425 research outputs found
Intense Source of Slow Positrons
We describe a novel design for an intense source of slow positrons based on
pair production with a beam of electrons from a 10 MeV accelerator hitting a
thin target at a low incidence angle. The positrons are collected with a set of
coils adapted to the large production angle. The collection system is designed
to inject the positrons in a Greaves-Surko trap [1]. Such a source could be the
basis for a series of experiments in fundamental and applied research and would
also be a prototype source for industrial applications which concern the field
of defect characterization in the nanometer scale.Comment: submitted to N.I.M.
Electronic Structures and Bonding of Oxygen on Plutonium Layers
Oxygen adsorption on delta-Pu (100) and (111) surfaces have been studied at
both non-spin-polarized and spin-polarized levels using the generalized
gradient approximation of density functional theory (GGA-DFT)with Perdew and
Wang functionals. The center position of the (100) surface is found to be the
most favorable site with chemisorption energies of 7.386 eV and 7.080 eV at the
two levels of theory. The distances of the oxygen adatom from the Pu surface
are found to be 0.92A and 1.02A, respectively. For the (111) surface
non-spin-polarized calculations, the center position is also the preferred site
with a chemisorption energy of 7.070 eV and the distance of the adatom being
1.31A, but for spin-polarized calculations the bridge and the center sites are
found to be basically degenerate, the difference in chemisorption energies
being only 0.021 eV. In general, due to the adsorption of oxygen, plutonium 5f
orbitals are pushed further below the Fermi energy, compared to the bare
plutonium layers. The work function, in general, increases due to oxygen
adsorption on plutonium surfaces.Comment: Spin-polarization is considered, and the paper is revised accordingl
Search for Superscreening effect in Superconductor
4 pages, 3 figures, Expérience au GANIL avec SPIRAL/EXOGAMThe decay of O() and Ne() implanted in niobium in its superconducting and metallic phase was measured using purified radioactive beams produced by the SPIRAL/GANIL facility. Half-lives and branching ratios measured in the two phases are consistent within one-sigma error bar. This measurement casts strong doubts on the predicted strong electron screening in superconductor, the so-called superscreening. The measured difference in screening potential energy is 110(90) eV for Ne and 400(320) eV for O. Precise determinations of the half-lives were obtained for O: 26.476(9)~s and Ne: 17.254(5)~s
Modeling the actinides with disordered local moments
A first-principles disordered local moment (DLM) picture within the
local-spin-density and coherent potential approximations (LSDA+CPA) of the
actinides is presented. The parameter free theory gives an accurate description
of bond lengths and bulk modulus. The case of -Pu is studied in
particular and the calculated density of states is compared to data from
photo-electron spectroscopy. The relation between the DLM description, the
dynamical mean field approach and spin-polarized magnetically ordered modeling
is discussed.Comment: 6 pages, 4 figure
A density functional study of molecular oxygen adsorption and reaction barrier on Pu (100) surface
Oxygen molecule adsorptions on a Pu (100) surface have been studied in
detail, using the generalized gradient approximation to density functional
theory. Dissociative adsorption with a layer by layer alternate spin
arrangement of the plutonium layer is found to be energetically more favorable
compared to molecular adsorption. Hor2 approach on a bridge site without spin
polarization was found to the highest chemisorbed site with energy of 8.787 eV
among all the cases studied. The second highest chemisorption energy of 8.236
eV, is the spin-polarized Hor2 or Ver approach at center site. Inclusion of
spin polarization affects the chemisorption processes significantly,
non-spin-polarized chemisorption energies being typically higher than the
spin-polarized energies. We also find that the 5f electrons to be more
localized in spin-polarized cases compared to the non-spin-polarized
counterparts. The ionic part of O-Pu bonding plays a significant role, while
the Pu 5f-O 2p hybridization was found to be rather week. Also, adsorptions of
oxygen push the top of 5f band deeper away from the Fermi level, indicating
further bonding by the 5f orbitals might be less probable. Except for the
interstitial sites, the work functions increase due to adsorptions of oxygen
Ground State Theory of delta-Pu
Correlation effects are important for making predictions in the delta phase
of Pu. Using a realistic treatment of the intra-atomic Coulomb correlations we
address the long-standing problem of computing ground state properties. The
equilibrium volume is obtained in good agreement with experiment when taking
into account Hubbard U of the order 4 eV. For this U, the calculation predicts
a 5f5 atomic-like configuration with L=5, S=5/2, and J=5/2 and shows a nearly
complete compensation between spin and orbital magnetic moments.Comment: 4 pages, 1 postscript figure, 1 jpg figure (viewable via Netscape,
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The Magnitude and Mechanism of Charge Enhancement of CH∙∙O H-bonds
Quantum calculations find that neutral methylamines and thioethers form complexes, with N-methylacetamide (NMA) as proton acceptor, with binding energies of 2–5 kcal/mol. This interaction is magnified by a factor of 4–9, bringing the binding energy up to as much as 20 kcal/mol, when a CH3+ group is added to the proton donor. Complexes prefer trifurcated arrangements, wherein three separate methyl groups donate a proton to the O acceptor. Binding energies lessen when the systems are immersed in solvents of increasing polarity, but the ionic complexes retain their favored status even in water. The binding energy is reduced when the methyl groups are replaced by longer alkyl chains. The proton acceptor prefers to associate with those CH groups that are as close as possible to the S/N center of the formal positive charge. A single linear CH··O hydrogen bond (H-bond) is less favorable than is trifurcation with three separate methyl groups. A trifurcated arrangement with three H atoms of the same methyl group is even less favorable. Various means of analysis, including NBO, SAPT, NMR, and electron density shifts, all identify the +CH··O interaction as a true H-bond
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