Transcript Slide 1

MACS –a New High Intensity Cold Neutron Spectrometer at NIST
C. Broholm1,2, P. C. Brand2, C. Brocker2, J. W. Lynn2, R. Barkhouser1, J. D. Orndorff1, T. D. Pike1,2, Y. Qiu1, T. Reeves1, G. Scharfstein1, S. A. Smee3
1Department
of Physics and Astronomy, Johns Hopkins University,
Baltimore, MD 21218
2NIST
Center for Neutron Research,
Gaithersburg, MD 20899
3Spallation
Neutron Source, ORNL,
Oak Ridge, TN 37830
Introduction: Inelastic neutron scattering is a unique probe of nano-scale dynamic
phenomena in solids. Unfortunately, current instrumentation often limits applicability to
cases where large crystalline samples can be produced. The Multi Axis Crystal
Spectrometer (MACS) now under development at NIST, aims to broaden the range of
materials that can be analyzed with this powerful technique. Two orders of magnitude
improvement in efficiency is achieved by focusing cold neutrons with a Bragg lens and
using a multiplexing detection system.
Science: Dynamic
short range order is important in many topical condensed matter
systems. While MACS will be a general purpose spectrometer for energies less than 20
meV, it will be particularly well suited for probing dynamic nano-scale structure. In a
matter of hours the instrument will deliver a map of the wave vector dependence of
inelastic neutron scattering from which real space short range order can be extracted by
Fourier inversion.
Frustrated Magnets: The crystalline lattice can define a frustrating pattern of interactions
between magnetic atoms that cannot be satisfied by any spin configuration. MACS will
be particularly well suited for probing short range order resulting from frustration. Fig. 1
shows elastic diffuse scattering from geometrically frustrated, SrCr9pGa12-9pO19. While
samples close to full concentration on the magnetic sub-lattice are of greatest interest,
the experiment was on magnetically diluted materials (p=60%) where sizeable single
crystals are available. MACS will help to probe the materials of greatest scientific interest
even when only milligram samples are available.
Shutter
Cryo filters
Figure 7. Isometric view of the MACS cold neutron
spectrometer. Along the beam line are seen shutter,
cooled Be, PG, and Al2O3 filters, 60’ and 40’ radial
collimators, variable aperture, monochromator,
super-mirror guide, cryostat and detector system.
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Conceptual Design: MACS is designed to maximize the efficiency for energy resolved surveys of Q-space in the
sub-thermal energy range. Two innovations should lead to performance enhancements of up to two orders of
magnitude over current instrumentation.
Figure 1. Crystal structure of SrCr9pGa12-9pO19. Where
geometrical frustration suppress long range magnetic order.
Figure 2. Coherent diffuse elastic scattering
from a geometrically frustrated magnet.
Colossal Magneto-resistance: The ferromagnetic transition in La1-xCaxMnO3 is
accompanied by a dramatic reduction in resistivity, which could be useful for magnetic
sensing. The reason is strong spin-lattice-charge coupling that leads to polarons whose
structure is revealed by anomalous diffuse scattering. MACS will be ideally suited for
probing the structure of such coupled degrees of freedom.
Figure 3. Diffraction from lattice polarons in
La0.7Ca0.3MnO3 in the resistive phase.
Figure 4. Temperature dependence of lattice and spin
polaron scattering indicating that polaron localization is
responsible for the T-dependent resistivity.
Quantum Impurities: In quantum systems with a macroscopic singlet ground state
impurities can have counterintuitive and potentially useful properties. For example a hole
in a spin-1 chain generates a complex spin polaron, the structure of which can be
probed by inelastic magnetic neutron scattering.
Incident beam line principles: The NG0 port of the NIST center for neutron research where MACS will be situated,
affords an unusually large 10-2 Steradian view of the NIST cold source. MACS will take advantage of the large solid
angle through a doubly focusing and monochromating Bragg lens. While this implies a greater angular divergence
at the sample, the longer wave lengths employed by MACS ensures that Q-resolution remains comparable to a
thermal triple axis machine with 60’ collimation. The combination of a bright cold source and a large divergence
angle yields a flux on the sample position that exceeds 108 n/cm2/s at 0.2 meV energy resolution.
