9,188 research outputs found
Resonant Elastic X-Ray Scattering from 5f Systems
The first REXS experiments on a uranium compound at the U M4 edge (3,728 keV) took place at BNL twenty years ago. An enormous enhancement of the scattering intensity was found. Since that time many other systems have been examined. This paper reviews some of the highlights of resonant scattering from actinide systems, and attempts to extrapolate what might be the future of this field.JRC.E.6-Actinides researc
5d-5f Electric-multipole Transitions in Uranium Dioxide Probed by Non-resonant Inelastic X-ray Scattering
Non-resonant inelastic x ray scattering (NIXS) experiments have been
performed to probe the 5d-5f electronic transitions at the uranium O(4,5)
absorption edges in uranium dioxide. For small values of the scattering vector
q, the spectra are dominated by dipole-allowed transitions encapsulated within
the giant resonance, whereas for higher values of q the multipolar transitions
of rank 3 and 5 give rise to strong and well-defined multiplet structure in the
pre-edge region. The origin of the observed non-dipole multiplet structures is
explained on the basis of many-electron atomic spectral calculations. The
results obtained demonstrate the high potential of NIXS as a bulk-sensitive
technique for the characterization of the electronic properties of actinide
materials.Comment: Submitted to Physical Review Letters on 31 December 200
Interaction between U/UO2 bilayers and hydrogen studied by in-situ X-ray diffraction
This paper reports experiments investigating the reaction of H with
uranium metal-oxide bilayers. The bilayers consist of 100 nm of
epitaxial -U (grown on a Nb buffer deposited on sapphire) with a
UO overlayer of thicknesses of between 20 and 80 nm. The oxides were made
either by depositing via reactive magnetron sputtering, or allowing the uranium
metal to oxidise in air at room temperature. The bilayers were exposed to
hydrogen, with sample temperatures between 80 and 200 C, and monitored via
in-situ x-ray diffraction and complimentary experiments conducted using
Scanning Transmission Electron Microscopy - Electron Energy Loss Spectroscopy
(STEM-EELS). Small partial pressures of H caused rapid consumption of the
U metal and lead to changes in the intensity and position of the diffraction
peaks from both the UO overlayers and the U metal. There is an
orientational dependence in the rate of U consumption. From changes in the
lattice parameter we deduce that hydrogen enters both the oxide and metal
layers, contracting the oxide and expanding the metal. The air-grown oxide
overlayers appear to hinder the H-reaction up to a threshold dose, but
then on heating from 80 to 140 C the consumption is more rapid than for the
as-deposited overlayers. STEM-EELS establishes that the U-hydride layer lies at
the oxide-metal interface, and that the initial formation is at defects or
grain boundaries, and involves the formation of amorphous and/or
nanocrystalline UH. This explains why no diffraction peaks from UH
are observed. {\textcopyright British Crown Owned Copyright 2017/AWE}Comment: Submitted for peer revie
Synchrotron Radiation Techniques and their Application to Actinide Materials
Research on actinide materials, both basic and applied, has been greatly
advanced by the general techniques available from high-intensity photon beams
from x-ray synchrotron sources. The most important single reason is that such
x-ray sources can work with minute (e.g., microgram) samples, and at this
level, the radioactive hazards of actinides are much reduced. We start by
discussing the form and encapsulation procedures used for different techniques,
then discuss the basic theory for interpreting the results. By reviewing a
selection of x-ray diffraction (XRD), resonant elastic x-ray scattering (REXS),
x-ray magnetic circular dichroism (XMCD), resonant and non-resonant inelastic
scattering (RIXS, NIXS), dispersive inelastic x-ray scattering (IXS), and
conventional and resonant photoemission experiments, we demonstrate the
potential of synchrotron radiation techniques in studying lattice and
electronic structure, hybridization effects, multipolar order, and lattice
dynamics in actinide materials.Comment: To be published in Reviews of Modern Physics; 57 pages, 36 figures,
475 reference
Unexpected phase locking of magnetic fluctuations in the multi-k magnet USb
The spin waves in the multi-k antiferromagnet USb soften and become quasielastic well below the antiferromagnetic ordering temperature TN. This occurs without a magnetic or structural transition. It has been suggested that this change is in fact due to dephasing of the different multi-k components: a switch from 3-k to 1-k behavior. In this work, we use inelastic neutron scattering with tridirectional polarization analysis to probe the quasielastic magnetic excitations and reveal that the 3-k structure does not dephase. More surprisingly, the paramagnetic correlations also maintain the same clear phase correlations well above TN (up to at least 1.4TN)
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