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, with a half-life of 30.04 years and 134Cs with a half-life of 2.0650 years. All other isotopes have half-lives less than 2 weeks, most under an hour.
Caesium is an abundant fission product (135 and 137 are directly produced) and various isotopes are of concern as such, see the sections below.
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
|-id=Caesium-112 | rowspan=2|112Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 57 | rowspan=2|111.950155(64) | rowspan=2 style="text-align:center" | 1994 | rowspan=2|490(30) μs | p (>99.74%) | 111Xe | rowspan=2|1+# | rowspan=2| |- | α (<0.26%) | 108I |-id=Caesium-113 | 113Cs | style="text-align:right" | 55 | style="text-align:right" | 58 | 112.9444285(92) | style="text-align:center" | 1984 | 16.94(9) μs | p | 112Xe | (3/2+) | |-id=Caesium-114 | rowspan=4|114Cs | rowspan=4 style="text-align:right" | 55 | rowspan=4 style="text-align:right" | 59 | rowspan=4|113.941292(91) | rowspan=4 style="text-align:center" | 1978 | rowspan=4|570(20) ms | β+ (91.1%) | 114Xe | rowspan=4|(1+) | rowspan=4| |- | β+, p (8.7%) | 113I |- | β+, α (0.19%) | 110Te |- | α (0.018%) | 110I |-id=Caesium-115 | rowspan=2|115Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 60 | rowspan=2|114.93591(11)# | rowspan=2 style="text-align:center" | 1978 | rowspan=2|1.4(8) s | β+ (99.93%) | 115Xe | rowspan=2|9/2+# | rowspan=2| |- | β+, p (0.07%) | 114I |-id=Caesium-116 | rowspan=3|116Cs | rowspan=3 style="text-align:right" | 55 | rowspan=3 style="text-align:right" | 61 | rowspan=3|115.93340(11)# | rowspan=3 style="text-align:center" | 1975 | rowspan=3|700(40) ms | β+ (99.67%) | 116Xe | rowspan=3|(1+) | rowspan=3| |- | β+, p (0.28%) | 115I |- | β+, α (0.049%) | 112Te |-id=Caesium-116m | rowspan=3 style="text-indent:1em" | 116mCsOrder of ground state and isomer is uncertain. | rowspan=3 colspan="3" style="text-indent:2em" | 100(60)# keV | rowspan=3 style="text-align:center" | 1977 | rowspan=3|3.85(13) s | β+ (99.56%) | 116Xe | rowspan=3|(7+) | rowspan=3| |- | β+, p (0.44%) | 115I |- | β+, α (0.0034%) | 112Te |-id=Caesium-117 | 117Cs | style="text-align:right" | 55 | style="text-align:right" | 62 | 116.928617(67) | style="text-align:center" | 1972 | 8.4(6) s | β+ | 117Xe | 9/2+# | |-id=Caesium-117m | style="text-indent:1em" | 117mCs | colspan="3" style="text-indent:2em" | 150(80)# keV | style="text-align:center" | 1986 | 6.5(4) s | β+ | 117Xe | 3/2+# | |-id=Caesium-118 | rowspan=3|118Cs | rowspan=3 style="text-align:right" | 55 | rowspan=3 style="text-align:right" | 63 | rowspan=3|117.926560(14) | rowspan=3 style="text-align:center" | 1969 | rowspan=3|14(2) s | β+ (99.98%) | 118Xe | rowspan=3|2(−) | rowspan=3| |- | β+, p (0.021%) | 117I |- | β+, α (0.0012%) | 114Te |-id=Caesium-118m1 | rowspan=3 style="text-indent:1em" | 118m1Cs | rowspan=3 colspan="3" style="text-indent:2em" | X keV | rowspan=3 style="text-align:center" | 1977 | rowspan=3|17(3) s | β+ (99.98%) | 118Xe | rowspan=3|(7−) | rowspan=3| |- | β+, p (0.021%) | 117I |- | β+, α (0.0012%) | 114Te |-id=Caesium-118m2 | style="text-indent:1em" | 118m2Cs | colspan="3" style="text-indent:2em" | Y keV | style="text-align:center" | (2021)Half-life not measured, not included in discovery database
