A 7Be nucleus absorbs a neutron and becomes 7Li, emitting a proton. After the Big Bang it was the main reaction that destroyed 7Be, and with it much of the future lithium.
Most of the primordial lithium was first made as 7Be, which turned into 7Li only after the nucleosynthesis was over. Any 7Be destroyed earlier lowers the lithium predicted today. This reaction removed the largest share. Because the predicted lithium is higher than the amount seen in old stars, its rate has been re-measured to check whether a larger value could close the gap.
Facts
- How well known
- Time-of-flight data now cover the BBN energies; the higher cross section they give is far from enough to solve the lithium problem.
- Live evaluation
- No live evaluation yet · Evaluations
- Q-value · energy released
- 1.644 MeV
- Half-life of 7Be
- 53.2 d
- Measured in the laboratory; in a star, thermally excited states and ionization can change it. It decides how often this nucleus decays before it captures a neutron.
- Thermal energy kT
- about 26–69 keV, the energy of a gas at 300–800 million kelvin
By stellar site
- 69 keVdeuterium bottleneck, 0.8 GK
- 26 keVend of BBN, 0.3 GK
- Why no Gamow window
- A neutron feels no Coulomb barrier, so there is no Gamow window: the relevant energies are those of the thermal distribution itself, around kT.
- Kind
- Particle-exchange reaction
How it was measured
3 measurements, 1988–2021Show the listHide the list
1988
LANSCE, Los Alamos National Laboratory, Los Alamos NM
Neutron beams and time of flight · 25 meV to 13.5 keV (neutron energy)
Time-of-flight measurement with a 7Be sample, normalized to the thermal cross section measured at the Omega West Reactor; it stops below the BBN energies. [1]
2018
n_TOF, CERNCERN
Neutron beams and time of flight · thermal to about 325 keV (neutron energy)
High-purity 7Be target implanted at ISOLDE, the EAR2 flight path and a silicon telescope; the cross section is higher than the evaluations at low energy and agrees with 7Li(p,n)7Be data. The effect on the lithium problem is minor. [2]
2021
CRIB radioactive-beam separator, Japan
Trojan Horse method · BBN energies (via 7Be(d,7Li p)1H and 7Be(d,αα)1H)
Trojan Horse measurement of 7Be(n,p)7Li and 7Be(n,α)4He at the same time; an improved (n,p0) cross section and the first quantification of the (n,p1) channel in the BBN energy region, which in the PRIMAT code lowers the predicted 7Li by about a tenth. [3]
Measurements (EXFOR)
Every dataset EXFOR holds for this reaction, drawn by the energies it covers, against the energies a star uses.
19 datasets in 8 EXFOR entries, 1955–2021 · 3 without an energy range, not drawn
- Lowest energy in EXFOR
- 17.9 meV
- Hanna, 1955; Gledenov et al., 1987; KOEHLER et al., 1989; Damone et al., 2018; Tomandl et al., 2019 · σ · 23291.002
- Maxwellian averages measured
- None in EXFOR
- Rate from energies not in EXFOR
- 4.1%
- at kT = 68.9 keV, smooth estimate
- Energies that matter at kT = 68.9 keV
- 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 | 8 | 8.74 meV – 1.75 MeV |
| σ→Eₓpartial | 5 | 22.1 meV – 2.45 MeV |
| ωγresonance strength | 1 | 11.4 keV – 3.1 MeV |
| …other | 2 | 22.1 meV – 22.1 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 reaction has a large cross section at thermal energies and follows a 1/v trend up to resonances in 8Be at higher neutron energies. BBN needs neutron energies of tens of keV. Direct measurements combine radioactive 7Be targets with neutron time-of-flight beams.
Before 2018 the only two data sets from thermal to keV energies dated from the 1980s, did not reach the BBN window and disagreed with each other. The n_TOF measurement at CERN, with a 7Be target implanted at ISOLDE, covered neutron energies from thermal up to about 325 keV. Its cross section is higher than the evaluations at low energy and agrees with values inferred from the time-reversed 7Li(p,n)7Be reaction, but the change reduces the predicted primordial lithium only slightly.
A Trojan Horse measurement with a 7Be beam (Hayakawa et al. 2021) separated the (n,p0) and (n,p1) channels and quantified the transition to the first excited state of 7Li at BBN energies for the first time; including it lowers the predicted 7Li by about a tenth, still far from the factor of about three that the lithium problem needs.
Evaluations
Compilations that combine the data into a recommended rate.
Open questions
- The contribution of transitions to the first excited state of 7Li at BBN energies, which the total cross-section measurements of 1988 and 2018 do not separate; the 2021 Trojan Horse data give a first value, still to be confirmed by a direct measurement.
Related reactions
References
- 1.
P. E. Koehler, C. D. Bowman et al.
7Be(n,p)7Li total cross section from 25 meV to 13.5 keV
Phys. Rev. C 37, 917 (1988) · 10.1103/PhysRevC.37.917
- 2.
L. Damone et al. (n_TOF Collaboration)
7Be(n,p)7Li reaction and the cosmological lithium problem
Phys. Rev. Lett. 121, 042701 (2018) · 10.1103/PhysRevLett.121.042701 · arXiv:1806.03050
- 3.
S. Hayakawa, M. La Cognata, L. Lamia et al.
Astrophys. J. Lett. 915, L13 (2021) · 10.3847/2041-8213/ac061f
- 4.
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
- 5.
C. Iliadis, A. Coc
Thermonuclear reaction rates and primordial nucleosynthesis
Astrophys. J. 901, 127 (2020) · arXiv:2008.12200
How to cite this page
J. Skowronski, 7Be(n,p)7Li, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/7Be-np-7Li.
Cite the date you used; it is the last change to this page.
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