Bibliothek

Notizen: Using a new technique to generate cold electron beams, an electron-beam positron-plasma experiment was performed in a previously unexplored range of energies. An electron beam, formed from a thermalized room-temperature electron plasma, is transmitted through a positron plasma stored in a quadrupole Penning trap geometry. The transit-time instability, which is excited by the beam, was previously studied using a hot-cathode electron gun. The large beam energies produced by the cathode did not permit an investigation of the instability in the interesting range of energies near its onset. Using a new 0.1 eV energy width electron beam, we have reinvestigated the system. The experimental data are compared with the results of a theoretical model, also described in this paper. The theory employs a linearized cold fluid and Vlasov approach to model the plasma and beam dynamics, respectively. The data and predictions are in good agreement over the broad range of energies and beam currents studied. © 2001 American Institute of Physics.

Notizen: Genetic differences (polymorphisms) among members of a population are thought to influence susceptibility to various environmental exposures. In practice, however, this information is rarely incorporated into quantitative risk assessment and risk management. We describe an analytic framework for predicting the risk reduction and value-of-information (VOI) resulting from specific risk management applications of genetic biomarkers, and we apply the framework to the example of occupational chronic beryllium disease (CBD), an immune-mediated pulmonary granulomatous disease. One described Human Leukocyte Antigen gene variant, HLA-DPβ1*0201, contains a substitution of glutamate for lysine at position 69 that appears to have high sensitivity (∼94%) but low specificity (∼70%) with respect to CBD among individuals occupationally exposed to respirable beryllium. The expected postintervention CBD prevalence rates for using the genetic variant (1) as a required job placement screen, (2) as a medical screen for semiannual in place of annual lymphocyte proliferation testing, or (3) as a voluntary job placement screen are 0.08%, 0.8%, and 0.6%, respectively, in a hypothetical cohort with 1% baseline CBD prevalence. VOI analysis is used to examine the reduction in total social cost, calculated as the net value of disease reduction and financial expenditures, expected for proposed CBD intervention programs based on the genetic susceptibility test. For the example cohort, the expected net VOI per beryllium worker for genetically based testing and intervention is $13,000, $1,800, and $5,100, respectively, based on a health valuation of $1.45 million per CBD case avoided. VOI results for alternative CBD valuations are also presented. Despite large parameter uncertainty, probabilistic analysis predicts generally positive utility for each of the three evaluated programs when avoidance of a CBD case is valued at $1 million or higher. Although the utility of a proposed risk management program may be evaluated solely in terms of risk reduction and financial costs, decisions about genetic testing and program implementation must also consider serious social, legal, and ethical factors.

Notizen: We have conducted a detailed study of the thermal stability of Bragg gratings written in hydrogen-loaded and unloaded germanium-doped optical fiber. Interference of either continuous-wave or pulsed ultraviolet light was used to induce the index modulation gratings. Some gratings were kept at room temperature and others were annealed at fixed temperatures for 10–20 h. For temperatures between room temperature and 350 °C, gratings in the hydrogen-loaded fiber showed significantly greater decay than those in the unloaded counterpart. The ultraviolet-induced index modulation in hydrogen-loaded fiber was reduced by 40% after 10 h at 176 °C, whereas it was reduced by only 5% in unloaded fiber under the same conditions. The annealing behavior of gratings written using the pulsed source was identical to that of gratings written with the continuous-wave source, and the thermal stability of gratings in hydrogen-loaded fiber did not depend on the magnitude of the index modulation. We also observed that the annealing of ultraviolet-induced OH absorption in the hydrogen-loaded fiber was not correlated with the grating decay. Our annealing results show that the species responsible for the index change in the hydrogen-loaded fiber are less stable than those in the unloaded fiber. © 1995 American Institute of Physics.

Notizen: Measurements of the temperature dependent transport properties of epitaxial BaTiO3 are reported. Electrical resistivity and thermoelectric power were measured over the temperature range of 77–300 K. Room temperature resistivities of the as-deposited, undoped films range from 105 to 108 Ω cm, while values as low as 55 Ω cm are obtained for the La-doped films. The resistivity shows an activated temperature dependence with the measured activation energies ranging between 0.11 and 0.50 eV. The activation energy depends strongly upon the thin film carrier concentration. Thermoelectric power measurements indicate that the films are n-type. The Seebeck coefficient for La-doped BaTiO3 exhibits metallike behavior, with its magnitude directly proportional to temperature. Temperature dependent resistivity and thermopower measurements indicate that the carrier mobility is activated. A transport model is proposed whereby conduction occurs in the La-doped films via hopping between localized states within a pseudogap formed between a lower Hubbard band and the BaTiO3 conduction band edge. © 1996 American Institute of Physics.

Notizen: We have developed a versatile, pulsed source of cold (ΔE=0.018 eV), low-energy positrons (E(approximate)0–9 eV). Multiple pulses of 105 positrons, each 10 μs in duration, are extracted from a thermalized, room temperature positron plasma stored in a Penning trap. The frequency, duration, and amplitude of the pulses can be varied over a wide range. © 1997 American Institute of Physics.