Notation: mass number as a left superscript (e.g., ³⁹Ar).
Columns: Isotope | Z | A | Stable? | Half‑life | Decay mode(s) | Daughter | Radiation | Origin | Uses | Notes.
Backbone: evaluated NUBASE/ENSDF values as compiled in Isotopes of argon; LiveChart/NuDat consulted; key application notes and precision t½ results cited individually. (Wikipedia, IAEA Nuclear Data Services)
| Isotope | Z | A | Stable? | Half‑life | Decay mode(s) | Daughter | Radiation | Origin | Uses | Notes |
|---|---|---|---|---|---|---|---|---|---|---|
| ²⁹Ar | 18 | 29 | No (unbound) | prompt | 2p | ²⁷S | protons (2p) | Synthetic (accelerators) | Research (extreme p‑rich) | Ground‑state 2p emitter. (Wikipedia) |
| ³⁰Ar | 18 | 30 | No (unbound) | < 10 ps | 2p | ²⁸S | protons (2p) | Synthetic | Proton‑dripline structure | Lifetime upper‑limit from width. (Wikipedia) |
| ³¹Ar | 18 | 31 | No | 15.0(3) ms | β⁺ with p/2p/3p branches | ³¹Cl; ³⁰S; ²⁹P; ²⁸Si | β⁺; delayed p/2p/3p | Synthetic | β‑delayed multi‑p emission benchmark | Rare β⁺3p observed. (Wikipedia) |
| ³²Ar | 18 | 32 | No | 98(2) ms | β⁺ (64.4%); β⁺p (35.6%) | ³²Cl; ³¹S | β⁺; delayed p | Synthetic | β‑delayed‑p statistics | — (Wikipedia) |
| ³³Ar | 18 | 33 | No | 173.0(20) ms | β⁺ (61.3%); β⁺p (38.7%) | ³³Cl; ³²S | β⁺; delayed p | Synthetic | β‑delayed‑p statistics | — (Wikipedia) |
| ³⁴Ar | 18 | 34 | No | 846.46(35) ms | β⁺ | ³⁴Cl | β⁺ | Synthetic | Reaction/decay studies | — (Wikipedia) |
| ³⁵Ar | 18 | 35 | No | 1.7756(10) s | β⁺ | ³⁵Cl | β⁺ | Synthetic | β⁺ standards | — (Wikipedia) |
| ³⁶Ar | 18 | 36 | Stable | — | — | — | — | Natural | Noble‑gas geochemistry (³⁶Ar/³⁸Ar ratios) | One of three stable Ar nuclides. (Wikipedia) |
| ³⁷Ar | 18 | 37 | No (natural trace) | 35.011(19) d | EC → ³⁷Cl | ³⁷Cl | X‑rays/Auger (EC) | Cosmogenic & synthetic | Solar‑neutrino detector product, CTBT OSI tracer, environmental studies | Classic Homestake chlorine detector product (³⁷Cl → ³⁷Ar); t½ ~35 d. (Wikipedia, PubMed) |
| ³⁸Ar | 18 | 38 | Stable | — | — | — | — | Natural | Planetary atmospheres; noble‑gas IRMS | Trace stable isotope. (Wikipedia) |
| ³⁹Ar | 18 | 39 | No (natural trace) | 302(10) y | β⁻ → ³⁹K | ³⁹K | β⁻ | Cosmogenic (air); subsurface production; synthetic | Groundwater & ocean dating (50–1000 y); background in LAr detectors (→ seek depleted Ar) | Modern direct half‑life measurement with DEAP‑3600: 302 ± 8(stat) ± 6(sys) y. (arXiv, AGU Publications, Nature) |
| ⁴⁰Ar | 18 | 40 | Stable | — | — | — | — | Natural (99.6%) | K–Ar / ⁴⁰Ar/³⁹Ar geochronology; LAr detectors | Product of ⁴⁰K decay; cornerstone of rock dating. (Wikipedia, U.S. Geological Survey) |
| ⁴¹Ar | 18 | 41 | No | 109.61(4) min | β⁻ → ⁴¹K | ⁴¹K | β⁻; γ 1293.6 keV | Activation ⁴⁰Ar(n,γ) in reactors | Stack/plume tracer; activation monitor; γ‑calibration | Dominant γ at 1293.6 keV. (wwwndc.jaea.go.jp, INIS) |
| ⁴²Ar | 18 | 42 | No (natural trace) | 32.9(1.1) y | β⁻ → ⁴²K (→ ⁴²Ca) | ⁴²K | β⁻; (daughter β⁻) | Cosmogenic/activation; synthetic | Low‑background physics: ⁴²Ar/⁴²K is a known background in LAr systems; mitigation studied | Concentration in air is extremely low; underground Ar useful to suppress it. (PubMed, SpringerLink, arXiv) |
| ⁴³Ar | 18 | 43 | No | 5.37(6) min | β⁻ → ⁴³K | ⁴³K | β⁻ | Synthetic | Decay‑scheme studies | — (Wikipedia) |
| ⁴⁴Ar | 18 | 44 | No | 11.87(5) min | β⁻ → ⁴⁴K | ⁴⁴K | β⁻ | Synthetic | β/γ spectroscopy | — (Wikipedia) |
