520 research outputs found
Elasticity Theory and Shape Transitions of Viral Shells
Recently, continuum elasticity theory has been applied to explain the shape
transition of icosahedral viral capsids - single-protein-thick crystalline
shells - from spherical to buckled/faceted as their radius increases through a
critical value determined by the competition between stretching and bending
energies of a closed 2D elastic network. In the present work we generalize this
approach to capsids with non-icosahedral symmetries, e.g., spherocylindrical
and conical shells. One key new physical ingredient is the role played by
nonzero spontaneous curvature. Another is associated with the special way in
which the energy of the twelve topologically-required five-fold sites depends
on the background local curvature of the shell in which they are embedded.
Systematic evaluation of these contributions leads to a shape phase diagram in
which transitions are observed from icosahedral to spherocylindrical capsids as
a function of the ratio of stretching to bending energies and of the
spontaneous curvature of the 2D protein network. We find that the transition
from icosahedral to spherocylindrical symmetry is continuous or weakly
first-order near the onset of buckling, leading to extensive shape degeneracy.
These results are discussed in the context of experimentally observed
variations in the shapes of a variety of viral capsids.Comment: 53 pages, 17 figure
Molybdenum targets for production of 99mTc by a medical cyclotron
Introduction
Alternative methods for producing the medical imaging isotope 99mTc are actively being developed around the world in anticipation of the imminent shutdown of the National Research Universal (NRU) reactor in Chalk River, Ontario, Canada and the high flux reactor (HFR) in Petten, Holland that together currently produce up to 80 % of the world’s supply through fission. The most promising alternative methods involve accelerators that focus Bremsstrahlung radiation or protons on metallic targets comprised of 100Mo and a supporting material used to conduct heat away during irradiation. As an example, the reaction 100Mo(p,2n)99mTc provides a direct route that can be incorporated into routine production in regional nuclear medicine centers that possess medical cyclotrons for production of other isotopes, such as those used for Positron Emission Tomography (PET).
The targets used to produce 99mTc are subject to a number of operational constraints. They must withstand the temperatures generated by the irradiation and be fashioned to accommodate temperature gradients from in situ cooling. The targets must be resilient, which means they cannot disintegrate during irradiation or post processing, because of the radioactive nature of the products. Yet, the targets must be easily post-processed to separate the 99mTc. In addition, the method used to manufacture the targets must not be wasteful of the 100Mo, because of its cost (~$2/mg). Any manufacturing process should be able to function remotely in a shielded space to accommodate the possibility of radioactive recycled target feedstock. There are a number of methods that have been proposed for large-scale target manufacturing including electrophoretic deposition, pressing and sinter-ing, electroplating and carburization [1]. How to develop these methods for routine production is an active business [2,3]. From the industrial perspective, plasma spraying showed promising results initially [4], but the process became very expensive requiring customized equipment in order to reduce losses because of overspray,which also required a large inventory of expen-sive feedstock. In this paper we report the ex-perimental validation of an industrial process for production of targets comprising a Mo layer and a copper support.
Materials and methods
Target Design
Targets have been manufactured for irradiation at 15 MeV. Two targets are shown in FIG. 1: one as-manufactured and another after irradiation; no visible changes were observed following irradiation. The supporting circular copper (C101) disks have diameters of 24 mm and thickness of 1.6 mm. The molybdenum in the center of the target is fully dense with thickness 230 μm determined from SEM cross-sections.Targets have also been manufactured for irradi-ation in a general-purpose target holder designed to be attached to all makes of cyclotrons found in regional nuclear medicine centers. The elliptical targets were designed for high-volume production of 99mTc with 15 MeV protons at currents of 400 µA with 15% collimation [4]. The elliptical shape reduces the heat flux associated with high current sources. The cooling channels on the back of the target are designed to with-stand the high temperature generated during Irradiation.
A thermal simulation of expected temperatures during irradiation is shown in FIG. 3. The center of the target is expected to reach 260 oC during irradiation. The elliptical targets were formed from a 27 mm C101 copper plate with width 22 mm and length 55 mm. The molybdenum in the center of the target is fully dense with thickness 60 m de-termined from SEM cross-sections. FIG. 4 shows the molybdenum deposition in the center of the target in a form of an ellipse (38×10 mm).
Results and Conclusions
Circular targets have been produced and suc-cessfully irradiated for up to 5 h with a proton beam with energy 15 MeV and current 50 µA. (FIG. 1). The targets were resilient. Before irradi-ation the targets were subjected to mechanical shock tests and thermal gradients with no ob-servable effect. After irradiation there was no indication of any degradation. The manufacturing process produced 20 consistently reproducible targets within an hour with a molybdenum loss of less than 2 %. After irradiation the targets were chemically processed and the products characterized by Ge-HP gamma spectrometry. Only Tc isotopes were found. No other contami-nants were identified after processing. The de-tails of the separation and purification are de-scribed elsewhere [5].
