The stellar destruction rates of 26Al^{26}\mathrm{Al} via 26Al(n,p)26Mg^{26}\mathrm{Al}(n,p)^{26}\mathrm{Mg} and 26Al(n,α)23Na^{26}\mathrm{Al}(n,\alpha)^{23}\mathrm{Na} set how much 26Al^{26}\mathrm{Al} survives in convective and explosive carbon/neon burning of massive stars and in the helium intershell of asymptotic giant branch stars, and are the dominant remaining nuclear-physics uncertainty on 26Al^{26}\mathrm{Al} production in these sites. The current evaluation is anchored to direct data only up to 150 keV neutron energy (roughly T9≈0.4T_9\approx0.4); above that it rests on Hauser–Feshbach extrapolations tied to a single 1986 activation dataset, producing a factor-of-2.4 spread in the predicted 26Al^{26}\mathrm{Al} abundance at the point of peak explosive-burning production. The problem is to constrain these stellar rates over T9≈0.3T_9\approx0.3–2.52.5 through new high-neutron-energy cross-section data and to quantify the resulting impact on 26Al^{26}\mathrm{Al} nucleosynthesis. No cross-section data above 150 keV neutron energy are currently available for these channels.

Background

The radionuclide aluminum-26 (26Al^{26}\mathrm{Al}) decays by positron emission with a half-life of 0.72 Myr; the de-excitation of its daughter 26Mg^{26}\mathrm{Mg} produces the 1.809 MeV gamma-ray line that has been mapped across the Galaxy by the COMPTEL and INTEGRAL/SPI telescopes, making 26Al^{26}\mathrm{Al} a direct tracer of ongoing nucleosynthesis (Diehl et al. 2021). In meteorites, excesses of 26Mg^{26}\mathrm{Mg} in calcium–aluminum-rich inclusions (the oldest Solar-System solids) show that live 26Al^{26}\mathrm{Al} was present when the Sun formed, and the 26Al/27Al^{26}\mathrm{Al}/^{27}\mathrm{Al} ratios measured in presolar silicon-carbide (SiC) stardust grains constrain the stars that produced those grains (Groopman et al. 2015; Liu et al. 2021). The dominant Galactic sources are massive stars — Wolf–Rayet winds (strong winds from massive stars that have stripped away their outer hydrogen envelopes) and core-collapse supernovae (CCSNe, the explosions that end the lives of massive stars) — with additional contributions from classical novae (thermonuclear explosions on the surfaces of accreting white dwarfs) and low-mass asymptotic giant branch (AGB) stars.

In all of these sites 26Al^{26}\mathrm{Al} is synthesized mainly by 25Mg(p,γ)26Al^{25}\mathrm{Mg}(p,\gamma)^{26}\mathrm{Al} during hydrogen burning, and destroyed by β+\beta^+ decay, by 26Al(p,γ)27Si^{26}\mathrm{Al}(p,\gamma)^{27}\mathrm{Si}, and — wherever free neutrons are abundant — by the neutron-induced reactions 26Al(n,p)26Mg^{26}\mathrm{Al}(n,p)^{26}\mathrm{Mg} and 26Al(n,α)23Na^{26}\mathrm{Al}(n,\alpha)^{23}\mathrm{Na}. Free neutrons are released by 22Ne(α,n)25Mg^{22}\mathrm{Ne}(\alpha,n)^{25}\mathrm{Mg}, activated in the helium-rich intershell of AGB stars and during helium and convective carbon/neon (C/Ne) burning in massive stars, so the two neutron channels control how much 26Al^{26}\mathrm{Al} survives in AGB ejecta, in the convective C-burning shells of massive stars, and in the explosive C/Ne burning that occurs when the supernova shock traverses those shells. Stellar temperatures are conventionally quoted as T9T_9, the temperature in units of 10910^9 K; convective C burning corresponds to T9∼1T_9\sim1–1.5 and the relevant explosive burning to T9∼0.3T_9\sim0.3–2.5.

