63 research outputs found
Effect of quantum confinement on exciton-phonon interactions
We investigate the homogeneous linewidth of localized type-I excitons in
type-II GaAs/AlAs superlattices. These localizing centers represent the
intermediate case between quasi-two-dimensional (Q2D) and
quasi-zero-dimensional localizations. The temperature dependence of the
homogeneous linewidth is obtained with high precision from
micro-photoluminescence spectra. We confirm the reduced interaction of the
excitons with their environment with decreasing dimensionality except for the
coupling to LO-phonons. The low-temperature limit for the linewidth of these
localized excitons is five times smaller than that of Q2D excitons. The
coefficient of exciton-acoustic-phonon interaction is 5 ~ 6 times smaller than
that of Q2D excitons. An enhancement of the average exciton-LO-phonon
interaction by localization is found in our sample. But this interaction is
very sensitive to the detailed structure of the localizing centers.Comment: 6 pages, 4 figure
Advances in Global and Local Helioseismology: an Introductory Review
Helioseismology studies the structure and dynamics of the Sun's interior by
observing oscillations on the surface. These studies provide information about
the physical processes that control the evolution and magnetic activity of the
Sun. In recent years, helioseismology has made substantial progress towards the
understanding of the physics of solar oscillations and the physical processes
inside the Sun, thanks to observational, theoretical and modeling efforts. In
addition to the global seismology of the Sun based on measurements of global
oscillation modes, a new field of local helioseismology, which studies
oscillation travel times and local frequency shifts, has been developed. It is
capable of providing 3D images of the subsurface structures and flows. The
basic principles, recent advances and perspectives of global and local
helioseismology are reviewed in this article.Comment: 86 pages, 46 figures; "Pulsation of the Sun and Stars", Lecture Notes
in Physics, Vol. 832, Rozelot, Jean-Pierre; Neiner, Coralie (Eds.), 201
Perspectives in Global Helioseismology, and the Road Ahead
We review the impact of global helioseismology on key questions concerning
the internal structure and dynamics of the Sun, and consider the exciting
challenges the field faces as it enters a fourth decade of science
exploitation. We do so with an eye on the past, looking at the perspectives
global helioseismology offered in its earlier phases, in particular the
mid-to-late 1970s and the 1980s. We look at how modern, higher-quality, longer
datasets coupled with new developments in analysis, have altered, refined, and
changed some of those perspectives, and opened others that were not previously
available for study. We finish by discussing outstanding challenges and
questions for the field.Comment: Invited review; to appear in Solar Physics (24 pages, 6 figures
Recommended from our members
Waste measurements at a plutonium facility
Solid plutonium contaminated wastes are often highly heterogeneous, span a wide range of chemical compositions and matrix types, and are packaged in a variety of container sizes. NDA analysis of this waste depends on operator knowledge of these parameters so that proper segregation, instrument selection, quality assurance, and uncertainty estimation can take place. This report describes current waste measurement practices and uncertainty estimates at a US plutonium scrap recovery facility and presents a program for determining reproducibility and bias in NDA measurements. Following this, an operator's perspective on desirable NDA upgrades is offered
Recommended from our members
Prototype fast neutron counter for the assay of impure plutonium
A fast coincident neutron counter using liquid scintillators and gamma-ray/neutron pulse-shape discrimination has been constructed for the analysis of plutonium samples with unknown self-multiplication and (..cap alpha..,n) production. The counter was used to measure plutonium-bearing materials that cover a range of masses and (..cap alpha..,n) reaction rates of importance to the safeguards community. Measured values of the /sup 240/Pu effective mass differed, on average, from their declared values by 0.4% for plutonium oxides and by -2.2% for metal and MgO-loaded samples. Poorer results were obtained for materials with large (..cap alpha..,n) reaction rates and low self-multiplication such as plutonium ash and plutonium fluoride
Recommended from our members
Non-destructive measurement of solid plutonium waste at Los Alamos National Laboratory
Los Alamos National Laboratory (LANL) is a national defense facility involved in the recovery and processing of plutonium. Wastes and residues are routinely generated here from many stages of plutonium metal fabrication and from pyrochemical and aqueous processing of plutonium scrap. Materials which require measurement include plutonium oxide from burned residues, Pu-bearing salts from production/reduction and metal purification processes, impure plutonium metal, metal reduction slags, ash, undissolved oxide heels, ceramics, and auxiliary implements such as HEPA filters, plastics, and cleaning rags. Nondestructive assays (NDA) of transuranic (TRU) waste from these materials are often troublesome and may pose formidable challenges to the measurement specialist. This document discusses these waste measurement issues at LANL. 25 figs., 2 tabs
- …