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Reaction atlas · Neutron sources

Particle-exchange reaction · Neutron sources of the s-process

22Ne(α,n)25Mg

: neon‑22 + helium‑4 → magnesium‑25 + neutronHow to read the notation

On this page

Neon-22 captures a helium nucleus and releases a neutron. It is the main neutron source in massive stars and a second source in the late pulses of giant stars.

Stars convert the carbon, nitrogen and oxygen they were born with into nitrogen-14 while they burn hydrogen. When helium burns, the nitrogen turns into neon-22. Near the end of helium burning in stars more than about ten times the mass of the Sun, and during the hottest helium flashes of giant stars, the temperature rises enough for neon-22 to capture a helium nucleus and release a neutron. These neutrons make most of the elements between iron and strontium, the weak s-process. Releasing a neutron costs energy, so the reaction cannot happen at all below a threshold. Below it, and in competition above it, neon-22 can capture helium through another reaction, 22Ne(α,γ)26Mg, which uses it up without giving a neutron. The balance between the two decides how many neutrons are released.

Facts

How well known
At the energies that matter in stars, direct experiments give only upper limits, and the indirect methods do not yet agree.
Live evaluation
No live evaluation yet · Evaluations
Reaction rates
Explore the rates →
Q-value · energy needed
-0.478 MeV
Gamow window
Kind
Particle-exchange reaction

On the timeline

  1. 1957Cameron's lectures on nucleogenesis
  2. 2022A 3.5 MV accelerator is installed under Gran SassoLUNA

How it was measured

6 measurements, 1993–2024Show the list
  1. 1993

    Direct measurement at the surface

    Direct measurement of both s-process neutron sources, 22Ne(α,n)25Mg and 13C(α,n)16O, with new rates for each. [1]

  2. 2001

    Dynamitron, StuttgartInstitut für Strahlenphysik, Universität Stuttgart

    Direct measurement at the surface · E(α,lab) = 0.57–1.47 MeV

    Windowless gas target Rhinoceros and a new neutron detector with active shielding, up to 50% efficiency and 0.02 counts/s background; the resulting rate lies well below earlier compilations for some stellar temperatures below 0.3 GK. [2]

  3. 2017

    n_TOF, CERNCERN

    Neutron beams and time of flight · 25Mg + n, thermal to about 300 keV neutron energy

    25Mg(n,γ) and total cross sections fitted together in an R-matrix analysis gave unambiguous spin-parity assignments for 26Mg states above the neutron threshold; the limits point to a larger (α,n)/(α,γ) ratio than previously evaluated. [3]

  4. 2020

    Cyclotron Institute, Texas A&MTexas A&M University

    Transfer reactions and ANCs · 22Ne(6Li,d)26Mg in inverse kinematics

    Deuterons and 26Mg recoils detected in coincidence with the TIARA array and the MDM spectrometer; a new neutron/γ decay branching ratio for a key resonance gives it a new α strength. [4]

  5. 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 derived (α,n) and (α,γ) rates differ significantly from earlier values. [5]

  6. 2024

    Nuclear Science Laboratory, Notre DameUniversity of Notre Dame

    Direct measurement at the surface · 830 keV resonance (E_α,lab; E_cm ≈ 702 keV)

    Strength of the key resonance remeasured with a stilbene detector; similar to the earlier direct results and about three times the (α,γ) strength, against transfer data that imply the two are about equal. [6]

Measurements (EXFOR)

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

16 datasets in 9 EXFOR entries, 1973–2024 · 3 without an energy range, not drawn

Temperature
Lowest energy in EXFOR
693 keV
Drotleff et al., 1991; Harms et al., 1991; DROTLEFF et al., 1993; Jaeger et al., 2001 · σ · O0967.002
Distance from the Gamow peak
Only the upper edge of the window
E₀ = 564 keV, window 478–710 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 MeVCentre-of-mass energy (keV)E₀ = 564 keVσSσ→Eₓωγlowest in EXFOR 693 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)
σcross section5476 keV – 4.34 MeV
SS-factor1476 keV – 1.96 MeV
σ→Eₓpartial3623 keV – 1.8 MeV
ωγresonance strength4537 keV – 2.54 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: Below the 830 keV resonance (E_α,lab; E_cm ≈ 702 keV) only upper limits exist from direct experiments, and the indirect constraints on the near-threshold 26Mg states do not yet agree. A new direct measurement is under way at LUNA within the SHADES project.

The reaction opens just above the neutron threshold, and the rate at T9 ≈ 0.2–0.3 is dominated by resonances in 26Mg above it, in particular the well-known resonance at 830 keV (E_α,lab; E_cm ≈ 702 keV). Below it, direct experiments give only upper limits, because the cross section is small and the neutron background in surface laboratories is large. The rate therefore rests on the properties of near-threshold 26Mg states obtained indirectly: neutron resonances in 25Mg + n measured at n_TOF (Massimi et al. 2017), α-transfer reactions (Ota et al. 2020; Jayatissa et al. 2020). The evaluation of Adsley et al. (2021), based on updated spin-parity assignments, lowered the (α,n) rate substantially while leaving the (α,γ) rate almost unchanged. The transfer data suggest that the neutron and γ strengths of the 830 keV resonance are about equal; direct measurements, most recently with a stilbene detector (Shahina et al. 2024), find the (α,n) strength about three times the (α,γ) strength.

A direct measurement below the 830 keV resonance is the aim of the SHADES project, with a hybrid array of EJ-309 scintillators and 3He counters and a recirculating gas target at the MV machine of the Bellotti Ion Beam Facility (BIBF); the expected sensitivity gain is more than two orders of magnitude (Rapagnani et al. 2022).

Peak energy E0 ± half the 1/e width, centre of mass, from the AME2020 masses. For an endothermic reaction the ordinary window would lie partly or wholly below the threshold, |Q|, where the reaction cannot happen; the range shown starts at the threshold and runs to where the product of the Boltzmann factor and the Coulomb penetration, exp(−E/kT − 2πη), has fallen by 1/e from its maximum above threshold.

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) [7]
  • —P. Adsley et al., Phys. Rev. C 103, 015805 (2021) [8]

Open questions

  • A direct measurement below the 830 keV resonance, which needs the low neutron background of an underground laboratory and a neutron detector with energy sensitivity.
  • Spin, parity and α width of the 26Mg states just above the neutron threshold, where transfer and neutron-resonance data still disagree.
  • The ratio of the (α,n) and (α,γ) strengths of the 830 keV resonance, about 3 in direct measurements and about 1 from transfer data.

Discussed on the open-questions page

Related reactions

References

  1. 1.

    H. W. Drotleff, A. Denker, H. Knee, M. Soiné, G. Wolf, J. W. Hammer, U. Greife, C. Rolfs, H. P. Trautvetter

    Reaction rates of the s-process neutron sources 13C(α,n)16O and 22Ne(α,n)25Mg

    Astrophys. J. 414, 735 (1993)

  2. 2.

    M. Jaeger, R. Kunz, A. Mayer, J. W. Hammer, G. Staudt, K. L. Kratz, B. Pfeiffer

    22Ne(α,n)25Mg: the key neutron source in massive stars

    Phys. Rev. Lett. 87, 202501 (2001) · 10.1103/PhysRevLett.87.202501

  3. 3.
  4. 4.
  5. 5.
  6. 6.
  7. 7.

    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)

  8. 8.

How to cite this page

J. Skowronski, 22Ne(α,n)25Mg, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/22Ne-an-25Mg.

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

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