Development of Nondestructive X-ray CT Imaging Techniques to Identify and Locate Presolar
Grains in Meteorite Sample Jbilet with Application to Ryugu Samples
D. Z. Shulaker1, M. Ferrucci2, B. M. Rogers2, M. R. Savina1, B. H. Isselhardt1, Q. Shollenberger1
1
Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, CA (shulaker1@llnl.gov)
2
Materials Engineering Division, Lawrence Livermore National Laboratory, CA.
Introduction: The development of non-destructive analytical methods of precious material from sample-return missions isnecessary to preserve as much material as possible, including presolar grains. Most commonly, an entire sample is dissolved toisolate presolar grains from the sample matrix. However, dissolving an entire meteorite or material from a sample-return
mission also dissolves essentially everything but presolar SiC, graphite, and nanodiamonds. This results in the loss of precious
material and presolar silicates. In attempt to save sample material, previous studies target presolar graphite and silicon carbide(SiC) grains using nanoscale secondary ion mass spectrometry (NanoSIMS), scanning electron microscopy (SEM), and/or
focused ion beam (FIB) to image regions of meteorite fragments or thin sections [1–6]. When compared to traditional
dissolution of meteorite fragments, these methods preserve grains and contextural information. However, these methods are
exceptionally time consuming, cover very little sample surface area, and often result in locating few presolar grains.
X-ray computed tomography (CT) is a non-destructive imaging technique that generates a three-dimensional
representation of a sample in terms of an X-ray attenuation image. X-ray CT is used extensively in many fields of research andin industry, though its application to locating and identifying graphite and SiC grains in meteorites and other precious samples
from sample-return missions is limited. X-ray CT is promising for this task because of its potential to locate and identify 10s to
100s of presolar grains while preserving the sample. The X-ray CT images can also be used to efficiently target specificregions in the sample for more precise characterization by NanoSIMS or for subsequent analyses, such as by resonanceionization mass spectrometry (RIMS). In this work, we present ongoing work in the development of an X-ray CT based
paradigm for imaging fragments of meteorite sample Jbilet Winselwan (hereafter referred to as Jbilet) to locate and identify
presolar grain types. Ultimately, X-ray CT will be used to image Ryugu fragments to locate and identify presolar grains andplace presolar grains in mineralogical context. The X-ray CT image of the full Ryugu fragment will be used to intelligently
target smaller regions of interest for subsequent isotopic analyses by NanoSIMS and RIMS.
Methods: To develop X-ray CT imaging techniques, a ~150 μm diameter fragment of Jbilet was mounted on carbon tape on
an SEM stub. This was imaged on a Zeiss Xradia 510 Versa using a conventional acquisition and subsequent tomographic
reconstruction by cone-beam filtered back projection. The reconstructed image consists of a three-dimensional array ofvolumetric picture elements (voxels), each assigned a gray value corresponding to the attenuation of X-rays by the material(s)contained within the voxel. The ability to distinguish features in the sample is dependent on the ability to detect changes in thereconstructed gray values. X-ray attenuation is a function of material composition (namely, atomic number, and physicaldensity) and photon energy. Conventional cabinet X-ray systems typically use vacuum tube X-ray sources, which generate
polychromatic ‘bremsstrahlung’ X-rays, i.e., characterized by a broad range of photon energies. However, since conventional
reconstruction algorithms assume monochromatic X-rays, changes in the reconstructed gray values are not linearly related tochanges in atomic number and/or physical density. The absence of sensitivity to photon energy in conventional X-ray CT
imaging means that it is difficult to distinguish features with similar material properties, such as presolar grains.
One of our long-term objectives is to perform multi-spectrum acquisitions for direct material characterization byleveraging so-called ‘dual-energy’ acquisitions: two datasets acquired under distinct X-ray spectra, and a home-grownalgorithm for decomposing the acquired data into voxel-wise effective atomic number and electron density [7]. As a precursor
to our long-term objective, in this study we performed a conventional ‘single-spectrum’ acquisition at 40 kV tube acceleration
voltage with a voxel size of 240 nm (Fig. 1). The reconstructed image was subsequently analyzed with VGStudio (VolumeGraphics GmbH) and its foam/powder analysis functionality to identify individual particles and to extract their morphologicaland gray value statistics.
Results: Despite the previous caveat on relating gray value changes to material differences, features with substantiallydifferent attenuation properties could be distinguished using the mean gray value of the voxels contained within the particle
regions. Furthermore, we can compare the extracted particle morphology, such as sphericity, diameter, and volume, toexpected morphology for each grain as an additional criterion for particle classification. Based on these criteria, we make thefollowing observations: (1) pores are typically less than 15 μm in diameter; (2) larger spherical voids approximately 20 μm indiameter are surrounded by purportedly concentric shells of high attenuation particles with a finer particle size compared to thematrix, and (3) particles with the highest mean attenuation values are highly spherical and generally less than 5 μm in diameter.
Using the data collected in this study in concert with previous studies of Jbilet [8,9], we hypothesize the identification ofvarious discrete particles. For instance, that the larger, spherical features could be chondrules or chondrules with alteration
phases and smaller high mean grey value spherical particles could be metal blebs.
Future work: We have identified several ways to overcome identified limitations with X-ray CT, such as reducing imaging
artifacts, improving image quality, and spatial and attenuation resolution. For instance, without energy sensitivity in theacquisition and subsequent reconstruction, it is difficult to separate grains with similar morphology but distinct material
properties, like SiC and graphite. Beam-hardening artifacts (such as false high gray value regions due in large part to theabsence of energy sensitivity observed at the exterior edge of the meteorite as shown in Fig. 2) can interfere with the correct
identification of features. By imaging a thinner fragment, we can use lower energy photons, which will result in reduced beamhardening artifacts and increased sensitivity to small changes in material properties within the sample.
We will also investigate the inclusion of external material references, such as synthetic SiC and graphite, into themeasurement volume to serve as gray value benchmarks for identifying the same materials inside the sample.
Acknowledgments: We thank G. Brennecka for the Jbilet Winselwan meteorite sample and A. Mohan for reconstruction
assistance. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National
Laboratory under Contract DE-DEAC52-07NA27344 and was supported by the Laboratory Directed Research andDevelopment Program at LLNL under project 20-ERD-030 and 23-LW-015. LLNL-ABS-854399.
References: [1] Nguyen A. N. et al. (2007) ApJ, 656, 1223–1249. [2] Keller L. et al. (2012) MicroSC, 18, 1704–1705. [3]Davidson J. et al. (2014) GCA, 139, 248–266. [4] Sanghani M. N. et al. (2021) ApJS, 352, 1–26. [5] Barosch J. et al. (2022)
ApJL, 935, 1–12. [6] Nguyen A. N. et al. (2023) Sci. Adv., 9, eadh1003. [7] Azevedo S. G. et al. (2016) IEEE Trans Nucl Sci,63, 341–350. [8] Friend P. et al. (2018) Meteorit. Planet. Sci., 53, 2470–2491. [9] King A. J. et al. Meteorit. Planet. Sci., 54,521–543.

Figure 1. X-ray CT image of a ~150 μm diameter

Figure 2. Cross section slice through a reconstructed

Jbilet fragment.

X-ray CT image of Jbilet fragment. Examples of observed
characteristics: square is a pore, triangle is metal bleb, and thecircle is a chondrule. Arrows point to the “beam hardening”
artifact that appears around the meteorite edge.