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Isotopes of krypton
Isotopes of krypton
Isotopes of krypton
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Isotopes of krypton (36Kr)
Main isotopes[1] Decay
Isotope abun­dance half-life (t1/2) mode pro­duct
78Kr 0.360% 9.2×1021 y[2] εε 78Se
79Kr synth 1.46 d β+ 79Br
80Kr 2.29% stable
81Kr trace 2.3×105 y ε 81Br
81mKr synth 13.10 s IT 81Kr
ε 81Br
82Kr 11.6% stable
83Kr 11.5% stable
84Kr 57.0% stable
85Kr trace 10.728 y β− 85Rb
86Kr 17.3% stable
Standard atomic weight Ar°(Kr)

There are 34 known isotopes of krypton (36Kr) with atomic mass numbers from 67 to 103. Naturally occurring krypton is made of five stable isotopes and one (78
Kr) which is slightly radioactive with an extremely long half-life, plus traces of radioisotopes that are produced by cosmic rays in the atmosphere. Atmospheric krypton today is, however, considerably radioactive due almost entirely to artificial 85Kr.[5]

List of isotopes

[edit]
Nuclide
[n 1] 		Z N Isotopic mass (Da)[6]
[n 2][n 3] 		Half-life[1]
[n 4][n 5] 		Decay
mode[1]
[n 6] 		Daughter
isotope
[n 7][n 8] 		Spin and
parity[1]
[n 9][n 5] 		Natural abundance (mole fraction)
Excitation energy Normal proportion[1] Range of variation
67Kr 36 31 66.98331(46)# 7.4(29) ms β+? (63%) 67Br 3/2-#
2p (37%) 65Se
68Kr 36 32 67.97249(54)# 21.6(33) ms β+, p (>90%) 67Se 0+
β+? (<10%) 68Br
p? 67Br
69Kr 36 33 68.96550(32)# 27.9(8) ms β+, p (94%) 68Se (5/2−)
β+ (6%) 69Br
70Kr 36 34 69.95588(22)# 45.00(14) ms β+ (>98.7%) 70Br 0+
β+, p (<1.3%) 69Se
71Kr 36 35 70.95027(14) 98.8(3) ms β+ (97.9%) 71Br (5/2)−
β+, p (2.1%) 70Se
72Kr 36 36 71.9420924(86) 17.16(18) s β+ 72Br 0+
73Kr 36 37 72.9392892(71) 27.3(10) s β+ (99.75%) 73Br (3/2)−
β+, p (0.25%) 72Se
73mKr 433.55(13) keV 107(10) ns IT 73Kr (9/2+)
74Kr 36 38 73.9330840(22) 11.50(11) min β+ 74Br 0+
75Kr 36 39 74.9309457(87) 4.60(7) min β+ 75Br 5/2+
76Kr 36 40 75.9259107(43) 14.8(1) h β+ 76Br 0+
77Kr 36 41 76.9246700(21) 72.6(9) min β+ 77Br 5/2+
77mKr 66.50(5) keV 118(12) ns IT 77Kr 3/2−
78Kr[n 10] 36 42 77.92036634(33) 9.2 +5.5
−2.6 ±1.3×1021 y[2] 		Double EC 78Se 0+ 0.00355(3)
79Kr 36 43 78.9200829(37) 35.04(10) h β+ 79Br 1/2−
79mKr 129.77(5) keV 50(3) s IT 79Kr 7/2+
80Kr 36 44 79.91637794(75) Stable 0+ 0.02286(10)
81Kr[n 11] 36 45 80.9165897(12) 2.29(11)×105 y EC 81Br 7/2+ 6×10−13[7]
81mKr 190.64(4) keV 13.10(3) s IT 81Kr 1/2−
EC (0.0025%) 81Br
82Kr 36 46 81.9134811537(59) Stable 0+ 0.11593(31)
83Kr[n 12] 36 47 82.914126516(9) Stable 9/2+ 0.11500(19)
83m1Kr 9.4053(8) keV 156.8(5) ns IT 83Kr 7/2+
83m2Kr 41.5575(7) keV 1.830(13) h IT 83Kr 1/2−
84Kr[n 12] 36 48 83.9114977271(41) Stable 0+ 0.56987(15)
84mKr 3236.07(18) keV 1.83(4) μs IT 84Kr 8+
85Kr[n 12] 36 49 84.9125273(21) 10.728(7) y β− 85Rb 9/2+ 1×10−11[7]
85m1Kr[n 12] 304.871(20) keV 4.480(8) h β− (78.8%) 85Rb 1/2−
IT (21.2%) 85Kr
85m2Kr 1991.8(2) keV 1.82(5) μs
 IT 85Kr (17/2+)
86Kr[n 13][n 12] 36 50 85.9106106247(40) Observationally Stable[n 14] 0+ 0.17279(41)
87Kr 36 51 86.91335476(26) 76.3(5) min β− 87Rb 5/2+
88Kr 36 52 87.9144479(28) 2.825(19) h β− 88Rb 0+
89Kr 36 53 88.9178354(23) 3.15(4) min β− 89Rb 3/2+
