Time-of-flight Diffractometer Research Articles

The performance of General Purpose Powder Diffractometer at CSNS

The first time of flight diffractometer at the China Spallation Neutron Source (CSNS), General Purpose Powder Diffractometer (GPPD), has entered the user program in 2018. GPPD is a flexible and versatile instrument to study the crystallographic and magnetic structures of polycrystalline materials. After more than 3 years of debugging and operation, the high performance of the diffractometer has been achieved for a good instrumental resolution 0.15% and a wide d-space detection range 0.05–50.48 Å together with the desirable signal-to-noise ratio characteristics, enabling a large number of user experiments and achievements. The typical scientific applications of GPPD demonstrate its outstanding potential for unraveling the precise structure of crystals.

ENGIN-X: a third-generation neutron strain scanner

ENGIN-X, a new time-of-flight (TOF) neutron diffractometer optimized to measure elastic strains at precise locations in bulky specimens recently commissioned at the ISIS Facility in the Rutherford Laboratory, UK, is described. Fast counting times, together with a flexible and accurate definition of the instrumental gauge volume are the main requirements of neutron strain scanning and have been addressed on ENGIN-X through the design of a novel TOF diffractometer with a tuneable resolution and interchangeable radial collimators. Further, the routine operation of the instrument has been optimized by creating a virtual instrument,i.e.a three-dimensional computer representation of the diffractometer and samples, which assists in the planning and execution of experiments. On comparing ENGIN-X with its predecessor ENGIN, a 25× gain in performance is found, which has allowed the determination of stresses up to 60 mm deep in steel specimens. For comparison with constant-wavelength diffractometers, special attention has been paid to the absolute number of counts recorded during the experiments. A simple expression is presented for the estimation of counting times in TOF neutron strain scanning experiments.

Quantitative Texture Analysis with the HIPPO TOF Diffractometer

One of the design goals of the neutron time-of-flight (TOF) diffractometer HIPPO (HIgh Pressure - Preferred Orientation) at LANSCE (Los Alamos Neutron Science Center) was efficient quantitative texture analysis. In this paper, the effects of the HIPPO detector geometry and layout on texture analysis, particularly the shape and dimensions of the detector panels, are investigated. An aluminum sample with a strong and asymmetric texture was used to determine the methodological limitations of various methods of quantitative texture analysis. Several algorithms for extracting the orientation distribution function (ODF) from the TOF-spectra are compared: discrete orientations at arbitrary positions, harmonic method in Rietveld codes (MAUD and GSAS) and discrete methods in MAUD. All methods provide a similar representation of the main texture component, but discrete methods have a fundamental advantage over harmonic methods in characterizing regions of the ODF with low orientation densities. For HIPPO data of the present sample, harmonic expansions beyond lmax= 12 introduce subsidiary maxima and minima, which are consistently identified as artifacts. The results of our analysis establishes HIPPO as an efficient instrument to quantitatively determine preferred orientations in relatively short measuring times, if the texture features are not exceedingly sharp (full-width at half-maximum (FWHM) in the ODF > 20-30°).

Quantitative texture analysis with the HIPPO neutron TOF diffractometer

One of the design goals of the neutron time-of-flight (TOF) diffractometer HIPPO (High Pressure–Preferred Orientation) at LANSCE (Los Alamos Neutron Science Center) was efficient quantitative texture analysis. In this paper, the effects of the HIPPO detector geometry and layout on texture analysis, particularly the shape and dimensions of the detector panels, are investigated in detail. An equal-channel angular-pressed (ECAP) aluminium sample with a strong texture was used to determine the methodological limitations of various methods of quantitative texture analysis. Several algorithms for extracting the orientation distribution function (ODF) from the TOF spectra are compared: discrete orientations at arbitrary positions, harmonic methods in Rietveld codes (MAUD and GSAS) and discrete methods in MAUD. Because of the detector geometry, the sharpest texture peaks that can be represented are 12–15° in width, resulting in an optimal texture resolution of 25–30°. Due to the limited resolution and incomplete pole-figure coverage, harmonic expansions beyond L = 12 (where L is the maximum degree of the harmonic expansion) introduce subsidiary oscillations, which are consistently identified as artifacts. Only discrete methods provide a quantitative representation of the texture. Harmonic methods are adequate for a qualitative description of the main texture component. The results of the analysis establish HIPPO as an efficient instrument to determine preferred orientations in relatively short measuring times.

