Isotopes of argon

Isotopes of argon  (₁₈Ar)

Main isotopes [1] Decay abun­dance half-life (t1/2) mode pro­duct 36Ar 0.334% stable 37Ar trace 35.01 d ε 37Cl 38Ar 0.0630% stable 39Ar trace 302 y β− 39K 40Ar 99.6% stable 41Ar trace 109.61 min β− 41K 42Ar synth 32.9 y β− 42K
Standard atomic weight Ar°(Ar)
	[39.792, 39.963] [2] 39.95±0.16 (abridged) [3]
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Argon (₁₈Ar) has 26 known isotopes, from ²⁹Ar to ⁵⁴Ar, of which three are stable (³⁶Ar, ³⁸Ar, and ⁴⁰Ar). On Earth, ⁴⁰Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are ³⁹Ar with a half-life of 302 years, ⁴²Ar with a half-life of 32.9 years, and ³⁷Ar with a half-life of 35.01 days. All other isotopes have half-lives of less than two hours, and most less than one minute. Isotopes lighter than ³⁸Ar decay to chlorine or lighter elements, while heavier ones beta decay to potassium.

The naturally occurring ⁴⁰K, with a half-life of 1.248×10⁹ years, decays to stable ⁴⁰Ar by electron capture (10.72%) and by positron emission (0.001%), and also to stable ⁴⁰Ca via beta decay (89.28%). These properties and ratios are used to determine the age of rocks through potassium–argon dating. ^[4]

Despite the trapping of ⁴⁰Ar in many rocks, it can be released by melting, grinding, and diffusion. Almost all argon in the Earth's atmosphere is the product of ⁴⁰K decay, since 99.6% of Earth's atmospheric argon is ⁴⁰Ar, whereas in the Sun and presumably in primordial star-forming clouds, argon consists of ~85% ³⁶Ar, ~15% ³⁸Ar and only trace ⁴⁰Ar. Similarly, the ratio of the isotopes ³⁶Ar:³⁸Ar:⁴⁰Ar in the atmospheres of the outer planets is measured to be 8400:1600:1. ^[5]

In the Earth's atmosphere, radioactive ³⁹Ar (and to a lesser extent ³⁷Ar) is made by cosmic ray activity, primarily from ⁴⁰Ar. In the subsurface environment, ³⁹Ar is also produced through neutron capture by ³⁹K or ⁴²Ca, with proton or alpha emission respectively; ³⁷Ar was created in subsurface nuclear explosions similarly from ⁴⁰Ca. ^[4] The content of ³⁹Ar in natural argon is measured to be of (8.6±0.4)×10⁻¹⁶ g/g, or (0.964±0.024) Bq/kg weight. ^[6]

The content of ⁴²Ar (half-life 33 years) in the Earth's atmosphere, though it had previously been reported as a cosmogenic isotope, ^[7] is lower than 6×10⁻²¹ of the element. ^[8] Many endeavors require argon depleted in the cosmogenic isotopes, known as depleted argon ^[9] and this may be obtained from underground sources that have been isolated from the atmosphere long enough for these isotopes to decay.

³⁶Ar, in the form of argon hydride, was detected in the Crab Nebula supernova remnant during 2013. ^{[10] [11]} This was the first time a noble molecule was detected in outer space. ^{[10] [11]}