90Kr 36 54 89.9195279(20) 32.32(9) s β− 90mRb 0+
91Kr 36 55 90.9238063(24) 8.57(4) s β− 91Rb 5/2+
β−, n? 90Rb
92Kr 36 56 91.9261731(29) 1.840(8) s β− (99.97%) 92Rb 0+
β−, n (0.0332%) 91Rb
93Kr 36 57 92.9311472(27) 1.287(10) s β− (98.05%) 93Rb 1/2+
β−, n (1.95%) 92Rb
94Kr 36 58 93.934140(13) 212(4) ms β− (98.89%) 94Rb 0+
β−, n (1.11%) 93Rb
95Kr 36 59 94.939711(20) 114(3) ms β− (97.13%) 95Rb 1/2+
β−, n (2.87%) 94Rb
β−, 2n? 93Rb
95mKr 195.5(3) keV 1.582(22) μs
 IT 95Kr (7/2+)
96Kr 36 60 95.942998(62)[8] 80(8) ms β− (96.3%) 96Rb 0+
β−, n (3.7%) 95Rb
97Kr 36 61 96.94909(14) 62.2(32) ms β− (93.3%) 97Rb 3/2+#
β−, n (6.7%) 96Rb
β−, 2n? 95Rb
98Kr 36 62 97.95264(32)# 42.8(36) ms β− (93.0%) 98Rb 0+
β−, n (7.0%) 97Rb
β−, 2n? 96Rb
99Kr 36 63 98.95878(43)# 40(11) ms β− (89%) 99Rb 5/2−#
β−, n (11%) 98Rb
β−, 2n? 97Rb
100Kr 36 64 99.96300(43)# 12(8) ms β− 100Rb 0+
β−, n? 99Rb
β−, 2n? 98Rb
101Kr 36 65 100.96932(54)# 9# ms
[>400 ns] 		β−? 101Rb 5/2+#
β−, n? 100Rb
β−, 2n? 99Rb
102Kr[9] 36 66 0+
103Kr[10] 36 67
This table header & footer:
  1. ^ mKr – Excited nuclear isomer.
  2. ^ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
  3. ^ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
  4. ^ Bold half-life – nearly stable, half-life longer than age of universe.
  5. ^ a b # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  6. ^ Modes of decay:
    n: Neutron emission
  7. ^ Bold italics symbol as daughter – Daughter product is nearly stable.
  8. ^ Bold symbol as daughter – Daughter product is stable.
  9. ^ ( ) spin value – Indicates spin with weak assignment arguments.
  10. ^ Primordial radionuclide
  11. ^ Used to date groundwater
  12. ^ a b c d e Fission product
  13. ^ Formerly used to define the meter
  14. ^ Believed to decay by β−β− to 86Sr
  • The isotopic composition refers to that in air.

Notable isotopes

[edit]

Krypton-81

[edit]

Krypton-81 (half-life 230,000 years) is useful in determining how old the water beneath the ground is. Radioactive krypton-81 is the product of spallation reactions with cosmic rays striking gases present in the Earth atmosphere, along with the six stable or nearly stable krypton isotopes.[11] The long half-life ensures that the isotope has a uniform concentration in the atmosphere and in surface water; when the water goes underground is supply is no longer replenished and decays, allowing dating of the residence time in deep aquifers in a range of 20,000 to a million years, bridging the gap where other isotopic methods (e.g. carbon-14 dating) lose sensitivity. The same long half-life renders detection of its decay impossible and, therefore, demands some form of mass spectrometry. Even so, technical limitations of the method have traditionally required the sampling of very large volumes of water: several hundred liters or a few cubic meters of water (about a milligram of krypton). This is particularly challenging for dating pore water in deep clay aquitards with very low hydraulic conductivity.[12] More recently, it has been announced[13] that samples an order of magnitude less can be used successfully.