A stellar reaction rate (reactivity) is the Maxwell–Boltzmann average of the reaction cross section over the thermal distribution of relative velocities at the stellar temperature. Iliadis et al. (2011) identified the two neutron-induced destruction reactions as among the strongest remaining rate uncertainties affecting massive-star 26Al^{26}\mathrm{Al} and recommended them as prime targets for measurement, motivating a high-precision measurement of both cross sections at the CERN n_TOF neutron time-of-flight facility, valid up to 150 keV neutron energy (roughly T9≈0.4T_9\approx0.4). Building on that dataset, Battino et al. (2023) re-evaluated the ground-state reactivities from T9=0.01T_9=0.01 to 10: up to 150 keV the rates follow the measured cross sections, while above they rely on statistical (Hauser–Feshbach) nuclear-reaction calculations with the EMPIRE code — whose predictions depend on the chosen optical-model potentials describing neutron, proton, and alpha scattering — anchored to the only pre-existing higher-energy data, the activation reactivities of Trautvetter et al. (1986) at T=0.36T=0.36, 0.82 and 3.6 GK, which lie below all theoretical predictions.

Propagating the upper and lower limits of the new rates through a 15-solar-mass supernova model, Battino et al. (2023) found that the 26Al^{26}\mathrm{Al} abundance at the mass coordinate of peak production (Tpeak=2.39T_\mathrm{peak}=2.39 GK) varies by a factor of 2.4 — the uncertainty being dominated by the extrapolation above 150 keV, exactly the regime of explosive C/Ne burning (T9≈0.3T_9\approx0.3–2.5). In parallel, the stellar rate at high temperature involves reactions on thermally populated excited states of 26Al^{26}\mathrm{Al} — an odd-odd nucleus with a long-lived 228-keV isomer and a dense low-lying level scheme — whose contribution (about 10% of the nuclei are thermally excited at 2.39 GK) was held fixed rather than varied. On the AGB side, the new rates let models reproduce the full observed range of 26Al/27Al^{26}\mathrm{Al}/^{27}\mathrm{Al} in 62 mainstream SiC grains, but that comparison is limited by a factor-of-two systematic arising from the measuring instrument's element-dependent sensitivity between Al and Mg, by stellar-model uncertainties (convective-boundary mixing, which controls intershell temperatures and neutron exposures), and by the still-uncertain 22Ne(α,n)25Mg^{22}\mathrm{Ne}(\alpha,n)^{25}\mathrm{Mg} source rate. The authors conclude that an improvement of the uncertainties from T9∼0.3T_9\sim0.3 to 2.5 is required for future studies, and report that a new measurement of both reactions at high neutron energy has been proposed with a new silicon-strip detector setup at n_TOF.

Problem Statement

The problem is to replace the extrapolation-dominated evaluation of the 26Al(n,p)26Mg^{26}\mathrm{Al}(n,p)^{26}\mathrm{Mg} and 26Al(n,α)23Na^{26}\mathrm{Al}(n,\alpha)^{23}\mathrm{Na} stellar destruction rates with a determinate, experimentally constrained determination valid over the explosive-burning range T9≈0.3T_9\approx0.3–2.52.5 (neutron energies of hundreds of keV, above the 150 keV limit of the current direct data). Concretely, a resolution requires the following jointly necessary components of one objective:

  1. Direct experimental cross-section data — or constraints of equivalent rigor, such as surrogate-reaction or spectroscopy-based determinations of the relevant resonance structure — for the ground-state 26Al(n,p)^{26}\mathrm{Al}(n,p) and 26Al(n,α)^{26}\mathrm{Al}(n,\alpha) reactions above roughly 150 keV neutron energy, together with re-evaluated Maxwellian-averaged stellar reactivities and their uncertainty bands over the target range T9≈0.3T_9\approx0.3–2.52.5.

  2. Quantification of the impact of the improved rates on 26Al^{26}\mathrm{Al} nucleosynthesis in explosive C/Ne burning and in the AGB intershell, where the nuclear-rate contribution to the uncertainty in predicted 26Al^{26}\mathrm{Al} abundances is currently a factor of 2.4 at the point of peak production in a 15-solar-mass supernova model.

The total stellar rate at these temperatures also involves reactions on thermally populated excited states of 26Al^{26}\mathrm{Al} (about 10% of the nuclei are thermally excited at 2.39 GK), whose contribution current nucleosynthesis studies hold fixed rather than vary; quantifying this contribution is not a required component of this problem. The stellar-model uncertainties (convective-boundary mixing, the 22Ne(α,n)25Mg^{22}\mathrm{Ne}(\alpha,n)^{25}\mathrm{Mg} neutron source, progenitor mass, rotation, and binary history) and the factor-of-two meteoritic SiC-grain 26Al/27Al^{26}\mathrm{Al}/^{27}\mathrm{Al} measurement systematics are likewise acknowledged limitations of the source analysis; reducing them in their own right is not part of this problem.

References

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