# Isotopes of caesium

Caesium ($_{55}$Cs) has 41 known isotopes, ranging in mass number from 112 to 152. Only one isotope, $^{133}$Cs, is stable. The longest-lived radioisotopes are $^{135}$Cs with a half-life of 1.33 million years, $^{137}$ Cs with a half-life of 30.1671 years and $^{134}$Cs 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

| Nuclide | Z | N | Isotopic mass (Da) | Half-life | Decay mode | Daughter isotope | Spin and parity | Isotopic abundance |
| Isotopic abundance |||
| $^{112}$Cs | 55 | 57 | 111.95017(12)# | 490(30) μs | p (>99.74%) | $^{111}$Xe | 1+# | |
| $^{112}$Cs | 55 | 57 | 111.95017(12)# | 490(30) μs | α (<0.26%) | $^{108}$I | 1+# | |
| $^{113}$Cs | 55 | 58 | 112.9444285(92) | 16.94(9) μs | p | $^{112}$Xe | (3/2+) | |
| $^{114}$Cs | 55 | 59 | 113.941292(91) | 570(20) ms | β$^+$ (91.1%) | $^{114}$Xe | (1+) | |
| $^{114}$Cs | 55 | 59 | 113.941292(91) | 570(20) ms | β$^+$, p (8.7%) | $^{113}$I | (1+) | |
| $^{114}$Cs | 55 | 59 | 113.941292(91) | 570(20) ms | β$^+$, α (0.19%) | $^{110}$Te | (1+) | |
| $^{114}$Cs | 55 | 59 | 113.941292(91) | 570(20) ms | α (0.018%) | $^{110}$I | (1+) | |
| $^{115}$Cs | 55 | 60 | 114.93591(11)# | 1.4(8) s | β$^+$ (99.93%) | $^{115}$Xe | 9/2+# | |
| $^{115}$Cs | 55 | 60 | 114.93591(11)# | 1.4(8) s | β$^+$, p (0.07%) | $^{114}$I | 9/2+# | |
| $^{116}$Cs | 55 | 61 | 115.93340(11)# | 700(40) ms | β$^+$ (99.67%) | $^{116}$Xe | (1+) | |
| $^{116}$Cs | 55 | 61 | 115.93340(11)# | 700(40) ms | β$^+$, p (0.28%) | $^{115}$I | (1+) | |
| $^{116}$Cs | 55 | 61 | 115.93340(11)# | 700(40) ms | β$^+$, α (0.049%) | $^{112}$Te | (1+) | |
| $^{116m}$Cs | 100(60)# keV | 100(60)# keV | 100(60)# keV | 3.85(13) s | β$^+$ (99.56%) | $^{116}$Xe | (7+) | |
| $^{116m}$Cs | 100(60)# keV | 100(60)# keV | 100(60)# keV | 3.85(13) s | β$^+$, p (0.44%) | $^{115}$I | (7+) | |
| $^{116m}$Cs | 100(60)# keV | 100(60)# keV | 100(60)# keV | 3.85(13) s | β$^+$, α (0.0034%) | $^{112}$Te | (7+) | |
| $^{117}$Cs | 55 | 62 | 116.928617(67) | 8.4(6) s | β$^+$ | $^{117}$Xe | 9/2+# | |
| $^{117m}$Cs | 150(80)# keV | 150(80)# keV | 150(80)# keV | 6.5(4) s | β$^+$ | $^{117}$Xe | 3/2+# | |
| $^{118}$Cs | 55 | 63 | 117.926560(14) | 14(2) s | β$^+$ (99.98%) | $^{118}$Xe | 2(−) | |