Because cosmic ray production in the atmosphere creates a globally fairly uniform 81Kr/Kr concentration, one can assume a known initial ratio in meteoric water before recharge. There are essentially no significant anthropogenic or in situ geological sources (in typical crustal settings) that would confound the decay clock, making krypton-81 a relatively "clean" choice for geological dating.[citation needed]

The short-lived isomer krypton-81m (half-life 13 seconds) has medical uses but is often considered impractical for use as it must be generated from the rare rubidium-81.[14] It almost entirely decays to the ground state with a monochromatic gamma ray.

Krypton-85

[edit]
Main article: Krypton-85

Krypton-85 (half-life 10.728 years) is produced by the nuclear fission of uranium and plutonium in nuclear weapons testing and in nuclear reactors, as well as by cosmic rays. An important goal of the Limited Nuclear Test Ban Treaty of 1963 was to eliminate the release of such radioisotopes into the atmosphere, and since 1963 much of that krypton-85 has had time to decay. However, it is almost inevitable that krypton-85 is released during the reprocessing of fuel rods from nuclear reactors,[15] which is far larger-volume than was ever nuclear testing.

Atmospheric concentration

[edit]
See also: Nuclear reprocessing

The atmospheric concentration of krypton-85 around the North Pole is about 30 percent higher than that at the South Pole because nearly all of the world's nuclear reactors and all of its major nuclear reprocessing plants are located in the Northern Hemisphere, well north of the equator[16] and transfer of air between the hemispheres is slow.

The nuclear reprocessing plants with significant capacities are located in the United States, the United Kingdom, the French Republic, the Russian Federation, Mainland China (PRC), Japan, India, and Pakistan.

Krypton-86

[edit]

Krypton-86 was formerly used to define the meter from 1960 until 1983, when the definition of the meter was based on the wavelength of the 606 nm (orange) spectral line of a krypton-86 atom.[17]

See also

[edit]

Daughter products other than krypton

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Isotopes of krypton

View on Grokipedia
from Grokipedia
Isotopes of are the nuclides of the , with 36, that differ in their number of neutrons while sharing the same number of protons, resulting in distinct mass numbers and nuclear properties. Naturally occurring in Earth's atmosphere comprises six isotopes—^{78}Kr (0.35% abundance), ^{80}Kr (2.28%), ^{82}Kr (11.58%), ^{83}Kr (11.49%), ^{84}Kr (57.00%), and ^{86}Kr (17.30%)—with ^{84}Kr dominating the isotopic mixture due to its prevalence in primordial processes. Trace quantities of the radioactive ^{85}Kr, produced mainly via and fission in nuclear reactors and weapons tests, contribute to atmospheric levels, exhibiting a of 10.73 years and decaying by beta emission to ^{85}Rb. Of the approximately 34 known krypton isotopes, ranging from mass number 67 to 103, the majority are artificially synthesized radioactive variants with short half-lives, typically decaying via beta emission, , or , and are studied in for insights into shell structures and fission yields. Notable among radioactive isotopes is ^{81}Kr, with a half-life of about 210,000 years, which serves as a geochemical tracer for residence times and dating due to its production from interactions with atmospheric . ^{85}Kr finds applications in non-destructive leak testing, luminescent lighting, and atmospheric monitoring of nuclear activities, highlighting its role in both practical technologies and . Stable krypton isotopes, enriched versions of which are commercially available, enable precise and NMR studies, underscoring 's utility in despite its chemical inertness.

