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EBS and in-beam $\gamma$ -ray investigation of FCVA-prepared Cr $_2$ O $_3$ targets for low-energy $^{16}$ O+ $^{16}$ O fusion experiments

This study characterizes FCVA-deposited Cr $_2$ O $_3$ targets for low-energy $^{16}$ O+ $^{16}$ O fusion experiments using EBS and in-beam $\gamma$ -ray spectroscopy, confirming their stoichiometry and uniformity while identifying carbon-induced background contributions to optimize future cross-section measurements.

Original authors: C. Wen, X. Chen, L. Wang, Z. An, F. Bai, Y. Chen, X. Fang, Y. X. Fan, B. S. Gao, Z. Y. Guo, J. F. Han, H. T. Hu, W. P. Lin, B. Liao, S. Lin, G. Liu, X. Q. Liu, P. P. Ren, J. Su, J. H. Tan, X. D. Tang

Published 2026-09-09

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: C. Wen, X. Chen, L. Wang, Z. An, F. Bai, Y. Chen, X. Fang, Y. X. Fan, B. S. Gao, Z. Y. Guo, J. F. Han, H. T. Hu, W. P. Lin, B. Liao, S. Lin, G. Liu, X. Q. Liu, P. P. Ren, J. Su, J. H. Tan, X. D. Tang, P. Wang, S. Wang, D. H. Xie, N. T. Zhang

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep inside massive stars, long after they have exhausted their primary fuel, a critical transformation occurs. The core, now rich in oxygen, reaches temperatures so extreme that oxygen nuclei begin to smash into one another. This process, known as oxygen burning, is a pivotal moment in a star's life, determining how it evolves and what heavier elements it creates before its final collapse. To understand this cosmic alchemy, scientists must know exactly how often these oxygen nuclei fuse together at the specific, low energies found in stellar interiors. However, measuring this is incredibly difficult. In a laboratory, the likelihood of two oxygen nuclei fusing drops off so sharply as the energy decreases that the signal becomes almost invisible, buried beneath a sea of noise from other, more common reactions. To see the faint glow of the true fusion event, researchers need targets made of pure oxygen compounds, prepared with such precision that no stray atoms interfere with the delicate measurement.

A team of researchers has taken a significant step toward solving this problem by creating and testing a new type of target material for these experiments. They focused on chromium oxide, a solid compound rich in oxygen, which they prepared using a specialized technique called filtered cathodic vacuum arc deposition. This method involves creating a stream of charged chromium atoms and guiding them through a magnetic filter that acts like a sieve, removing large, unwanted droplets and ensuring only a smooth, dense film settles onto a metal base. The goal was to create a layer of chromium oxide that was perfectly uniform and free of contaminants, specifically carbon, which is the primary enemy in these low-energy fusion studies. Even a tiny amount of carbon can trigger its own fusion reactions with the oxygen beam, producing gamma rays that mimic the signal scientists are trying to find, leading to false conclusions about how often fusion occurs.

To verify their work, the team subjected their new targets to rigorous testing. First, they fired a beam of protons at the chromium oxide films and measured how the protons bounced back. This technique, known as elastic backscattering spectrometry, allowed them to count the atoms within the film with high precision. They found that the films were remarkably consistent, with a thickness and chemical balance that matched their expectations. The oxygen content was high and uniform across the surface, with a density of roughly 3.24 to 3.25 times ten to the seventeenth atoms per square centimeter. The ratio of chromium to oxygen was also very close to the ideal chemical formula, suggesting the film was well-formed. However, the analysis also revealed a small but measurable amount of carbon, about 1.25 to 1.29 percent, embedded within the oxide layer itself. More surprisingly, they discovered a much larger concentration of carbon on the surface of the metal base underneath the film, a residue that the cleaning process had failed to remove completely.

The researchers then moved to the next stage of validation, placing these targets in a beam of oxygen ions to see what happened when they were bombarded with the same particles used in actual fusion experiments. They listened for the specific flashes of gamma radiation that would indicate a successful fusion event. The results were a mix of success and caution. The detectors clearly picked up the unique signatures of the oxygen-oxygen fusion reaction, specifically the gamma rays emitted when the resulting nuclei shed their excess energy. These signals appeared at energies of 1249, 1266, 1779, and 2235 kiloelectron volts, confirming that the fusion process was occurring and could be measured. However, the spectra were also dominated by strong signals from aluminum and magnesium, which are produced when the oxygen beam hits carbon atoms. Because the carbon contamination on the metal base was so significant, these background signals were intense, effectively drowning out the specific gamma rays that would have come from oxygen fusing with carbon to form those same elements.

This study demonstrates that the filtered cathodic vacuum arc technique is a viable and powerful method for creating high-quality oxide targets for nuclear physics. The films produced are dense, uniform, and chemically stable, providing a solid foundation for future experiments. Yet, the work also highlights a critical bottleneck: while the film itself is clean, the substrate it sits on remains a source of interference. The carbon contamination on the metal base is substantial enough to generate a background that complicates the measurement of certain fusion pathways. The researchers conclude that for the next generation of experiments, which aim to map the full range of fusion reactions with maximum precision, the focus must shift to purifying the base materials even further. By eliminating the carbon on the substrate, scientists can finally clear the view, allowing them to measure the total fusion rate of oxygen nuclei without the shadow of impurities, bringing us one step closer to understanding the fiery hearts of dying stars.

