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Reaction atlas · Proton–proton chains

Particle-exchange reaction · pp-II

7Li(p,α)4He

: lithium‑7 + proton → helium‑4 + helium‑4How to read the notation

On this page

A 7Li nucleus captures a proton and splits into two 4He nuclei. It is the last step of the pp-II chain, and it destroys lithium in stars.

The reaction closes the pp-II chain in the Sun but never limits its speed. It matters more as a destroyer of lithium: 7Li burns already at temperatures of a few million kelvin, so stars lose their lithium as soon as it is carried deep enough by mixing. The lithium seen at the surface of a star is therefore a clue to how its outer layers mix. After the Big Bang the reaction also removed part of the 7Li made directly.

Facts

How well known
Measured down to low energies. The remaining uncertainty is the correction for the atomic electrons of the target, which make the reaction easier in the laboratory than it is for bare nuclei.
Live evaluation
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Reaction rates
Explore the rates →
Q-value · energy released
17.346 MeV
Gamow window
  • 15 ± 5.2 keVcentre of the Sun, 15.7 MK (pp-II)
Kind
Particle-exchange reaction

How it was measured

2 measurements, 1992–2001Show the list
  1. 1992

    Ruhr-Universität Bochum and Universität Münster

    Direct measurement at the surface · 10–1450 keV (whole data set)

    Proton, deuteron and lithium beams with atomic and molecular targets, to compare electron screening in 6Li and 7Li reactions; the low-energy cross sections are enhanced in all cases, somewhat more for atomic than for molecular targets. [1]

  2. 2001

    Trojan Horse method · 10–400 keV

    Bare S-factor from the Trojan Horse method in a distorted-wave Born approximation analysis: S_b(0) = 55 ± 3 keV b and a screening potential Uₑ = 330 ± 40 eV, above the adiabatic limit of 175 eV. [2]

Measurements (EXFOR)

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

67 datasets in 40 EXFOR entries, 1938–2025 · 2 without an energy range, not drawn

centre of the Sun, T₉ = 0.0157 (15.7 MK)

Lowest energy in EXFOR
10.1 keV
Schroeder et al., 1989 · S · A1291.005
Distance from the Gamow peak
Inside the window
E₀ = 15.3 keV, window 10–20.5 keV
Rate from energies not in EXFOR
3.3%
at T₉ = 0.0157, smooth estimate · 3.3% of it lies below the lowest energy in EXFOR
1 keV10 keV100 keV1 MeV10 MeV100 MeVCentre-of-mass energy (keV)E₀ = 15.3 keVσSσ→Eₓdσ/dΩAyYotherlowest in EXFOR 10.1 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 section138.74 keV – 10.5 MeV
SS-factor108.8 keV – 1.52 MeV
σ→Eₓpartial1370 keV – 2.1 MeV
dσ/dΩangular distribution3520.7 keV – 19.2 MeV
Aypolarization17.87 MeV – 19.2 MeV
Yyield319.7 keV – 887 keV
…other276 keV – 6.87 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 cross section is large enough to be measured with good statistics at low energies, which made it a standard case for studying electron screening in laboratory targets: at the lowest energies the measured S-factor rises above the bare-nucleus trend. Engstler et al. (1992) measured it with proton beams on lithium targets and with lithium beams on hydrogen gas, and found the enhancement in all cases. The Trojan Horse method, which is free of screening effects, gave a bare S(0) = 55 ± 3 keV b; compared with the direct data it implies a screening potential of 330 ± 40 eV, well above the adiabatic limit of 175 eV.

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.

  • —P. Descouvemont et al., At. Data Nucl. Data Tables 88, 203 (2004) [3]
  • —Y. Xu et al., Nucl. Phys. A 918, 61 (2013) [4]

Open questions

  • The electron-screening potential extracted from low-energy data, which exceeds the atomic-physics estimate.

Related reactions

References

  1. 1.

    S. Engstler, G. Raimann, C. Angulo, U. Greife, C. Rolfs, U. Schröder, E. Somorjai, B. Kirch, K. Langanke

    Isotopic dependence of electron screening in fusion reactions

    Z. Phys. A 342, 471 (1992) · 10.1007/BF01294958

  2. 2.

    M. Lattuada, R. G. Pizzone, S. Typel et al.

    The bare astrophysical S(E) factor of the 7Li(p,α)α reaction

    Astrophys. J. 562, 1076 (2001)

  3. 3.

    P. Descouvemont, A. Adahchour, C. Angulo, A. Coc, E. Vangioni-Flam

    Compilation and R-matrix analysis of Big Bang nuclear reaction rates

    At. Data Nucl. Data Tables 88, 203 (2004) · arXiv:astro-ph/0407101

  4. 4.

How to cite this page

J. Skowronski, 7Li(p,α)4He, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/7Li-pa-4He.

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

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