| (6+) | |-id=Caesium-118m3 | style="text-indent:1em" | 118m3Cs | colspan="3" style="text-indent:2em" | 65.9 keV | style="text-align:center" | (2021) | | IT | 118Cs | (3−) | |-id=Caesium-118m4 | style="text-indent:1em" | 118m4Cs | colspan="3" style="text-indent:2em" | 125.9+X keV | style="text-align:center" | 2021 | 550(60) ns | IT | 118m1Cs | (7+) | |-id=Caesium-118m5 | style="text-indent:1em" | 118m5Cs | colspan="3" style="text-indent:2em" | 195.2+X keV | style="text-align:center" | (2021) | <500 ns | IT | 118m4Cs | (8+) | |-id=Caesium-119 | rowspan=2|119Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 64 | rowspan=2|118.9223 77(15) | rowspan=2 style="text-align:center" | 1969 | rowspan=2|43.0(2) s | β+ | 119Xe | rowspan=2|9/2+ | rowspan=2| |- | β+, α (<2×10−6%) | 115Te |-id=Caesium-119m1 | style="text-indent:1em" | 119m1Cs | colspan="3" style="text-indent:2em" | 50(30)# keV | style="text-align:center" | 1978 | 30.4(1) s | β+ | 119Xe | 3/2+ | |-id=Caesium-119m2 | style="text-indent:1em" | 119m2Cs | colspan="3" style="text-indent:2em" | 110.0(7)# keV | style="text-align:center" | 2021 | 55(5) μs | IT | 119Cs | 11/2− | |-id=Caesium-120 | rowspan=3|120Cs | rowspan=3 style="text-align:right" | 55 | rowspan=3 style="text-align:right" | 65 | rowspan=3|119.920677(11) | rowspan=3 style="text-align:center" | 1969 | rowspan=3|60.4(6) s | β+ | 120Xe | rowspan=3|2+ | rowspan=3| |- | β+, α (<2×10−5%) | 116Te |- | β+, p (<7×10−6%) | 119I |-id=Caesium-120m | rowspan=3 style="text-indent:1em" | 120mCs | rowspan=3 colspan="3" style="text-indent:2em" | 100(60)# keV | rowspan=3 style="text-align:center" | 1977 | rowspan=3|57(6) s | β+ | 120Xe | rowspan=3|(7−) | rowspan=3| |- | β+, α (<2×10−5%) | 116Te |- | β+, p (<7×10−6%) | 119I |-id=Caesium-121 | 121Cs | style="text-align:right" | 55 | style="text-align:right" | 66 | 120.917227(15) | style="text-align:center" | 1969 | 155(4) s | β+ | 121Xe | 3/2+ | |-id=Caesium-121m | rowspan=2 style="text-indent:1em" | 121mCs | rowspan=2 colspan="3" style="text-indent:2em" | 68.5(3) keV | rowspan=2 style="text-align:center" | 1981 | rowspan=2|122(3) s | β+ (83%) | 121Xe | rowspan=2|9/2+ | rowspan=2| |- | IT (17%) | 121Cs |-id=Caesium-122 | rowspan=2|122Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 67 | rowspan=2|121.916108(36) | rowspan=2 style="text-align:center" | 1969 | rowspan=2|21.18(19) s | β+ | 122Xe | rowspan=2|1+ | rowspan=2| |- | β+, α (<2×10−7%) | 118Te |-id=Caesium-122m1 | style="text-indent:1em" | 122m1Cs | colspan="3" style="text-indent:2em" | 45.87(12) keV | style="text-align:center" | (1987)Only published in a conference proceeding and not a refereed journal | >1 μs | IT | 122Cs | 3+ | |-id=Caesium-122m2 | style="text-indent:1em" | 122m2Cs | colspan="3" style="text-indent:2em" | 140(30) keV | style="text-align:center" | 1983 | 3.70(11) min | β+ | 122Xe | 8− | |-id=Caesium-122m3 | style="text-indent:1em" | 122m3Cs | colspan="3" style="text-indent:2em" | 127.07(16) keV | style="text-align:center" | 1969 | 360(20) ms | IT | 122Cs | 5− | |-id=Caesium-123 | 123Cs | style="text-align:right" | 55 | style="text-align:right" | 68 | 122.912996(13) | style="text-align:center" | 1954 | 5.88(3) min | β+ | 123Xe | 1/2+ | |-id=Caesium-123m1 | style="text-indent:1em" | 123m1Cs | colspan="3" style="text-indent:2em" | 156.27(5) keV | style="text-align:center" | 1972 | 1.64(12) s | IT | 123Cs | 11/2− | |-id=Caesium-123m2 | style="text-indent:1em" | 123m2Cs | colspan="3" style="text-indent:2em" | 252(6) keV | style="text-align:center" | 2000 | 114(5) ns | IT | 123Cs | (9/2+) | |-id=Caesium-124 | 124Cs | style="text-align:right" | 55 | style="text-align:right" | 69 | 123.9122474(98) | style="text-align:center" | 1969 | 30.9(4) s | β+ | 124Xe | 1+ | |-id=Caesium-124m | rowspan=2 style="text-indent:1em" | 124mCs | rowspan=2 colspan="3" style="text-indent:2em" | 462.63(14) keV | rowspan=2 style="text-align:center" | 1983 | rowspan=2|6.41(7) s | IT (99.89%) | 124Cs | rowspan=2|(7)+ | rowspan=2| |- | β+ (0.11%) | 124Xe |-id=Caesium-125 | 125Cs | style="text-align:right" | 55 | style="text-align:right" | 70 | 124.9097260(83) | style="text-align:center" | 1954 | 44.35(29) min | β+ | 125Xe | 1/2+ | |-id=Caesium-125m | style="text-indent:1em" | 125mCs | colspan="3" style="text-indent:2em" | 266.1(11) keV | style="text-align:center" | 1998 | 900(30) ms | IT | 125Cs | (11/2−) | |-id=Caesium-126 | 126Cs | style="text-align:right" | 55 | style="text-align:right" | 71 | 125.909446(11) | style="text-align:center" | 1954 | 1.64(2) min | β+ | 126Xe | 1+ | |-id=Caesium-126m1 | style="text-indent:1em" | 126m1Cs | colspan="3" style="text-indent:2em" | 273.0(7) keV | style="text-align:center" | (1991) | ~1 μs | IT | 126Cs | (4−) | |-id=Caesium-126m2 | style="text-indent:1em" | 126m2Cs | colspan="3" style="text-indent:2em" | 596.1(11) keV | style="text-align:center" | 2003 | 171(14) μs | IT | 126Cs | 8−# | |-id=Caesium-127 | 127Cs | style="text-align:right" | 55 | style="text-align:right" | 72 | 126.9074175(60) | style="text-align:center" | 1950 | 6.25(10) h | β+ | 127Xe | 1/2+ | |-id=Caesium-127m | style="text-indent:1em" | 127mCs | colspan="3" style="text-indent:2em" | 452.23(21) keV | style="text-align:center" | 1971 | 55(3) μs | IT | 127Cs | (11/2)− | |-id=Caesium-128 | 128Cs | style="text-align:right" | 55 | style="text-align:right" | 73 | 127.9077485(57) | style="text-align:center" | 1951 | 3.640(14) min | β+ | 128Xe | 1+ | |-id=Caesium-129 | 129Cs | style="text-align:right" | 55 | style="text-align:right" | 74 | 128.9060659(49) | style="text-align:center" | 1950 | 32.06(6) h | β+ | 129Xe | 1/2+ | |-id=Caesium-129m | style="text-indent:1em" | 129mCs | colspan="3" style="text-indent:2em" | 575.40(14) keV | style="text-align:center" | 1977 | 718(21) ns | IT | 129Cs | (11/2−) | |-id=Caesium-130 | rowspan=2|130Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 75 | rowspan=2|129.9067093(90) | rowspan=2 style="text-align:center" | 1952 | rowspan=2|29.21(4) min | β+ (98.4%) | 130Xe | rowspan=2|1+ | rowspan=2| |- | β− (1.6%) | 130Ba |-id=Caesium-130m | rowspan=2 style="text-indent:1em" | 130mCs | rowspan=2 colspan="3" style="text-indent:2em" | 163.25(11) keV | rowspan=2 style="text-align:center" | 1977 | rowspan=2|3.46(6) min | IT (99.84%) | 130Cs | rowspan=2|5− | rowspan=2| |- | β+ (0.16%) | 130Xe |-id= | 131Cs | style="text-align:right" | 55 | style="text-align:right" | 76 | 130.90546846(19) | style="text-align:center" | 1947 | 9.689(16) d | EC | 131Xe | 5/2+ | |-id=Caesium-132 | rowspan=2|132Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 77 | rowspan=2|131.9064378(11) | rowspan=2 style="text-align:center" | 1953 | rowspan=2|6.480(6) d | β+ (98.13%) | 132Xe | rowspan=2|2+ | rowspan=2| |- | β− (1.87%) | 132Ba |-id= | 133CsUsed to define the secondFission product | style="text-align:right" | 55 | style="text-align:right" | 78 | 132.905451958(8) | style="text-align:center" | 1921 | colspan=3 align=center|Stable | 7/2+ | 1.0000 |-id= | rowspan=2|134Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 79 | rowspan=2|133.906718501(17) | rowspan=2 style="text-align:center" | 1940 | rowspan=2|2.0650(4) y | β− | 134Ba | rowspan=2|4+ | rowspan=2| |- | EC (3.0×10−4%) | 134Xe |-id=Caesium-134m | style="text-indent:1em" | 134mCs | colspan="3" style="text-indent:2em" | 138.7441(26) keV | style="text-align:center" | 1940 | 2.912(2) h | IT | 134Cs | 8− | |-id= | 135CsLong-lived fission product | style="text-align:right" | 55 | style="text-align:right" | 80 | 134.90597691(39) | style="text-align:center" | 1949 | 1.33(19)×106 y | β− | 135Ba | 7/2+ | |-id=Caesium-135m | style="text-indent:1em" | 135mCs | colspan="3" style="text-indent:2em" | 1632.9(15) keV | style="text-align:center" | 1962 | 53(2) min | IT | 135Cs | 19/2− | |-id= | 136Cs | style="text-align:right" | 55 | style="text-align:right" | 81 | 135.9073114(20) | style="text-align:center" | 1951 | 13.01(5) d | β− | 136Ba | 5+ |