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Reaction atlas · Helium burning

Radiative capture · Helium burning

14N(α,γ)18F

: nitrogen‑14 + helium‑4 → fluorine‑18 + gamma rayHow to read the notation

On this page

Nitrogen-14 captures a helium nucleus and becomes fluorine-18. It is the first step that turns the nitrogen left over from hydrogen burning into neon-22, the fuel of a stellar neutron source.

Hydrogen burning through the CNO cycle leaves most of a star's carbon, nitrogen and oxygen in the form of nitrogen-14. When helium starts to burn, this nitrogen captures helium and becomes fluorine-18, which decays within hours to oxygen-18. Oxygen-18 captures another helium nucleus and becomes neon-22. The amount of neon-22 that later releases neutrons is therefore set by the nitrogen that went through this reaction.

Facts

How well known
The resonances that set the rate have been measured directly; the lowest one is known only from a theoretical argument.
Live evaluation
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Reaction rates
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Q-value · energy released
4.415 MeV
Gamow window
  • 355 ± 90 keVcore helium burning, 0.2 GK (Helium burning)
Product 18F
radioactive, t½ = 1.83 h
Kind
Radiative capture

How it was measured

1 measurement, 2000Show the list
  1. 2000

    Forschungszentrum Karlsruhe, Karlsruhe, and University of Notre Dame

    Direct measurement at the surface · low-energy resonances

    Strengths of the low-energy resonances with thin TiN targets and a Ge clover detector; the lowest resonance was not observed and was argued to be far too weak to measure. [1]

Measurements (EXFOR)

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

4 datasets in 3 EXFOR entries, 1971–2000 · 1 without an energy range, not drawn

core helium burning, T₉ = 0.2 (200 MK)

Lowest energy in EXFOR
446 keV
Goerres et al., 2000 · ωγ · C0843.002
Distance from the Gamow peak
0.51 Gamow widths above
E₀ = 354 keV, window 264–444 keV
Rate from energies not in EXFOR
88%
at T₉ = 0.2, smooth estimate · 88% 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 = 354 keVYωγlowest in EXFOR 446 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)
Yyield14.84 MeV – 5.65 MeV
ωγresonance strength2241 keV – 887 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

At helium-burning temperatures the rate is dominated by a few narrow resonances in 18F, and the compiled rates are sums of their strengths. Görres et al. (2000) measured the low-energy resonances with thin TiN targets and a Ge clover detector at Karlsruhe and Notre Dame. The lowest resonance was not observed; the authors argued that its strength lies far below the reach of present experiments.

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]
  • —C. Iliadis et al., Nucl. Phys. A 841, 31 (2010) [3]

Open questions

  • The strength of the lowest resonances, which control the rate at the start of helium burning.

Related reactions

References

  1. 1.

    J. Görres, C. Arlandini, U. Giesen, M. Heil, F. Käppeler, H. Leiste, E. Stech, M. Wiescher

    Low-energy resonances in 14N(α,γ)18F and their astrophysical implications

    Phys. Rev. C 62, 055801 (2000) · 10.1103/PhysRevC.62.055801

  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)

  3. 3.

How to cite this page

J. Skowronski, 14N(α,γ)18F, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/14N-ag-18F.

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

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