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Direct measurement of $^{18}$ F( $\alpha,p$ ) $^{21}$ Ne cross sections with ANASEN

This paper presents the first direct measurement of the $^{18}$ F( $\alpha,p$ ) $^{21}$ Ne reaction cross sections using a radioactive $^{18}$ F beam at TRIUMF-ISAC, which significantly reduces uncertainties in the reaction rate and reveals a 45% increase in predicted $^{19}$ F production for Asymptotic Giant Branch nucleosynthesis models.

Original authors: K. S. Davis, J. C. Blackmon, C. M. Deibel, G. L. Wilson, M. Alcorta, L. T. Baby, D. W. Bardayan, S. Carmichael, S. Chakraborty, C. Esparza, J. Glorius, A. I. Karakas, A. Lennarz, B. Kay, J. Henning, G

Published 2026-09-09

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CC BY 4.0

Original authors: K. S. Davis, J. C. Blackmon, C. M. Deibel, G. L. Wilson, M. Alcorta, L. T. Baby, D. W. Bardayan, S. Carmichael, S. Chakraborty, C. Esparza, J. Glorius, A. I. Karakas, A. Lennarz, B. Kay, J. Henning, G. W. McCann, S. Pain, C. Ruiz, R. Russell, V. Sitaraman, B. Sudarsan, I. Tolstukhin, L. Wagner, I. Wiedenhöver

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

Stars are not just distant points of light; they are vast, churning factories that forge the elements making up our world and our bodies. For stars like our Sun, and for those slightly larger, the final chapter of their lives is spent as Asymptotic Giant Branch stars. These aging giants pulse with heat, creating layers where helium fuses into heavier elements. In these fiery shells, a specific nuclear reaction acts as a gatekeeper, deciding how much fluorine is created and released into the universe. For decades, astronomers have tried to predict how much fluorine these stars should produce, but their calculations have been stuck in the dark. The reason is a missing piece of data: scientists have never directly measured how a specific radioactive atom, fluorine-18, interacts with helium nuclei to release protons. Without this direct measurement, models of how stars evolve and how they enrich the cosmos with elements have relied on guesses and indirect clues, leaving large gaps in our understanding of the chemical history of the galaxy.

A team of researchers has finally filled this gap by performing the first direct measurement of this elusive reaction. Working at the TRIUMF laboratory in Canada, they created a beam of radioactive fluorine-18 atoms and fired them into a chamber filled with pure helium gas. This setup allowed them to watch the reaction happen in real time. As the fluorine atoms collided with the helium, they transformed, knocking out protons and leaving behind neon atoms. By tracking the paths and energies of these ejected protons with a sophisticated array of silicon detectors, the scientists could reconstruct exactly what happened during the collision. They covered a wide range of collision energies, from 2 to 4 million electron volts, a range that corresponds to the intense temperatures found inside the hearts of aging stars and during the violent explosions of white dwarfs.

The results of this experiment were clear and direct. The total rate at which the reaction occurred matched well with the predictions made by statistical models, which are computer simulations used to estimate nuclear behavior when direct data is missing. However, the team found a notable difference in the details: the reaction produced more neon atoms in a specific excited state than the models had predicted. This excited state is a higher-energy version of the neon atom that settles down quickly, but its increased production suggests that the reaction is slightly more complex than previously thought. Crucially, the researchers also confirmed that a competing reaction involving a different isotope, neon-18, did not interfere with their measurements, ensuring that their data was pure and reliable.

With these new, direct measurements in hand, the team recalculated the speed of this reaction across the temperatures found in stars. They combined their new data with previous, indirect measurements to create a refined rate that is far more certain than anything available before. The uncertainty in this rate has been reduced significantly, narrowing the range of possible outcomes by a factor of roughly two. This precision allows astrophysicists to run models of stellar evolution with much greater confidence. When they applied this new, faster reaction rate to models of Asymptotic Giant Branch stars, the results showed a substantial increase in the production of fluorine. Specifically, the models now predict that these stars produce about 45% more fluorine than previously estimated.

This finding has immediate implications for our understanding of the universe's chemical makeup. The new rate helps explain the abundance of fluorine observed in the solar system and in the stars around us. It also clarifies the conditions inside white dwarfs that might lead to specific types of supernova explosions. While the reaction rate is higher than the old recommendation, it is not so high as to explain certain unusual ratios of neon isotopes found in stardust grains, suggesting that other physical processes, such as mixing within the star, play a larger role in those specific anomalies. By removing this major source of uncertainty, the study provides a solid foundation for future research, allowing scientists to focus on other variables to solve the remaining puzzles of how stars create the elements that make up our world.

Technical Summary: Direct Measurement of 18F(α, p)21Ne Cross Sections with ANASEN

Problem Statement
The $^{18}\text{F}(\alpha, p)^{21}\text{Ne}$ reaction plays a critical role in astrophysical nucleosynthesis, specifically within Asymptotic Giant Branch (AGB) stars and accreting white dwarfs (WDs) leading to Type Ia supernovae. In AGB stars, this reaction acts as a proton source for the $^{18}\text{O}(p, \alpha)^{15}\text{N}(\alpha, \gamma)^{19}\text{F}$ chain, significantly influencing fluorine production. In WD double-detonation scenarios, it facilitates the $^{12}\text{C}(p, \gamma)^{13}\text{N}(\alpha, p)^{16}\text{O}$ chain, potentially accelerating helium burning.