Figure 8. Flux on sample versus incident energy
calculated using Monte Carlo Simulation (MCSTAS).
Numbers for IN14 are from the ILL web page.
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Figure 6. Wave vector dependence of inelastic
scattering that reveals the structure of a complex
spin polaron surrounding holes in Y2-xCaxBaNiO5.
Details on Incident beam line: The reflecting surface of the monochromator is
curved for vertical focusing. Focusing in the horizontal plane is accomplished by
varying the projection of the monochromator surface normal so it bisects the angle
formed by lines from the given point on the monochromator to the sample and source
respectively. The distribution of scattering angles varies as the monochromator is
rotated around a vertical axis. The distribution is minimized, as is the range of incident
energies, when the monochromator is tangent to the Rowland circle shown on Fig. 11.
The mean incident energy is varied by translating the monochromator along the beam
tube and rotating the cylindrical drum and sample to intercept the reflected beam. A
super-mirror guide with adjustable sides increases the angular size of the sample as
viewed from the monochromator to match that of the cold source for a 20% intensity
gain.
The Double focusing monochromator: The MACS monochromator has been built and
it has passed optical testing. The reflecting surface consists of 357 Pyrolytic Graphite
(002) platelets with a total area of 1428 cm2. These are mounted on 21 aluminum
fingers that can rotate under stepping motor control about parallel vertical axes for
horizontal focusing. The fingers can be bent so as to form the arc of a circle with
variable radius for vertical focusing.
Figure 9. Energy resolution versus incident energy for
three collimation configurations.
On a conventional triple axis spectrometer, energy and wave vector resolution are coupled. Not on MACS, where
energy resolution is controlled by radial collimators that define the active source area and wave vector resolution is
controlled by the variable aperture before the monochromator.
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Figure 5. Chain structure of a hole doped
quantum spin liquid.
Variable aperture
Figure 11. top view of MACS. The detector system is shown in a single setting while the monochromator is indicated in
three different positions. The dashed line is the Rowland Circle that goes through the neutron source, the monochromator
and the sample. Horizontal monochromatic focusing requires that the monochromator is tangent to this circle.
SrCr9pGa12-9pO19
Cr
Collimators
Q
Figure 10. Sketch of the region of wave vector space that
can be mapped using MACS at Ef=3.7 meV and 1 meV
energy transfer. The ellipses show the areas probed by the
21 detection channels in one setting of sample and detectors.
Detection System Principles: The conventional triple axis
spectrometer has a single detection channel that can rotate
around the sample to map inelastic scattering. MACS will have
21 channels in simultaneous operation to increase efficiency for
surveys by more than an order of magnitude. With a different
data acquisition protocol, MACS will be complementary to the
advanced pulsed neutron instrumentation now being developed
for the Spallation Neutron Source. The ability to probe excitation
spectra over a wide range of energies at the SNS and to zoom in
on specific energy ranges of interest on MACS will be a powerful
combination that will help scientists and engineers to understand
and design the materials that will fuel the technologies of the
twenty first century.
Figure 12. Picture of the MACS doubly
focusing monochromator mounted with
mirrors for optical testing.
Figure 13. Top view of the MACS 21 channel detection system. The emphasis
in the design is high reliability and efficiency and ultra low background.
Shielding thickness averages 33 cm of moderating and absorbing material.
Details on MACS detection System: The 21 channels of the MACS detector are
separated by 8 degrees. Each channel has a vertically focusing double bounce analyzer
with 2o by 8o maximum. acceptance. There is also a “two-axis” detector in each channel
providing high intensity diffraction and energy integrated data. Each channel will also
have three cooled filter options (PG, Be, and BeO) to suppress higher order
contamination and elastically scattered neutrons from the sample during inelastic
experiments. There are also three collimation options to allow variation of detection
system energy and wave vector resolution.