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Reaction atlas · Neutron sources

Radiative capture · Neutron sources of the s-process

22Ne(α,γ)26Mg

: neon‑22 + helium‑4 → magnesium‑26 + gamma rayHow to read the notation

On this page

Neon-22 captures a helium nucleus and keeps it, becoming magnesium-26. It competes with the neutron-releasing reaction on the same nucleus.

Every neon-22 nucleus that captures helium through this reaction is lost as a neutron source. At lower temperatures this capture is faster than the neutron-producing reaction, so it decides how much neon-22 is still left when the star becomes hot enough to release neutrons. It also adds magnesium-26 to the gas the star returns to the Galaxy.

Facts

How well known
The 830 keV resonance is measured to about 6%, but the lower-lying resonances are known only from indirect data and direct upper limits.
Live evaluation
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Reaction rates
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Q-value · energy released
10.615 MeV
Gamow window
  • 565 ± 130 keVAGB thermal pulse, 0.27 GK (Neutron sources of the s-process)
  • 605 ± 145 keVmassive star, He core end, 0.3 GK (Neutron sources of the s-process)
Kind
Radiative capture

How it was measured

4 measurements, 2019–2022, 1 at LUNAShow the list
  1. 2019

    LENA, TUNLTriangle Universities Nuclear Laboratory

    Direct measurement at the surface · E(α,lab) = 0.83 MeV resonance

    Implanted 22Ne target and γγ-coincidence detection; E(α,lab) = 835.2 ± 3.0 keV and ωγ = (4.6 ± 1.2)×10−5 eV, higher than earlier values but overlapping within the uncertainties. [1]

  2. 2020

    Cyclotron Institute, Texas A&MTexas A&M University

    Transfer reactions and ANCs · 22Ne(6Li,d) and 22Ne(7Li,t), 1.0 MeV/u 22Ne beam

    Partial α widths of 26Mg resonances in the Gamow window; the (α,γ) rate differs significantly from earlier values. [2]

  3. 2022

    LUNA 400 kVLaboratori Nazionali del Gran Sasso (INFN)

    Direct measurement underground · E = 334 keV resonance

    First direct limit, ωγ < 4.0×10−11 eV (90% CL), higher than most indirect limits but free of their model assumptions. [3]

  4. 2022

    Nuclear Science Laboratory, Notre DameUniversity of Notre Dame

    Direct measurement at the surface · E(α,lab) = 650 and 830 keV resonances

    Implanted 22Ne targets; ωγ(830 keV) = 35 ± 4 µeV and the first direct upper limit for the 650 keV resonance. [4]

Measurements (EXFOR)

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

7 datasets in 6 EXFOR entries, 1975–2022 · 1 without an energy range, not drawn

Temperature
Lowest energy in EXFOR
683 keV
Wolke et al., 1989 · σ · O1122.002
Distance from the Gamow peak
Only the upper edge of the window
E₀ = 564 keV, window 432–696 keV
Rate from energies not in EXFOR
85%
at T₉ = 0.27, smooth estimate · 85% of it lies below the lowest energy in EXFOR

1 resonance-strength datasets reach into this window: the smooth estimate does not count resonances.

100 keV1 MeV10 MeV100 MeVCentre-of-mass energy (keV)E₀ = 564 keVσ→Eₓdσ/dΩωγlowest in EXFOR 683 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)
σ→Eₓpartial2683 keV – 10.4 MeV
dσ/dΩangular distribution13.75 MeV – 13.3 MeV
ωγresonance strength3283 keV – 707 keV
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

The rate is dominated by low-energy resonances in 26Mg, chiefly those at E(α,lab) = 650 and 830 keV. For the 830 keV resonance Hunt et al. (2019) found ωγ = (4.6 ± 1.2)×10−5 eV at LENA, and Shahina et al. (2022) measured ωγ = 35 ± 4 µeV and give a weighted average with earlier measurements of 35 ± 2 µeV; for the 650 keV resonance they report an upper limit, ωγ < 0.15 µeV. At lower energies the resonance strengths come only from indirect data, and the literature values for the resonance at E = 334 keV (E(α,lab) ≈ 395 keV, Ex = 10949 keV) span a wide range.

LUNA set the first direct upper limit on it, ωγ < 4.0×10−11 eV at 90% confidence, with a windowless enriched 22Ne gas target and a 4π BGO summing detector at the 400 kV accelerator; the background in the region of interest was below 0.5 counts per day (Piatti et al. 2022). The authors propose to extend the approach to higher energies at the 3.5 MV machine of the Bellotti Ion Beam Facility (BIBF).

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) [5]
  • —P. Adsley et al., Phys. Rev. C 103, 015805 (2021) [6]

Open questions

  • The strengths of the resonances below 830 keV, in particular at E = 334 keV, where direct sensitivity must improve by orders of magnitude to reach the indirect estimates.
  • Spin-parity assignments of the 26Mg states in the Gamow window, which decide whether a state can be populated in α capture.

Discussed on the open-questions page

Related reactions

References

  1. 1.

    S. Hunt, C. Iliadis, A. E. Champagne, L. N. Downen, A. Cooper

    New measurement of the E(α,lab) = 0.83 MeV resonance in 22Ne(α,γ)26Mg

    Phys. Rev. C 99, 045804 (2019) · 10.1103/PhysRevC.99.045804

  2. 2.
  3. 3.
  4. 4.

    Shahina, J. Görres, D. Robertson, M. Wiescher et al.

    Direct measurement of the low-energy resonances in 22Ne(α,γ)26Mg reaction

    Phys. Rev. C 106, 025805 (2022) · 10.1103/PhysRevC.106.025805 · arXiv:2208.07885

  5. 5.

    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)

  6. 6.

How to cite this page

J. Skowronski, 22Ne(α,γ)26Mg, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/22Ne-ag-26Mg.

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

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