Caesium-133

Caesium (₅₅Cs) has 41 known isotopes, ranging in mass number from 112 to 152. Only one isotope, ¹³³Cs, is stable. The longest-lived radioisotopes are ¹³⁵Cs with a half-life of 1.33 million years, ¹³⁷
Cs
with a half-life of 30.1671 years and ¹³⁴Cs with a half-life of 2.0652 years. All other isotopes have half-lives less than 2 weeks, most under an hour.

Isotopes of caesium (₅₅Cs)

Main isotopes Decay abun­dance half-life (t1/2) mode pro­duct ^131Cs synth 9.7 d ε ^131Xe ^133Cs 100% stable ^134Cs synth 2.0648 y ε ^134Xe β^− ^134Ba ^135Cs trace 1.33×10^6 y β^− ^135Ba ^137Cs synth 30.04 y β^− ^137Ba
Standard atomic weight Ar°(Cs)
	132.90545196±0.00000006 132.91±0.01 (abridged)
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Beginning in 1945 with the commencement of nuclear testing, caesium radioisotopes were released into the atmosphere where caesium is absorbed readily into solution and is returned to the surface of the Earth as a component of radioactive fallout. Once caesium enters the ground water, it is deposited on soil surfaces and removed from the landscape primarily by particle transport. As a result, the input function of these isotopes can be estimated as a function of time.

