Cold Neutron Spectroscopy on MACS

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Cold Neutron Spectroscopy on MACS
Collin Broholm The Johns Hopkins University and
NIST Center for Neutron Research

• Virtues and limitations of INS
• Enhancing INS at the NCNR
• Description of MACS
• Science on MACS
• Summary

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Neutron Spectroscopy

• A central tool in condensed matter physics
• Unique information about dynamic correlations
• Model independent access to interaction strength
• Access microscopic structure of dynamic systems
• Limited scope on current instruments
• Need cm3 sized crystals
• Need weeks of beam time
• Need neutron scattering expert
• Increased sensitivity will broaden impact
• Comprehensive surveys to test theory
• Parametric studies
• Smaller samples

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Interesting samples come in all sizes
Bound spinons in spin-1 chain
Free spinons in spin-1/2 chain
10 mm
40 mm
Y2BaNiO5
Cu(C4D4N2)(NO3)2
Bound spinons in spin-1/2 ladder
Frustrated Magnetism
0.1 mm
4 mm
ZnCr2O4
Cu2(quinox)2Cl4
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Resolution requirements to probe magnets

• Q and E resolved spectroscopy

• Energy scale J varies more than length scale a

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Comparing TOF to TAS

• Can focus neutrons with Bragg optics
• Freely select range of energy transfer
• Can use reactor CW flux

TAS like

• Large detector solid angle is possible
• E-scan without moving parts
• Can use spallation source peak flux

TOF like
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TOF/TAS complementarity
TOF Data from MAPS/ISIS
Y2BaNiO5
Xu et al science (2000)
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Unique Opportunities for INS at the NCNR
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Characteristics of a TAS at NG0

• Wave vector resolution using full beam
• Energy Resolution
• Flux on sample
• Incident Energy Range 2.5-20 meV

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Maximizing the potential for new science

• Beam delivery system
• Focus full beam onto small sample
• High rejection rate for non-Ei neutrons
• Variable Q and Ei resolution
• Detection system
• Maximize solid angle of detection system
• Offer variable resolution
• Maximize S/N through shielding and geometry
• User interface
• Fast, accurate, and safe setup
• Data Acquisition Planning Tools
• Click for access to all features
• Comprehensive visualization and analysis tools

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Overview of MACS
Shielding
Helium
6.2 m
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MACS beam shutter

• Four position rotating shutter
• Closed 70 mm thick neutron shielding
• 50 mm vertical slit
• Conical full opening
• 100 mm vertical slit

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Cooled Incident Beam filters

• Reject non-Ei neutrons
• Eilt 5 meV 10 cm beryllium
• Eilt15 meV 5 cm PG
• Eilt20 meV 8 cm Sapphire
• Cold filters move in vacuum
• Pneumatic actuation in lt15 s
• Closed cycle helium cooling

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The Monochromator Cask
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Four-position Radial collimator

• Control source size hence DE
• Gd2O3 coated Stainless foils
• Two aligned segments 4 settings
• Pneumatic actuation in lt15 s

Randy Hammond JHU
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Variable incident beam aperture

• Control beam envelope hence DQ
• Independent control of Q-resolution
• Trim beam to match monochromator
• Full range actuation in 5 s
• 10 cm moderatingabsorbing shutters

Scott Spangler JHU
High density Polyethylene
B4C
10 cm
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The MACS monochromator
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Projected Performance (McStas)
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5
F (108 n/cm2/s)
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open
3
60
2
40
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1
0
Y. Qiu and Y. Dong (2004)
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Actual Measured Performance
2 cm
4 cm
Jose A. Rodriguez, NIST
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2020 Channel MACS detection system
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Internal B4C Poly Urethane shielding
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The Double Crystal Analyzer Unit

• Variable energy 2.5 meVltElt15 meV
• Vertically focusing compound lense
• Background suppressing collimator
• Motion controlled by a single motor
• Patent pending design by Tim Pike

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Vertical focusing to reduce background
Double analyzer is compound lens
efficient vertical focusing
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Multi Analyzer Crystal Spectrometer
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Constant energy transfer slice
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Assembling slices to probe Q-E volume
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Data Acquisition

• Virtual spectroscopic arc detector
• Counting incorporates 2Q scan for continuous
  coverage along arc in Q-space
• Range of sample rotations determines area covered
• Virtual TAS
• Specify which set of channels can be used
• Software decides on appropriate active channel
• Operation indistinguishable from conventional TAS
• All detector information nonetheless recorded
• Graphical data acquisition planning tools
• Real time images of data

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Elements of Scientific Program for MACS

• Expand the scope for Inelastic scattering from
  crystals
• 0.5 mm3 samples
• Impurities at the 1 level
• Extreme environments pressure and fields to tune
  correlated systems
• Complete surveys to reveal spin-wave-conduction
  electron interactions
• Probing short range order
• Solid ionic conductors, spin glasses,
  quasi-crystals, ferroelectrics, charge and spin
  polarons, quantum magnets, frustrated magnets.
• Excitations in artificially structured solids
• Spin waves in magnetic super-lattices
• Magnetic fluctuations in nano-structured
  materials
• Weak broken symmetry phases
• Incommensurate charge, lattice, and spin order in
  correlated electron systems

Lee et al. (2002)
J. Rodriquez et al (2004)
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Summary

• MACS makes use of unique aspect of the NBSR
  large solid angle access to intense cold neutron
  source
• World class flux on sample
• 2020 channel low background detector system
• Ability to tailor energy range and resolution
  makes MACS complementary to TOF spectrometers
• First experiments anticipated in 2007

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Contributors to MACS project

• NIST Center for Neutron Research
• G. Baltic, P. C. Brand, C. Brocker, M. English,
  P. D. Gallagher, C. J. Glinka, Z. Huang, P.
  Kopetka, J. G. LaRock, J. W. Lynn, J. Moyer, N.
  Maliszewskyj, D. J. Pierce, J. Rodriguez, M.
  Rowe, J. Rush, and others
• Johns Hopkins University
• R. Barkhouser, C. Broholm, R. Hammond, P. K.
  Hundertmark, R. Lavender, J. Orndorff, T. D.
  Pike, G. Scharfstein, S. A. Smee, and others

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