List of isotopes

Nuclide [n 1]	Z	N	Isotopic mass (Da) [12] [n 2] [n 3]	Half-life [1]	Decay mode [1] [n 4]	Daughter isotope [n 5]	Spin and parity [1] [n 6] [n 7]	Natural abundance (mole fraction)
	Excitation energy							Normal proportion [1]	Range of variation
29Ar [13]	18	11	29.04076(47)#		2p	27S	5/2+#
30Ar	18	12	30.02369(19)#	<10 ps	2p	28S	0+
31Ar	18	13	31.01216(22)#	15.0(3) ms	β+, p (68.3%)	30S	5/2+
					β+ (22.63%)	31Cl
					β+, 2p (9.0%)	29P
					β+, 3p (0.07%)	28Si
					β+, p, α? (<0.38%)	26Si
					β+, α? (<0.03%)	27P
					2p? (<0.03%)	29S
32Ar	18	14	31.9976378(19)	98(2) ms	β+ (64.42%)	32Cl	0+
					β+, p (35.58%)	31S
33Ar	18	15	32.98992555(43)	173.0(20) ms	β+ (61.3%)	33Cl	1/2+
					β+, p (38.7%)	32S
34Ar	18	16	33.980270092(83)	846.46(35) ms	β+	34Cl	0+
35Ar	18	17	34.97525772(73)	1.7756(10) s	β+	35Cl	3/2+
36Ar	18	18	35.967545106(28)	Observationally Stable [n 8]			0+	0.003336(210)
37Ar	18	19	36.96677630(22)	35.011(19) d	EC	37Cl	3/2+	Trace [n 9]
38Ar	18	20	37.96273210(21)	Stable			0+	0.000629(70)
39Ar [n 10]	18	21	38.9643130(54)	302(10) y [14] [15]	β−	39K	7/2−	8×10−16 [16] [n 9]
40Ar [n 11]	18	22	39.9623831220(23)	Stable			0+	0.996035(250) [n 12]
41Ar	18	23	40.96450057(37)	109.61(4) min	β−	41K	7/2−	Trace [n 9]
42Ar	18	24	41.9630457(62)	32.9(11) y	β−	42K	0+
43Ar	18	25	42.9656361(57)	5.37(6) min	β−	43K	5/2(−)
44Ar	18	26	43.9649238(17)	11.87(5) min	β−	44K	0+
45Ar	18	27	44.96803973(55)	21.48(15) s	β−	45K	(5/2−,7/2−)
46Ar	18	28	45.9680392(25)	8.4(6) s	β−	46K	0+
47Ar	18	29	46.9727671(13)	1.23(3) s	β− (>99.8%)	47K	(3/2)−
					β−, n? (<0.2%)	46K
48Ar	18	30	47.976001(18)	415(15) ms	β− (62%)	48K	0+
					β−, n (38%)	47K
49Ar	18	31	48.98169(43)#	236(8) ms	β−	49K	3/2−#
					β−, n (29%)	48K
					β−, 2n?	47K
50Ar	18	32	49.98580(54)#	106(6) ms	β− (63%)	50K	0+
					β−, n (37%)	49K
					β−, 2n?	48K
51Ar	18	33	50.99303(43)#	30# ms [>200 ns]	β−?	51K	1/2−#
					β−, n?	50K
					β−, 2n?	49K
52Ar	18	34	51.99852(64)#	40# ms [>620 ns]	β−?	52K	0+
					β−, n?	51K
					β−, 2n?	50K
53Ar	18	35	53.00729(75)#	20# ms [>620 ns]	β−?	53K	5/2−#
					β−, n?	52K
					β−, 2n?	51K
54Ar	18	36	54.01348(86)#	5# ms [>400 ns]	β−?	54K	0+
					β−, n?	53K
					β−, 2n?	52K
This table header & footer: view

1. ^mAr – 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. Modes of decay:

   EC:	Electron capture
   n:	Neutron emission
   p:	Proton emission

5. Bold symbol as daughter – Daughter product is stable.
6. ( ) spin value – Indicates spin with weak assignment arguments.
7. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
8. Believed to undergo double electron capture to ³⁶S (lightest theoretically unstable nuclide for which no evidence of radioactivity has been observed)
9. Cosmogenic nuclide
10. Used in argon–argon dating
11. Used in argon–argon dating and potassium–argon dating
12. Generated from ⁴⁰K in rocks. These ratios are terrestrial. Cosmic abundance is far less than ³⁶Ar.