| $^{118}$Cs | 55 | 63 | 117.926560(14) | 14(2) s | β$^+$, p (0.021%) | $^{117}$I | 2(−) | |
| $^{118}$Cs | 55 | 63 | 117.926560(14) | 14(2) s | β$^+$, α (0.0012%) | $^{114}$Te | 2(−) | |
| $^{118m1}$Cs | X keV | X keV | X keV | 17(3) s | β$^+$ (99.98%) | $^{118}$Xe | (7−) | |
| $^{118m1}$Cs | X keV | X keV | X keV | 17(3) s | β$^+$, p (0.021%) | $^{117}$I | (7−) | |
| $^{118m1}$Cs | X keV | X keV | X keV | 17(3) s | β$^+$, α (0.0012%) | $^{114}$Te | (7−) | |
| $^{118m2}$Cs | Y keV | Y keV | Y keV | | | | (6+) | |
| $^{118m3}$Cs | 65.9 keV | 65.9 keV | 65.9 keV | | IT | $^{118}$Cs | (3−) | |
| $^{118m4}$Cs | 125.9+X keV | 125.9+X keV | 125.9+X keV | 550(60) ns | IT | $^{118m1}$Cs | (7+) | |
| $^{118m5}$Cs | 195.2+X keV | 195.2+X keV | 195.2+X keV | <500 ns | IT | $^{118m4}$Cs | (8+) | |
| $^{119}$Cs | 55 | 64 | 118.9223 77(15) | 43.0(2) s | β$^+$ | $^{119}$Xe | 9/2+ | |
| $^{119}$Cs | 55 | 64 | 118.9223 77(15) | 43.0(2) s | β$^+$, α (<2×10$^{−6}$%) | $^{115}$Te | 9/2+ | |
| $^{119m}$Cs | 50(30)# keV | 50(30)# keV | 50(30)# keV | 30.4(1) s | β$^+$ | $^{119}$Xe | 3/2+ | |
| $^{120}$Cs | 55 | 65 | 119.920677(11) | 60.4(6) s | β$^+$ | $^{120}$Xe | 2+ | |
| $^{120}$Cs | 55 | 65 | 119.920677(11) | 60.4(6) s | β$^+$, α (<2×10$^{−5}$%) | $^{116}$Te | 2+ | |
| $^{120}$Cs | 55 | 65 | 119.920677(11) | 60.4(6) s | β$^+$, p (<7×10$^{−6}$%) | $^{119}$I | 2+ | |
| $^{120m}$Cs | 100(60)# keV | 100(60)# keV | 100(60)# keV | 57(6) s | β$^+$ | $^{120}$Xe | (7−) | |
| $^{120m}$Cs | 100(60)# keV | 100(60)# keV | 100(60)# keV | 57(6) s | β$^+$, α (<2×10$^{−5}$%) | $^{116}$Te | (7−) | |
| $^{120m}$Cs | 100(60)# keV | 100(60)# keV | 100(60)# keV | 57(6) s | β$^+$, p (<7×10$^{−6}$%) | $^{119}$I | (7−) | |
| $^{121}$Cs | 55 | 66 | 120.917227(15) | 155(4) s | β$^+$ | $^{121}$Xe | 3/2+ | |
| $^{121m}$Cs | 68.5(3) keV | 68.5(3) keV | 68.5(3) keV | 122(3) s | β$^+$ (83%) | $^{121}$Xe | 9/2+ | |
| $^{121m}$Cs | 68.5(3) keV | 68.5(3) keV | 68.5(3) keV | 122(3) s | IT (17%) | $^{121}$Cs | 9/2+ | |
| $^{122}$Cs | 55 | 67 | 121.916108(36) | 21.18(19) s | β$^+$ | $^{122}$Xe | 1+ | |
| $^{122}$Cs | 55 | 67 | 121.916108(36) | 21.18(19) s | β$^+$, α (<2×10$^{−7}$%) | $^{118}$Te | 1+ | |
| $^{122m1}$Cs | 45.87(12) keV | 45.87(12) keV | 45.87(12) keV | >1 μs | IT | $^{122}$Cs | 3+ | |
