Process for the manufacture of carbon or graphite fibers

Carbon or graphite fibers are manufactured by heating polyacrylonitrile fiber materials in various solutions and gases. They are characterized in that the materials are heated to temperatures from 150 to 300 C in a solution containing one or more acids from the group of carbonic acids, sulfonic acids, and/or phenols. The original molecular orientation of the fibers is preserved by the cyclization that occurs before interlacing, which gives very strong and stiff carbon or graphite fibers without additional high temperature stretching treatments

Performance analysis of grazing incidence imaging systems

An exact expression relating the coordinates of a point on the incident ray, a point of reflection from an arbitrary surface, and a point on the reflected ray is derived. The exact relation is then specialized for the case of grazing incidence, and first order and third order systematic analyses are carried out for a single reflective surface and then for a combination of two surfaces. The third order treatment yields a complete set of primary aberrations for single element and two element systems. The importance of a judicious choice for a coordinate system in showing field curvature to clearly be the predominant aberration for a two element system is discussed. The validity of the theory is verified through comparisons with the exact ray trace results for the case of the telescope

Analysis of electric-field-induced spin splitting in wide modulation-doped quantum wells

We analyze the proper inclusion of electric-field-induced spin splittings in the framework of the envelope function approximation. We argue that the Rashba effect should be included in the form of a macroscopic potential as diagonal terms in a multiband approach rather than the commonly used Rashba term dependent on k and electric field. It is pointed out that the expectation value of the electric field in a subband is sometimes not unique because the expectation values can even have opposite signs for the spin-split subband components. Symmetric quantum wells with Dresselhaus terms and the influence of the interfaces on the spin splitting are also discussed. We apply a well established multiband approach to wide modulation-doped InGaSb quantum wells with strong built-in electric fields in the interface regions. We demonstrate an efficient mechanism for switching on and off the Rashba splitting with an electric field being an order of magnitude smaller than the local built-in field that determines the Rashba splitting. The implications of our findings for spintronic devices, in particular the Datta-Das spin transistor and proposed modifications of it, are discussed.Comment: Modified version, now published. 10 pages, 3 figures, 2 table

Does affirmative action work?

After four decades, we are still debating how much impact affirmative action can and should have on opportunities and outcomes at work.Discrimination in employment ; Sex discrimination against women ; Affirmative action programs

A program for calculating optimum dimensions of alpha radioisotope capsules exposed to varying stress and temperature

Method and computer program for calculating creep and optimizing dimensions of capsules filled with alpha-emitting radioisotopes and exposed to varying stress and temperatur

Electron spin orientation under in-plane optical excitation in GaAs quantum wells

We study the optical orientation of electron spins in GaAs/AlGaAs quantum wells for excitation in the growth direction and for in-plane excitation. Time- and polarization-resolved photoluminescence excitation measurements show, for resonant excitation of the heavy-hole conduction band transition, a negligible degree of electron spin polarization for in-plane excitation and nearly 100% for excitation in the growth direction. For resonant excitation of the light-hole conduction band transition, the excited electron spin polarization has the same (opposite) direction for in-plane excitation (in the growth direction) as for excitation into the continuum. The experimental results are well explained by an accurate multiband theory of excitonic absorption taking fully into account electron-hole Coulomb correlations and heavy-hole light-hole coupling.Comment: 10 pages, 4 figures, final versio

Sum rules for an atomic hyperfine structure in a magnetic field

Sum rules for the energy levels of a hyperfine multiplet in a constant uniform magnetic field is presented. It is found that for any values of the electron angular moment and the nuclear spin there are certain linear combinations of energy levels which do not depend on the magnetic field and can be used to determine the unperturbated hfs separation in the presence of perturbing magnetic field. It is also demonstrated that there are other linear combinations which are linear with the external magnetic field and hence can be used to determine bound values of the electron and nuclear magnetic moments. The accuracy of the approximation within which the result is valid is also discussed

Determination of the Krypton-Impurity Evolution in the XENON1T Detector and Optimization of Separation Columns for Krypton in Xenon Assays

The XENON experiment aims for the direct detection of dark matter with liquid xenon as target material for so-called WIMPs (Weakly Interacting Massive Particles), which represent one of the main candidates for particle dark matter. As the expected signal rate is less than a couple of events per year, it is mandatory to understand and reduce possible background contributions. The radioactive krypton isotope 85Kr is among the most serious internal background contributions, which can mimic a WIMP signal. However, krypton traces are successfully removed from liquid xenon by cryogenic distillation. The external monitoring of the remaining krypton traces using a rare gas mass spectrometer (RGMS) sets the framework of this work. First, the krypton concentration evolution of the XENON1T detector was determined. A change in the signal shape of RGMS required a new analysis procedure, including a thorough and much improved error treatment. Within this analysis, the lowest krypton in xenon concentration ever measured in a running detector is set. Second, different adsorbents were examined and characterized to optimize the separation of krypton from xenon, necessary for the present analysis of concentrations in the ppq regime. Due to the posed requirements, such as resolution, peak width, and tailing, the activated carbon ShinCarbon turned out to be the best candidate. The ShinCarbon column will be an integral part of a planned system called AUTO-RGMS and will lead to a significantly improved separation efficiency, a better reproducibility, and a lower detection limit