A proton capture on the rare neon isotope 21Ne that makes radioactive 22Na. It is the second proton capture of the NeNa cycle.
21Ne is the rarest of the stable neon isotopes, and in hydrogen burning it is destroyed quickly by this reaction. Its rate therefore sets how much 21Ne survives in stars and how much 22Na is made. In nova explosions, 22Na is of special interest because its decay gives a γ-ray line that space telescopes search for; the amount predicted depends in part on this reaction.
Facts
- How well known
- The main low-energy resonances were remeasured underground in 2025; two of them came out more than 50% stronger than in the earlier literature, with uncertainties of about 5%.
- Live evaluation
- No live evaluation yet · Evaluations
- Reaction rates
- Explore the rates →
- Q-value · energy released
- 6.739 MeV
- Product 22Na
- radioactive, t½ = 2.6 yr
- Kind
- Radiative capture
How it was measured
1 measurement, 2025, 1 at LUNAShow the listHide the list
2025
LUNA 400 kVLaboratori Nazionali del Gran Sasso (INFN)
Direct measurement underground · Eₚ = 127.3–352.6 keV (five resonances)
Resonance strengths measured deep underground. The 272.3 and 352.6 keV resonances are more than 1.5 times stronger than the literature values; new γ-ray branching ratios for three resonances and an updated rate for nova conditions. [1]
Measurements (EXFOR)
Every dataset EXFOR holds for this reaction, drawn by the energies it covers, against the energies a star uses.
3 datasets in 2 EXFOR entries, 1977–2005
- Lowest energy in EXFOR
- 247 keV
- Engel et al., 2005 · ωγ · C1403.003
- Distance from the Gamow peak
- 2.3 Gamow widths above
- E₀ = 104 keV, window 72.9–135 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) |
|---|---|---|
| ωγresonance strength | 3 | 247 keV – 1.94 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 rate at the temperatures of hydrogen shell burning and of novae is given by a few narrow resonances. The LUNA measurement of Sidhu et al. (2025) remeasured the resonances at Eₚ = 127.3, 271.4, 272.3, 291.5 and 352.6 keV deep underground. The strengths of the 127.3, 271.4 and 291.5 keV resonances agree with earlier values within 1σ, while those of the 272.3 and 352.6 keV resonances, ωγ = 129.9 ± 5.8 meV and 14.9 ± 0.8 meV, are more than a factor 1.5 above the literature values. New branching ratios were obtained for the 127.3, 272.3 and 352.6 keV resonances, and the thermonuclear rate was updated for nova temperatures.
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]
Open questions
- The origin of the discrepancy between the new and the earlier strengths of the 272.3 and 352.6 keV resonances.
- An independent measurement of the 272.3 and 352.6 keV strengths with a different target and detection technique.
Related reactions
References
- 1.
R. S. Sidhu et al. (LUNA Collaboration)
Improved direct measurement of low-energy resonances in the 21Ne(p,γ)22Na reaction
Phys. Rev. C 112, L052801 (2025) · 10.1103/vqt1-jxpw · arXiv:2505.22426LUNA
- 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)
How to cite this page
J. Skowronski, 21Ne(p,γ)22Na, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/21Ne-pg-22Na.
Cite the date you used; it is the last change to this page.
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