Caesium (₅₅Cs) has 41 known isotopes, the atomic masses of these isotopes range 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.

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[edit]

Nuclide
^{[n 1]}

Isotopic mass (Da)
^{[n 2]}^{[n 3]}

Half-life

Decay
mode
^{[n 4]}

Daughter
isotope
^{[n 5]}^{[n 6]}

Spin and
parity
^{[n 7]}^{[n 8]}

Isotopic
abundance

Excitation energy^{[n 8]}

¹¹²Cs

111.95030(33)#

500(100) μs

¹¹¹Xe

1+#

¹⁰⁸I

¹¹³Cs

112.94449(11)

16.7(7) μs

p (99.97%)

¹¹²Xe

5/2+#

β⁺ (.03%)

¹¹³Xe

¹¹⁴Cs

113.94145(33)#

0.57(2) s

β⁺ (91.09%)

¹¹⁴Xe

(1+)

β⁺, p (8.69%)

¹¹³I

β⁺, α (.19%)

¹¹⁰Te

α (.018%)

¹¹⁰I

¹¹⁵Cs

114.93591(32)#

1.4(8) s

β⁺ (99.93%)

¹¹⁵Xe

9/2+#

β⁺, p (.07%)

¹¹⁴I

¹¹⁶Cs

115.93337(11)#

0.70(4) s

β⁺ (99.67%)

¹¹⁶Xe

(1+)

β⁺, p (.279%)

¹¹⁵I

β⁺, α (.049%)

¹¹²Te

^116mCs

100(60)# keV

3.85(13) s

β⁺ (99.48%)

¹¹⁶Xe

4+, 5, 6

β⁺, p (.51%)

¹¹⁵I

β⁺, α (.008%)

¹¹²Te

¹¹⁷Cs

116.92867(7)

8.4(6) s

β⁺

¹¹⁷Xe

(9/2+)#

^117mCs

150(80)# keV

6.5(4) s

β⁺

¹¹⁷Xe

3/2+#

¹¹⁸Cs

117.926559(14)

14(2) s

β⁺ (99.95%)

¹¹⁸Xe

β⁺, p (.042%)

¹¹⁷I

β⁺, α (.0024%)

¹¹⁴Te

^118mCs

100(60)# keV

17(3) s

β⁺ (99.95%)

¹¹⁸Xe

(7−)

β⁺, p (.042%)

¹¹⁷I

β⁺, α (.0024%)

¹¹⁴Te

¹¹⁹Cs

118.922377(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

119.920677(11)

61.2(18) 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

120.917229(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

121.91611(3)

21.18(19) s

β⁺

¹²²Xe

β⁺, α (2×10⁻⁷%)

¹¹⁸Te

^122m1Cs

45.8 keV

>1 μs

(3)+

^122m2Cs

140(30) keV

3.70(11) min

β⁺

¹²²Xe

8−

^122m3Cs

127.0(5) keV

360(20) ms

(5)−

¹²³Cs

122.912996(13)

5.88(3) min

β⁺

¹²³Xe

1/2+

^123m1Cs

156.27(5) keV

1.64(12) s

¹²³Cs

(11/2)−

^123m2Cs

231.63+X keV

114(5) ns

(9/2+)

¹²⁴Cs

123.912258(9)

30.9(4) s

β⁺

¹²⁴Xe

^124mCs

462.55(17) keV

6.3(2) s

¹²⁴Cs

(7)+

¹²⁵Cs

124.909728(8)

46.7(1) min

β⁺

¹²⁵Xe

1/2(+)

^125mCs

266.6(11) keV

900(30) ms

(11/2−)

¹²⁶Cs

125.909452(13)

1.64(2) min

β⁺

¹²⁶Xe

^126m1Cs

273.0(7) keV

>1 μs

^126m2Cs

596.1(11) keV

171(14) μs

¹²⁷Cs

126.907418(6)

6.25(10) h

β⁺

¹²⁷Xe

1/2+

^127mCs

452.23(21) keV

55(3) μs

(11/2)−

¹²⁸Cs

127.907749(6)

