Capabilities of the ARCS Instrument

ARCS Overview

The wide angular-range chopper spectrometer ARCS at the Spallation Neutron Source (SNS) is optimized to provide a high neutron flux at the sample position with a large solid angle of detector coverage. The instrument incorporates modern neutron instrumentation, such as an elliptically focused neutron guide, high speed magnetic bearing choppers, and a massive array of 3He linear position sensitive detectors. Novel features of the spectrometer include the use of a large gate valve between the sample and detector vacuum chambers and the placement of the detectors within the vacuum, both of which provide a window-free final flight path to minimize background scattering while allowing rapid changing of the sample and sample environment equipment. ARCS views the SNS decoupled ambient temperature water moderator, using neutrons with incident energy typically in the range from 15 to 1500 meV. This range, coupled with the large detector coverage, allows a wide variety of studies of excitations in condensed matter, such as lattice dynamics and magnetism, in both powder and single-crystal samples.

Typical sample environment equipment used at ARCS includes closed-cycle refrigerators (bottom and top loading, 5K – 300K), cryostats (liquid He 1.8K – 300K) and furnaces (resistive heater 300K – 900K, radiative heater (ILL style) 300K – 1500K). Other types of equipment that may be accommodated, including a large capacity dilution refrigerator (40 mK base temperature). Reference: D. L. Abernathy, M. B. Stone, M. J. Loguillo, M. S. Lucas, O. Delaire, X. Tang, J. Y. Y. Lin, and B. Fultz, “Design and operation of the wide angular-range chopper spectrometer ARCS at the Spallation Neutron Source,” Review of Scientific Instruments 83, 15114 (2012)

Measurements of single crystal phonons in large volumes of reciprocal space

Phonon excitations in single crystals provide a detailed look at the atomic interactions in a material. By measuring large volumes of reciprocal space, one may find unusual features that might be overlooked by more focused triple-axis measurements. Examples include the effects of electron-phonon interactions and anharmonicity. Due to its large detector coverage ARCS can acquire such data quickly, allowing for more detailed studies as a function of temperature or other parameters. The sample environments used provide a vertical axis of rotatation of the sample and are optimized for the very low background required for single crystal studies.

Examples of Relevant Research
O. Delaire, K. Marty, M. B. Stone, P. R. C. Kent, M. S. Lucas, D. L. Abernathy, D. Mandrus, and B. C. Sales, “Phonon softening and metallization of a narrow-gap semiconductor by thermal disorder.”Proceedings of the National Academy of Science (PNAS), 108, 4725 (2011).
  • Sample size: ~30 g (single crystal FeSi)
  • Typical measurement time: ~15 minutes per temperature, angle
  • Total experiment time: 4 days

Single-crystal phonon dispersions of FeSi, measured by time-of-flight inelastic neutron scattering (ARCS), illustrating the change in phonon frequencies between 10 K (A–C) and 300 K (D–F).

M. E. Manley, D. L. Abernathy, N. I. Agladze, and A. J. Sievers, “Symmetry-breaking dynamical pattern and localization observed in the high-temperature vibrational spectrum of NaI.”Scientific Reports 1, 4 (2011).
  • Sample size: ~35 g (single crystal NaI)
  • Typical measurement time: ~20 minutes per temperature, angle
  • Total experiment time: 5 days

Parameteric studies of phonon density-of-states

Measurements of the phonon density-of-states in powder samples can be used to study how atomic vibrations contribute to the thermodynamics and bulk properties of materials. By doing parametric studies with variations of temperature and composition, the nature of the interactions in the sample can be related to lattice expansion, thermal conductivity and stability of alloys, for example. ARCS allows such studies to be performed quickly due to its large flux and wide incident energy and angular ranges. These measurements combined with computational studies and bulk characterization can lead to better understanding of the thermodynamics of alloys, negative thermal expansion materials, and thermoelectric properties.

