Review: Pair distribution functions from neutron total scattering for the study of local structure in disordered materials

This chapter focuses on scintillation detectors for thermal neutron detection at neutron scattering facilities. The chapter includes two parts. The first part is on Anger camera-based thermal neutron detectors, and the second part is dedicated to thermal neutron detectors using Wavelength Shifting Fibers (WLSF). Neutron sensitive Anger camerasAnger cameras offer many properties that are attractive for scattering applications such as single crystal diffractionSingle crystal diffraction. They can be tiled sufficiently in both planar and spherical geometries and offer a good spatial resolutionSpatial resolutions and uniform active surface. This chapter describes the structure and operation of PMT-based Anger camerasAnger cameras in use at Oak Ridge National Laboratory neutron beamlines and discusses a new version that uses SiPMs for improved performance and capabilities. The WLSFWavelength shifting fiber (WLSF) based detectors can provide effective alternative to other detectors, particularly He-3 tube based detectors. The WLSFWavelength shifting fiber (WLSF) can provide large area detectors with high detection efficiencyEfficiency, spatial resolutionSpatial resolutions, timing resolutionTiming resolutions, and at substantially lower costs. This chapter describes physical principles of WLSFWavelength shifting fiber (WLSF) method, detector design concepts, and details of the position encodingPosition encoding methods. The current status, potential limitations, and possible solutions are also described. Finally, an example of the first results achieved with a new WLSFWavelength shifting fiber (WLSF) detector design is presented. 

Scintillator-Based Detectors

We present a magnetometry, muon spin relaxation ($\mu$SR), and neutron scattering study of the insulating face-centered-cubic spin glass Zn$_{0.5}$Mn$_{0.5}$Te. The magnetometry and $\mu$SR results confirm a spin freezing transition around $T_f \approx 23$ K, with the spin fluctuation rate decreasing gradually and somewhat inhomogeneously through the sample volume as the temperature decreases toward $T_f$. Characteristic spin correlation times well above $T_f$ are on the order of 10$^{-10}$ s, in line with expectations for a cluster spin glass. Using magnetic pair distribution function (mPDF) analysis and reverse Monte Carlo (RMC) modeling of the magnetic diffuse neutron scattering data, we show that the spin-glass ground state consists of clusters of spins exhibiting short-range-ordered type-III antiferromagnetic correlations, with a locally ordered moment of 3.1(1) $\mu_{\mathrm{B}}$ between nearest-neighbor spins. The type-III correlations decay exponentially as a function of spin separation distance with a correlation length of approximately 5 \AA. The diffuse magnetic scattering and corresponding mPDF show no significant changes across $T_f$, indicating that the dynamically fluctuating short-range spin correlations in the paramagnetic state retain the same basic type-III configuration; the only change apparent from the neutron scattering data is a gradual reduction of the correlation length and locally ordered moment with increasing temperature. Taken together, these results paint a unique and detailed picture of the local magnetic structure and dynamics in Zn$_{0.5}$Mn$_{0.5}$Te and show that this material is best described as a cluster spin glass. In addition, this work showcases a statistical method for extracting diffuse scattering signals from neutron powder diffraction data.

pdf/cluster-spin-glass-correlations-and-dynamics-in-zn-0-5-mn-0-hy57gow4858g.pdf

Cluster spin glass correlations and dynamics in Zn$_{0.5}$Mn$_{0.5}$Te

The neutron sources at Oak Ridge National Laboratory use a wide suite of sample environment equipment to deliver extreme conditions for a number of experiments. Much of this instrumentation focuses on extremes of temperature, such as cryostats, closed-cycle refrigerators in both low and high temperature configurations, and radiant heating furnaces. When the temperature is controlled across a large range, thermal deflection effects can notably move the sample and affect its alignment in the beam. Here, we combine these sample environments with neutron imaging and machine vision to determine the motion of a representative sample with respect to the neutron beam. We find vertical sample displacement on the order of 1-2 mm and horizontal displacement that varies from near-negligible to 1.2 mm. While these deflections are not relevant for some of the beamlines at the first target station at the spallation neutron source and the high flux isotope reactor, they will become critical for upcoming instrumentation at the second target station, as well as any instruments targeting sub-mm samples, as neutron sources and optics evolve to smaller and more focused beams. We discuss mitigation protocols and potential modifications to the environment to minimize the effect of misalignment due to thermal deflection.

Thermal deflection in neutron scattering sample environments at Oak Ridge National Laboratory.

We characterize the suitability of two additive manufactured materials for use in shielding thermal neutrons for neutron scattering instrumentation. We compare two different boron based additive manufactured masks to a traditionally manufactured boron nitride mask in a real-use case scenario. The first additive manufactured mask is a composite of aluminum and boron carbide. The second additive manufactured mask is a composite of boron carbide and cyanoacrylate. We report the thermal conductivity of the aluminum boron carbide mask in comparison to other relevant materials. The aluminum boron carbide material may find use in shielding applications where thermal conductivity, a low total neutron scattering cross section, and low toxicity are criteria. 

Characterization of aluminum and boron carbide based additive manufactured material for thermal neutron shielding

This article reviews some current trends that can be observed in the development of neutron scattering instrument technologies. While the number of neutron scattering facilities worldwide and the number of beam days they offer are largely stable, their scientific impact is increasing through improving instrumental capabilities, new and more versatile instruments, and more efficient data collection protocols. Neutron beams are becoming smaller but more intense, and instruments are being designed to utilize more ‘useful’ neutrons in unit time. This article picks and discusses a few recent developments in the areas of integrated source and instrument design, use of computational tools, new detectors, and experiment automation.

/pdf/modern-trends-in-neutron-scattering-instrument-technologies-18ol2sef.pdf

Modern Trends in Neutron Scattering Instrument Technologies

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Modern Trends in Neutron Scattering Instrument Technologies