Radioactive krypton-85 captures a neutron. Whether it does so or decays first tells how dense the neutrons were in the star, and it has never been measured directly.
Krypton-85 lives about ten years in the laboratory. In the 13C pocket of an AGB star a nucleus waits a few centuries for its next neutron, and in the brief 22Ne bursts only a few months, so whether krypton-85 decays or captures depends on where it is made. If neutrons are few, it decays into rubidium-85; if they are many, it captures another one and becomes krypton-86, and the flow then reaches rubidium-87 through the next capture. The amounts of krypton-86 and of the two rubidium isotopes therefore record the neutron density, and so tell the two neutron sources apart. Krypton-86 in stardust grains from AGB stars, found in meteorites, is one of the clues read in this way.
Facts
- How well known
- Never measured directly: the rate of this radioactive gas is inferred from calculations and indirect experiments.
- Live evaluation
- No live evaluation yet · Evaluations
- Reaction rates
- Explore the rates →
- Q-value · energy released
- 9.857 MeV
- Half-life of 85Kr
- 10.7 yr
- 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 8–90 keV, the energy of a gas at 90–1000 million kelvin
By stellar site
- 7.8 keVAGB 13C pocket, 90 MK
- 23 keVAGB thermal pulse, 0.27 GK
- 26 keVmassive star, core helium burning, 0.3 GK
- 90 keVmassive star, shell carbon burning, 1.04 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
- Radiative capture
On the timeline
How it was measured
1 measurement, 2013Show the listHide the list
2013
HIγS, Triangle Universities Nuclear Laboratory
Photonuclear reactions · 86Kr(γ,n), photons from 9.86 to 13 MeV
Photodisintegration of 86Kr with monoenergetic photons; with 86Kr(γ,γ′) it fixes the dipole strength used to predict 85Kr(n,γ). [1]
Measurements (EXFOR)
Every dataset EXFOR holds for this reaction, drawn by the energies it covers, against the energies a star uses.
7 datasets in 4 EXFOR entries, 1950–2006 · 4 without an energy range, not drawn
- Lowest energy in EXFOR
- No measurement counts
- No dataset here passes the rule for a measurement (“How these numbers are made”).
- Maxwellian averages measured
- 2 sets
- kT from 30 keV to 30 keV
- Rate from energies not in EXFOR
- 100%
- at kT = 7.76 keV, smooth estimate
- Energies that matter at kT = 7.76 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 | 1 | 25 meV – 25 meV |
| MACSMaxwellian average | 2 | 30 keV – 30 keV |
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
How well known, in detail: No direct measurement exists; the stellar cross section comes from statistical-model calculations constrained by 86Kr photodisintegration.
85Kr sits at the branching that feeds the s-process side of the N = 50 closed shell, where 86Kr, 87Rb and 88Sr have magic neutron number; the branching is a classic neutron-density indicator (Käppeler et al. 2011). Not all the flow reaches the 10.7 yr ground state: about 40–60% of the captures on 84Kr make 85Kr in its 4.5 h isomeric state, and about 80% of those decay straight to 85Rb, bypassing the branching (Lugaro and Chieffi 2011). There is no direct measurement, and the stellar cross section comes from statistical-model calculations.
Raut et al. (2013) constrained it indirectly: they measured the photodisintegration 86Kr(γ,n) at HIγS with monoenergetic photons from the neutron separation energy (9.86 MeV) to 13 MeV, combined it with 86Kr(γ,γ′) data into the dipole response of 86Kr, and used it to predict the 85Kr(n,γ) cross section. With it, AGB models of less than about 1.5 M☉, in which the 13C burns convectively, are a possible explanation for the highest 86Kr/82Kr ratios of presolar SiC grains.
Evaluations
Compilations that combine the data into a recommended rate.
Open questions
- A direct measurement of the capture cross section on a 85Kr sample.
Related reactions
References
- 1.
R. Raut, A. P. Tonchev, G. Rusev, W. Tornow, C. Iliadis, M. Lugaro, J. Buntain, S. Goriely, J. H. Kelley, R. Schwengner, A. Banu, N. Tsoneva
Cross-section measurements of the 86Kr(γ,n) reaction to probe the s-process branching at 85Kr
Phys. Rev. Lett. 111, 112501 (2013) · 10.1103/PhysRevLett.111.112501 · arXiv:1309.4159
- 2.
Z. Y. Bao, H. Beer, F. Käppeler, F. Voss, K. Wisshak, T. Rauscher
Neutron cross sections for nucleosynthesis studies
At. Data Nucl. Data Tables 76, 70 (2000) · 10.1006/adnd.2000.0838
- 3.
I. Dillmann et al.
Proc. Int. Conf. on Nuclear Data for Science and Technology 2007 (EDP Sciences, 2008), p. 575 (2008) · 10.1051/ndata:07303 · arXiv:0806.2076
- 4.
F. Käppeler, R. Gallino, S. Bisterzo, W. Aoki
The s process: nuclear physics, stellar models, and observations
Rev. Mod. Phys. 83, 157 (2011) · 10.1103/RevModPhys.83.157 · arXiv:1012.5218
- 5.
M. Lugaro, A. Chieffi
Radioactivities in low- and intermediate-mass stars
Lect. Notes Phys. 812, 83 (2011) · arXiv:1010.1304
How to cite this page
J. Skowronski, 85Kr(n,γ)86Kr, in Reaction atlas, 2026-10-12, https://jskowron.eu/atlas/85Kr-ng-86Kr.
Cite the date you used; it is the last change to this page.
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