List of isotopes

Nuclide	Z	N	Isotopic mass (Da)	Half-life	Decay mode	Daughter isotope	Spin and parity	Isotopic abundance
	Excitation energy
^112Cs	55	57	111.95017(12)#	490(30) μs	p (>99.74%)	^111Xe	1+#
					α (<0.26%)	^108I
^113Cs	55	58	112.9444285(92)	16.94(9) μs	p	^112Xe	(3/2+)
^114Cs	55	59	113.941292(91)	570(20) ms	β^+ (91.1%)	^114Xe	(1+)
					β^+, p (8.7%)	^113I
					β^+, α (0.19%)	^110Te
					α (0.018%)	^110I
^115Cs	55	60	114.93591(11)#	1.4(8) s	β^+ (99.93%)	^115Xe	9/2+#
					β^+, p (0.07%)	^114I
^116Cs	55	61	115.93340(11)#	700(40) ms	β^+ (99.67%)	^116Xe	(1+)
					β^+, p (0.28%)	^115I
					β^+, α (0.049%)	^112Te
^116mCs	100(60)# keV			3.85(13) s	β^+ (99.56%)	^116Xe	(7+)
					β^+, p (0.44%)	^115I
					β^+, α (0.0034%)	^112Te
^117Cs	55	62	116.928617(67)	8.4(6) s	β^+	^117Xe	9/2+#
^117mCs	150(80)# keV			6.5(4) s	β^+	^117Xe	3/2+#
^118Cs	55	63	117.926560(14)	14(2) s	β^+ (99.98%)	^118Xe	2(−)
					β^+, p (0.021%)	^117I
					β^+, α (0.0012%)	^114Te
^118m1Cs	X keV			17(3) s	β^+ (99.98%)	^118Xe	(7−)
					β^+, p (0.021%)	^117I
					β^+, α (0.0012%)	^114Te
^118m2Cs	Y keV						(6+)
^118m3Cs	65.9 keV				IT	^118Cs	(3−)
^118m4Cs	125.9+X keV			550(60) ns	IT	^118m1Cs	(7+)
^118m5Cs	195.2+X keV			<500 ns	IT	^118m4Cs	(8+)
^119Cs	55	64	118.9223 77(15)	43.0(2) s	β^+	^119Xe	9/2+
					β^+, α (<2×10^−6%)	^115Te
^119mCs	50(30)# keV			30.4(1) s	β^+	^119Xe	3/2+
^120Cs	55	65	119.920677(11)	60.4(6) s	β^+	^120Xe	2+
					β^+, α (<2×10^−5%)	^116Te
					β^+, p (<7×10^−6%)	^119I
^120mCs	100(60)# keV			57(6) s	β^+	^120Xe	(7−)
					β^+, α (<2×10^−5%)	^116Te
					β^+, p (<7×10^−6%)	^119I
^121Cs	55	66	120.917227(15)	155(4) s	β^+	^121Xe	3/2+
^121mCs	68.5(3) keV			122(3) s	β^+ (83%)	^121Xe	9/2+
					IT (17%)	^121Cs
^122Cs	55	67	121.916108(36)	21.18(19) s	β^+	^122Xe	1+
					β^+, α (<2×10^−7%)	^118Te
^122m1Cs	45.87(12) keV			>1 μs	IT	^122Cs	3+
^122m2Cs	140(30) keV			3.70(11) min	β^+	^122Xe	8−
^122m3Cs	127.07(16) keV			360(20) ms	IT	^122Cs	5−
^123Cs	55	68	122.912996(13)	5.88(3) min	β^+	^123Xe	1/2+
^123m1Cs	156.27(5) keV			1.64(12) s	IT	^123Cs	11/2−
^123m2Cs	252(6) keV			114(5) ns	IT	^123Cs	(9/2+)
^124Cs	55	69	123.9122474(98)	30.9(4) s	β^+	^124Xe	1+
^124mCs	462.63(14) keV			6.41(7) s	IT (99.89%)	^124Cs	(7)+
					β^+ (0.11%)	^124Xe
^125Cs	55	70	124.9097260(83)	44.35(29) min	β^+	^125Xe	1/2+
^125mCs	266.1(11) keV			900(30) ms	IT	^125Cs	(11/2−)
^126Cs	55	71	125.909446(11)	1.64(2) min	β^+	^126Xe	1+
^126m1Cs	273.0(7) keV			~1 μs	IT	^126Cs	(4−)
^126m2Cs	596.1(11) keV			171(14) μs	IT	^126Cs	8−#
^127Cs	55	72	126.9074175(60)	6.25(10) h	β^+	^127Xe	1/2+
^127mCs	452.23(21) keV			55(3) μs	IT	^127Cs	(11/2)−
^128Cs	55	73	127.9077485(57)	3.640(14) min	β^+	^128Xe	1+
^129Cs	55	74	128.9060659(49)	32.06(6) h	β^+	^129Xe	1/2+
^129mCs	575.40(14) keV			718(21) ns	IT	^127Cs	(11/2−)
^130Cs	55	75	129.9067093(90)	29.21(4) min	β^+ (98.4%)	^130Xe	1+
					β^− (1.6%)	^130Ba
^130mCs	163.25(11) keV			3.46(6) min	IT (99.84%)	^130Cs	5−
					β^+ (0.16%)	^130Xe
^131Cs	55	76	130.90546846(19)	9.689(16) d	EC	^131Xe	5/2+
^132Cs	55	77	131.9064378(11)	6.480(6) d	β^+ (98.13%)	^132Xe	2+
					β^− (1.87%)	^132Ba
^133Cs	55	78	132.905451958(8)	Stable			7/2+	1.0000
^134Cs	55	79	133.906718501(17)	2.0650(4) y	β^−	^134Ba	4+
					EC (3.0×10^−4%)	^134Xe
^134mCs	138.7441(26) keV			2.912(2) h	IT	^134Cs	8−
^135Cs	55	80	134.90597691(39)	1.33(19)×10^6 y	β^−	^135Ba	7/2+
^135mCs	1632.9(15) keV			53(2) min	IT	^135Cs	19/2−
^136Cs	55	81	135.9073114(20)	13.01(5) d	β^−	^136Ba	5+
^136mCs	517.9(1) keV			17.5(2) s	β^−?	^136Ba	8−
					IT?	^136Cs
^137Cs	55	82	136.90708930(32)	30.04(4) y	β^− (94.70%)	^137mBa	7/2+
					β^− (5.30%)	^137Ba
^138Cs	55	83	137.9110171(98)	33.5(2) min	β^−	^138Ba	3−
^138mCs	79.9(3) keV			2.91(10) min	IT (81%)	^138Cs	6−
					β^− (19%)	^138Ba
^139Cs	55	84	138.9133638(34)	9.27(5) min	β^−	^139Ba	7/2+
^140Cs	55	85	139.9172837(88)	63.7(3) s	β^−	^140Ba	1−
^140mCs	13.931(21) keV			471(51) ns	IT	^140Cs	(2)−
^141Cs	55	86	140.9200453(99)	24.84(16) s	β^− (99.97%)	^141Ba	7/2+
					β^−, n (0.0342%)	^140Ba
^142Cs	55	87	141.9242995(76)	1.687(10) s	β^− (99.91%)	^142Ba	0−
					β^−, n (0.089%)	^141Ba
^143Cs	55	88	142.9273473(81)	1.802(8) s	β^− (98.38%)	^143Ba	3/2+
					β^−, n (1.62%)	^142Ba
^144Cs	55	89	143.932075(22)	994(6) ms	β^− (97.02%)	^144Ba	1−
					β^−, n (2.98%)	^143Ba
^144mCs	92.2(5) keV			1.1(1) μs	IT	^144Cs	(4−)
^145Cs	55	90	144.9355289(97)	582(4) ms	β^− (87.2%)	^145Ba	3/2+
					β^−, n (12.8%)	^144Ba
^145mCs	762.9(4) keV			0.5(1) μs	IT	^145Cs	13/2#
^146Cs	55	91	145.9406219(31)	321.6(9) ms	β^− (85.8%)	^146Ba	1−
					β^−, n (14.2%)	^145Ba
^146mCs	46.7(1) keV			1.25(5) μs	IT	^146Cs	4−#
^147Cs	55	92	146.9442615(90)	230.5(9) ms	β^− (71.5%)	^147Ba	(3/2+)
					β^−, n (28.5%)	^146Ba
^147mCs	701.4(4) keV			190(20) ns	IT	^147Cs	13/2#
^148Cs	55	93	147.949639(14)	151.8(10) ms	β^− (71.3%)	^148Ba	(2−)
					β^−, n (28.7%)	^147Ba
^148mCs	45.2(1) keV			4.8(2) μs	IT	^148Cs	4−#
^149Cs	55	94	148.95352(43)#	112.3(25) ms	β^− (75%)	^149Ba	3/2+#
					β^−, n (25%)	^148Ba
^150Cs	55	95	149.95902(43)#	81.0(26) ms	β^− (~56%)	^150Ba	(2−)
					β^−, n (~44%)	^149Ba
^151Cs	55	96	150.96320(54)#	59(19) ms	β^−	^151Ba	3/2+#
This table header & footer: view

