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Reaction atlas · CNO cycles

Radiative capture · CNO-II

15N(p,γ)16O

: nitrogen‑15 + proton → oxygen‑16 + gamma rayHow to read the notation

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The branch point of the CNO cycles. Rarely, nitrogen-15 keeps the captured proton and becomes oxygen-16, and the material passes from the first CNO cycle into the second.

Every time this reaction happens instead of the much more frequent 15N(p,α)12C, a nucleus leaves the CN cycle and enters the cycle that runs through oxygen and fluorine. Its rate therefore sets how much oxygen-16, oxygen-17 and fluorine hydrogen burning produces in red giants, in AGB stars and in nova explosions. Measurements deep underground at Gran Sasso, by the LUNA collaboration, showed that the rate is about half the value used in the older compilations.

Facts

How well known
Measurements deep underground reach 70 keV and fix the rate to about 10%. The remaining uncertainty is in extending the data to lower energies, where several contributions to the reaction interfere with each other.
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Reaction rates
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Q-value · energy released
12.127 MeV
Gamow window
Kind
Radiative capture

How it was measured

5 measurements, 2008–2012, 2 at LUNAShow the list
  1. 2008

    Transfer reactions and ANCs · transfer reactions; R-matrix fit

    Asymptotic normalization coefficients for 15N + p → 16O fix the direct capture to the 16O ground state. The R-matrix fit gives about one CN catalyst lost to the second cycle every 2200 ± 300 cycles, instead of about 1000 in the NACRE compilation. [1]

  2. 2009

    LUNA 400 kVLaboratori Nazionali del Gran Sasso (INFN)

    Direct measurement underground · 90–230 keV

    Total cross section with a windowless nitrogen gas target and a 4π BGO summing detector; the cross section at nova energies is a factor of 2 lower than previously believed. [2]

  3. 2010

    Nuclear Science Laboratory, Notre DameUniversity of Notre Dame

    Direct measurement at the surface · 130–1800 keV (proton energy)

    Measurement across both broad resonances, at Ep = 338 and 1028 keV, and below them; a multilevel R-matrix analysis gives S(0) = 39.6 keV b. [3]

  4. 2011

    LUNA 400 kVLaboratori Nazionali del Gran Sasso (INFN)

    Direct measurement underground · 70–370 keV

    Extends the LUNA data and confirms the earlier R-matrix extrapolation. The rate is about a factor of 2 below NACRE and CF88, with a 10% uncertainty. [4]

  5. 2012

    Nuclear Science Laboratory, Notre DameUniversity of Notre Dame

    Direct measurement at the surface

    Exclusive cross sections for the γ-ray cascades through excited states of 16O, measured together with 15N(p,α1γ)12C. [5]

Measurements (EXFOR)

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

77 datasets in 18 EXFOR entries, 1952–2012 · 13 without an energy range, not drawn

core of a 15 M☉ star, T₉ = 0.035 (35 MK)

Lowest energy in EXFOR
72.8 keV
Caciolli et al., 2011 · S · O2205.002
Distance from the Gamow peak
0.94 Gamow widths above
E₀ = 46.9 keV, window 33.1–60.6 keV
Rate from energies not in EXFOR
98%
at T₉ = 0.035, smooth estimate · 98% of it lies below the lowest energy in EXFOR
10 keV100 keV1 MeV10 MeV100 MeVCentre-of-mass energy (keV)E₀ = 46.9 keVσSσ→Eₓdσ/dΩAyotherlowest in EXFOR 72.8 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 section2292 keV – 1.33 MeV
SS-factor272.8 keV – 368 keV
σ→Eₓpartial19123 keV – 37.2 MeV
dσ/dΩangular distribution345.25 MeV – 88.4 MeV
Aypolarization65.18 MeV – 46.5 MeV
…other15.72 MeV – 12.8 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

The capture at low energy is dominated by the two broad 1− resonances in 16O that also govern 15N(p,α)12C, interfering with each other and with direct capture to the 16O ground state. The non-resonant contribution and the interference sign between the resonances determine the S-factor below the first resonance. Asymptotic normalization coefficients (ANCs) from transfer reactions constrain the direct-capture component. The LUNA data, from 70 to 370 keV, and the Notre Dame data, from 130 keV to 1.8 MeV, give a rate about a factor of 2 below the NACRE and CF88 compilations, with an uncertainty near 10%.

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.

  • —E. G. Adelberger et al., Rev. Mod. Phys. 83, 195 (2011) [6]
  • —Y. Xu et al., Nucl. Phys. A 918, 61 (2013) [7]
  • —B. Acharya et al., Rev. Mod. Phys. 97, 035002 (2025) [8]

Open questions

  • The direct-capture contribution to the ground state, which sets the S-factor below the lower resonance.

Related reactions

References

  1. 1.
  2. 2.
  3. 3.

    P. J. LeBlanc, G. Imbriani, J. Görres, M. Wiescher et al.

    Constraining the S factor of 15N(p,γ)16O at astrophysical energies

    Phys. Rev. C 82, 055804 (2010) · 10.1103/PhysRevC.82.055804 · arXiv:1011.2518

  4. 4.

    A. Caciolli et al. (LUNA Collaboration)

    Revision of the 15N(p,γ)16O reaction rate and oxygen abundance in H-burning zones

    Astron. Astrophys. 533, A66 (2011) · 10.1051/0004-6361/201117475 · arXiv:1107.4514LUNA

  5. 5.

    G. Imbriani, R. J. deBoer, A. Best, M. Couder, G. Gervino, J. Görres, P. J. LeBlanc, H. Leiste, A. Lemut, E. Stech, F. Strieder, E. Uberseder and M. Wiescher

    Measurement of γ rays from 15N(p,γ)16O cascade and 15N(p,α1γ)12C reactions

    Phys. Rev. C 85, 065810 (2012); erratum Phys. Rev. C 86, 039902 (2012) · 10.1103/PhysRevC.85.065810

  6. 6.

    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

  7. 7.
  8. 8.

How to cite this page

J. Skowronski, 15N(p,γ)16O, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/15N-pg-16O.

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

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