The first and slowest step of the NeNa cycle. A 20Ne nucleus captures a proton and becomes 21Na, giving off the extra energy as a γ-ray.
20Ne is the most abundant neon isotope, and in the NeNa cycle nuclei wait longest as 20Ne before the next proton arrives. This reaction therefore sets how fast neon is turned into sodium in the hydrogen-burning layers of red giants, of massive stars on the asymptotic giant branch (AGB stars, in the late stage where hydrogen and helium burn in thin shells), and in nova explosions. Because so much neon is available, even a slow leak through this step matters for the sodium made in these stars.
Facts
- How well known
- Measured underground at low energies, and its main resonance is known to about 5%; the extension to the lowest energies still depends on a model.
- Live evaluation
- Plot the data · 21Na →
- Reaction rates
- Explore the rates →
- Q-value · energy released
- 2.432 MeV
- Product 21Na
- radioactive, t½ = 22.5 s
- Kind
- Radiative capture
How it was measured
3 measurements, 2018–2025, 2 at LUNAShow the listHide the list
2018
Nuclear Science Laboratory, Notre DameUniversity of Notre Dame
Direct measurement at the surface · Eₚ = 500–2000 keV
Cross sections for the primary transitions to the bound states of 21Na. An R-matrix fit separated direct capture from the subthreshold resonance; the resulting rate is lower than those in the libraries of the time by up to 20% in the temperature range of interest. [1]
2023
LUNA 400 kVLaboratori Nazionali del Gran Sasso (INFN)
Direct measurement underground · E_cm below 352 keV, and the 368 keV resonance
First measurement of direct capture below E_cm = 352 keV, with a windowless gas target and two HPGe detectors. Resonance strength ωγ = 0.112 ± 0.002 (stat) ± 0.005 (sys) meV, three times more precise than before; the rate is 20% lower than previously adopted below 0.1 GK and unchanged above. [2]
2025
LUNA 400 kVLaboratori Nazionali del Gran Sasso (INFN)
Direct measurement underground · E_cm ≈ 368 keV resonance (366 keV in the report) and direct capture below 400 keV
Conference report on the LUNA campaign with a windowless gas target and two HPGe detectors, the campaign published in Masha et al. (2023); it describes the experiment and gives preliminary results on the resonance and on direct capture at the lowest energies. [3]
Measurements (EXFOR)
Every dataset EXFOR holds for this reaction, drawn by the energies it covers, against the energies a star uses.
20 datasets in 8 EXFOR entries, 1959–2023 · 3 without an energy range, not drawn
- Lowest energy in EXFOR
- 248 keV
- Masha et al., 2023 · S · O2598.003
- Distance from the Gamow peak
- 2.3 Gamow widths above
- E₀ = 104 keV, window 72.8–135 keV
- Rate from energies not in EXFOR
- > 99.99%
- at T₉ = 0.08, 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 | 1 | 1.05 MeV – 1.05 MeV |
| SS-factor | 1 | 1.05 MeV – 1.05 MeV |
| σ→Eₓpartial | 6 | 248 keV – 1.84 MeV |
| dσ/dΩangular distribution | 2 | 348 keV – 2.15 MeV |
| Yyield | 1 | 1.11 MeV – 1.11 MeV |
| ωγresonance strength | 5 | 350 keV – 1.11 MeV |
| …other | 1 | 350 keV – 350 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.
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: Underground data now reach below E_cm = 352 keV and the 368 keV resonance strength is known to about 5%; the low-energy extrapolation still rests on the separation of direct capture from the subthreshold state.
Below about 0.4 MeV the cross section is non-resonant. It is dominated by direct capture and by the tail of a 21Na state just below the proton threshold, the 2425 keV level, whose contribution has to be separated from direct capture in an R-matrix fit. The first resonance above threshold, at E_cm = 368 keV, is narrow and contributes at higher temperatures.
The Notre Dame data of Lyons et al. (2018) between Eₚ = 0.5 and 2.0 MeV fixed the primary transitions to the bound states for the R-matrix extrapolation. The LUNA measurement of Masha et al. (2023) used a windowless neon gas target and two HPGe detectors deep underground to measure direct capture for the first time below E_cm = 352 keV, and remeasured the energy and strength of the 368 keV resonance.
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) [4]
Open questions
- The contribution of the subthreshold 2425 keV state relative to direct capture at E_cm below 300 keV, which sets the rate at the lowest temperatures.
- The LUNA strength of the 368 keV resonance is three times more precise than earlier values and has not yet been checked by an independent measurement.
- A combined R-matrix analysis of the capture data at all energies, to fix the shape of the S-factor near the threshold.
Related reactions
References
- 1.
S. Lyons et al.
Determination of 20Ne(p,γ)21Na cross sections from Eₚ = 500–2000 keV
Phys. Rev. C 97, 065802 (2018) · 10.1103/PhysRevC.97.065802
- 2.
E. Masha, L. Barbieri, J. Skowronski et al. (LUNA Collaboration)
Phys. Rev. C 108, L052801 (2023) · 10.1103/PhysRevC.108.L052801 · arXiv:2311.04089LUNA
- 3.
A. Caciolli (LUNA Collaboration)
Study of the 20Ne(p,γ)21Na reaction at LUNA
(2025) · arXiv:2510.01884LUNA
- 4.
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)
How to cite this page
J. Skowronski, 20Ne(p,γ)21Na, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/20Ne-pg-21Na.
Cite the date you used; it is the last change to this page.
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