The ²⁷Al(p, α)²⁴Mg and ²⁷Al(p, γ)²⁸Si reactions at astrophysical energies: First time observation of the 84 keV resonance

Laboratori Nazionali del Sud, Istituto Nazionale di Fisica Nucleare, 95123 Catania, Italy

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Published online: 13 May 2026

Abstract

The ²⁶Al abundance represents a key diagnostic tool, serving as a tracer of ongoing nucleosynthesis throughout the Galaxy and providing constraints on the rate of Galactic core-collapse supernovae. Quantification of this abundance relies on the analysis of the ²⁶Mg excess relative to ²⁴Mg in meteorites and on the detection of the 1809 keV γ-ray line using space-borne telescopes, and is usually normalized to the ²⁷ Al abundance. Consequently, precise isotopic measurements of stable aluminum and magnesium are essential. These elements also participate in the Mg-Al cycle, responsible for synthesizing Al and Mg in stellar interiors. Spectroscopic observations of globular-cluster red giants have revealed systematic Mg-Al anticorrelations, providing evidence for multiple stellar populations and necessitating additional observational and theoretical work. Central to understanding these astrophysical phenomena is the characterization of the ²⁷Al(p, α)²⁴Mg and ²⁷Al(p, γ)²⁸Si nuclear reactions, which constitute the primary destruction channels for ²⁷Al. The extraordinarily small cross sections at astrophysically relevant energies present significant challenges for direct laboratory measurements, resulting in substantial uncertainties. The Trojan Horse Method provides an alternative experimental approach, enabling the investigation of these reactions at energies below 100 keV. This technique has led to the determination of the 84 keV resonance strength and the establishment of improved upper limits for nearby resonances. The resulting reaction rates are lower by factors approaching 3 compared to the values currently implemented in astrophysical models, with significant implications for nucleosynthesis calculations in massive stars.