the oars over the shell would improve performance.

Ergonomics↗

PubMed2002

Free interfacial vibrations in cylindrical shells.

The 2D equations in the Kirchhoff-Love theory are subjected to asymptotic analysis in the case of free interfacial vibrations of a longitudinally inhomogeneous infinite cylindrical shell. Three types of interfacial vibrations, associated with bending, super low-frequency semi-membrane, and extensional motions, are investigated. It is remarkable that for extensional modes natural frequencies have asymptotically small imaginary parts caused by a weak coupling with propagating bending waves. Bending and extensional vibrations correspond to Stonely-type plate waves, while semi-membrane ones are strongly dependent on shell curvature and do not allow flat plate interpretation. The paper represents generalization of the recent authors' publication [Kaplunov et al., J. Acoust. Soc. Am. 107, 1383-1393 (2000)] dealing with edge vibrations of a semi-infinite cylindrical shell.

Journal Article↗

PubMed2002

Leaky helical flexural wave backscattering contributions from tilted cylindrical shells in water: observations and modeling.

For tilt angles smaller than the meridional ray coupling condition previously investigated [S. F. Morse et al., J. Acoust. Soc. Am. 103, 785-794 (1998)], flexural helical waves on cylindrical shells can significantly enhance the backscattering. These contributions are compared and modeled here for an empty cylinder. Experiments using tone bursts were performed on a tilted stainless steel shell to investigate the contributions caused by flexural leaky Lamb waves above the coincidence frequency of the shell. In some of the measurements the tone bursts were of sufficient duration to superpose helical wave contributions of successive circumnavigations, along with the meridional contribution near the critical tilt, to arrive at a quasi-steady-state backscattering amplitude for the cylinder. These measurements are compared with an approximate numerical partial-wave series solution and a ray theory as a function of the tilt angle. The data for ka = 20 follow the basic shape of the ray theory and the relevant features of the partial-wave model. They illustrate the importance of the interference of successive helical wave contributions. Measurements (also as a function of the tilt angle) using tone bursts that were sufficiently short to separate the earliest helical wave contribution from later contributions also support the ray theory.

Journal Article↗