Oxygen-16 captures a proton and becomes radioactive fluorine-17. It is the slowest step of the second CNO cycle.
Because this capture is slow, most of the oxygen-16 in a star survives hydrogen burning. The small part that burns makes oxygen-17, whose amount relative to oxygen-16 is measured in red giants and in grains of stardust found in meteorites, and is used to test stellar models.
Facts
- How well known
- Known to about 4%: how the cross section changes with energy is well understood from theory, and the remaining uncertainty is in the overall scale of the measurements.
- Live evaluation
- Plot the data · 17F →
- Reaction rates
- Explore the rates →
- Q-value · energy released
- 0.600 MeV
- Product 17F
- radioactive, t½ = 1.07 min
- Kind
- Radiative capture
How it was measured
2 measurements, 1997–2022Show the listHide the list
1997
Direct measurement at the surface
Cross sections for capture to the ground state and to the first excited state. The S-factor for the excited-state transition rises as the energy decreases, which the authors explained by the loose binding of the proton in the 1/2+ state. [1]
2022
Calculation from theory · all reliable data sets, refitted
All reliable cross sections fitted with a single-particle potential model, the ANCs of the 17F ground and first excited states used as scaling parameters, with statistical and systematic uncertainties treated separately; the rate uncertainty is ±4.2%, about half of the previously reported 7.5%. [2]
Measurements (EXFOR)
Every dataset EXFOR holds for this reaction, drawn by the energies it covers, against the energies a star uses.
26 datasets in 11 EXFOR entries, 1954–2012 · 5 without an energy range, not drawn
core of a 15 M☉ star, T₉ = 0.035 (35 MK)
- Lowest energy in EXFOR
- 132 keV
- Hester et al., 1958 · σ · C0174.002
- Distance from the Gamow peak
- 2.8 Gamow widths above
- E₀ = 51.3 keV, window 36.9–65.7 keV
- Rate from energies not in EXFOR
- > 99.99%
- at T₉ = 0.035, smooth estimate · > 99.99% of it lies below the lowest energy in EXFOR
- 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 | 3 | 132 keV – 580 keV |
| SS-factor | 3 | 249 keV – 2.82 MeV |
| σ→Eₓpartial | 3 | 470 keV – 2.4 MeV |
| dσ/dΩangular distribution | 9 | 225 keV – 41.8 MeV |
| Aypolarization | 1 | 19.6 MeV – 19.6 MeV |
| …other | 2 | 941 keV – 28.2 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.
Try it: an R-matrix evaluation
This system has a live AZURE2 model on this site: change the energies and widths of its levels and see how the fit to the data and the extrapolation to stellar energies change.
Details
How well known, in detail: A non-resonant direct-capture reaction whose energy dependence is fixed by theory; a Bayesian analysis (2022) gives the rate to ±4.2%, about half the earlier 7%, limited by the normalization of the surface data.
There are no resonances at stellar energies. The cross section is direct capture to the 17F ground state and to its first excited state, a weakly bound 1/2+ level whose extended wave function makes the capture to it dominant at low energy and gives the S-factor a steep rise towards zero energy. The energy dependence is therefore well described by direct-capture and microscopic models, and the rate rests on surface data scaled with such a model.
Solar Fusion II adopts S(0) = 10.6 ± 0.8 keV b from model calculations normalized to the data of 1973 and 1997. A 2008 evaluation, which corrected the data sets since 1958 for coincidence summing and stopping powers, gives the rate with an uncertainty of about 7%. A Bayesian fit of all reliable data, with a potential model whose scaling parameters are the asymptotic normalization coefficients of the two bound states, halves that uncertainty to ±4.2% (Iliadis et al. 2022).
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
- Direct data closer to the Gamow window, to test the predicted rise of the S-factor at low energy.
Related reactions
References
- 1.
R. Morlock, R. Kunz, A. Mayer, M. Jaeger et al.
Halo properties of the first 1/2+ state in 17F from the 16O(p,γ)17F reaction
Phys. Rev. Lett. 79, 3837 (1997) · 10.1103/PhysRevLett.79.3837
- 2.
C. Iliadis, V. Palanivelrajan, R. S. de Souza
Bayesian estimation of the S factor and thermonuclear reaction rate for 16O(p,γ)17F
Phys. Rev. C 106, 055802 (2022) · 10.1103/PhysRevC.106.055802 · arXiv:2210.14354
- 3.
E. G. Adelberger et al.
Solar fusion cross sections II: the pp chain and CNO cycles
Rev. Mod. Phys. 83, 195 (2011) · 10.1103/RevModPhys.83.195
- 4.
C. Iliadis, C. Angulo, P. Descouvemont, M. Lugaro and P. Mohr
Phys. Rev. C 77, 045802 (2008) · 10.1103/PhysRevC.77.045802 · arXiv:0803.2757
- 5.
B. Acharya et al.
Solar fusion III: New data and theory for hydrogen-burning stars
Rev. Mod. Phys. 97, 035002 (2025) · 10.1103/8lm7-gs18 · arXiv:2405.06470
How to cite this page
J. Skowronski, 16O(p,γ)17F, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/16O-pg-17F.
Cite the date you used; it is the last change to this page.
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