Search results for "RADIATION"
showing 10 items of 5298 documents
Electrophoretic mobility of electrostatically interacting colloidal spheres
2004
We have measured the electrophoretic mobility μ = vE/E (where E is the electric field strength and vE the electrophoretic velocity) of highly charged colloidal spheres in deionized aqueous suspension at particle number densities n between 0.15 and 150 µm−3. Under these conditions the system exhibits fluid or crystalline order. We used laser Doppler velocimetry to determine the electrophoretic velocities vE as spatially averaged particle velocities from both integral and spatially resolved measurements. With this approach we were for the first time able to extend measurements far into the crystalline region of the phase diagram. We found μ to be constant at low n while at large n we observe …
Photoluminescence at 1.9 eV in synthetic wet silica
2001
We report the effects of γ-irradiation on the optical activity of wet synthetic silica samples. As a function of γ-dose, the growth of a composite structure in the 4–6 eV spectral region of the absorption spectrum is observed. This structure can be resolved into two main contributions centered at 5.8 and 4.8 eV, respectively. The first component is usually attributed to an optical transition of the E′ centers. The second one is able to excite an emission band centered at 1.9 eV. The analysis of the growth kinetics, in the γ-dose range 20–1000 Mrad, of both emission at 1.9 eV and absorption at 4.8 eV shows that these two bands change in a similar way, reaching constant amplitudes, after an i…
Solvatochromism Unravels the Emission Mechanism of Carbon Nanodots
2016
High quantum yield, photoluminescence tunability, and sensitivity to the environment are hallmarks that make carbon nanodots interesting for fundamental research and applications. Yet, the underlying electronic transitions behind their bright photoluminescence are strongly debated. Despite carbon-dot interactions with their environment should provide valuable insight into the emitting transitions, they have hardly been studied. Here, we investigate these interactions in a wide range of solvents to elucidate the nature of the electronic transitions. We find remarkable and systematic dependence of the emission energy and kinetics on the characteristics of the solvent, with strong response of …
Creation of paramagnetic defects by gamma irradiation in amorphous silica
2000
An electron spin resonance (ESR) study of the defects induced by γ-rays in various types of natural and synthetic silica is reported. Three main structures were identified: the E′ center and two doublets with field splitting of 7.4 and 11.8 mT, respectively, both centered around the E′ center signal. Another structure partially overlapping the E′ center line was also detected, consisting in three peaks with a maximum field splitting of 1.36 mT. We have investigated the growth kinetics of these centers on increasing the y-ray accumulated dose. In all investigated materials the growth of E′ centers can be interpreted as caused by γ-activated conversion of one or more precursors. The 1.36 mT s…
Single Semiconductor Nanocrystals under Compressive Stress: Reversible Tuning of the Emission Energy
2017
The photoluminescence of individual CdSe/CdS/ZnS core/shell nanocrystals has been investigated under external forces. After mutual alignment of a correlative atomic force and confocal microscope, individual particles were colocalized and exposed to a series of force cycles by using the tip of the AFM cantilever as a nanoscale piston. Thus, force-dependent changes of photophysical properties could be tracked on a single particle level. Remarkably, individual nanocrystals either shifted to higher or to lower emission energies with no indications of multiple emission lines under applied force. The direction and magnitude of these reversible spectral shifts depend on the orientation of nanocrys…
Temperature dependent photoluminescence of photocatalytically active titania nanopowders
2007
Abstract Temperature photoluminescence (PL) in the sub-ambient range has been developed as a tool for characterizing photocatalytic materials. The use of well-characterized TiO2 nanoparticles with calibrated particle sizes allowed to face the temperature dependent PL results with the photocatalytic activity and several important physico-chemical parameters. In the relaxation of the photoexcited electron/hole pairs, the transfer towards surface sites is in competition with radiative and non-radiative recombinations. Temperature dependent PL appears thus to be a very sensitive technique to study the efficiency of the electron or the hole trapping at the surface of titania nanoparticles where …
Luminescence of γ-radiation-induced defects in α-quartz
2004
Optical transitions associated with γ-radiation-induced defects in crystalline α-quartz were investigated by photoluminescence excited by both pulsed synchrotron radiation and steady-state light. After a 10 MGy γ-dose we observed two emissions at 4.9 eV (ultraviolet band) and 2.7 eV (blue band) excitable in the range of the induced absorption band at 7.6 eV. These two luminescence bands show a different temperature dependence: the ultraviolet band becomes bright below 80 K; the blue band increases below 180 K, but drops down below 80 K. Both emissions decay in a timescale of a few ns under pulsed excitation, however the blue band could also be observed in slow recombination processes and it…
Conformations of Star-Branched Polyelectrolytes
1996
Scaling theory describing the conformations of weakly charged star-branched polyelectrolytes in dilute and semi-dilute salt-free solutions is developed. The dependence of the star size on the number of branches as well as on the solution concentration is analyzed. It is shown that the star size increases with an increase in the number of branches, f, at small f and tends to a constant value at large f. An increase in the concentration of stars in a solution results in a decrease in the star size R according to the power low : R ∼ c -1/3 in the range of a moderate concentration and R ∼ c -1/4 at larger concentration. For stars with a small number of branches the behavior R ∼ c -1/2 in a cert…
A rotating wheel system for the detection of spontaneously fissioning nuclides from heavy ion reactions
1981
Abstract A rotating wheel system for the detection of spontaneously fissioning reaction products from heavy ion reactions was developed. In this system products recoiling from a rotating target wheel are stopped in a catcher foil stack which can be rotated at various velocities up to 80 rotations per second. All products emitted within a wide emission cone are stopped in the catcher foils and are rotated into shielded positions with stationary fission-track detectors positioned on both sides of each rotating foil. This technique allows a sensitive detection of spontaneously fissioning nuclides over a broad range of half-lives from 1 ms to about 1 d. By post-bombardment exposure of the catch…
Oxygen diffusivity in tumor tissue (DS-Carcinosarcoma) under temperature conditions within the range of 20?40�C
1977
The O2 diffusion constants D and K of tumor tissue (DS-Carcinosarcoma in the rat kidney) were determined at temperatures of 20, 30, 37, and 40 degrees C. The following mean values were obtained for the conditions of 37 degrees C: D = 1.75-10(-5) cm2/s and K = 1.9-10(-5) mlO2/cm-min-atm. Within the range of 20-40 degrees C, temperature variations in tumor tissue cause changes in the O2 diffusion coefficient D of 2.0-2.5%/C and in the Krogh O2 diffusion constant K of 0.5-1.5%/C. The measured O2 diffusion constants for tumor tissue correspond to values of normal tissue with similar water content. This indicates that the insufficient O2 supply in DS-Carcinosarcoma is due not to unfavorable O2 d…