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Reaction atlas · s-process

Radiative capture · s-process

85Kr(n,γ)86Kr

: krypton‑85 + neutron → krypton‑86 + gamma rayHow to read the notation

On this page

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

  1. 1987Stardust from AGB stars in a meteorite

How it was measured

1 measurement, 2013Show the list
  1. 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

Temperature
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
10 meV100 meV1 eV10 eV100 eV1 keV10 keV100 keVEnergy, or kT for a Maxwellian average (keV)kT = 7.76 keVσMACS
  • 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
The datasets of the gap map, by quantity
QuantityDatasetsE_cm range (keV)
σcross section125 meV – 25 meV
MACSMaxwellian average230 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.

  • —Z. Y. Bao et al., At. Data Nucl. Data Tables 76, 70 (2000) [2]
  • —I. Dillmann et al., Proc. Int. Conf. on Nuclear Data for Science and Technology 2007 (EDP Sciences, 2008), p. 575 [3]
  • —F. Käppeler et al., Rev. Mod. Phys. 83, 157 (2011) [4]
  • —M. Lugaro, A. Chieffi, Lect. Notes Phys. 812, 83 (2011) [5]

Open questions

  • A direct measurement of the capture cross section on a 85Kr sample.

Related reactions

References

  1. 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. 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. 3.

    I. Dillmann et al.

    New stellar (n,γ) cross sections and the "Karlsruhe Astrophysical Database of Nucleosynthesis in Stars"

    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. 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. 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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