The reaction that makes radioactive 26Al in hydrogen-burning stars. The decay of 26Al gives the 1809 keV γ-ray line seen across the Milky Way.
26Al lives long enough to spread through the Galaxy but not so long that it piles up, so the 1809 keV γ-ray line from its decay shows where nuclei were made in the last few million years. Space telescopes map this line, and the amount of 26Al they see is compared with the predicted output of massive stars, Wolf–Rayet stars, AGB stars and novae. Most of that 26Al is made by this reaction. The reaction can leave 26Al in its ground state, which gives the γ-ray line, or in a short-lived excited state, the isomer 26ᵐAl, which decays straight to 26Mg without it. How the captures divide between the two decides how much of the line a star contributes. The same reaction also matters for the 26Al once present in the early Solar System, recorded in meteorites, and for the magnesium and aluminium abundances of globular-cluster stars.
Facts
- How well known
- The key resonances were measured directly underground, at LUNA and at JUNA. The main uncertainty left is how much of the product is made in the long-lived form of aluminium-26, the one that γ-ray telescopes see in the Galaxy.
- Live evaluation
- No live evaluation yet · Evaluations
- Reaction rates
- Explore the rates →
- Q-value · energy released
- 6.306 MeV
- Product 26Al
- radioactive, t½ = 717 000 yr
- Kind
- Radiative capture
On the timeline
How it was measured
4 measurements, 1990–2022, 2 at LUNAShow the listHide the list
1990
Direct measurement at the surface · low-energy (p,γ) resonances
γ-ray decay schemes of the low-energy resonances, measured together with resonances in 26Mg(p,γ) and 27Al(p,γ). [1]
2010
LUNA 400 kVLaboratori Nazionali del Gran Sasso (INFN)
Direct measurement underground · E_cm = 304 keV resonance
Strength of the 304 keV resonance with higher accuracy than before, to serve as a normalization for the low-energy resonances. [2]
2012
LUNA 400 kVLaboratori Nazionali del Gran Sasso (INFN)
Direct measurement underground · E_cm = 92 and 189 keV resonances
First direct observation of the 92 keV resonance with a 4π BGO summing detector, ωγ = (2.9 ± 0.6) × 10−10 eV. γ-ray branchings and strength of the 189 keV resonance with an HPGe detector, 20% higher than previous values. [3]
2022
JUNAChina Jinping Underground Laboratory
Direct measurement underground · E_cm = 92 keV resonance
First experiment at JUNA in the China Jinping Underground Laboratory: a precise measurement of the 92 keV resonance, independent of the LUNA one. [4]
Measurements (EXFOR)
Every dataset EXFOR holds for this reaction, drawn by the energies it covers, against the energies a star uses.
24 datasets in 13 EXFOR entries, 1962–2021 · 3 without an energy range, not drawn
- Lowest energy in EXFOR
- 88.6 keV
- Strieder et al., 2012 · ωγ · O1976.003
- Distance from the Gamow peak
- Inside the window
- E₀ = 117 keV, window 84.6–150 keV
- Rate from energies not in EXFOR
- 6.5%
- at T₉ = 0.08, smooth estimate · 5.8% of it lies below the lowest energy in EXFOR
2 resonance-strength datasets reach into this window: the smooth estimate does not count resonances.
- 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
| Quantity | Datasets | E_cm range (keV) |
|---|---|---|
| dσ/dΩangular distribution | 4 | 298 keV – 1.77 MeV |
| ωγresonance strength | 12 | 88.6 keV – 1.98 MeV |
| …other | 5 | 182 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 key low-energy resonances were measured directly underground, at LUNA and at JUNA; the split between ground state and isomer, especially for the 92 keV resonance, is the largest remaining uncertainty for the 26Al ground-state yield.
Between 50 and 150 MK the rate is given by a handful of narrow resonances, of which the one at E_cm = 92 keV dominates around 100 MK. Each resonance decays by a γ-ray cascade that ends either in the ground state or in the isomer of 26Al, so the rate has to be split using the measured ground-state fraction of each resonance. At LUNA, Limata et al. (2010) measured the strength of the 304 keV resonance as a reference for the weaker ones, and Strieder et al. (2012) observed the 92 keV resonance directly for the first time with a 4π BGO summing detector, ωγ = (2.9 ± 0.6) × 10−10 eV, and studied the γ-ray branchings and strength of the 189 keV resonance with an HPGe detector, finding a strength 20% larger than earlier works.
The rate derived from these data (Straniero et al. 2013) increases the production of the isomer by up to a factor of 5 between 50 and 150 MK and doubles the total rate at 100 MK, so less 26Al is made in the ground state. The JUNA collaboration chose the 92 keV resonance for the first experiment at the China Jinping Underground Laboratory and reported a precise measurement of its strength (Su et al. 2022). The γ-ray decay schemes of the low-energy resonances were first studied by Iliadis et al. (1990).
For the 92 keV resonance, Kankainen et al. (2021) identified four γ-ray transitions and obtained a ground-state fraction f0 = 0.52(2)(6), the most precise value so far and at the low end of those adopted before, which lowers the production of 26Al in its ground state.
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.
Open questions
- The ground-state fraction of the 92 keV resonance, which sets how much of the rate produces 26Al in its ground state and therefore the 1809 keV line.
- The comparison of the LUNA and JUNA strengths of the 92 keV resonance, the two direct measurements in deep underground laboratories.
- The effect of the higher isomer production on the 26Al yields of Wolf–Rayet and massive AGB stars.
Discussed on the open-questions page
Related reactions
References
- 1.
C. Iliadis et al.
Low-energy resonances in 25Mg(p,γ)26Al, 26Mg(p,γ)27Al and 27Al(p,γ)28Si
Nucl. Phys. A 512, 509 (1990)
- 2.
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
- 3.
F. Strieder et al. (LUNA Collaboration)
The 25Mg(p,γ)26Al reaction at low astrophysical energies
Phys. Lett. B 707, 60 (2012) · 10.1016/j.physletb.2011.12.029 · arXiv:1112.3313LUNA
- 4.
J. Su, H. Zhang, Z. H. Li et al. (JUNA Collaboration)
First result from the Jinping Underground Nuclear Astrophysics experiment JUNA: precise measurement of the 92 keV 25Mg(p,γ)26Al resonance
Sci. Bull. 67, 125 (2022)
- 5.
O. Straniero et al. (LUNA Collaboration)
Impact of a revised 25Mg(p,γ)26Al reaction rate on the operation of the Mg-Al cycle
Astrophys. J. 763, 100 (2013) · arXiv:1211.6661LUNA
- 6.
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)
- 7.
A. Kankainen et al.
Phys. Lett. B 813, 136033 (2021) · 10.1016/j.physletb.2020.136033
- 8.
C. Lederer-Woods et al. (n_TOF Collaboration)
Destruction of the cosmic γ-ray emitter 26Al in massive stars: study of the key 26Al(n,p) reaction
Phys. Rev. C 104, L022803 (2021) · 10.1103/PhysRevC.104.L022803
How to cite this page
J. Skowronski, 25Mg(p,γ)26Al, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/25Mg-pg-26Al.
Cite the date you used; it is the last change to this page.
Something wrong or missing? Report an error (opens a new issue on GitHub) · Suggest a reaction (opens a new issue on GitHub)