Fundamental properties

Nuclear characteristics and stability

Krypton isotopes possess 36 protons, with neutron numbers determining their mass (A) and stability. The known isotopes span mass numbers approximately from 69 to 100, comprising six stable isotopes and approximately 26 radioactive ones. The stable isotopes—^{78}Kr, ^{80}Kr, ^{82}Kr, ^{83}Kr, ^{84}Kr, and ^{86}Kr—have neutron numbers N = 42 to 50, positioning them along the valley of β-stability for Z = 36. Stability is enhanced by nuclear pairing effects in even-even configurations (even N and Z), yielding ground-state spins of 0⁺, and by the neutron shell closure at N = 50 in ^{86}Kr, which increases binding energy and resistance to decay. The odd-neutron ^{83}Kr (N = 47) remains stable with spin-parity 9/2⁺, reflecting the ground-state configuration of the unpaired neutron in the 2d_{5/2} orbital, though such odd-N stability is less common beyond this range.
IsotopeAtomic mass (u)Natural abundance (%)Spin-parity
^{78}Kr77.920365(5)0.355(3)0⁺
^{80}Kr79.916378(5)2.286(10)0⁺
^{82}Kr81.913483(6)11.593(31)0⁺
^{83}Kr82.914127(2)11.500(19)9/2⁺
^{84}Kr83.91149773(3)56.987(15)0⁺
^{86}Kr85.91061063(3)17.279(41)0⁺
Radioactive isotopes deviate from this stability window: proton-rich ones (low A) exhibit neutron-proton imbalance favoring β⁺ decay or , while neutron-rich ones (high A) favor β⁻ decay, with half-lives shortening farther from the stable line due to increased and asymmetry energies in the . No α decay is observed in isotopes, as the Q-value is negative owing to the high Z and resulting .

Decay modes and half-lives

Radioactive isotopes of krypton primarily decay through beta processes, with the specific mode depending on whether the isotope is neutron-rich or proton-rich relative to the stable isotopes. Neutron-excess isotopes (typically A > 86) undergo β⁻ decay, converting a to a proton, , and antineutrino, leading to daughters. Proton-excess isotopes (typically A < 78) decay via β⁺ emission or (EC), converting a proton to a , (for β⁺), and , resulting in daughters. is negligible for krypton isotopes due to insufficient Q-values and high barriers for this mass region. Half-lives span over 15 orders of magnitude, from sub-millisecond for neutron-deficient or highly neutron-rich extremes produced in accelerators or fission, to ~10⁵ years for primordial or cosmogenic tracers. The longest-lived radioactive isotope, ⁸¹Kr, decays 100% by EC to ⁸¹Br with a half-life of (2.29 ± 0.11) × 10⁵ years and Q-value of 0.472 MeV. ⁸⁵Kr, a major anthropogenic fission product, decays ~99.6% by β⁻ to ground-state ⁸⁵Rb (half-life 10.756 ± 0.010 years, endpoint energy 687 keV) and ~0.4% to an excited state followed by gamma emission. Most other radioactive isotopes have half-lives under 50 days; for instance, ⁸⁷Kr (β⁻, 76.3 min), ⁸⁸Kr (β⁻, 2.84 h), and ⁷⁹Kr (EC/β⁺, 35.0 h). Isomeric states, such as ⁸⁵Krᵐ (half-life 4.48 h, β⁻ decay), contribute minor branches but do not alter dominant ground-state modes. Delayed occurs in some neutron-rich decays (e.g., ⁸⁷Br → ⁸⁷Kr → n emission), but is limited to <10% branching for krypton-relevant chains. Empirical trends show decreasing toward drip lines, with β-decay rates governed by phase-space factors and forbidden transitions in odd-A nuclei.

Isotopic inventory

Table of known isotopes

Krypton has six stable isotopes and approximately 28 known radioactive isotopes, with mass numbers ranging from 67 to 101. The table below lists the stable isotopes with their natural abundances and nuclear properties, as well as selected radioactive isotopes of significance due to longer half-lives or applications. Comprehensive nuclear for all isotopes, including short-lived ones, are maintained in such as those from the National Nuclear Data Center.
IsotopeNatural abundance (%)Half-lifeDecay mode(s)Nuclear spin (I)
^{78}Kr0.35Stable—0
^{80}Kr2.28Stable—0
^{82}Kr11.58Stable—0
^{83}Kr11.49Stable—9/2+
^{84}Kr57.00Stable—0
^{86}Kr17.30Stable—0
^{81}Kr—210,000 yEC7/2+
^{85}Kr—10.73 yβ⁻9/2+
Additional radioactive isotopes, such as ^{76}Kr (14.8 h, EC) and ^{87}Kr (1.27 h, β⁻), have shorter half-lives and are primarily produced artificially.

