Oxygen-16 captures a neutron. Oxygen is so abundant that this tiny cross section makes it one of the main neutron poisons in massive stars.
In the helium core and the carbon-burning shells of a massive star, oxygen-16 is among the most abundant nuclei. Every neutron it captures is taken from the iron seeds of the weak s-process. The loss is not always final: the oxygen-17 it makes can capture a helium nucleus and give the neutron back through 17O(α,n)20Ne, so the two reactions together decide how strongly oxygen poisons the process.
Facts
- How well known
- Measured, but small and hard to measure; a 2016 review of all the data changed how the recommended value varies with temperature.
- Live evaluation
- No live evaluation yet · Evaluations
- Reaction rates
- Explore the rates →
- Q-value · energy released
- 4.143 MeV
- Thermal energy kT
- about 26 keV, the energy of a gas at 300 million kelvin
- Why no Gamow window
- A neutron feels no Coulomb barrier, so there is no Gamow window: the relevant energies are those of the thermal distribution itself, around kT.
- Kind
- Radiative capture
How it was measured
1 measurement, 1995Show the listHide the list
1995
Neutron beams and time of flight · 10–80 keV (neutron energy)
Cross section measured at stellar energies; it is dominated by nonresonant direct p-wave capture, and the Maxwellian average was derived. [1]
Measurements (EXFOR)
Every dataset EXFOR holds for this reaction, drawn by the energies it covers, against the energies a star uses.
31 datasets in 12 EXFOR entries, 1963–2020 · 4 without an energy range, not drawn
massive star, core helium burning, kT = 25.9 keV
- Lowest energy in EXFOR
- 23.8 meV
- McDonald et al., 1977; Firestone et al., 2016 · σ · 10668.004
- Maxwellian averages measured
- 4 sets
- kT from 30 keV to 30 keV
- Rate from energies not in EXFOR
- > 99.99%
- at kT = 25.9 keV, smooth estimate
- Energies that matter at kT = 25.9 keV
- 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 | 7 | 23.8 meV – 531 keV |
| σ→Eₓpartial | 12 | 23.8 meV – 531 keV |
| dσ/dΩangular distribution | 1 | 6.58 MeV – 13.2 MeV |
| ωγresonance strength | 1 | 408 keV – 408 keV |
| MACSMaxwellian average | 4 | 30 keV – 30 keV |
| …other | 2 | 23.8 meV – 6.98 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: The 2016 re-evaluation of all data changed the energy dependence of the recommended stellar cross section.
At stellar energies the cross section is dominated by non-resonant direct p-wave capture, which makes it rise with energy rather than fall as 1/v; this was established by measurements between 10 and 80 keV (Igashira et al. 1995). Mohr et al. (2016) re-evaluated all data and recommended Maxwellian-averaged cross sections from kT = 5 to 100 keV that are lower than the previous recommendation up to kT = 60 keV and up to 14% higher at 100 keV, and studied the effect on the weak s-process in core helium burning (kT = 26 keV) and shell carbon burning (kT = 90 keV).
Evaluations
Compilations that combine the data into a recommended rate.
Related reactions
References
- 1.
M. Igashira, Y. Nagai, K. Masuda, T. Ohsaki, H. Kitazawa
Measurement of the 16O(n,γ)17O reaction cross section at stellar energy and the critical role of nonresonant p-wave neutron capture
Astrophys. J. 441, L89 (1995)
- 2.
Z. Y. Bao, H. Beer, F. Käppeler, F. Voss, K. Wisshak, T. Rauscher
Neutron cross sections for nucleosynthesis studies
At. Data Nucl. Data Tables 76, 70 (2000) · 10.1006/adnd.2000.0838
- 3.
P. Mohr, C. Heinz, M. Pignatari, I. Dillmann, A. Mengoni, F. Käppeler
Astrophys. J. 827, 29 (2016) · arXiv:1605.02639
How to cite this page
J. Skowronski, 16O(n,γ)17O, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/16O-ng-17O.
Cite the date you used; it is the last change to this page.
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