1. ^mCs – 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
   IT:	Isomeric transition
   n:	Neutron emission
   p:	Proton emission

5. Bold italics symbol as daughter – Daughter product is nearly stable.
6. Bold symbol as daughter – Daughter product is stable.
7. ( ) spin value – Indicates spin with weak assignment arguments.
8. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
9. Order of ground state and isomer is uncertain.
10. Used to define the second
11. Fission product

Caesium-131

Caesium-131, introduced in 2004 for brachytherapy by Isoray, has a half-life of 9.7 days and 30.4 keV energy.

Caesium-133

Caesium-133 is the only stable isotope of caesium. The SI base unit of time, the second, is defined by a specific caesium-133 transition. Since 1967, the official definition of a second is:

The second, symbol s, is defined by taking the fixed numerical value of the caesium frequency, Δν_Cs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 Hz, which is equal to s⁻¹.

Caesium-134

Caesium-134 has a half-life of 2.0652 years. It is produced both directly (at a very small yield because ¹³⁴Xe is stable) as a fission product and via neutron capture from nonradioactive ¹³³Cs (neutron capture cross section 29 barns), which is a common fission product. Caesium-134 is not produced via beta decay of other fission product nuclides of mass 134 since beta decay stops at stable ¹³⁴Xe. It is also not produced by nuclear weapons because ¹³³Cs is created by beta decay of original fission products only long after the nuclear explosion is over.

