395 research outputs found
A fixed angle double mirror filter for preparing a pink undulator beam at the Advanced Photon Source
Recent advances in X-ray Photon Correlation Spectroscopy (XPCS) use the full bandwidth of an undulator harmonic in order to maximize the coherent flux for small angle X-ray scattering experiments. X-ray mirrors and filters are typically used to select a given harmonic of the spectrum. At the University of Michigan/Howard University/Lucent Technologies, Bell Labs, Collaborative Access Team (MHATT-CAT) undulator beamline of the Advanced Photon Source, we have designed a fixed-angle Double Mirror Filter which will provide a âpink beamâ (i.e., 2â3% bandwidth) for XPCS experiments. This device uses two small mirrors which vertically reflect a 0.1âmmĂ0.1âmm0.1mmĂ0.1mm white beam in a symmetric geometry. The doubly reflected beam propagates parallel to the incident white beam, but is offset vertically by 35 mm. Using the standard offset of the APS allows one to stop the white beam with a standard APS beam stop. In this report, we will describe our design considerations for this instrument. We also report the results of preliminary tests of the performance. The mirrors preserve the transverse coherence of the source, and filter the undulator spectrum as expected. © 2000 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87429/2/238_1.pd
Effects of pressure on spin fluctuations and the exchange interaction in La2CuO4 as determined by twoâmagnon Raman scattering (abstract)
We have measured the twoâmagnon Raman scattering spectrum of magnetic La2CuO4 at pressures of up to 100 kbar. Analysis of the moments of the twoâmagnon line shape indicates that the renormalization parameters resulting from spin fluctuations are essentially pressure independent in this pressure range. Our results provide the first direct determination of the pressure dependence of the inâplane exchange coupling constant J. The pressure dependence of J is compared with that of the NĂ©el temperature and discussed in the context of recent theories for quasiâtwoâdimensional magnetic systems.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71024/2/JAPIAU-69-8-5392-1.pd
Lithium metal for x-ray refractive optics
Lithium metal is the best material for refractive lenses that must focus x-rays with energies below 15 keV, but to date no lens from Li has been reported. This letter demonstrates focusing of 10 keV x-rays with a one-dimensional sawtooth lens made from Li. The lensâ theoretical gain is 4.5, with manufacturing imperfections likely responsible for the threefold gain that is observed. Despite the Li reactivity the lens is stable over months of operation if kept under vacuum. © 2001 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69644/2/APPLAB-79-25-4085-1.pd
Refractive optics using lithium metal
Thanks to its low x-ray absorption, lithium should be the material of choice for x-ray refractive lenses. This article discusses some of the measurements done to verify lithiumâs relevant properties. Both x-ray transmission and refraction are consistent with expectations. The lens gain suffers from broadening that is related to small-angle scattering. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69944/2/RSINAK-73-3-1492-1.pd
X-ray Near Field Speckle: Implementation and Critical Analysis
We have implemented the newly-introduced, coherence-based technique of x-ray
near-field speckle (XNFS) at 8-ID-I at the Advanced Photon Source. In the near
field regime of high-brilliance synchrotron x-rays scattered from a sample of
interest, it turns out, that, when the scattered radiation and the main beam
both impinge upon an x-ray area detector, the measured intensity shows
low-contrast speckles, resulting from interference between the incident and
scattered beams. We built a micrometer-resolution XNFS detector with a high
numerical aperture microscope objective and demonstrate its capability for
studying static structures and dynamics at longer length scales than
traditional far field x-ray scattering techniques. Specifically, we
characterized the structure and dynamics of dilute silica and polystyrene
colloidal samples. Our study reveals certain limitations of the XNFS technique,
which we discuss.Comment: 53 pages, 16 figure
Experience with a fluorescenceâbased beam position monitor at the APS
We have recently redesigned the first crystal mount of our cryogenically cooled monochromator to reduce its sensitivity to pressure fluctuations in the cryogenic lines feeding the Si (111) crystal. With the use of a fluorescenceâbased Xâray beam position monitor (BPM) placed 19 m away from the monochromator, much operational experience has been gained on the sensitivity of the beam position and intensity to small changes in the cooling system. In this presentation, we will describe our Xâray BPM design and performance and will provide examples of changes that have made the beam position more stable on our beamline. One such change for example has been the topâup operation of the Advanced Photon Source (APS), which has reduced the thermal drifts associated with the ring current decay. © 2004 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87659/2/679_1.pd
Design and performance of a stable first crystal mount for a cryogenically cooled Si monochromator at the Advanced Photon Source
We present a new design for mounting a cryogenically cooled Si crystal which gives greatly improved beam stability. The design has been successfully implemented at the University of Michigan, Howard University, Bell Laboratories-Lucent Technologies Collaborative Access Team (MHATT-CAT) 7ID Beamline of the Advanced Photon Source. Before the installation of the new crystal mount, our Si (lll) cryogenically cooled monochromator was sensitive to the pressure fluctuations of the liquid nitrogen coolant, such that the angle of incidence on the first crystal varied linearly with the applied pressure in the cooling lines, causing beam motion of about 250âÎŒm,â60âm250ÎŒm,60m from the source. The key element of the design is a symmetrically positioned cooling manifold which balances the forces caused by pressure fluctuations. With this new mount, the typical beam stability is now about 10 ÎŒm, comparable to the source stability. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69891/2/RSINAK-73-3-1511-1.pd
Evidence for Two Superconducting Gaps in
We have measured the Raman spectra of polycrystalline MgB from 25 {\cm}
to 1200 {\cm}. When the temperature was decreased below the superconducting
transition temperature , we observed a superconductivity-induced
redistribution in the electronic Raman continuum. Two pair-breaking peaks
appear in the spectra, suggesting the presence of two superconducting gaps.
Furthermore, we have analyzed the measured spectra using a quasi
two-dimensional model in which two s-wave superconducting gaps open on two
sheets of Fermi surface. For the gap values we have obtained (2.7 meV) and (6.2 meV). Our results suggest
that a conventional phonon-mediated pairing mechanism occurs in the planar
boron bands and is responsible for the superconductivity of MgB.Comment: 3 figure
Observation of the Smectic C -- Smectic I Critical Point
We report the first observation of the smectic C--smectic I (C--I) critical
point by Xray diffraction studies on a binary system. This is in confirmity
with the theoretical idea of Nelson and Halperin that coupling to the molecular
tilt should induce hexatic order even in the C phase and as such both C and I
(a tilted hexatic phase) should have the same symmetry. The results provide
evidence in support of the recent theory of Defontaines and Prost proposing a
new universality class for critical points in layered systems.Comment: 9 pages Latex and 5 postscript figures available from
[email protected] on request, Phys.Rev.Lett. (in press
Effects of Pore Walls and Randomness on Phase Transitions in Porous Media
We study spin models within the mean field approximation to elucidate the
topology of the phase diagrams of systems modeling the liquid-vapor transition
and the separation of He--He mixtures in periodic porous media. These
topologies are found to be identical to those of the corresponding random field
and random anisotropy spin systems with a bimodal distribution of the
randomness. Our results suggest that the presence of walls (periodic or
otherwise) are a key factor determining the nature of the phase diagram in
porous media.Comment: REVTeX, 11 eps figures, to appear in Phys. Rev.
- âŠ