Stable isotopes

Abundances and natural occurrence

The stable isotopes of —^{78}Kr, ^{80}Kr, ^{82}Kr, ^{83}Kr, ^{84}Kr, and ^{86}Kr—comprise the entirety of naturally occurring , with no significant contribution from products to the total inventory under ambient conditions. Krypton itself is a trace constituent of the Earth's atmosphere, present at a volume mixing ratio of 1.14 parts per million, from which commercial extraction via of liquefied air is performed. Smaller quantities occur dissolved in natural waters, brines, and certain minerals such as , but these represent negligible fractions compared to the atmospheric reservoir. The isotopic abundances in atmospheric , which serve as the standard reference due to global mixing and homogeneity, are:
IsotopeNatural abundance (%)
^{78}Kr0.35
^{80}Kr2.28
^{82}Kr11.58
^{83}Kr11.49
^{84}Kr57.00
^{86}Kr17.30
These ratios originate primarily from processes, including in asymptotic giant branch stars and explosions, with the material incorporated into the solar nebula and subsequently accreted into during its formation approximately 4.54 billion years ago. Isotopic variations due to mass-dependent or cosmogenic production are minimal for these nuclides, as cosmic-ray interactions contribute primarily to lighter radioactive isotopes like ^{81}Kr rather than altering the bulk composition. The heaviest isotope, ^{86}Kr, shows evidence of enrichment from late accretion of outer solar system material, as inferred from mantle-derived samples, but this does not appreciably deviate from atmospheric values.

Long-lived radioactive isotopes

Cosmogenic and primordial isotopes

^{81}Kr, with a of (2.3 \pm 0.4) \times 10^5 years, represents the primary long-lived radioactive of in Earth's natural inventory, produced exclusively through cosmogenic in the atmosphere. High-energy protons and neutrons interact with stable krypton isotopes—predominantly ^{80}Kr, but also ^{78}Kr, ^{82}Kr, ^{83}Kr, and ^{84}Kr—yielding ^{81}Kr via processes such as (p,2n) and (n,p) . Production occurs mainly in the upper atmosphere, with global rates estimated at approximately 1.3 \times 10^6 atoms per square centimeter per year, leading to a steady-state atmospheric abundance of about 1 ^{81}Kr atom per 10^8 stable Kr atoms. This isotope's concentration integrates flux over timescales comparable to its , providing a record insensitive to solar activity cycles shorter than ~10^4 years or climatic variations, as ^{81}Kr diffuses globally via atmospheric mixing before decay. Underground production of ^{81}Kr has been detected in subsurface fluids at rates up to 10^{-15} to 10^{-14} atoms per liter per year, but remains negligible compared to atmospheric input for most terrestrial reservoirs. No primordial long-lived radioactive isotopes of krypton persist in significant quantities on , as any such nuclides would require half-lives exceeding 4.5 billion years to survive from planetary accretion; ^{81}Kr's shorter ensures its inventory is dominated by ongoing cosmogenic replenishment rather than relic primordial contributions. Other potentially long-lived Kr isotopes, such as ^{85}Kr ( 10.76 years), exhibit trace cosmogenic production but are overwhelmingly anthropogenic from , rendering natural cosmogenic sources undetectable in modern samples.

Fission-derived isotopes

Krypton-85 (^{85}Kr) is the primary long-lived radioactive of krypton generated as a direct fission product or via of short-lived precursors in nuclear reactions. It possesses a of 10.76 years and undergoes beta-minus decay to stable rubidium-85 (^{85}Rb), emitting electrons with a maximum energy of 687 keV and accompanying gamma rays. This isotope arises predominantly from the fission of and in nuclear reactors and, historically, from atmospheric . In thermal neutron-induced fission of ^{235}U, the cumulative fission yield for ^{85}Kr is 0.273 ± 0.004%, equivalent to roughly 2.73 atoms per 100 fissions, while the yield from ^{239}Pu fission is lower at approximately 0.099%. Approximately 0.3% of all fissions in uranium-based fuels produce ^{85}Kr atoms, contributing to its accumulation in . Release occurs mainly during fuel reprocessing, where gaseous fission products are vented, leading to elevated atmospheric concentrations since the mid-20th century, peaking around 1980 due to reprocessing activities and declining thereafter as practices evolved. Shorter-lived krypton isotopes such as ^{87}Kr (half-life 76 minutes), ^{88}Kr (half-life 2.84 hours), and ^{85m}Kr (half-life 4.48 hours) are also fission products but decay rapidly, feeding into ^{85}Kr or other chains without significant long-term persistence. These contribute to the initial independent yields but are not classified as long-lived, with their production yields varying by fissioning nucleus; for instance, relative yields of ^{85}Kr, ^{87}Kr, and ^{88}Kr have been measured in uranium fission spectra. Unlike cosmogenic or primordial krypton radioisotopes, fission-derived ^{85}Kr dominates anthropogenic inventories, enabling its use as a tracer for nuclear fuel age determination through decay chronometry.

