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Design and operation of the wide angular-range chopper spectrometer ARCS at the Spallation Neutron Source
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Image of FIG. 1.
FIG. 1.

Overview rendering of ARCS with components labeled. Instrument components are described in the text and in Table I. The instrument's primary shutter is not illustrated and the first radial shielding baffle is not illustrated for clarity of the figure.

Image of FIG. 2.
FIG. 2.

(a) Line drawing and pictures of T0 chopper rotors. The 175 kg rotor is supported and rotated by a vertical-axis magnetic bearing system. (b) Photograph of the T0 chopper system. The height of the full housing is 1.1 m.

Image of FIG. 3.
FIG. 3.

View from above of the neutron optics for ARCS just before the sample chamber. Neutrons travel from the guide system at the top, pass through one of two Fermi choppers mounted on a translation table, a variable aperture, beam monitor, and removable and fixed neutron guides. Guide 3 is a removable guide section. Guide 4 shares its vacuum with the sample chamber.

Image of FIG. 4.
FIG. 4.

View from the low angles toward the sample and high angle detectors. The neutron guide enters the sample chamber at the left, transporting neutrons to the sample position. A large semi-circular gate valve (shown in the open position) can be raised to isolate the sample volume for rapid changes of sample environment. The vertical gap in detector coverage shows the location of one of the scattered beam radial baffle positions which has subsequently been installed.

Image of FIG. 5.
FIG. 5.

ARCS beam monitor data compared to absolute intensity Monte Carlo calculations with and without the ARCS neutron guide. Symbols are shown for every 50 data points. The data from the monitor below ∼1 ms are not reliable due to the high prompt pulse radiation.

Image of FIG. 6.
FIG. 6.

ARCS monitor and vanadium scattering data (symbols) compared to simulation (dashed lines). All data correspond to measurements with a 6.4 mm diameter 5 cm tall vanadium rod with an incident energy of 100 meV. The time-of-flight spectra are measured/calculated at the (a) first beam monitor, (b) the middle detector row, and (c) the second or downstream beam monitor. The inset in (c) is the same data shown in panel (c) on a logarithmic intensity scale.

Image of FIG. 7.
FIG. 7.

Inelastic scattering spectra for 2,5-diiodothiophene (C4H2I2S) versus incident energy. Data were acquired at several incident energies, and are offset vertically for presentation. Symbols are shown for every 10 datapoints. Details of chopper configurations are described in the text.

Image of FIG. 8.
FIG. 8.

Measured FWHM of excitations in 2,5-diiodothiophene (C4H2I2S) for different incident energies. Solid lines are calculations of the resolution function as described in the text. The peak near 30meV seen in Figure 7 is not used for comparison due to intrinsic broadening.

Image of FIG. 9.
FIG. 9.

Inelastic scattering spectrum for 2,5-diiodothiophene (C4H2I2S) for E i = 100 meV measured on ARCS. Data (open circles) are compared to a MC instrument simulation (solid line) of an ideal scatterer with the same dimensions and orientation as the sample, as described in the text. A Lorenzian plus second order polynomial background (dashed line) is shown.

Image of FIG. 10.
FIG. 10.

Inelastic scattering from liquid 4He. Data correspond to constant wave vector cuts through the helium recoil line integrated between 23 < Q < 23.5 inverse Angstroms. Inset shows the measured scattering intensity as a function of energy and wave vector transfer. An empty cell measurement was used as a measure of the background and subtracted from these data. Measurements are further described in the text. Solid lines correspond to simple Gaussian fits.

Image of FIG. 11.
FIG. 11.

Inelastic neutron scattering spectra examining the phonon scattering for a single crystal of FeSi. Details of the measurement are described in the text. Panel (b) corresponds to integrating data between 15 and 17 meV and plotting the result in the (HK3) plane for L = 3 ± 0.1. Panel (a) corresponds to a cut through the data in (b) for K = 3 ± 0.1 and L = 3 ± 0.1.

Image of FIG. 12.
FIG. 12.

ARCS radial collimator renderings. (a) Oblique view of radial collimator from above the beam position. Absorbing foils are not being installed in the vicinity of the through beam to avoid any additional scattering by the direct beam. (b) Installed view of radial collimator as viewed from within the detector chamber. Individual is shown for scale. Material around the sample and detector chamber interface is boron carbide based neutron shielding.


Generic image for table
Table I.

ARCS instrument parameters. Detector coverage and location of instrument components are further discussed in the text and shown in Fig. 1.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Design and operation of the wide angular-range chopper spectrometer ARCS at the Spallation Neutron Source