2,233 research outputs found
Physiological Characterization of Stolon Regression in a Colonial Hydroid
As with many colonial animals, hydractiniid hydroids display a range of morphological variation. Sheet-like forms exhibit feeding
polyps close together with short connecting stolons, whereas runner-like forms have more distant polyps and longer connecting
stolons. These morphological patterns are thought to derive from rates of stolon growth and polyp formation. Here, stolon
regression is identified and characterized as a potential process underlying this variation. Typically, regression can be observed
in a few stolons of a normally growing colony. For detailed studies, many stolons of a colony can be induced to regress by
pharmacological manipulations of reactive oxygen species (e.g. hydrogen peroxide) or reactive nitrogen species (e.g. nitric
oxide). The regression process begins with a cessation of gastrovascular flow to the distal part of the stolon. High levels of
endogenous H2O2 and NO then accumulate in the regressing stolon. Remarkably, exogenous treatments with either H2O2 or an NO
donor equivalently trigger endogenous formation of both H2O2 and NO. Cell death during regression is suggested by both
morphological features, detected by transmission electron microscopy, and DNA fragmentation, detected by TUNEL. Stolon
regression may occur when colonies detect environmental signals that favor continued growth in the same location rather than
outward growth
Advanced Radiation Panel design for applications in National Security and Food Safety
We describe a new concept for a basic radiation detection panel based on
conventional scintillator technology and commercially available solid-state
photo-detectors. The panels are simple in construction, robust, very efficient
and cost-effective and are easily scalable in size, from tens of cm to tens
of m. We describe two possible applications: flagging radioactive food
coontamination and detection of illicit radio nucleides, such as those
potentially used in a terrorist attack with a dirty bomb.Comment: 10 pages, 11 figure
A model of rf breakdown arcs
This paper presents a rst iteration of a model that attempts to describe all aspects of breakdown in rf cavities and provides some estimates of the parameters and parameter ranges involved, as an aid to producing more precise models and more useful experiments. The model describes how breakdown events can be triggered, how they grow, it identi es the power source for their rapid growth, mechanisms that limit their growth, how they are extinguished and how they can be mitigated. We also discuss applications to superconducting rf and high pressure gas structures. The model relies heavily on previous experiments with 805 and 201 MHz warm copper cavities, and pre-liminary plasma modeling using the code OOPIC Pro. We compare estimates from the model with experimental data where this is possible. Because of the geometrical dependence of all parameters, the wide range of experiments being performed, the wide range of experimental parameters in a given breakdown event and the lack of extensive systematic parameter searches at this stage in our studies, it is diffcult to present precise results. We are constrained to showing what mechanisms are involved, the strength of these mechanisms and how they interact to produce the experimental data. We are primarily interested in the development and dynamics of the arc, magnetic and gas effects and insights on how to avoid arcing in all environments
Differential Transfer Ionization Cross Sections for 50175-keV Proton-Helium Collisions
We have measured coincidences between neutralized projectiles and He recoil ions for 50175-keV proton-helium collisions. From the data we obtained transfer ionization (TI) cross sections differential in the projectile scattering angle. Laboratory scattering angles range from 0 to 2.0 mrad. The experimental method allowed separation of the postcollision charge states of the target atoms. The ratio of the cross sections for TI to the sum of TI and single capture, F, is presented as a function of projectile scattering angle. Comparison is made to previous measurements of this ratio where data is available. The differential cross sections are compared to dynamical classical trajectory Monte Carlo (dCTMC) calculations. Agreement in the shape of the differential cross sections is good between the theory and measurement over the entire energy range
Valence band photoemission from the GaN(0001) surface
A detailed investigation by one-step photoemission calculations of the
GaN(0001)-(1x1) surface in comparison with recent experiments is presented in
order to clarify its structural properties and electronic structure. The
discussion of normal and off-normal spectra reveals through the identified
surface states clear fingerprints for the applicability of a surface model
proposed by Smith et al. Especially the predicted metallic bonds are confirmed.
In the context of direct transitions the calculated spectra allow to determine
the valence band width and to argue in favor of one of two theoretical bulk
band structures. Furthermore a commonly used experimental method to fix the
valence band maximum is critically tested.Comment: 8 pages, 11 eps files, submitted to PR
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