| $^{122m2}$Cs | 140(30) keV | 140(30) keV | 140(30) keV | 3.70(11) min | β$^+$ | $^{122}$Xe | 8− | |
| $^{122m3}$Cs | 127.07(16) keV | 127.07(16) keV | 127.07(16) keV | 360(20) ms | IT | $^{122}$Cs | 5− | |
| $^{123}$Cs | 55 | 68 | 122.912996(13) | 5.88(3) min | β$^+$ | $^{123}$Xe | 1/2+ | |
| $^{123m1}$Cs | 156.27(5) keV | 156.27(5) keV | 156.27(5) keV | 1.64(12) s | IT | $^{123}$Cs | 11/2− | |
| $^{123m2}$Cs | 252(6) keV | 252(6) keV | 252(6) keV | 114(5) ns | IT | $^{123}$Cs | (9/2+) | |
| $^{124}$Cs | 55 | 69 | 123.9122474(98) | 30.9(4) s | β$^+$ | $^{124}$Xe | 1+ | |
| $^{124m}$Cs | 462.63(14) keV | 462.63(14) keV | 462.63(14) keV | 6.41(7) s | IT (99.89%) | $^{124}$Cs | (7)+ | |
| $^{124m}$Cs | 462.63(14) keV | 462.63(14) keV | 462.63(14) keV | 6.41(7) s | β$^+$ (0.11%) | $^{124}$Xe | (7)+ | |
| $^{125}$Cs | 55 | 70 | 124.9097260(83) | 44.35(29) min | β$^+$ | $^{125}$Xe | 1/2+ | |
| $^{125m}$Cs | 266.1(11) keV | 266.1(11) keV | 266.1(11) keV | 900(30) ms | IT | $^{125}$Cs | (11/2−) | |
| $^{126}$Cs | 55 | 71 | 125.909446(11) | 1.64(2) min | β$^+$ | $^{126}$Xe | 1+ | |
| $^{126m1}$Cs | 273.0(7) keV | 273.0(7) keV | 273.0(7) keV | ~1 μs | IT | $^{126}$Cs | (4−) | |
| $^{126m2}$Cs | 596.1(11) keV | 596.1(11) keV | 596.1(11) keV | 171(14) μs | IT | $^{126}$Cs | 8−# | |
| $^{127}$Cs | 55 | 72 | 126.9074175(60) | 6.25(10) h | β$^+$ | $^{127}$Xe | 1/2+ | |
| $^{127m}$Cs | 452.23(21) keV | 452.23(21) keV | 452.23(21) keV | 55(3) μs | IT | $^{127}$Cs | (11/2)− | |
| $^{128}$Cs | 55 | 73 | 127.9077485(57) | 3.640(14) min | β$^+$ | $^{128}$Xe | 1+ | |
| $^{129}$Cs | 55 | 74 | 128.9060659(49) | 32.06(6) h | β$^+$ | $^{129}$Xe | 1/2+ | |
| $^{129m}$Cs | 575.40(14) keV | 575.40(14) keV | 575.40(14) keV | 718(21) ns | IT | $^{127}$Cs | (11/2−) | |
| $^{130}$Cs | 55 | 75 | 129.9067093(90) | 29.21(4) min | β$^+$ (98.4%) | $^{130}$Xe | 1+ | |
| $^{130}$Cs | 55 | 75 | 129.9067093(90) | 29.21(4) min | β$^−$ (1.6%) | $^{130}$Ba | 1+ | |
| $^{130m}$Cs | 163.25(11) keV | 163.25(11) keV | 163.25(11) keV | 3.46(6) min | IT (99.84%) | $^{130}$Cs | 5− | |
| $^{130m}$Cs | 163.25(11) keV | 163.25(11) keV | 163.25(11) keV | 3.46(6) min | β$^+$ (0.16%) | $^{130}$Xe | 5− | |
| $^{131}$Cs | 55 | 76 | 130.90546846(19) | 9.689(16) d | EC | $^{131}$Xe | 5/2+ | |