| ⁴⁵Ar | 18 | 45 | No | 21.48(15) s | β⁻ → ⁴⁵K | ⁴⁵K | β⁻ | Synthetic | Structure near N=27 | — (Wikipedia) |
| ⁴⁶Ar | 18 | 46 | No | 8.4(6) s | β⁻ → ⁴⁶K | ⁴⁶K | β⁻ | Synthetic | Magic‑number tests (N=28) | — (Wikipedia) |
| ⁴⁷Ar | 18 | 47 | No | 1.23(3) s | β⁻ (> 99.8%); β⁻,n (< 0.2%) | ⁴⁷K; ⁴⁶K | β⁻; delayed n (minor) | Synthetic | β‑delayed‑n limits | — (Wikipedia) |
| ⁴⁸Ar | 18 | 48 | No | 415(15) ms | β⁻ (62%); β⁻,n (38%) | ⁴⁸K; ⁴⁷K | β⁻; delayed n | Synthetic | β‑delayed‑n benchmarks | — (Wikipedia) |
| ⁴⁹Ar | 18 | 49 | No | 236(8) ms | β⁻ (± n 29%) | ⁴⁹K; ⁴⁸K | β⁻; delayed n | Synthetic | Dripline approach | — (Wikipedia) |
| ⁵⁰Ar | 18 | 50 | No | 106(6) ms | β⁻ (63%); β⁻,n (37%) | ⁵⁰K; ⁴⁹K | β⁻; delayed n | Synthetic | Multineutron emission | — (Wikipedia) |
| ⁵¹Ar | 18 | 51 | No | ≈ 30 ms (#; > 200 ns) | β⁻? (± n/2n?) | ⁵¹K (…⁵⁰/⁴⁹K) | β⁻; delayed n? | Synthetic | Edge‑of‑stability tests | Trend/evaluation (#). (Wikipedia) |
| ⁵²Ar | 18 | 52 | No | ≈ 40 ms (#; > 620 ns) | β⁻? (± n/2n?) | ⁵²K (…⁵¹/⁵⁰K) | β⁻; delayed n? | Synthetic | Edge‑of‑stability tests | Trend/evaluation (#). (Wikipedia) |
| ⁵³Ar | 18 | 53 | No | ≈ 20 ms (#; > 620 ns) | β⁻? (± n/2n?) | ⁵³K (…⁵²/⁵¹K) | β⁻; delayed n? | Synthetic | Heaviest well‑documented | Trend/evaluation (#). (Wikipedia) |
| ⁵⁴Ar | 18 | 54 | No | ≈ 5 ms (#; > 400 ns) | β⁻? (± n/2n?) | ⁵⁴K (…⁵³/⁵²K) | β⁻; delayed n? | Synthetic | Most neutron‑rich Ar observed | Trend/evaluation (#). (Wikipedia) |
Radiation key: β⁺/EC → annihilation γ (511 keV) and X‑rays (EC); β⁻ (electron); β⁻,n/β⁻,2n = β‑decay with 1/2 delayed neutron(s); 2p = prompt two‑proton emission from unbound states.
Applied highlights: ³⁹Ar is a premier age tracer for waters and ice (50–1000 y), now measurable on litre‑scale samples by Atom‑Trap Trace Analysis; ⁴¹Ar is the familiar 1293.6 keV γ activation product used for stack‑plume studies; ⁴²Ar/⁴²K forms a background in liquid‑argon detectors, mitigated by design and by sourcing underground Ar; ⁴⁰Ar underpins K–Ar / ⁴⁰Ar/³⁹Ar geochronology. (AGU Publications, Nature, wwwndc.jaea.go.jp, SpringerLink, arXiv, U.S. Geological Survey)
Totals — Argon (Z = 18)
- Stable3 (³⁶Ar, ³⁸Ar, ⁴⁰Ar)
- Unstable23 (²⁹–³⁵Ar, ³⁷Ar, ³⁹Ar, ⁴¹–⁵⁴Ar)
- Total isotopes26. (Wikipedia)
Running cumulative totals (H → Ar, Z = 1…18)
- Cumulative total isotopes325 (prior through Cl: 299 • Ar: +26)
- Cumulative stable38 (prior through Cl: 35 • Ar: +3)
- Cumulative unstable287
Sources & cross‑checks
- Isotopes of argon — complete per‑nuclide table (A = 29–54), half‑lives, branches, daughters; also natural abundances and counts. (Wikipedia)
- IAEA LiveChart of Nuclides / ENSDF — authoritative decay & structure look‑ups used for verification. (IAEA Nuclear Data Services)
- Precision half‑lives: classic ³⁷Ar/³⁹Ar/⁴²Ar measurements; new ³⁹Ar direct measurement (DEAP‑3600, 2025). (PubMed, arXiv)
- K–Ar and ⁴⁰Ar/³⁹Ar geochronology: USGS review. (U.S. Geological Survey)
- ³⁹Ar as an environmental tracer: foundational and modern A‐TTA dating papers. (Cambridge University Press & Assessment, AGU Publications, Nature)
- ⁴¹Ar 1293.6 keV γ line & activation context: nuclide sheets and plume/monitoring literature. (wwwndc.jaea.go.jp, INIS)
- ⁴²Ar/⁴²K backgrounds & mitigation in LAr experiments (GERDA/LEGEND and related). (SpringerLink, arXiv)
Next: Potassium — K (Z = 19).
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