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
- 480–710 keVAbove the 0.48 MeV thresholdAGB thermal pulse, 0.27 GK (Neutron sources of the s-process)
- 480–765 keVAbove the 0.48 MeV thresholdmassive star, He core end, 0.3 GK (Neutron sources of the s-process)
- Kind
- Particle-exchange reaction
On the timeline
How it was measured
- Direct measurement at the surface
- Neutron beams and time of flight
- Transfer reactions and ANCs
- Direct measurement underground
6 measurements, 1993–2024Show the listHide the list
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]
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]
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]
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]
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]
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
- 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.
- 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) |
|---|---|---|
| σcross section | 5 | 476 keV – 4.34 MeV |
| SS-factor | 1 | 476 keV – 1.96 MeV |
| σ→Eₓpartial | 3 | 623 keV – 1.8 MeV |
| ωγresonance strength | 4 | 537 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.
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.
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.
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.
C. Massimi et al. (n_TOF Collaboration)
Neutron spectroscopy of 26Mg states: constraining the stellar neutron source 22Ne(α,n)25Mg
Phys. Lett. B 768, 1 (2017) · 10.1016/j.physletb.2017.02.025 · arXiv:1702.04520
- 4.
S. Ota, G. Christian et al.
Decay properties of 22Ne + α resonances and their impact on s-process nucleosynthesis
Phys. Lett. B 802, 135256 (2020) · 10.1016/j.physletb.2020.135256 · arXiv:2001.08206
- 5.
H. Jayatissa, G. V. Rogachev et al.
Phys. Lett. B 802, 135267 (2020) · 10.1016/j.physletb.2020.135267 · arXiv:2001.08316
- 6.
Shahina, R. J. deBoer, M. Wiescher et al.
Phys. Rev. C 110, 015801 (2024) · 10.1103/PhysRevC.110.015801
- 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.
P. Adsley et al.
Reevaluation of the 22Ne(α,γ)26Mg and 22Ne(α,n)25Mg reaction rates
Phys. Rev. C 103, 015805 (2021) · 10.1103/PhysRevC.103.015805 · arXiv:2005.14482
How to cite this page
J. Skowronski, 22Ne(α,n)25Mg, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/22Ne-an-25Mg.
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