| $^{132}$Cs | 55 | 77 | 131.9064378(11) | 6.480(6) d | β$^+$ (98.13%) | $^{132}$Xe | 2+ | |
| $^{132}$Cs | 55 | 77 | 131.9064378(11) | 6.480(6) d | β$^−$ (1.87%) | $^{132}$Ba | 2+ | |
| $^{133}$Cs | 55 | 78 | 132.905451958(8) | Stable | Stable | Stable | 7/2+ | 1.0000 |
| $^{134}$Cs | 55 | 79 | 133.906718501(17) | 2.0650(4) y | β$^−$ | $^{134}$Ba | 4+ | |
| $^{134}$Cs | 55 | 79 | 133.906718501(17) | 2.0650(4) y | EC (3.0×10$^{−4}$%) | $^{134}$Xe | 4+ | |
| $^{134m}$Cs | 138.7441(26) keV | 138.7441(26) keV | 138.7441(26) keV | 2.912(2) h | IT | $^{134}$Cs | 8− | |
| $^{135}$Cs | 55 | 80 | 134.90597691(39) | 1.33(19)×10$^6$ y | β$^−$ | $^{135}$Ba | 7/2+ | |
| $^{135m}$Cs | 1632.9(15) keV | 1632.9(15) keV | 1632.9(15) keV | 53(2) min | IT | $^{135}$Cs | 19/2− | |
| $^{136}$Cs | 55 | 81 | 135.9073114(20) | 13.01(5) d | β$^−$ | $^{136}$Ba | 5+ | |
| $^{136m}$Cs | 517.9(1) keV | 517.9(1) keV | 517.9(1) keV | 17.5(2) s | β$^−$? | $^{136}$Ba | 8− | |
| $^{136m}$Cs | 517.9(1) keV | 517.9(1) keV | 517.9(1) keV | 17.5(2) s | IT? | $^{136}$Cs | 8− | |
| $^{137}$Cs | 55 | 82 | 136.90708930(32) | 30.04(4) y | β$^−$ (94.70%) | $^{137m}$Ba | 7/2+ | |
| $^{137}$Cs | 55 | 82 | 136.90708930(32) | 30.04(4) y | β$^−$ (5.30%) | $^{137}$Ba | 7/2+ | |
| $^{138}$Cs | 55 | 83 | 137.9110171(98) | 33.5(2) min | β$^−$ | $^{138}$Ba | 3− | |
| $^{138m}$Cs | 79.9(3) keV | 79.9(3) keV | 79.9(3) keV | 2.91(10) min | IT (81%) | $^{138}$Cs | 6− | |
| $^{138m}$Cs | 79.9(3) keV | 79.9(3) keV | 79.9(3) keV | 2.91(10) min | β$^−$ (19%) | $^{138}$Ba | 6− | |
| $^{139}$Cs | 55 | 84 | 138.9133638(34) | 9.27(5) min | β$^−$ | $^{139}$Ba | 7/2+ | |
| $^{140}$Cs | 55 | 85 | 139.9172837(88) | 63.7(3) s | β$^−$ | $^{140}$Ba | 1− | |
| $^{140m}$Cs | 13.931(21) keV | 13.931(21) keV | 13.931(21) keV | 471(51) ns | IT | $^{140}$Cs | (2)− | |
| $^{141}$Cs | 55 | 86 | 140.9200453(99) | 24.84(16) s | β$^−$ (99.97%) | $^{141}$Ba | 7/2+ | |
| $^{141}$Cs | 55 | 86 | 140.9200453(99) | 24.84(16) s | β$^−$, n (0.0342%) | $^{140}$Ba | 7/2+ | |
| $^{142}$Cs | 55 | 87 | 141.9242995(76) | 1.687(10) s | β$^−$ (99.91%) | $^{142}$Ba | 0− | |
| $^{142}$Cs | 55 | 87 | 141.9242995(76) | 1.687(10) s | β$^−$, n (0.089%) | $^{141}$Ba | 0− | |
| $^{143}$Cs | 55 | 88 | 142.9273473(81) | 1.802(8) s | β$^−$ (98.38%) | $^{143}$Ba | 3/2+ | |