Texture analysis with the new HIPPO TOF diffractometer

The new neutron time-of-flight (TOF) diffractometer HIPPO (High-Pressure-Preferred Orientation) at LANSCE (Los Alamos Neutron Science Center) is described and results for quantitative texture analysis of a standard sample are discussed. HIPPO overcomes the problem of weak neutron scattering intensities by taking advantage of the improved source at LANSCE, a short flight path (9m) and a novel three-dimensional arrangement of detector banks with 1360 3He tubes, on five conical rings with scattering angles ranging from 2θ=10° to 150°. Flux at the sample is on the order of 107neutronscm−2s−1. A large sample chamber (75cm diameter well) can accommodate ancillary equipment such as an automatic sample changer/goniometer used in this study. This instrument was used to measure the texture of a round-robin limestone standard and extract orientation distribution data from TOF diffraction spectra with different methods (Rietveld harmonic method, Rietveld direct method, automatic fitting of individual peak intensities), and the results compare favorably. Also, there is good agreement with results obtained on the same sample measured at other facilities, but with greatly reduced measuring time for HIPPO.

A high resolution TOF diffractometer and spectrometer

We propose a combined high-resolution time-of-flight (TOF) diffractometer and spectrometer for the new Munich reactor FRM-II. The instrument uses a long flight path and a fast first chopper for the primary spectrometer. The setup for the secondary spectrometer consists of a back scattering detector for the diffractometer and analyser crystals on an arc around the sample position for the spectrometer. The relative resolution of the diffractometer will be Δ d/ d≈2×10 −4 and the energy resolution of the spectrometer will be 2 μeV with a silicon analyser system in near back scattering geometry. The dynamic range can be shifted with the chopper system by several meV, i.e. it exceeds a crystal back scattering spectrometer by a factor 50.

A new neutron diffractometer with multiple detectors combining the advantages of time-of-flight and double-axis diffractometers

In this paper, the principle of a new neutron diffractometer is presented. In this diffractometer are combined the advantages of a high-resolution time-of-flight (TOF) diffractometer and some essential characteristics of a classical double-axis spectrometer (DAS), specific disadvantages of both instruments being eliminated. Its name, MARTIN (multiple-angle high-resolution time-of-flight instrument for neutrons), is derived from the particular instrumental set-up. With the MARTIN diffractometer, a neutron pulse from a narrow wavelength band (0.2–0.5 A) impinges on the sample. The diffracted radiation is registered and time-analyzed by detectors located at several scattering angles. The angular arrangement of the detectors is such that each detector samples a different portion of the diffractogram, with the portions detected by adjacent detectors partially overlapping. These partial diffractograms can be conjoined to yield the complete diffractogram of interest. This new machine has all the advantages of a TOF diffractometer: no contamination of the primary beam by higher orders, higher counting rates because of the possible utilization of the entire Debye–Scherrer ring, higher flexibility because no monochromators are needed and less strict requirements of mechanical precision as a result of the fixed detector arrangement. In addition, the use of a narrow wavelength band reduces the problem of absorption and extinction corrections in TOF diffractometry to one solvable by the well known data treatment of two-axis diffractometry. A prototype of the MARTIN diffractometer was designed for powder diffraction and installed as an extension of the TOF diffractometer at the Munich Research Reactor (FRM) in Garching, where its characteristics were tested on a powder sample of Al2O3.

A method for observing the ferroelectric domain structure by using a time-of-flight neutron diffractometer

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Equal-path time-of-flight neutron diffraction

Measurements of the neutron diffraction patterns of liquids and non-crystalline solids with crystal spectrometer facilities and time-of-flight diffraction techniques are discussed. It is shown that measurements with an equal-path time-of flight diffractometer generally provide the most accurate representation of a total diffraction pattern. The results of neutron diffraction measurements of vitreous silica with a crystal spectrometer and an equal-path time-of-flight diffractometer are contrasted.