The combined yield of ¹³³Cs and ¹³⁴Cs is given as 6.7896%. The proportion between the two will change with continued neutron irradiation. ¹³⁴Cs also captures neutrons with a cross section of 140 barns, becoming long-lived radioactive ¹³⁵Cs.

Caesium-134 undergoes beta decay (β⁻), producing ¹³⁴Ba directly and emitting on average 2.23 gamma ray photons (mean energy 0.698 MeV).

Caesium-135

Long-lived fission products

• v
• t
• e

Nuclide	t1⁄2	Yield	Q	βγ
	(Ma)	(%)	(keV)
^99Tc	0.211	6.1385	294	β
^126Sn	0.230	0.1084	4050	βγ
^79Se	0.327	0.0447	151	β
^135Cs	1.33	6.9110	269	β
^93Zr	1.53	5.4575	91	βγ
^107Pd	6.5	1.2499	33	β
^129I	16.14	0.8410	194	βγ
Decay energy is split among β, neutrino, and γ if any. Per 65 thermal neutron fissions of ^235U and 35 of ^239Pu. Has decay energy 380 keV, but its decay product ^126Sb has decay energy 3.67 MeV. Lower in thermal reactors because ^135Xe, its predecessor, readily absorbs neutrons.

Caesium-135 is a mildly radioactive isotope of caesium with a half-life of 1.33 million years. It decays via emission of a low-energy beta particle into the stable isotope barium-135. Caesium-135 is one of the seven long-lived fission products and the only alkaline one. In most types of nuclear reprocessing, it stays with the medium-lived fission products (including ¹³⁷
Cs which can only be separated from ¹³⁵
Cs via isotope separation) rather than with other long-lived fission products. Except in the Molten salt reactor, where ¹³⁵
Cs is created as a completely separate stream outside the fuel (after the decay of bubble-separated ¹³⁵
Cs). The low decay energy, lack of gamma radiation, and long half-life of ¹³⁵Cs make this isotope much less hazardous than ¹³⁷Cs or ¹³⁴Cs.

Its precursor ¹³⁵Xe has a high fission product yield (e.g., 6.3333% for ²³⁵U and thermal neutrons) but also has the highest known thermal neutron capture cross section of any nuclide. Because of this, much of the ¹³⁵Xe produced in current thermal reactors (as much as >90% at steady-state full power) will be converted to extremely long-lived (half-life on the order of 10²¹ years) ¹³⁶
Xe before it can decay to ¹³⁵
Cs despite the relatively short half life of ¹³⁵
Xe. Little or no ¹³⁵
Xe will be destroyed by neutron capture after a reactor shutdown, or in a molten salt reactor that continuously removes xenon from its fuel, a fast neutron reactor, or a nuclear weapon. The xenon pit is a phenomenon of excess neutron absorption through ¹³⁵
Xe buildup in the reactor after a reduction in power or a shutdown and is often managed by letting the ¹³⁵
Xe decay away to a level at which neutron flux can be safely controlled via control rods again.

A nuclear reactor will also produce much smaller amounts of ¹³⁵Cs from the nonradioactive fission product ¹³³Cs by successive neutron capture to ¹³⁴Cs and then ¹³⁵Cs.

The thermal neutron capture cross section and resonance integral of ¹³⁵Cs are 8.3 ± 0.3 and 38.1 ± 2.6 barns respectively. Disposal of ¹³⁵Cs by nuclear transmutation is difficult, because of the low cross section as well as because neutron irradiation of mixed-isotope fission caesium produces more ¹³⁵Cs from stable ¹³³Cs. In addition, the intense medium-term radioactivity of ¹³⁷Cs makes handling of nuclear waste difficult.

