Imaging: The Neutron and X-ray Tomography (NeXT) System

Summary

The NIST NeXT detector is the first imaging system to combine the power of both neutron imaging and x-ray
3-D tomography into a powerful, single-pass probe for materials structure and composition analysis. This novel technology has been applied to research in lithium-ion batteries, automotive hydrogen fuel cells, concrete, geochemistry, rock mechanics, and soil science, among others. The continued development of analysis tools for the method will push the speed and accuracy at which samples can be segmented facilitating its use in more materials science and engineering research.

Description

Figure 1: Die-cast toy car with a zinc alloy body, steel axles, and plastic wheels and interior components. This example shows the nearly opposite views seen by neutrons and X-rays.

Neutrons and X-rays are complementary, nondestructive, penetrating probes of matter. X-rays interact with the electron cloud of atoms and therefore see increasing attenuation with atomic number while neutrons interact with the nuclei resulting in a very scattered pattern with atomic number. The differences in material interaction can be seen using a die-cast toy car in Figure 1.

For samples that are evolving with time or undergoing stochastic processes, it is not possible to image with neutrons and X-rays at different facilities as the sample will change between scans. To solve this issue, the BT-2 Neutron Imaging Facility offers simultaneous Neutron and X-ray Tomography, the NIST-NeXT system. NeXT orients a microfocus X-ray tube perpendicular to the neutron beam which allows both modalities to image the sample at the same time. When captured simultaneously, it is possible to directly fuse the two datasets to leverage the full complementarity of the two probes. Figure 2 shows the position of the NeXT system at the 6 m sample position in BT-2 and provides a detailed view of the system and its components. The paper describing the system can be found here Review of Scientific Instruments 88, 113702 (2017).

System specifications:

• X-ray generator: 90 keV peak energy microfocus X-ray tube, tungsten anode, 10 µm minimum spot size, 80 W maximum power
• X-ray detector: P43 gadolinium oxysulfide scintillator screen mounted in mirror box, various CMOS cameras available, final resolution set by photo lens
• Typical operation the X-ray final resolution is matched to the neutron detector, 3-5 projection images are taken at each projection angle during a tomography scan
• Neutron and X-ray detectors can be independently operated with independent control of resolution, exposure time, and number of images taken to fit needs of experiment
• X-ray system can be operated during reactor outages allowing X-ray only work to be conducted during reactor refueling
• The NeXT system is due to be upgraded for fiscal year 2022

Figure 2: Engineering schematic of the NeXT system. The model people are 183 cm tall for scale.

Tools for neutron/X-ray data fusion:

In support of the NeXT system, the neutron imaging team continuously develops tools for data fusion, the process by which the neutron and X-ray datasets are combined and analyzed together. A registration tool provides volume registration to account for the 90° offset in orientation and small differences in translational position and pixel pitch due to how the reconstructions are performed. For high resolution scans or dynamic scans that require short exposure times that may suffer from higher levels of noise in the reconstructed volumes, a tool allows for optimization of multiple filtering algorithms to denoise the volumes prior to segmentation. Finally, a bivariate histogram segmentation tool allows the user to load two registered volumes and then draw polygon regions of interest on the histogram to quickly label phases in the 3D volume as shown in Figure 3 below. The histogram data was generated from a AAA alkaline battery that is shown in more detail in Figure 4 below. The paper discussing the data fusion tools can be found here https://doi.org/10.1117/12.2569666.

Figure 3: Bivariate histogram where regions of interest have been identified with polygons. Colored polygons correspond to identified materials in the slice image to the right.

Figure 4: AAA alkaline battery. Three scans along the height of the battery stitched together to obtain high spatial resolution and capture full length of battery. (a) neutron cross-section across diameter of battery, (b) corresponding X-ray cross-section, (c) neutron cross-section along length of battery, (d) corresponding X-ray slice, (e) segmented slice produced from (c) and (d), (f) visualization of segmented components of the AAA battery produced with the bivariate histogram segmentation tool. As captured in the visualization: stainless steel can (magenta), manganese oxide cathode (red), polymer separator (yellow), Zinc anode (blue), electrolyte (green), anode current collector spike (cyan), gasket (gray). https://doi.org/10.1117/12.2569666.

NIST-NeXT has been applied to research in lithium-ion batteries, automotive hydrogen fuel cells, concrete, geochemistry, rock mechanics, and soil science, among others. The continued development of analysis tools for the method will push the speed and accuracy at which samples can be segmented facilitating its use in more materials science and engineering research.