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Reaction atlas · MgAl cycle

Radiative capture · MgAl cycle

24Mg(p,γ)25Al

: magnesium‑24 + proton → aluminium‑25 + gamma rayHow to read the notation

Well knownMeasured at LUNA
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The first step of the MgAl cycle. 24Mg, the most abundant magnesium isotope, captures a proton and becomes 25Al, which quickly decays to 25Mg.

This is one of the slowest proton captures in the MgAl cycle, so it controls how fast the most common magnesium isotope is converted into 25Mg and, after one more capture, into radioactive 26Al and stable 27Al. In the hottest hydrogen-burning stars it destroys 24Mg; some stars in globular clusters show less magnesium and more aluminium than their neighbours, a pattern attributed to this kind of burning.

Facts

How well known
The resonance that matters most in hydrogen-burning stars has been measured underground with good precision; at lower energies the rate still relies on an extrapolation.
Live evaluation
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Reaction rates
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Q-value · energy released
2.271 MeV
Gamow window
  • 115 ± 33 keVAGB hot-bottom burning, 80 MK (MgAl cycle)
  • 215 ± 70 keVclassical nova, 0.2 GK (MgAl cycle)
Product 25Al
radioactive, t½ = 7.17 s
Kind
Radiative capture

How it was measured

1 measurement, 2010, 1 at LUNAShow the list
  1. 2010

    LUNA 400 kVLaboratori Nazionali del Gran Sasso (INFN)

    Direct measurement underground · E_cm = 214 keV resonance

    Resonance strength measured underground with several techniques, more precise than earlier values; also intended as a reference for lower-energy resonances. [1]

Measurements (EXFOR)

Every dataset EXFOR holds for this reaction, drawn by the energies it covers, against the energies a star uses.

18 datasets in 9 EXFOR entries, 1975–2021 · 1 without an energy range, not drawn

Temperature
Lowest energy in EXFOR
204 keV
Trautvetter et al., 1975 · σ · C2509.002
Distance from the Gamow peak
1.3 Gamow widths above
E₀ = 117 keV, window 84.6–150 keV
Rate from energies not in EXFOR
99.74%
at T₉ = 0.08, smooth estimate · 99.74% of it lies below the lowest energy in EXFOR
10 keV100 keV1 MeV10 MeVCentre-of-mass energy (keV)E₀ = 117 keVSσ→Eₓdσ/dΩωγotherlowest in EXFOR 204 keV
  • Gamow window, hatched where no measurement reaches
  • Outlined bars are shown but not counted (the rule is under “How these numbers are made”).
  • A diamond is a dataset at a single energy. Bars running off the right edge continue to higher energies.
The datasets of the gap map, by quantity
The datasets of the gap map, by quantity
QuantityDatasetsE_cm range (keV)
SS-factor1580 keV – 1.49 MeV
σ→Eₓpartial1204 keV – 2.21 MeV
dσ/dΩangular distribution1773 keV – 2.19 MeV
ωγresonance strength10205 keV – 2.31 MeV
…other4205 keV – 1.63 MeV
How these numbers are made

Lowest measured energy, everywhere in the data explorer: the lowest centre-of-mass energy at which a dataset in EXFOR measures the reaction itself (cross section, S-factor, angular distribution, yield or resonance strength), judged point by point. A point counts when it is a value, not an upper or lower limit, when it lies at least one standard deviation above zero, and when its energy is physical: at least 1 keV for charged particles, and not below the threshold of an endothermic reaction. Not counted: extrapolated, calculated, evaluated or derived values; superseded, deleted or preliminary data; ratios; a single S-factor value, which in EXFOR is a fit result; and indirect methods (Trojan horse, Coulomb dissociation, ANC, surrogate reactions), recognized from EXFOR's method and analysis codes, their text and the title. Frame and unit errors of EXFOR are corrected first, and values the literature shows to be upper limits are read as limits, from a list that gives the evidence for each. Datasets that reach the same energy within 0.5% are all credited, earliest first. Rate from energies not in EXFOR: the share of the rate integrand at that temperature outside the energy ranges the counted datasets cover (from their lowest measured point to their highest), with a constant S-factor (σ ∝ S(E₀)/E · e^−2πη, so the integrand is e^(−E/kT − 2πη)); for a neutron capture, σ ∝ 1/v. Resonances make the real integrand lumpy, so read it as a smooth baseline.

Details

How well known, in detail: The resonance that dominates at hydrogen-burning temperatures has been measured directly underground with good precision; below it the rate relies on extrapolated direct capture.

The rate at the temperatures of hot-bottom burning is determined by direct capture and by the narrow resonance at E_cm = 214 keV. Limata et al. (2010) measured its strength at LUNA, together with resonances in 25Mg + p and 26Mg + p, with a precision higher than previously achieved; these resonances also serve as normalization references for weaker, lower-energy resonances in the magnesium isotopes, where absolute measurements suffer from beam-induced background and changing target stoichiometry.

Peak energy E0 ± half the 1/e width, centre of mass, from the AME2020 masses.

Evaluations

Compilations that combine the data into a recommended rate.

  • —C. Angulo, M. Arnould, M. Rayet et al. (NACRE Collaboration), Nucl. Phys. A 656, 3 (1999) [2]

Open questions

  • The direct capture S-factor below the 214 keV resonance, which so far comes from extrapolation of higher-energy data.

Related reactions

References

  1. 1.

    B. Limata et al. (LUNA Collaboration)

    New experimental study of low-energy (p,γ) resonances in magnesium isotopes

    Phys. Rev. C 82, 015801 (2010) · 10.1103/PhysRevC.82.015801 · arXiv:1006.5281LUNA

  2. 2.

    C. Angulo, M. Arnould, M. Rayet et al. (NACRE Collaboration)

    A compilation of charged-particle induced thermonuclear reaction rates

    Nucl. Phys. A 656, 3 (1999)

How to cite this page

J. Skowronski, 24Mg(p,γ)25Al, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/24Mg-pg-25Al.

Cite the date you used; it is the last change to this page.

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