3.640(14) min

β⁺

¹²⁸Xe

¹²⁹Cs

128.906064(5)

32.06(6) h

β⁺

¹²⁹Xe

1/2+

¹³⁰Cs

129.906709(9)

29.21(4) min

β⁺ (98.4%)

¹³⁰Xe

β⁻ (1.6%)

¹³⁰Ba

^130mCs

163.25(11) keV

3.46(6) min

IT (99.83%)

¹³⁰Cs

5−

β⁺ (.16%)

¹³⁰Xe

¹³¹Cs

130.905464(5)

9.689(16) d

¹³¹Xe

5/2+

¹³²Cs

131.9064343(20)

6.480(6) d

β⁺ (98.13%)

¹³²Xe

β⁻ (1.87%)

¹³²Ba

¹³³Cs^{[n 9]}^{[n 10]}

132.905451933(24)

Stable

7/2+

1.0000

¹³⁴Cs^{[n 10]}

133.906718475(28)

2.0652(4) y

β⁻

¹³⁴Ba

EC (3×10⁻⁴%)

¹³⁴Xe

^134mCs

138.7441(26) keV

2.912(2) h

¹³⁴Cs

8−

¹³⁵Cs^{[n 10]}

134.9059770(11)

2.3 x10⁶ y

β⁻

¹³⁵Ba

7/2+

^135mCs

1632.9(15) keV

53(2) min

¹³⁵Cs

19/2−

¹³⁶Cs

135.9073116(20)

13.16(3) d

β⁻

¹³⁶Ba

^136mCs

518(5) keV

19(2) s

β⁻

¹³⁶Ba

8−

¹³⁶Cs

¹³⁷Cs^{[n 10]}

136.9070895(5)

30.1671(13) y

β⁻ (95%)

^137mBa

7/2+

β⁻ (5%)

¹³⁷Ba

¹³⁸Cs

137.911017(10)

33.41(18) min

β⁻

¹³⁸Ba

3−

^138mCs

79.9(3) keV

2.91(8) min

IT (81%)

¹³⁸Cs

6−

β⁻ (19%)

¹³⁸Ba

¹³⁹Cs

138.913364(3)

9.27(5) min

β⁻

¹³⁹Ba

7/2+

¹⁴⁰Cs

139.917282(9)

63.7(3) s

β⁻

¹⁴⁰Ba

1−

¹⁴¹Cs

140.920046(11)

24.84(16) s

β⁻ (99.96%)

¹⁴¹Ba

7/2+

β⁻, n (.0349%)

¹⁴⁰Ba

¹⁴²Cs

141.924299(11)

1.689(11) s

β⁻ (99.9%)

¹⁴²Ba

0−

β⁻, n (.091%)

¹⁴¹Ba

¹⁴³Cs

142.927352(25)

1.791(7) s

β⁻ (98.38%)

¹⁴³Ba

3/2+

β⁻, n (1.62%)

¹⁴²Ba

¹⁴⁴Cs

143.932077(28)

994(4) ms

β⁻ (96.8%)

¹⁴⁴Ba

1(−#)

β⁻, n (3.2%)

¹⁴³Ba

^144mCs

300(200)# keV

<1 s

β⁻

¹⁴⁴Ba

(>3)

¹⁴⁴Cs

¹⁴⁵Cs

144.935526(12)

582(6) ms

β⁻ (85.7%)

¹⁴⁵Ba

3/2+

β⁻, n (14.3%)

¹⁴⁴Ba

¹⁴⁶Cs

145.94029(8)

0.321(2) s

β⁻ (85.8%)

¹⁴⁶Ba

1−

β⁻, n (14.2%)

¹⁴⁵Ba

¹⁴⁷Cs

146.94416(6)

0.235(3) s

β⁻ (71.5%)

¹⁴⁷Ba

(3/2+)

β⁻, n (28.49%)

¹⁴⁶Ba

¹⁴⁸Cs

147.94922(62)

146(6) ms

β⁻ (74.9%)

¹⁴⁸Ba

β⁻, n (25.1%)

