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

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

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.

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.