A 3He nucleus captures a proton and becomes 4He directly, emitting a positron and a neutrino. It is the rarest branch of the pp chain and makes the most energetic solar neutrinos, called hep neutrinos.
Only a tiny fraction of the pp-chain terminations goes this way, so the reaction adds nothing to the energy of the Sun. Its neutrinos, however, reach higher energies than any other solar neutrinos, beyond the end of the 8B spectrum. Large water and scintillator detectors search for them as a test of solar models; so far only upper limits have been set.
Facts
- How well known
- Known only from theory, and only roughly; solar-neutrino detectors have so far set only upper limits on the neutrinos it makes.
- Live evaluation
- No live evaluation yet · Evaluations
- Reaction rates
- Explore the rates →
- Q-value · energy released
- 19.795 MeV
- Q includes positron annihilation (2mec² = 1.022 MeV). Without it, 18.773 MeV is shared by the positron and the neutrino.
- Gamow window
- 11 ± 4.5 keVcentre of the Sun, 15.7 MK (hep)
- Kind
- Weak interaction
How it was measured
1 measurement, 2000Show the listHide the list
2000
Calculation from theory · 0–10 keV
Ab initio calculation with the AV18 two-nucleon and Urbana IX three-nucleon interactions; S(0) = 9.64 × 10−20 keV b and 10.1 × 10−20 keV b at 10 keV, about 4.5 times the earlier standard value of 2.3 × 10−20 keV b. P-wave transitions give about 40% of the result. [1]
Details
How well known, in detail: Known only from theory, with large relative uncertainty because of cancellations in the weak matrix element; solar-neutrino detectors give only upper limits on the flux.
The reaction is a weak capture like p + p, but the leading one-body Gamow–Teller matrix element is strongly suppressed by the symmetries of the 3He and 4He wave functions. Two-body axial currents and small components of the wave functions therefore contribute at the same level as the one-body term, and the result is sensitive to cancellations between them. The cross section cannot be measured; the S-factor is taken from ab initio calculations, and the predicted hep flux carries a much larger relative uncertainty than any other solar-neutrino flux.
The calculation of Marcucci et al. (2000) with correlated hyperspherical-harmonics wave functions gave an S-factor about 4.5 times the value then used in the standard solar model, with P-wave captures contributing about 40%. On the observational side, the first phase of SNO set an upper limit of 2.3 × 104 cm−2 s−1 (90% C.L.) on the total hep-neutrino flux (Aharmim et al., Astrophys. J. 653, 1545, 2006).
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.
Open questions
- A first detection of hep neutrinos, which would test the calculated S-factor.
- The convergence of the two-body current contributions in ab initio calculations.
Related reactions
References
- 1.
L. E. Marcucci, R. Schiavilla, M. Viviani, A. Kievsky, S. Rosati, J. F. Beacom
Phys. Rev. C 63, 015801 (2000) · 10.1103/PhysRevC.63.015801 · arXiv:nucl-th/0006005
- 2.
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
- 3.
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, 3He(p,e+ν)4He, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/3He-hep-4He.
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