Production mechanisms

Natural nucleosynthesis and cosmic ray production

The stable isotopes of krypton are synthesized primarily through neutron-capture processes during stellar evolution. The slow neutron-capture process (s-process), which occurs in the helium-burning shells of asymptotic giant branch stars via neutron sources like 13^{13}C(α\alpha,n)16^{16}O, contributes significantly to the production of isotopes such as 80^{80}Kr, 82^{82}Kr, 84^{84}Kr, and 86^{86}Kr, with neutron capture cross-sections on seed nuclei like 84^{84}Kr and 86^{86}Kr playing a key role in branching points that influence the final isotopic yields. The rapid neutron-capture process (r-process), driven by extreme neutron fluxes in events such as neutron star mergers or core-collapse supernovae, accounts for neutron-rich isotopes including substantial fractions of 83^{83}Kr and 86^{86}Kr, as inferred from solar system abundances after subtracting s-process contributions. Proton-rich isotopes like 78^{78}Kr originate mainly from the p-process (or γ\gamma-process), involving proton captures or photodisintegrations in supernova envelopes, with minor shielding effects from r-process paths. These primordial isotopes were incorporated into the solar nebula during its formation approximately 4.6 billion years ago, with isotopic ratios reflecting a mix of s-, r-, and p-process contributions calibrated against meteoritic and data. Variations in these ratios, observed in and cometary samples, indicate heterogeneous nucleosynthetic sources across the early solar system, though solar aligns closely with averaged stellar yields. Cosmic ray-induced spallation provides an ongoing natural production mechanism for certain radioactive krypton isotopes in Earth's atmosphere and extraterrestrial materials. 81^{81}Kr (half-life 229,000 years) forms predominantly through high-energy cosmic ray protons and neutrons interacting with atmospheric argon-40 and other constituents, yielding spallation products that integrate cosmic ray flux over millennial timescales. Production rates, calculated using cross-sections such as those from Silberberg and Tsao models, estimate atmospheric inventories of 81^{81}Kr at levels traceable for geochronology, with fluxes modulated by geomagnetic and solar activity. Similarly, 85^{85}Kr (half-life 10.76 years) arises from cosmic ray spallation, though its natural yield is dwarfed by anthropogenic sources; early calculations confirm spallation on lighter targets contributes minor fractions to its pre-industrial abundance. In meteorites and lunar , cosmogenic krypton isotopes (e.g., 78^{78}Kr to 83^{83}Kr) result from galactic of heavier target elements like , , and , with measured yields from proton irradiations (e.g., 730 MeV) revealing isotopic patterns distinct from stellar origins, such as enhanced light-isotope ratios due to fragmentation. These production rates vary with exposure age and depth, enabling corrections for trapped components in analyses.

Anthropogenic production in reactors and accelerators

Krypton isotopes are primarily produced anthropogenically in nuclear reactors through the fission of heavy nuclei such as and , yielding a range of fission fragments including neutron-rich krypton isotopes from mass numbers approximately 83 to 97. These include short-lived isotopes like ^{87}Kr (half-life 76 minutes) and ^{88}Kr (half-life 2.84 hours), as well as longer-lived ones such as ^{85}Kr ( 10.76 years), which accumulates due to its relatively high fission yield of around 0.3% in thermal neutron-induced fission of ^{235}U. During operation, these isotopes remain largely contained within rods, but a fraction—estimated at about 1% for ^{85}Kr—escapes into the coolant or atmosphere via cladding leaks; larger releases occur during spent reprocessing, where gaseous fission products are separated and vented. Minor contributions come from reactions, such as ^{84}Kr(n,γ)^{85}Kr, though these are overshadowed by direct fission pathways in power reactors. In particle accelerators, particularly , select short-lived krypton isotopes are synthesized via charged-particle-induced nuclear reactions on enriched targets, enabling production for research and medical applications where reactor-derived isotopes are unsuitable due to or purity constraints. For instance, ^{77}Kr (half-life 1.24 hours) has been generated through proton bombardment for use in regional cerebral blood flow imaging, with yields achieved via reactions on or targets. Similarly, ^{81m}Kr ( 13 seconds), valuable for pulmonary ventilation studies, is indirectly produced by of natural targets via the ^{nat}Kr(p,n)^{81}Rb reaction, followed by decay of the parent rubidium-81 ( 4.58 hours) to the krypton . Early experiments at facilities like CERN's synchrocyclotron in the demonstrated on-line production and separation of short-lived krypton isotopes (e.g., under minutes) using high-energy protons or deuterons on suitable targets, facilitating spectroscopic studies of neutron-deficient species not accessible via fission. Accelerator methods typically yield microcurie to millicurie quantities, prioritizing high over bulk production.

