193 research outputs found
Studies of particle beam overlap and electron deposition in thin foils
This report summarizes a somewhat diverse set of theoretical studies carried out in 1978 which were aimed at increasing our understanding of the physics of multiple-beam overlap and enhanced deposition in thin foils. The studies reported here involve electron and ion beam overlap in single and multiple cylindrical disks of channels, and single and multiple electron-beam deposition in thin foils. Some of the important consequences of these studies which affect ongoing research are the scaling formula derived for overlap current density gain in cylindrical geometry, an understanding of the importance of electron drift motion in thin-foil-enhanced deposition, and the necessity of providing non-axial return current paths and magnetic isolation of disks in multiple-disk configurations
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Thermal response of ceramic components during electron beam brazing
Ceramics are being used increasingly in applications where high temperatures are encountered such as automobile and gas turbine engines. However, the use of ceramics is limited by a lack of methods capable of producing strong, high temperature joints. This is because most ceramic-ceramic joining techniques, such as brazing, require that the entire assembly be exposed to high temperatures in order to assure that the braze material melts. Alternatively, localized heating using high energy electron beams may be used to selectively heat the braze material. In this work, high energy electron beam brazing of a ceramic part is modeled numerically. The part considered consists of a ceramic cylinder and disk between which is sandwiched an annular washer of braze material. An electron beam impinges on the disk, melting the braze metal. The resulting coupled electron and thermal transport equations are solved using Monte Carlo and finite element techniques. Results indicate that increased electron beam current decreases time to melt as well as required cooling time. Vacuum furnace brazing was also simulated and predicted results indicate increased processing times relative to electron beam brazing
Study of odd-mass N=82 isotones with realistic effective interactions
The microscopic quasiparticle-phonon model, MQPM, is used to study the energy
spectra of the odd , N=82 isotones. The results are compared with
experimental data, with the extreme quasiparticle-phonon limit and with the
results of an unrestricted shell model (SM)
calculation. The interaction used in these calculations is a realistic two-body
G-matrix interaction derived from modern meson-exchange potential models for
the nucleon-nucleon interaction. For the shell model all the two-body matrix
elements are renormalized by the -box method whereas for the MQPM the
effective interaction is defined by the G-matrix.Comment: Elsevier latex style espart, 26 pages, submitted to Nuclear Physics
Neutrino capture by r-process waiting-point nuclei
We use the Quasiparticle Random Phase Approximation to include the effects of
low-lying Gamow-Teller and first forbidden strength in neutrino capture by very
neutron-rich nuclei with N = 50, 82, or 126. For electron neutrinos in what is
currently considered the most likely r-process site the capture cross sections
are two or more times previous estimates. We briefly discuss the reliability of
our calculations and their implications for nucleosynthesis.Comment: 9 pages, 4 figure
Ground and excited states Gamow-Teller strength distributions of iron isotopes and associated capture rates for core-collapse simulations
This paper reports on the microscopic calculation of ground and excited
states Gamow-Teller (GT) strength distributions, both in the electron capture
and electron decay direction, for Fe. The associated electron and
positron capture rates for these isotopes of iron are also calculated in
stellar matter. These calculations were recently introduced and this paper is a
follow-up which discusses in detail the GT strength distributions and stellar
capture rates of key iron isotopes. The calculations are performed within the
framework of the proton-neutron quasiparticle random phase approximation
(pn-QRPA) theory. The pn-QRPA theory allows a microscopic
\textit{state-by-state} calculation of GT strength functions and stellar
capture rates which greatly increases the reliability of the results. For the
first time experimental deformation of nuclei are taken into account. In the
core of massive stars isotopes of iron, Fe, are considered to be
key players in decreasing the electron-to-baryon ratio () mainly via
electron capture on these nuclide. The structure of the presupernova star is
altered both by the changes in and the entropy of the core material.
Results are encouraging and are compared against measurements (where possible)
and other calculations. The calculated electron capture rates are in overall
good agreement with the shell model results. During the presupernova evolution
of massive stars, from oxygen shell burning stages till around end of
convective core silicon burning, the calculated electron capture rates on
Fe are around three times bigger than the corresponding shell model
rates. The calculated positron capture rates, however, are suppressed by two to
five orders of magnitude.Comment: 18 pages, 12 figures, 10 table
Beta-decay in odd-A and even-even proton-rich Kr isotopes
Beta-decay properties of proton-rich odd-A and even-even Krypton isotopes are
studied in the framework of a deformed selfconsistent Hartree-Fock calculation
with density-dependent Skyrme forces, including pairing correlations between
like nucleons in BCS approximation. Residual spin-isospin interactions are
consistently included in the particle-hole and particle-particle channels and
treated in Quasiparticle Random Phase Approximation. The similarities and
differences in the treatment of even-even and odd-A nuclei are stressed.