Prior to this work, the reaction rate had never been directly measured. Existing constraints relied on a time-inverse measurement of $^{21}\text{Ne}(p, \alpha)^{18}\text{F}$ by Lee et al. [3], which only constrained reactions populating the ground state of $^{21}\text{Ne}$ . Consequently, the contribution from excited states remained uncertain, leading to large uncertainties in reaction rates (spanning two orders of magnitude at $T \sim 0.3$ GK). These uncertainties hindered robust predictions for isotopic abundances, such as the high $^{21}\text{Ne}/^{22}\text{Ne}$ ratios observed in stellar silicon carbide grains, and limited the precision of AGB and supernova models.

Methodology
The authors performed the first direct measurement of the $^{18}\text{F}(\alpha, p)^{21}\text{Ne}$ excitation function using inverse kinematics at the TRIUMF-ISAC facility. The experiment utilized the Array for Nuclear Astrophysics and Structure with Exotic Nuclei (ANASEN), an upgraded system featuring an extended $^4\text{He}$ gas target and charged-particle detector arrays.

Beam and Target: A radioactive $^{18}\text{F}$ beam (1.5 MeV/u, $10^5$ – $10^6$ pps, 75% purity) was directed into a chamber containing 175 Torr of pure $^4\text{He}$ gas.
Detection: Outgoing protons were detected by two silicon detector arrays: a $\Delta E$ array (two hexagonal barrels of 100- $\mu$ m silicon strips) near the beam axis and a residual $E$ array (QQQ3 and SX3 detectors) further out.
Kinematics and Reconstruction: Proton trajectories were reconstructed from position measurements on the silicon arrays and extrapolated to the beam axis to determine the interaction point. This allowed for the calculation of the center-of-mass energy ( $E_{cm}$ ) based on the beam's energy loss in the gas.
Energy Range: The experiment covered a center-of-mass energy range of $2 \le E_{cm} \le 4$ MeV.
Background Suppression: Kinematic analysis confirmed that background from the $^{18}\text{Ne}(\alpha, p)^{21}\text{Na}$ reaction (due to $^{18}\text{Ne}$ impurities in the beam) was negligible, as the proton energies for the background reaction were distinct from the signal.

Key Results

Cross Sections: The total cross section was measured and found to be in good agreement (within 30%) with statistical model calculations (TALYS) using the default alpha optical potential ("alphaomp 6").
State Populations: The ground state and first excited state (351 keV) were observed across the full energy range. The second excited state (1746 keV) was resolved cleanly at $E_{cm} \gtrsim 3$ MeV. Notably, the population of the second excited state was observed to be greater than predicted by TALYS at high energies ( $E_{cm} \ge 3.75$ MeV), while the sum of the ground and first excited states was slightly lower than predicted in the same region.
Uncertainty Reduction: By combining these direct measurements with the previous time-inverse data (Lee et al. [3]) and statistical model extrapolations, the authors calculated a new reaction rate for temperatures $0.1 $–$ 3$ GK. The uncertainty in the rate was reduced to a factor of $\sim 1.8$ –$2.4$ across the temperature range, a significant improvement over previous constraints.
Comparison to Models: The new recommended rate is larger than the previous REACLIB rate in the range $0.3 < T < 0.6$ GK (up to 4 times larger) but aligns well with the REACLIB rate at $T \lesssim 0.3$ GK and $T \gtrsim 1$ GK.

Significance and Conclusions
The primary significance of this work is the reduction of uncertainties in the $^{18}\text{F}(\alpha, p)^{21}\text{Ne}$ reaction rate to a level that permits robust modeling of AGB nucleosynthesis.

AGB Models: Using the new recommended rate, stellar evolution models predict a 45% increase in $^{19}\text{F}$ production compared to previous rates. While this enhances fluorine yields, the authors note that the rate is still not high enough to explain the observed $^{21}\text{Ne}/^{22}\text{Ne}$ ratios in silicon carbide grains via this reaction alone, suggesting other factors (such as non-convective mixing) may be involved.
Supernova Models: For Type Ia supernova progenitors involving double detonations, the impact of the new rate at high temperatures ( $T > 1$ GK) is small, though it may slightly increase the proton supply for the $^{12}\text{C}(p, \gamma)^{13}\text{N}(\alpha, p)^{16}\text{O}$ chain. The authors suggest that the increased rate in the $0.3 < T < 0.6$ GK range could affect the availability of $^{18}\text{F}$ at the onset of thermonuclear runaway, warranting further full network calculations.

In conclusion, this direct measurement removes the $^{18}\text{F}(\alpha, p)^{21}\text{Ne}$ reaction as a major source of uncertainty in AGB modeling, allowing for tighter constraints on other input physics to match observed isotopic ratios.

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