⁷Be(p,γ)⁸B · Reaction atlas

A ⁷Be nucleus captures a proton and becomes ⁸B, giving off a γ-ray. The ⁸B then decays and emits the high-energy solar neutrinos that started the solar neutrino problem.

Only a small fraction of the ⁷Be in the Sun captures a proton; most of it captures an electron instead. Yet this rare branch makes the most energetic of the common solar neutrinos, the ones seen by the first chlorine detector in the Homestake mine and later measured precisely by Super-Kamiokande and SNO. The predicted number of these neutrinos is directly proportional to the rate of this reaction, so it has to be known well to use them as a probe of the solar core and of neutrino properties.

Facts

How well known: The best direct experiments, all with radioactive ⁷Be targets, differ by up to about 15%; Solar Fusion III, combining them with Coulomb-dissociation data in one theoretical framework, recommends S(0) = 20.5 ± 0.7 eV b (3.4%).
Live evaluation: No live evaluation yet · Evaluations
Reaction rates: Explore the rates →
Q-value · energy released: 0.136 MeV
Gamow window: 18 ± 5.8 keVcentre of the Sun, 15.7 MK (pp-III)
Product ⁸B: radioactive, t_½ = 772 ms
Kind: Radiative capture

On the timeline

How it was measured

8 measurements, 1983–2010Show the list

1983
Argonne National Laboratory, Argonne IL
Direct measurement at the surface · low energies and the 1⁺ resonance
Radioactive ⁷Be target, ⁸B detected through the delayed α particles from its β decay; the ⁷Be areal density determined with two independent methods, which removed the dependence of earlier data on the ⁷Li(d,p)⁸Li normalization. [1]
1994
RIKEN, WakoRIKEN (today the RIKEN Nishina Center for Accelerator-Based Science)
Coulomb dissociation · ⁸B beam at about 50 MeV/u
First Coulomb-dissociation experiment: break-up of ⁸B into ⁷Be + p in the Coulomb field of a heavy target, converted to the low-energy capture cross section. [2]
1998
CENBG, Bordeaux-GradignanCentre d'Études Nucléaires de Bordeaux Gradignan (CNRS/IN2P3)
Direct measurement at the surface · 0.35–1.4 MeV
Two independent measurements with different beam conditions, targets and detectors, in agreement with each other; the extrapolated S(0) is 15–20% below the value recommended at the time. [3]
2001
CSNSM, OrsayCentre de Sciences Nucléaires et de Sciences de la Matière (CNRS/IN2P3 and Université Paris-Sud), since 2020 part of IJCLab
Direct measurement at the surface · 111.7, 134.7 and 185.8 keV
A 132 mCi ⁷Be target, intense proton beams and a large-acceptance solenoidal spectrometer; S(0) = 18.5 ± 2.4 eV b from these data and 18.8 ± 1.7 eV b combined with the 1998 results. [4]
2003
Weizmann Institute accelerator laboratoryWeizmann Institute of Science
Direct measurement at the surface
⁷Be implanted in the backing and a raster-scanned proton beam, which removed the loss of ⁸B by backscattering; extrapolated S(0) = 21.2 ± 0.7 eV b in the arXiv version (an erratum followed in 2004). [5]
2003
GSI, DarmstadtGSI Helmholtzzentrum für Schwerionenforschung
Coulomb dissociation · ⁸B beam at 254 MeV/u
Exclusive Coulomb-dissociation measurement at high beam energy; the full analysis followed in 2006. [6, 7]
2003
CENPA, University of WashingtonCenter for Experimental Nuclear Physics and Astrophysics, University of Washington
Direct measurement at the surface
Precision measurement with a ⁷Be target made at TRIUMF; S(0) = 22.1 ± 0.6 (expt) ± 0.6 (theor) eV b, and a recommended best value of 21.4 eV b. [8]
2010
CENPA, University of WashingtonCenter for Experimental Nuclear Physics and Astrophysics, University of Washington
Direct measurement at the surface · below the 1⁺ resonance
Reanalysis of the earlier data with a more detailed target analysis and improved stopping powers: S(0) = 21.5 ± 0.6 (expt) ± 0.6 (theor) eV b, and 20.9 ± 0.6 ± 0.6 eV b from a fit of all modern low-energy data with the same theory. [9]

Measurements (EXFOR)

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

32 datasets in 22 EXFOR entries, 1960–2010 · 4 without an energy range, not drawn

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

Lowest energy in EXFOR: 112 keV; Hammache et al., 2001 · S · F0502.002
Distance from the Gamow peak: 8.1 Gamow widths above; E₀ = 18.5 keV, window 12.7–24.3 keV
Rate from energies not in EXFOR: 100%; at T₉ = 0.0157, smooth estimate · 100% 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

The datasets of the gap map, by quantity
Quantity	Datasets	E_cm range (keV)
σcross section	11	116 keV – 8.75 MeV
SS-factor	17	112 keV – 2.99 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.