Comparison to available experimental information is done for Gamow-Teller
strength distributions, summed strengths, and half-lives. The dependence of
these observables on deformation is particularly emphasized in a search for
signatures of the shape of the parent nucleus.Comment: 29 pages, 16 figure
The merit of high-frequency data in portfolio allocation
This paper addresses the open debate about the usefulness of high-frequency (HF) data in large-scale portfolio allocation. Daily covariances are estimated based on HF data of the S&P 500 universe employing a blocked realized kernel estimator. We propose forecasting covariance matrices using a multi-scale spectral decomposition where volatilities, correlation eigenvalues and eigenvectors evolve on different frequencies. In an extensive out-of-sample forecasting study, we show that the proposed approach yields less risky and more diversified portfolio allocations as prevailing methods employing daily data. These performance gains hold over longer horizons than previous studies have shown
High Precision Measurement of the Superallowed 0^+ to 0^+ Beta Decay of ^{22}Mg
The half-life, 3.8755(12) s, and superallowed branching ratio, 0.5315(12),
for ^{22}Mg beta-decay have been measured with high precision. The latter
depended on gamma-ray intensities being measured with an HPGe detector
calibrated for relative efficiencies to an unprecedented 0.15%. Previous
precise measurements of 0^+ to 0^+ transitions have been restricted to the nine
that populate stable daughter nuclei. No more such cases exist, and any
improvement in a critical CKM unitarity test must depend on precise
measurements of more exotic nuclei. With this branching-ratio measurement, we
show those to be possible for T_z = -1 parents. We obtain a corrected Ft-value
of 3071(9) s, in good agreement with expectations.Comment: 4 pages, 2 figures, revtex
Fine-Grid Calculations for Stellar Electron and Positron Capture Rates on Fe-Isotopes
The acquisition of precise and reliable nuclear data is a prerequisite to
success for stellar evolution and nucleosynthesis studies. Core-collapse
simulators find it challenging to generate an explosion from the collapse of
the core of massive stars. It is believed that a better understanding of the
microphysics of core-collapse can lead to successful results. The weak
interaction processes are able to trigger the collapse and control the
lepton-to-baryon ratio () of the core material. It is suggested that the
temporal variation of within the core of a massive star has a pivotal
role to play in the stellar evolution and a fine-tuning of this parameter at
various stages of presupernova evolution is the key to generate an explosion.
During the presupernova evolution of massive stars, isotopes of iron, mainly
Fe, are considered to be key players in controlling ratio
via electron capture on these nuclide. Recently an improved microscopic
calculation of weak interaction mediated rates for iron isotopes was introduced
using the proton-neutron quasiparticle random phase approximation (pn-QRPA)
theory. The pn-QRPA theory allows a microscopic \textit{state-by-state}
calculation of stellar capture rates which greatly increases the reliability of
calculated rates. The results were suggestive of some fine-tuning of the
ratio during various phases of stellar evolution. Here we present for
the first time the fine-grid calculation of the electron and positron capture
rates on Fe. Core-collapse simulators may find this calculation
suitable for interpolation purposes and for necessary incorporation in the
stellar evolution codes.Comment: 21 pages, 6 ps figures and 2 table
Developmental expression of orphan g protein-coupled receptor 50 in the mouse brain
[Image: see text] Mental disorders have a complex etiology resulting from interactions between multiple genetic risk factors and stressful life events. Orphan G protein-coupled receptor 50 (GPR50) has been identified as a genetic risk factor for bipolar disorder and major depression in women, and there is additional genetic and functional evidence linking GPR50 to neurite outgrowth, lipid metabolism, and adaptive thermogenesis and torpor. However, in the absence of a ligand, a specific function has not been identified. Adult GPR50 expression has previously been reported in brain regions controlling the HPA axis, but its developmental expression is unknown. In this study, we performed extensive expression analysis of GPR50 and three protein interactors using rt-PCR and immunohistochemistry in the developing and adult mouse brain. Gpr50 is expressed at embryonic day 13 (E13), peaks at E18, and is predominantly expressed by neurons. Additionally we identified novel regions of Gpr50 expression, including brain stem nuclei involved in neurotransmitter signaling: the locus coeruleus, substantia nigra, and raphe nuclei, as well as nuclei involved in metabolic homeostasis. Gpr50 colocalizes with yeast-two-hybrid interactors Nogo-A, Abca2, and Cdh8 in the hypothalamus, amygdala, cortex, and selected brain stem nuclei at E18 and in the adult. With this study, we identify a link between GPR50 and neurotransmitter signaling and strengthen a likely role in stress response and energy homeostasis
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