Examples of Relevant Research
C.W. Li, X. Tang, J.A. Muñoz, J.B. Keith, S.J. Tracy, D.L. Abernathy and B. Fultz, “The structural relationship between negative thermal expansion and anharmonicity of cubic ScF3,” Physical Review Letters 107, 195504 (2011).
  • Sample size: ~15 g (powder ScF3)
  • Typical measurement time: ~45 minutes per temperature, energy
  • Total experiment time: 3 days

A. Möchel, I. Sergueev, H.-C. Wille, J. Voigt, M. Prager, M. B. Stone, B. C. Sales, Z. Guguchia, A. Shengelaya, V. Keppens, and R. P. Hermann, “Lattice dynamics and anomalous softening in the YbFe4Sb12 skutterudite.”,Physical Review B 84 194306 (2011).

  • Sample size: ~11 g (powder YbFe4Sb12
  • Typical measurement time: ~2 hours per temperature, energy
  • Total experiment time: 4 days
  1. Neutron-weighted ScF3 phonon DOS.
  2. Shifts of phonon peak centers relative to 7 K data

Studies of quantum liquids and solids

ARCS is well-suited to the study of the momentum distributions in quantum liquids and solids. The high flux of the SNS source combined with the large detector coverage allow for detailed measurements of 4He in confined geometries or under pressure, for example. Bose-Einstein condensation in such systems or the excitations in 3He-4He mixtures may be studied. A special high-resolution Fermi chopper is available for incident energies near 700meV with a momentum transfer range up to Q ~ 30 Å-1.

Examples of Relevant Research
S.O. Diallo, R.T. Azuah, D.L. Abernathy, R. Rota, J. Boronat, and H.R. Glyde, “Bose-Einstein Condensation in liquid 4He near the liquid-solid transition line,” Physical Review B 85, 140505(R) (2012)
  • Sample size: ~100cc
  • Typical measurement time: ~8 hours per temperature, pressure
  • Total experiment time: 7 days

    Observed scattering intensity S(Q,ω) as a function of energy transfer E = ħω and momentum transfer ħQ from liquid 4He at p = 24 bars and T = 40 mK. The signal from the empty Al container has been subtracted. The black dashed line is the calculated 4He recoil line Er = ħ2Q2/2m, shown as a guide to the eye.

Magnetic excitations

The high neutron flux at thermal and epithermal wavelengths makes ARCS appropriate for single crystal and powder measurements of magnetic materials with strong magnetic interactions. Single crystal measurements of magnetic systems often make use of the ability to measure large volumes of reciprocal space. The higher energies available at ARCS makes the instrument complimentary to lower energy measurements performed using thermal triple axis spectrometers. The large detector coverage of the instrument allows one to systematically measure the temperature dependence in powder samples as a function of composition.

Examples of Relevant Research
M. B. Stone, M. D. Lumsden, S. E. Nagler, D. J. Singh, J. He, B. C. Sales, and D. Mandrus, “Quasi-one-dimensional magnons in an intermetallic marcasite,” Physical Review Letters 108, 167202 (2012).
  • Sample size: 3.2 g single crystal CrSb2 (~14 mmol of S=1)
  • Typical measurement time: ~45 min per angle
  • Total experiment time: 4 days

Observed scattering intensity S(Q,ω) as a function of energy transfer E = ħω and momentum transfer from S=1 spins in CrSb2. Solid line is the determined spin-wave dispersion based upon a Heisenberg model.

O. J. Lipscombe, G. F. Chen, C. Fang, T. G. Perring, D. L. Abernathy, A. D. Christianson, T. Egami, N. Wang, J. Hu, and P. Dai, “Spin waves in the (p,0) magnetically ordered iron chalcogenide Fe1.05Te”Physical Review Letters 106, 57004 (2011).

  • Sample size: 6 g single crystal Fe1.05Te (~32 mmol of Fe)
  • Typical measurement time: ~16 hours per temperature and energy.
  • Total experiment time: 5 days

Observed scattering intensity S(Q,ω) as a function of momentum transfer for four different ranges of energy transfer for the magnetic scattering in Fe1.05Te.