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Title: Caesium-133  
Author: World Heritage Encyclopedia
Language: English
Subject: Stable nuclide, Unit of time
Publisher: World Heritage Encyclopedia


Caesium (Cs) has 40 known isotopes. The atomic masses of these isotopes range from 112 to 151. Only one isotope, 133Cs, is stable. The longest-lived radioisotopes are 135Cs with a half-life of 2.3 million years, 137Cs 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 isotopes were released into the atmosphere where it 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.

Standard atomic mass: 132.9054519(2) u


Caesium-133 is the only naturally occurring and only stable isotope of caesium. It is also produced by nuclear fission in nuclear reactors. One specific quantum transition in the Caesium-133 atom is used to define the second, a unit of time.


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 Cs-133 (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), producing Ba 134 directly and emitting a (1.6 MeV) gamma ray.


fission products
Q *
99Tc 0.211 6.1385 294 β
126Sn 0.230 0.1084 4050 βγ
79Se 0.327 0.0447 151 β
93Zr 1.53 5.4575 91 βγ
135Cs 2.3  6.9110 269 β
107Pd 6.5  1.2499 33 β
129I 15.7  0.8410 194 βγ
Hover underlined: more info

Caesium-135 is a mildly radioactive isotope of caesium, undergoing low-energy beta decay to barium-135 with a half-life of 2.3 million years. It is one of the 7 long-lived fission products and the only alkaline one. In nuclear reprocessing, it stays with Cs-137 and other medium-lived fission products rather than with other long-lived fission products. 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 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)[1] will be converted to stable 136Xe before it can decay to 135Cs. Little or no 135Xe 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.

A nuclear reactor will also produce much smaller amounts of 135Cs from the nonradioactive fission product Cs-133 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.[2] Disposal of Cs-135 by nuclear transmutation is difficult, because of the low cross section as well as because neutron irradiation of mixed-isotope fission caesium produces more Cs-135 from stable Cs-133. In addition, the intense medium-term radioactivity of Cs-137 makes handling of nuclear waste difficult.[3]

  • ANL factsheet


Caesium-136 has a half-life of 13.16 days. It is produced both directly (at a very small yield because 136Xe is stable) as a fission product and via neutron capture from long-lived Cs-135 (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 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 Ba-136 directly.


Main article: Caesium-137

137Cs 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 major health concern for decontaminating land near the Fukushima nuclear power plant.[4] 137Cs beta decays to barium-137m (a short-lived nuclear isomer) then to nonradioactive barium-137, and is also a strong emitter of gamma radiation. 137Cs has a very low rate of neutron capture and cannot be feasibly disposed of in this way, but must 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.