¹⁴⁷Ba

¹⁴⁹Cs

148.95293(21)#

150# ms [>50 ms]

β⁻

¹⁴⁹Ba

3/2+#

β⁻, n

¹⁴⁸Ba

¹⁵⁰Cs

149.95817(32)#

100# ms [>50 ms]

β⁻

¹⁵⁰Ba

β⁻, n

¹⁴⁹Ba

¹⁵¹Cs

150.96219(54)#

60# ms [>50 ms]

β⁻

¹⁵¹Ba

3/2+#

β⁻, n

¹⁵⁰Ba

This table header & footer:

^ ^mCs – Excited nuclear isomer.

^ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.

^ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).

^ Modes of decay:

EC:

IT:

^ Bold italics symbol as daughter – Daughter product is nearly stable.

^ Bold symbol as daughter – Daughter product is stable.

^ ( ) spin value – Indicates spin with weak assignment arguments.

^ ^a ^b # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).

^ Used to define the second

^ ^a ^b ^c ^d Fission product

Caesium-131[edit]

Caesium-131, introduced in 2004 for brachytherapybyIsoray,^[5] has a half-life of 9.7 days and 30.4 keV energy.

Caesium-133[edit]

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, $Δ ν Cs$ , the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom,^[6] to be 9192631770 when expressed in the unit Hz, which is equal to s⁻¹.

Caesium-134[edit]

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 section29barns), 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).^[7]

Caesium-135[edit]

Nuclide

t_1⁄2

Yield

Q^{[a 1]}

βγ

(Ma)

(%)^{[a 2]}

(keV)

⁹⁹Tc

0.211

6.1385

294

¹²⁶Sn

0.230

0.1084

4050^{[a 3]}

βγ

⁷⁹Se

0.327

0.0447

151

¹³⁵Cs

1.33

6.9110^{[a 4]}

269

⁹³Zr

1.53

5.4575

βγ

¹⁰⁷Pd

6.5

1.2499

¹²⁹I

15.7

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 2.3 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 Cs-135 via isotope separation) rather than with other long-lived fission products. Except in the Molten salt reactor, where Cs-135 is created as a completely separate stream outside the fuel (after the decay of bubble-separated Xe-135). The low decay energy, lack of gamma radiation, and long half-life of ¹³⁵Cs make this isotope much less hazardous than ¹³⁷Csor¹³⁴Cs.

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)^[8] 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 integralof¹³⁵Cs are 8.3 ± 0.3 and 38.1 ± 2.6 barns respectively.^[9] 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.^[10]

ANL factsheet

Caesium-136[edit]

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Caesium-136 has a half-life of 13.16 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 (neutron capture cross section 8.702 barns), which is a common fission product. Caesium-136 is not produced via beta decay of other fission product nuclides of mass 136 since beta decay stops at almost-stable ¹³⁶Xe. 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. ¹³⁶Cs also captures neutrons with a cross section of 13.00 barns, becoming medium-lived radioactive ¹³⁷Cs. Caesium-136 undergoes beta decay (β−), producing ¹³⁶Ba directly.

Caesium-137[edit]

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 radioactivityofspent 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.^[11] 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[edit]

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[edit]

^ 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.

^ "NIST Radionuclide Half-Life Measurements". NIST. Retrieved 2011-03-13.

^ "Standard Atomic Weights: Caesium". CIAAW. 2013.

^ 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. (2022-05-04). "Standard atomic weights of the elements 2021 (IUPAC Technical Report)". Pure and Applied Chemistry. doi:10.1515/pac-2019-0603. ISSN 1365-3075.

^ Isoray. "Why Cesium-131".

^ 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."

^ "Characteristics of Caesium-134 and Caesium-137". Japan Atomic Energy Agency. Archived from the original on 2016-03-04. Retrieved 2014-10-23.

^ 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.

^ 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.

^ 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.

^ Dennis (1 March 2013). "Cooling a Hot Zone". Science. 339 (6123): 1028–1029. doi:10.1126/science.339.6123.1028. PMID 23449572.