| $^{143}$Cs | 55 | 88 | 142.9273473(81) | 1.802(8) s | β$^−$, n (1.62%) | $^{142}$Ba | 3/2+ | |
| $^{144}$Cs | 55 | 89 | 143.932075(22) | 994(6) ms | β$^−$ (97.02%) | $^{144}$Ba | 1− | |
| $^{144}$Cs | 55 | 89 | 143.932075(22) | 994(6) ms | β$^−$, n (2.98%) | $^{143}$Ba | 1− | |
| $^{144m}$Cs | 92.2(5) keV | 92.2(5) keV | 92.2(5) keV | 1.1(1) μs | IT | $^{144}$Cs | (4−) | |
| $^{145}$Cs | 55 | 90 | 144.9355289(97) | 582(4) ms | β$^−$ (87.2%) | $^{145}$Ba | 3/2+ | |
| $^{145}$Cs | 55 | 90 | 144.9355289(97) | 582(4) ms | β$^−$, n (12.8%) | $^{144}$Ba | 3/2+ | |
| $^{145m}$Cs | 762.9(4) keV | 762.9(4) keV | 762.9(4) keV | 0.5(1) μs | IT | $^{145}$Cs | 13/2# | |
| $^{146}$Cs | 55 | 91 | 145.9406219(31) | 321.6(9) ms | β$^−$ (85.8%) | $^{146}$Ba | 1− | |
| $^{146}$Cs | 55 | 91 | 145.9406219(31) | 321.6(9) ms | β$^−$, n (14.2%) | $^{145}$Ba | 1− | |
| $^{146m}$Cs | 46.7(1) keV | 46.7(1) keV | 46.7(1) keV | 1.25(5) μs | IT | $^{146}$Cs | 4−# | |
| $^{147}$Cs | 55 | 92 | 146.9442615(90) | 230.5(9) ms | β$^−$ (71.5%) | $^{147}$Ba | (3/2+) | |
| $^{147}$Cs | 55 | 92 | 146.9442615(90) | 230.5(9) ms | β$^−$, n (28.5%) | $^{146}$Ba | (3/2+) | |
| $^{147m}$Cs | 701.4(4) keV | 701.4(4) keV | 701.4(4) keV | 190(20) ns | IT | $^{147}$Cs | 13/2# | |
| $^{148}$Cs | 55 | 93 | 147.949639(14) | 151.8(10) ms | β$^−$ (71.3%) | $^{148}$Ba | (2−) | |
| $^{148}$Cs | 55 | 93 | 147.949639(14) | 151.8(10) ms | β$^−$, n (28.7%) | $^{147}$Ba | (2−) | |
| $^{148m}$Cs | 45.2(1) keV | 45.2(1) keV | 45.2(1) keV | 4.8(2) μs | IT | $^{148}$Cs | 4−# | |
| $^{149}$Cs | 55 | 94 | 148.95352(43)# | 112.3(25) ms | β$^−$ (75%) | $^{149}$Ba | 3/2+# | |
| $^{149}$Cs | 55 | 94 | 148.95352(43)# | 112.3(25) ms | β$^−$, n (25%) | $^{148}$Ba | 3/2+# | |
| $^{150}$Cs | 55 | 95 | 149.95902(43)# | 81.0(26) ms | β$^−$ (~56%) | $^{150}$Ba | (2−) | |
| $^{150}$Cs | 55 | 95 | 149.95902(43)# | 81.0(26) ms | β$^−$, n (~44%) | $^{149}$Ba | (2−) | |
| $^{151}$Cs | 55 | 96 | 150.96320(54)# | 59(19) ms | β$^−$ | $^{151}$Ba | 3/2+# | |
| This table header & footer: view |||||||||
* ↑ $^m$Cs – 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:Electron capture IT:Isomeric transition n:Neutron emission p:Proton emission
* ↑ 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.