All measurements (32)Compound nucleus ⁸B

Details

The measurement is difficult because the target is radioactive: ⁷Be decays by electron capture, so targets must be produced, characterized and used within months, and their areal density and uniformity become the main systematic uncertainty. The capture is an external, non-resonant E1 process at low energy, with a narrow M1 resonance (the 1⁺ state of ⁸B) near 630 keV, and the extrapolation to solar energies relies on models of the energy dependence. Since 1998 the direct experiments with ⁷Be targets at Orsay and Bordeaux, at the Weizmann Institute and in Seattle give S(0) between about 18.5 and 22 eV b; the Seattle group quotes a model uncertainty of the extrapolation as large as its experimental one.

The S-factor has also been inferred from Coulomb dissociation of ⁸B beams on heavy targets, at RIKEN and at GSI. Solar Fusion II based its recommended value on the direct data alone. Solar Fusion III recommends S(0) = 20.5 ± 0.7 eV b, from a halo effective field theory analysis that includes the Coulomb-dissociation data as well as the direct captures.

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) [10]
—B. Acharya et al., Rev. Mod. Phys. 97, 035002 (2025) [11]

Open questions

The theoretical energy dependence used to extrapolate the direct data to solar energies.
The spread of about 15% between the S(0) values of the modern direct experiments.

Discussed on the open-questions page

⁷Be(p,γ)⁸B and the boron-8 solar neutrinos

Related reactions

References

1.
B. W. Filippone, A. J. Elwyn, C. N. Davids, D. D. Koetke
Proton capture cross section of ⁷Be and the flux of high energy solar neutrinos
Phys. Rev. C 28, 2222 (1983) · 10.1103/PhysRevC.28.2222
2.
T. Motobayashi et al.
Coulomb dissociation of ⁸B and the ⁷Be(p,γ)⁸B reaction at low energies
Phys. Rev. Lett. 73, 2680 (1994) · 10.1103/PhysRevLett.73.2680
3.
F. Hammache et al.
New measurement and analysis of the ⁷Be(p,γ)⁸B cross section
Phys. Rev. Lett. 80, 928 (1998) · 10.1103/PhysRevLett.80.928 · arXiv:nucl-ex/9712003
4.
F. Hammache et al.
Low-energy measurement of the ⁷Be(p,γ)⁸B cross section
Phys. Rev. Lett. 86, 3985 (2001) · 10.1103/PhysRevLett.86.3985 · arXiv:nucl-ex/0101014
5.
L. T. Baby, M. Hass et al.
Precision measurement of the ⁷Be(p,γ)⁸B cross section with an implanted ⁷Be target
Phys. Rev. Lett. 90, 022501 (2003); erratum Phys. Rev. Lett. 92, 029901 (2004) · 10.1103/PhysRevLett.90.022501 · arXiv:nucl-ex/0208005
6.
F. Schümann, F. Hammache, S. Typel et al.
Coulomb dissociation of ⁸B and the low-energy cross section of the ⁷Be(p,γ)⁸B solar fusion reaction
Phys. Rev. Lett. 90, 232501 (2003) · 10.1103/PhysRevLett.90.232501 · arXiv:nucl-ex/0304011
7.
F. Schümann, S. Typel et al.
Low-energy cross section of the ⁷Be(p,γ)⁸B solar fusion reaction from the Coulomb dissociation of ⁸B
Phys. Rev. C 73, 015806 (2006) · 10.1103/PhysRevC.73.015806
8.
A. R. Junghans et al.
Precise measurement of the ⁷Be(p,γ)⁸B S factor
Phys. Rev. C 68, 065803 (2003) · 10.1103/PhysRevC.68.065803 · arXiv:nucl-ex/0308003
9.
A. R. Junghans, K. A. Snover, E. C. Mohrmann, E. G. Adelberger, L. Buchmann
Updated S factors for the ⁷Be(p,γ)⁸B reaction
Phys. Rev. C 81, 012801(R) (2010) · 10.1103/PhysRevC.81.012801
10.
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
11.
B. Acharya et al.
Solar fusion III: New data and theory for hydrogen-burning stars
Rev. Mod. Phys. 97, 035002 (2025) · 10.1103/8lm7-gs18 · arXiv:2405.06470

How to cite this page

J. Skowronski, ⁷Be(p,γ)⁸B, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/7Be-pg-8B.

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