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At least 307 records · Page 17Linked to original sources
PubMed2006

Electrophoretic motion of a spherical particle in a converging-diverging nanotube.

A charged spherical particle is concentrically positioned in a converging-diverging nanotube filled with an electrolyte solution, resulting in an electric double layer (EDL) forming around the particle's surface. In the presence of an axially applied electric field, the particle electrophoretically migrates along the axis of the nanotube due to the electrostatic and hydrodynamic forces acting on the particle. In contrast to a cylindrical nanotube with a constant cross-sectional area in which the electric field is almost uniform, the presence of a converging-diverging section in a nanotube alters the electric field, perturbs the charge distribution, and induces a pressure gradient and a recirculating flow that affect the electrostatic and hydrodynamic forces acting on both the particle and the fluid. Depending on the magnitude of the surface charge density along the nanotube's wall, the particle's electrophoretic motion may be significantly accelerated as the particle transverses the converging-diverging section. A continuum model consisting of the Nernst-Planck, Poisson, and Navier-Stokes equations for the ionic concentrations, electric potential, and flow field is implemented to compute the particle's velocity as a function of the particle's size, the nanotube's geometry, surface charges, electric field intensity, bulk electrolyte concentration, and the particle's location. When the particle is negatively charged and the wall of the nanotube is uncharged, the particle migrates in the direction opposite to that of the applied electric field and the presence of the converging-diverging section significantly accelerates the particle's motion. This, however, is not always true when the nanotube's wall is charged with the same sign as that of the particle. Once the ratio of the surface charge density of the nanotube's wall to that of the particle exceeds a certain value, the negatively charged particle will not translocate through the tube toward the anode and does not attain the maximum velocity at the throat of the converging-diverging section. One can envision such a device to be a nanofilter that allows molecules with surface charge densities much higher than that of the wall to go through the nanofilter, while preventing molecules with surface charge densities lower than that of the wall from passing through the nanofilter. The induced recirculating flow may be used to enhance mixing and to stretch, fold, and trap molecules in nanofluidic detectors and reactors.

Journal Article↗
PubMedDate not supplied

Argon/propane ionization-chamber dosimetry for mixed x-ray/neutron fields.

The photoneutrons produced by high-energy x-ray machines can diffuse through the mazes usually employed at the treatment-room entrance and readily penetrate the lead-lined doors used for x-ray shielding. The measurement of these neutrons in the presence of x-rays and the determination of dose equivalent poses a problem for which there is currently no standard method of solution. In order to separate x-ray dose from neutron dose, the author employed an ionization chamber alternately filled with argon or propane. The response characteristics of this chamber to x-ray and neutrons are described. Quality factors were determined from a calculated neutron spectrum. As a result of these measurements, a 10-in. polyethylene door was added to the entranceway of a 25-MV linear accelerator.

Argon↗
PubMed1978

Comparative study of lesions in the brain of rats exposed to heavy ions: accelerator and stratospheric flight research.

Irradiations of rat's brain with heavy ionized particles (12/6C, 291 Mev/nucleon, 100 to 300 ions/mm2) have been carried out at the Lawrence Berkeley Laboratory. Exposures were made with slowed ions which stopped within the brain as well as with more energetic particles which passed through. Histological examination of irradiated encephalons by optical microscopy showed zones of glial reaction and an increased number of darkly staining cells. By electron microscopy, intercellular edema, various alterations of nervous cells and macrophagy were observed, especially with stopped ions.

Animals↗
PubMed2006

Laser ion-acceleration scaling laws seen in multiparametric particle-in-cell simulations.

The ion acceleration driven by a laser pulse at intensity I= 10(20)-10(22) W/cm(2) x (microm/lambda)(2) from a double layer target is investigated with multiparametric particle-in-cell simulations. For targets with a wide range of thickness l and density n(e), at a given intensity, the highest ion energy gain occurs at certain electron areal density of the target sigma = n(e)l, which is proportional to the square root of intensity. In the case of thin targets and optimal laser pulse duration, the ion maximum energy scales as the square root of the laser pulse power. When the radiation pressure of the laser field becomes dominant, the ion maximum energy becomes proportional to the laser pulse energy.

Journal Article↗