* 1 2 # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
* 1 2 3 4 5 6 7 Order of ground state and isomer is uncertain.
* ↑ Used to define the second
* 1 2 3 4 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 $^{134}$Xe is stable) as a fission product and via neutron capture from nonradioactive $^{133}$Cs (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 $^{134}$Xe. It is also not produced by nuclear weapons because $^{133}$Cs is created by beta decay of original fission products only long after the nuclear explosion is over.
The combined yield of $^{133}$Cs and $^{134}$Cs is given as 6.7896%. The proportion between the two will change with continued neutron irradiation. $^{134}$Cs also captures neutrons with a cross section of 140 barns, becoming long-lived radioactive $^{135}$Cs.
Caesium-134 undergoes beta decay (β$^−$), producing $^{134}$Ba directly and emitting on average 2.23 gamma ray photons (mean energy 0.698 MeV).

## Caesium-135

Long-lived fission productsvte :
| Nuclide | t$_{1⁄2}$ | Yield | Q | βγ |
| | (Ma) | (%) | (keV) | |
| $^{99}$Tc | 0.211 | 6.1385 | 294 | β |
| $^{126}$Sn | 0.230 | 0.1084 | 4050 | βγ |
| $^{79}$Se | 0.327 | 0.0447 | 151 | β |
| $^{135}$Cs | 1.33 | 6.9110 | 269 | β |
| $^{93}$Zr | 1.53 | 5.4575 | 91 | βγ |
| $^{107}$Pd | 6.5 | 1.2499 | 33 | β |
| $^{129}$I | 16.14 | 0.8410 | 194 | βγ |
| ↑ Decay energy is split among β, neutrino, and γ if any. ↑ Per 65 thermal neutron fissions of $^{235}$U and 35 of $^{239}$Pu. ↑ Has decay energy 380 keV, but its decay product $^{126}$Sb has decay energy 3.67 MeV. ↑ Lower in thermal reactors because $^{135}$Xe, 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 $^{135}$Cs make this isotope much less hazardous than $^{137}$Cs or $^{134}$Cs.
Its precursor $^{135}$Xe has a high fission product yield (e.g., 6.3333% for $^{235}$U and thermal neutrons) but also has the highest known thermal neutron capture cross section of any nuclide. Because of this, much of the $^{135}$Xe 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 10$^{21}$ 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 $^{135}$Cs from the nonradioactive fission product $^{133}$Cs by successive neutron capture to $^{134}$Cs and then $^{135}$Cs.
The thermal neutron capture cross section and resonance integral of $^{135}$Cs are 8.3 ± 0.3 and 38.1 ± 2.6 barns respectively. Disposal of $^{135}$Cs by nuclear transmutation is difficult, because of the low cross section as well as because neutron irradiation of mixed-isotope fission caesium produces more $^{135}$Cs from stable $^{133}$Cs. In addition, the intense medium-term radioactivity of $^{137}$Cs 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 $^{136}$Xe is beta-stable) as a fission product and via neutron capture from long-lived $^{135}$Cs, which is a common fission product. It is also not produced by nuclear weapons because $^{135}$Cs is created by beta decay of original fission products only long after the nuclear explosion is over. Caesium-136 undergoes beta decay (β−), producing $^{136}$Ba 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 $^{90}$Sr, 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. $^{137}$Cs 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.
$^{137}$Cs 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, $^{137}$Cs must simply be allowed to decay.
$^{137}$Cs has been used as a tracer in hydrologic studies, analogous to the use of $^3$H.

## 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.