See also

• Banana equivalent dose

Daughter products other than argon

• Isotopes of potassium
• Isotopes of chlorine
• Isotopes of sulfur
• Isotopes of phosphorus

References

1. Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
2. "Standard Atomic Weights: Argon". CIAAW. 2017.
3. Prohaska, Thomas; Irrgeher, Johanna; Benefield, Jacqueline; Böhlke, John K.; Chesson, Lesley A.; Coplen, Tyler B.; Ding, Tiping; Dunn, Philip J. H.; Gröning, Manfred; Holden, Norman E.; Meijer, Harro A. J. (4 May 2022). "Standard atomic weights of the elements 2021 (IUPAC Technical Report)". Pure and Applied Chemistry. doi:10.1515/pac-2019-0603. ISSN   1365-3075.
4. "⁴⁰Ar/³⁹Ar dating and errors". Archived from the original on 9 May 2007. Retrieved 7 March 2007.
5. Cameron, A.G.W. (1973). "Elemental and isotopic abundances of the volatile elements in the outer planets". Space Science Reviews. 14 (3–4): 392–400. Bibcode:1973SSRv...14..392C. doi:10.1007/BF00214750. S2CID   119861943.
6. P. Adhikari; et al. (2023). "Precision Measurement of the Specific Activity of ³⁹Ar in Atmospheric Argon with the DEAP-3600 Detector". The European Physical Journal C . 83 (7): 642. doi:10.1140/epjc/s10052-023-11678-6.
7. For example in Barabash, A.S.; Saakyan, R.R.; Umatov, V.I. (2016). "On concentration of 42Ar in the Earth's atmosphere". Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. arXiv: 1609.08890 . doi:10.1016/j.nima.2016.09.042.
8. V. D. Ashitkov; et al. (1998). "New experimental limit on the ⁴²Ar content in the Earth's atmosphere". Nuclear Instruments and Methods A . 416 (1): 179–181. Bibcode:1998NIMPA.416..179A. doi:10.1016/S0168-9002(98)00740-2.
9. H. O. Back; et al. (2012). "Depleted Argon from Underground Sources". Physics Procedia . 37: 1105–1112. Bibcode:2012PhPro..37.1105B. doi: 10.1016/j.phpro.2012.04.099 .
10. Quenqua, Douglas (13 December 2013). "Noble Molecules Found in Space". The New York Times . Retrieved 13 December 2013.
11. Barlow, M. J.; et al. (2013). "Detection of a Noble Gas Molecular Ion, ³⁶ArH+, in the Crab Nebula". Science . 342 (6164): 1343–1345. arXiv: 1312.4843 . Bibcode:2013Sci...342.1343B. doi:10.1126/science.1243582. PMID   24337290. S2CID   37578581.
12. Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*". Chinese Physics C. 45 (3): 030003. doi:10.1088/1674-1137/abddaf.
13. Mukha, I.; et al. (2018). "Deep excursion beyond the proton dripline. I. Argon and chlorine isotope chains". Physical Review C. 98 (6): 064308–1–064308–13. arXiv: 1803.10951 . Bibcode:2018PhRvC..98f4308M. doi:10.1103/PhysRevC.98.064308. S2CID   119384311.
14. P. Adhikari; et al. (2025). "Direct Measurement of the Half-Life of ³⁹Ar from 3.4 Years of Data with the DEAP-3600 Detector". The European Physical Journal C . 85 (7): 728. doi:10.1140/epjc/s10052-025-14289-5.
15. This sppears reliable but the Nubase2020 value is 268(8) years and the discrepancy is yet to be explained.
16. Lu, Zheng-Tian (1 March 2013). "What trapped atoms reveal about global groundwater". Physics Today. 66 (3): 74–75. Bibcode:2013PhT....66c..74L. doi:10.1063/PT.3.1926.

External links

• Argon isotopes data from The Berkeley Laboratory Isotopes Projects