Z(p) N(n)  
isotopic mass (u)
half-life decay
mode(s)[5][n 1]
isotope(s)[n 2]
(mole fraction)
range of natural
(mole fraction)
excitation energy
112Cs 55 57 111.95030(33)# 500(100) µs p 111Xe 1+#
α 108I
113Cs 55 58 112.94449(11) 16.7(7) µs p (99.97%) 112Xe 5/2+#
β+ (.03%) 113Xe
114Cs 55 59 113.94145(33)# 0.57(2) s β+ (91.09%) 114Xe (1+)
β+, p (8.69%) 113I
β+, α (.19%) 110Te
α (.018%) 110I
115Cs 55 60 114.93591(32)# 1.4(8) s β+ (99.93%) 115Xe 9/2+#
β+, p (.07%) 114I
116Cs 55 61 115.93337(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 55 62 116.92867(7) 8.4(6) s β+ 117Xe (9/2+)#
117mCs 150(80)# keV 6.5(4) s β+ 117Xe 3/2+#
118Cs 55 63 117.926559(14) 14(2) s β+ (99.95%) 118Xe 2
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 55 64 118.922377(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 55 65 119.920677(11) 61.2(18) s β+ 120Xe 2(-#)
β+, α (2×10−5%) 116Te
β+, p (7×10−6%) 118I
120mCs 100(60)# keV 57(6) s β+ 120Xe (7-)
β+, α (2×10−5%) 116Te
β+, p (7×10−6%) 118I
121Cs 55 66 120.917229(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 55 67 121.91611(3) 21.18(19) s β+ 122Xe 1+
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 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 231.63+X keV 114(5) ns (9/2+)
124Cs 55 69 123.912258(9) 30.9(4) s β+ 124Xe 1+
124mCs 462.55(17) keV 6.3(2) s IT 124Cs (7)+
125Cs 55 70 124.909728(8) 46.7(1) min β+ 125Xe 1/2(+)
125mCs 266.6(11) keV 900(30) ms (11/2-)
126Cs 55 71 125.909452(13) 1.64(2) min β+ 126Xe 1+
126m1Cs 273.0(7) keV >1 µs
126m2Cs 596.1(11) keV 171(14) µs
127Cs 55 72 126.907418(6) 6.25(10) h β+ 127Xe 1/2+
127mCs 452.23(21) keV 55(3) µs (11/2)-
128Cs 55 73 127.907749(6) 3.640(14) min β+ 128Xe 1+
129Cs 55 74 128.906064(5) 32.06(6) h β+ 129Xe 1/2+
130Cs 55 75 129.906709(9) 29.21(4) min β+ (98.4%) 130Xe 1+
β- (1.6%) 130Ba
130mCs 163.25(11) keV 3.46(6) min IT (99.83%) 130Cs 5-
β+ (.16%) 130Xe
131Cs 55 76 130.905464(5) 9.689(16) d EC 131Xe 5/2+
132Cs 55 77 131.9064343(20) 6.480(6) d β+ (98.13%) 132Xe 2+
β- (1.87%) 132Ba
133Cs[n 3][n 4] 55 78 132.905451933(24) Stable[n 5] 7/2+ 1.0000
134Cs[n 4] 55 79 133.906718475(28) 2.0652(4) a β- 134Ba 4+
EC (3×10−4%) 134Xe
134mCs 138.7441(26) keV 2.912(2) h IT 134Cs 8-
135Cs[n 4] 55 80 134.9059770(11) 2.3 x106 a β- 135Ba 7/2+
135mCs 1632.9(15) keV 53(2) min IT 135Cs 19/2-
136Cs 55 81 135.9073116(20) 13.16(3) d β- 136Ba 5+
136mCs 518(5) keV 19(2) s β- 136Ba 8-
IT 136Cs
137Cs[n 4] 55 82 136.9070895(5) 30.1671(13) a β- (95%) 137mBa 7/2+
β- (5%) 137Ba
138Cs 55 83 137.911017(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 55 84 138.913364(3) 9.27(5) min β- 139Ba 7/2+
140Cs 55 85 139.917282(9) 63.7(3) s β- 140Ba 1-
141Cs 55 86 140.920046(11) 24.84(16) s β- (99.96%) 141Ba 7/2+
β-, n (.0349%) 140Ba
142Cs 55 87 141.924299(11) 1.689(11) s β- (99.9%) 142Ba 0-
β-, n (.091%) 141Ba
143Cs 55 88 142.927352(25) 1.791(7) s β- (98.38%) 143Ba 3/2+
β-, n (1.62%) 142Ba
144Cs 55 89 143.932077(28) 994(4) ms β- (96.8%) 144Ba 1(-#)
β-, n (3.2%) 143Ba
144mCs 300(200)# keV <1 s β- 144Ba (>3)
IT 144Cs
145Cs 55 90 144.935526(12) 582(6) ms β- (85.7%) 145Ba 3/2+
β-, n (14.3%) 144Ba
146Cs 55 91 145.94029(8) 0.321(2) s β- (85.8%) 146Ba 1-
β-, n (14.2%) 145Ba
147Cs 55 92 146.94416(6) 0.235(3) s β- (71.5%) 147Ba (3/2+)
β-, n (28.49%) 147Ba
148Cs 55 93 147.94922(62) 146(6) ms β- (74.9%) 148Ba
β-, n (25.1%) 147Ba
149Cs 55 94 148.95293(21)# 150# ms [>50 ms] β- 149Ba 3/2+#
β-, n 148Ba
150Cs 55 95 149.95817(32)# 100# ms [>50 ms] β- 150Ba
β-, n 149Ba
151Cs 55 96 150.96219(54)# 60# ms [>50 ms] β- 151Ba 3/2+#
β-, n 150Ba


  • Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
  • Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC which use expanded uncertainties.


  • Isotope masses from:
  • Isotopic compositions and standard atomic masses from:
  • Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.

Isotopes of xenon Isotopes of caesium Isotopes of barium
Table of nuclides
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