Circular targets suitable for low-volume produc-tion of 99mTc have been manufactured and test-ed. The targets have been shown to meet the required operation constraints: the targets are resilient withstanding mechanical shock and irradiation conditions; they are readily produced with minimal losses; and post-processing after irradiation for 5 h has been shown to produce 99mTc.
Elliptical targets suitable for high-volume pro-duction of 99mTc with high power cyclotrons have been manufactured (FIG. 4). Like the circular targets, the elliptical targets are readily pro-duced with minimal losses and are able to with-stand mechanical shock and thermal gradients; however, they have yet to be irradiated
Uniqueness of Bessel models: the archimedean case
In the archimedean case, we prove uniqueness of Bessel models for general
linear groups, unitary groups and orthogonal groups.Comment: 22 page
Induced Crystallization of Polyelectrolyte-Surfactant Complexes at the Gas-Water Interface
Synchrotron-X-ray and surface tension studies of a strong polyelectrolyte
(PE) in the semi-dilute regime (~ 0.1M monomer-charges) with varying surfactant
concentrations show that minute surfactant concentrations induce the formation
of a PE-surfactant complex at the gas/solution interface. X-ray reflectivity
and grazing angle X-ray diffraction (GIXD) provide detailed information of the
top most layer, where it is found that the surfactant forms a two-dimensional
liquid-like monolayer, with a noticeable disruption of the structure of water
at the interface. With the addition of salt (NaCl) columnar-crystals with
distorted-hexagonal symmetry are formed.Comment: 4 pages, 5 eps figure
Dynamical Ordering of Driven Stripe Phases in Quenched Disorder
We examine the dynamics and stripe formation in a system with competing short
and long range interactions in the presence of both an applied dc drive and
quenched disorder. Without disorder, the system forms stripes organized in a
labyrinth state. We find that, when the disorder strength exceeds a critical
value, an applied dc drive can induce a dynamical stripe ordering transition to
a state that is more ordered than the originating undriven, unpinned pattern.
We show that signatures in the structure factor and transport properties
correspond to this dynamical reordering transition, and we present the dynamic
phase diagram as a function of strengths of disorder and dc drive.Comment: 4 pages, 4 postscript figure
FlyBase: genomes by the dozen
FlyBase () is the primary database of genetic and genomic data for the insect family Drosophilidae. Historically, Drosophila melanogaster has been the most extensively studied species in this family, but recent determination of the genomic sequences of an additional 11 Drosophila species opens up new avenues of research for other Drosophila species. This extensive sequence resource, encompassing species with well-defined phylogenetic relationships, provides a model system for comparative genomic analyses. FlyBase has developed tools to facilitate access to and navigation through this invaluable new data collection
Isotropic-nematic phase transition in suspensions of filamentous virus and the neutral polymer Dextran
We present an experimental study of the isotropic-nematic phase transition in
an aqueous mixture of charged semi-flexible rods (fd virus) and neutral polymer
(Dextran). A complete phase diagram is measured as a function of ionic strength
and polymer molecular weight. At high ionic strength we find that adding
polymer widens the isotropic-nematic coexistence region with polymers
preferentially partitioning into the isotropic phase, while at low ionic
strength the added polymer has no effect on the phase transition. The nematic
order parameter is determined from birefringence measurements and is found to
be independent of polymer concentration (or equivalently the strength of
attraction). The experimental results are compared with the existing
theoretical predictions for the isotropic-nematic transition in rods with
attractive interactions.Comment: 8 Figures. To be published in Phys. Rev. E. For more information see
http://www.elsie.brandeis.ed
All-Metal water target with spherical window
Introduction
The use of a prefabricated target window assembly greatly simplifies the window installation. The window module is sealed by metal knife-edges, thus eliminating any elestomers in the target construction.
Spherical Havar window offers high strength at reduced thickness and does not require helium cooling.
The target body is of platinum-plated silver.
The target assembly includes an integral beam collimator and a four-sector mask
Phase Diagram of Chiral Biopolymer Wigner Crystals
We study the statistical mechanics of counterion Wigner crystals associated
with hexagonal bundles of chiral biopolymers. We show that, due to spontaneous
chiral symmetry breaking induced by frustration, these Wigner crystals would be
chiral even if the biopolymers themselves were not chiral. Using a duality
transformation of the model onto a "spin-charge" Hamiltonian, we show that
melting of the Wigner crystal is due to the unbinding of screw dislocations and
that the melting temperature has a singular dependence on the intrinsic
chirality of the biopolymers. Finally, we report that, if electrostatic
interactions are strongly screened, the counterions can condense in the form of
an intermediate achiral Wigner solid phase that melts by the unbinding of
fractional topological charges.Comment: 43 pages, 13 figure
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