• ANL factsheet

Caesium-136

Caesium-136 has a half-life of 13.01 days. It is produced both directly (at a very small yield because ¹³⁶Xe is beta-stable) as a fission product and via neutron capture from long-lived ¹³⁵Cs, which is a common fission product. It is also not produced by nuclear weapons because ¹³⁵Cs is created by beta decay of original fission products only long after the nuclear explosion is over. Caesium-136 undergoes beta decay (β−), producing ¹³⁶Ba directly.

Caesium-137

Caesium-137, with a half-life of 30.17 years, is one of the two principal medium-lived fission products, along with ⁹⁰Sr, which are responsible for most of the radioactivity of spent nuclear fuel after several years of cooling, up to several hundred years after use. It constitutes most of the radioactivity still left from the Chernobyl accident and is a major health concern for decontaminating land near the Fukushima nuclear power plant.¹³⁷Cs beta decays to barium-137m (a short-lived nuclear isomer) then to nonradioactive barium-137. Caesium-137 does not emit gamma radiation directly, all observed radiation is due to the daughter isotope barium-137m.

¹³⁷Cs has a very low rate of neutron capture and cannot yet be feasibly disposed of in this way unless advances in neutron beam collimation (not otherwise achievable by magnetic fields), uniquely available only from within muon catalyzed fusion experiments (not in the other forms of Accelerator Transmutation of Nuclear Waste) enables production of neutrons at high enough intensity to offset and overcome these low capture rates; until then, therefore, ¹³⁷Cs must simply be allowed to decay.

¹³⁷Cs has been used as a tracer in hydrologic studies, analogous to the use of ³H.

Other isotopes of caesium

The other isotopes have half-lives from a few days to fractions of a second. Almost all caesium produced from nuclear fission comes from beta decay of originally more neutron-rich fission products, passing through isotopes of iodine then isotopes of xenon. Because these elements are volatile and can diffuse through nuclear fuel or air, caesium is often created far from the original site of fission.

References

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2. "NIST Radionuclide Half-Life Measurements". NIST. Retrieved 2011-03-13.
3. "Standard Atomic Weights: Caesium". CIAAW. 2013.
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9. Browne, E.; Tuli, J.K. (October 2007). "Nuclear Data Sheets for A = 137". Nuclear Data Sheets. 108 (10): 2173–2318. doi:10.1016/j.nds.2007.09.002.
10. Isoray. "Why Cesium-131". Archived from the original on 2019-06-30. Retrieved 2017-12-05.
11. Although the phase used here is more terse than in the previous definition, it still has the same meaning. This is made clear in the 9th SI Brochure, which almost immediately after the definition on p. 130 states: "The effect of this definition is that the second is equal to the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the unperturbed ground state of the ¹³³Cs atom."
12. John L. Groh (2004). "Supplement to Chapter 11 of Reactor Physics Fundamentals" (PDF). CANTEACH project. Archived from the original (PDF) on 10 June 2011. Retrieved 14 May 2011.
13. Hatsukawa, Y.; Shinohara, N; Hata, K.; et al. (1999). "Thermal neutron cross section and resonance integral of the reaction of135Cs(n,γ)136Cs: Fundamental data for the transmutation of nuclear waste". Journal of Radioanalytical and Nuclear Chemistry. 239 (3): 455–458. doi:10.1007/BF02349050. S2CID 97425651.
14. Ohki, Shigeo; Takaki, Naoyuki (2002). "Transmutation of Cesium-135 With Fast Reactors" (PDF). Proceedings of the Seventh Information Exchange Meeting on Actinide and Fission Product Partitioning & Transmutation, Cheju, Korea.
15. "NGAtlas/ZV". www-nds.iaea.org. Retrieved 2025-02-22.
16. Dennis (1 March 2013). "Cooling a Hot Zone". Science. 339 (6123): 1028–1029. doi:10.1126/science.339.6123.1028. PMID 23449572.

• Isotope masses from:
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
• Half-life, spin, and isomer data selected from the following sources.
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
  • National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory.
  • Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.). CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9.