Historical and metrological significance

Krypton-86 as a standard

Krypton-86 served as the basis for the international definition of the meter from 1960 to 1983, specifically through the measurement of its orange-red emission line. The meter was defined as exactly 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the 2p102p_{10} and 5d55d_5 energy levels of the krypton-86 atom. This definition was adopted by the 11th General Conference on Weights and Measures (CGPM) on October 14, 1960, replacing the previous platinum-iridium artifact standard, which had shown signs of wear and instability over time. The selection of krypton-86 stemmed from its stable isotopic properties and the exceptional sharpness and reproducibility of its at approximately 605.780 nm, enabling precise interferometric measurements with uncertainties below 10 . As a , krypton facilitated the production of monoisotopic lamps through isotopic enrichment, minimizing by operating at low temperatures near the triple point of (63.14 K). These lamps, excited by radio-frequency discharge, provided a highly intense and stable light source superior to earlier atomic standards like mercury-198, which suffered from complications. This standard achieved metrological precision that realized the meter with reproducibility on the order of 4 parts per million relative to the prior artifact, advancing length metrology toward atomic-scale accuracy. However, limitations emerged, including isotopic impurities in lamps and sensitivity to environmental factors like magnetic fields, prompting its replacement in by the 17th CGPM with a definition tying the meter to the in : the travels in 1/299,792,458 of a second. The krypton-86 standard's legacy persists in historical calibrations and as a benchmark for verifying the continuity of the metric system's evolution.

Scientific applications

Geochronology and hydrology

Krypton-81 (^81Kr), with a half-life of approximately 229,000 years, serves as a geochemical tracer for dating ancient groundwater in aquifers, enabling age determinations spanning 50,000 to over 1 million years, a range inaccessible to shorter-lived isotopes like tritium or carbon-14. Produced primarily by cosmic-ray spallation in the upper atmosphere, ^81Kr enters groundwater via precipitation and remains inert due to its noble gas nature, decaying minimally over relevant timescales and avoiding fractionation or adsorption issues common to other tracers. Atom trap trace analysis (ATTA) facilitates detection at ultra-low atmospheric concentrations (~1 × 10^{-15}), allowing precise ^81Kr/^Kr ratios to infer residence times without assumptions about recharge rates or diffusion. Applications include constraining flow in deep systems, such as the Continental Intercalaire aquifer in North Africa, where ^81Kr ages of 150,000–630,000 years exceed those from ^14C or ^4He methods, revealing minimal modern recharge. In regional studies, ^81Kr has quantified old water fractions in basins like , , identifying million-year-old components and informing sustainable extraction limits by distinguishing from active circulation. It complements by calibrating accumulation models in low-permeability zones, as ^81Kr production is cosmogenic and uniform, independent of in-situ decay sources like /. For instance, in cratonic settings, ^81Kr has shown meteoric flushing of saline brines occurred primarily 100,000–800,000 years ago, with residual ancient fluids persisting due to slow . For , cosmogenic krypton isotopes (^78Kr, ^80Kr, ^82Kr, ^83Kr, ^84Kr, and ^81Kr) produced in crystals by cosmic-ray interactions provide exposure ages and rates on geological timescales, targeting surfaces older than those dated by ^10Be or ^26Al due to krypton’s higher production thresholds in resistant minerals. These stable isotopes accumulate via and muon-induced reactions, with production rates calibrated against depth profiles (e.g., ~10–100 atoms/g/yr near surface), enabling burial dating or low- landscape evolution studies where diffusion losses are negligible in dense lattices. Radioactive ^81Kr in this context extends to atmospheric integration of past cosmic-ray fluxes, indirectly supporting paleoclimate chronologies by linking ^81Kr inventories to geomagnetic variations over 10^5–10^6 years. Analytical challenges include extraction purity, addressed via stepped heating and , yielding uncertainties of 5–10% for ^10^5-year exposures.