Technical Summary: EBS and In-Beam $\gamma$ -Ray Investigation of FCVA-Prepared Cr $_2$ O $_3$ Targets

Problem Statement
The precise measurement of low-energy $^{16}$ O+ $^{16}$ O fusion cross sections is critical for understanding stellar nucleosynthesis, particularly oxygen burning and the production of intermediate-mass nuclei. However, experimental progress is hindered by the rapid decrease in fusion cross sections at sub-barrier energies and significant systematic uncertainties arising from target properties. Specifically, impurity elements within solid oxide targets can induce parasitic nuclear reactions under oxygen beam bombardment. Carbon contamination is identified as a critical issue; the $^{12}$ C+ $^{16}$ O fusion reaction produces the same residual nuclei ( $^{24}$ Mg and $^{27}$ Al) as the $^{16}$ O+ $^{16}$ O reaction but with cross sections 2–3 orders of magnitude larger. Consequently, even trace carbon contamination can generate substantial $\gamma$ -ray backgrounds, leading to potential overestimation of fusion yields. There is a need for high-purity oxide targets with well-characterized stoichiometry and impurity levels to enable low-background measurements.

Methodology
This study employs a two-pronged approach: the fabrication of chromium oxide (Cr $_2$ O $_3$ ) targets using Filtered Cathodic Vacuum Arc (FCVA) deposition and their subsequent characterization via Elastic Backscattering Spectrometry (EBS) and in-beam $\gamma$ -ray spectroscopy.

Target Fabrication: Cr $_2$ O $_3$ targets were prepared using an FCVA system at Beijing Normal University. The process involved depositing a metallic Cr transition layer on polished Cr or Si substrates, followed by the co-deposition of Cr and O to form the oxide layer. A curved magnetic filter was utilized to remove macroparticles, ensuring dense, uniform films.
EBS Characterization: Elemental composition, areal density, and impurity content were analyzed using a 3 MV tandetron accelerator at Sichuan University. Proton beams (2.488 MeV) were directed at the targets, and scattered ions were detected by silicon detectors. Data were analyzed using the SIMNRA simulation program, incorporating SRIM stopping powers and specific cross-section data for O, C, Si, and Cr.
In-Beam $\gamma$ -Ray Spectroscopy: Targets were bombarded with $^{16}$ O $^{5+}$ beams at laboratory energies of 14.9 MeV and 15.9 MeV. $\gamma$ -ray emissions were measured using a high-purity germanium (HPGe) detector. Background contributions were evaluated by comparing spectra from Cr $_2$ O $_3$ targets against pure Cr substrates and environmental backgrounds.

Key Results

Target Uniformity and Stoichiometry: EBS analysis confirmed that the FCVA-deposited Cr $_2$ O $_3$ films exhibit excellent spatial uniformity and stable stoichiometry. The $^{16}$ O areal densities were measured to be in the range of $(3.24–3.25) \times 10^{17}$ atoms/cm $^2$ . The Cr/O atomic ratios were found to be slightly lower than the ideal 2:3 value (approximately 0.58–0.60), indicating a minor oxygen enrichment, likely due to deposition conditions.
Impurity Content: The Cr $_2$ O $_3$ layer contained a carbon atomic fraction of approximately 1.25–1.29% (areal density $\sim 6.5 \times 10^{15}$ atoms/cm $^2$ ). However, a significantly larger carbon contamination was identified on the surface of the Cr substrate, with an areal density of $2.55 \times 10^{17}$ atoms/cm $^2$ (roughly 40 times higher than in the oxide layer).
Background Analysis: In-beam $\gamma$ -ray spectra revealed prominent transitions at 844, 1015, and 1369 keV, corresponding to $^{27}$ Al and $^{24}$ Mg. Comparison with pure Cr substrate spectra indicated that these peaks are primarily driven by $^{12}$ C+ $^{16}$ O fusion reactions induced by carbon impurities in the substrate, rather than the $^{16}$ O+ $^{16}$ O reaction.
Fusion Signatures: Despite the background, characteristic $\gamma$ -ray peaks associated with $^{16}$ O+ $^{16}$ O evaporation channels were observed at 1249, 1266, 1779, and 2235 keV (corresponding to $^{31}$ S, $^{31}$ P, and $^{28}$ Si). These specific transitions were found to be free from carbon-induced backgrounds and showed the expected energy dependence, decreasing in intensity at lower beam energies.

Significance and Claims
The paper claims to provide an experimental basis for the development of high-purity oxide targets and the optimization of target configurations for future low-background $^{16}$ O+ $^{16}$ O fusion measurements. The study demonstrates that FCVA is a viable technique for producing Cr $_2$ O $_3$ targets with controlled thickness and stoichiometry. However, the authors emphasize that while the oxide layer itself is relatively pure, the carbon contamination on the substrate remains a dominant source of background noise.

The work concludes that future improvements for low-energy fusion studies must focus on reducing carbon contamination in both the substrate and the target material to improve the signal-to-background ratio. This reduction is necessary to enable the measurement of total fusion cross sections across all channels. The identification of specific $\gamma$ -ray peaks (1249–2235 keV) that are free from carbon interference offers a pathway for extracting $^{16}$ O+ $^{16}$ O fusion cross sections, provided substrate purity is enhanced.

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