Environmental and atmospheric tracing

Krypton-85 (¹⁸⁵Kr), a beta-emitting isotope with a half-life of 10.76 years, functions as a prominent anthropogenic tracer in atmospheric and environmental investigations owing to its production via nuclear fission and its noble gas properties, which prevent chemical reactions or deposition. Generated primarily from uranium and plutonium fission in reactors, nuclear fuel reprocessing, and historical weapons testing, ¹⁸⁵Kr enters the atmosphere in steadily increasing quantities, with global inventories rising due to ongoing nuclear activities and minimal natural sinks beyond radioactive decay. Its uniform mixing and detectability at low levels (e.g., atmospheric concentrations around 1–2 Bq/m³ in recent decades) enable tracing of long-range atmospheric transport, interhemispheric exchange, and ventilation timescales in the troposphere and stratosphere. In environmental contexts, ¹⁸⁵Kr monitoring detects emissions, serving as an indicator for compliance with non-proliferation treaties by identifying undeclared facilities through elevated local plumes that disperse globally. Automated sampling and analysis systems, including those achieving 1.5-hour resolution via online purification and , quantify dispersion patterns and source attribution, revealing transport dynamics from point releases. These measurements have documented hemispheric asymmetries, with northern latitudes showing higher concentrations (up to 20–30% gradient) due to predominant emissions from facilities in and . Krypton-81 (⁸¹Kr), produced cosmogenically at trace levels (half-life 229,000 years), exhibits near-uniform atmospheric distribution with negligible anthropogenic input (<2.5% of total), limiting its role in tracing modern pollution but supporting baseline studies of natural noble gas cycles and long-term air mass equilibration. Stable krypton isotopes (e.g., ⁸⁴Kr, ⁸⁶Kr) maintain invariant ratios in uncontaminated air (total Kr ~1 ppmv), offering auxiliary constraints on atmospheric fractionation processes only when deviations occur from industrial or volcanic influences, though such applications remain secondary to more variable noble gases like xenon.

Planetary and mantle studies

Krypton isotopes serve as tracers for volatile delivery and mantle evolution on , with analyses of plume-derived samples from hotspots like the Galápagos and revealing primitive isotopic ratios that differ markedly from atmospheric values. These ratios, including deficits in lighter isotopes such as 78^{78}Kr and 80^{80}Kr, indicate early accretion of outer solar system planetesimals rich in materials during Earth's formation, prior to significant atmospheric incorporation. Such findings challenge models of late-stage volatile addition, supporting a scenario where volatile-rich bodies contributed to the proto-Earth's inventory within the first few million years of solar system history. In gases from Yellowstone, isotopic compositions align with chondritic patterns, mirroring those inferred for (MORB) sources and pointing to a deep, undegassed preserving primordial solar nebula signatures. This chondritic affinity extends to heavier like , reinforcing evidence for heterogeneous mantle domains that retain pre-subduction volatile elemental ratios, with -to- fractionation suggesting minimal early degassing relative to atmospheric s. Beyond , krypton isotopes in the Chassigny demonstrate chondritic volatile sources in Mars' mantle, with measured 80^{80}Kr/83^{83}Kr and 82^{82}Kr/83^{83}Kr ratios matching CI chondrites rather than or expectations, implying rapid accretion of primitive materials during Mars' formation around 4.5 billion years ago. measurements by the rover's Sample Analysis at Mars instrument detected atmospheric abundances and isotopic ratios on Mars, with 84^{84}Kr/83^{83}Kr values elevated due to non-radiogenic mass-dependent from , providing constraints on hydrodynamic loss and crustal interactions over billions of years. Krypton isotopic data from comet 67P/Churyumov-Gerasimenko, obtained via the Rosetta mission, exhibit near-solar compositions for 80^{80}Kr/84^{84}Kr and related ratios, alongside solar argon-to-krypton elemental abundances, which inform models of volatile trapping in the and delivery to inner solar system bodies. These extraterrestrial applications highlight krypton's utility in distinguishing between chondritic, solar, and fractionated volatile end-members across planetary mantles and atmospheres.

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