Isotopes of caesium

{{Short description|none}} {{Infobox caesium isotopes}} 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, {{SimpleNuclide|Caesium|137|link=y}} 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{{clarify|date=April 2025}} of these isotopes can be estimated as a function of time.

== List of isotopes == {{Anchor|Caesium-111|Caesium-144m1|Caesium-144m2}}

<!--Please delete anchor(s) from the list above or table below if adding a dedicated isotope section(s).-->

{{Isotopes table | symbol = Cs | refs = NUBASE2020, AME2020 II, <!-- updated 2024–09–29 --> IsotopeFRIB, IsomerFRIB | notes = m, unc(), mass#, exen#, spin(), spin#, daughter-nst, daughter-st, p, IT, EC, n, discoveryname }} |-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)<ref name="nies2025">{{cite journal |last1=Nies |first1=L. |last2=Atanasov |first2=D. |last3=Athanasakis-Kaklamanakis |first3=M. |last4=Au |first4=M. |last5=Bernerd |first5=C. |last6=Blaum |first6=K. |last7=Chrysalidis |first7=K. |last8=Fischer |first8=P. |last9=Heinke |first9=R. |last10=Klink |first10=C. |last11=Lange |first11=D. |last12=Lunney |first12=D. |last13=Manea |first13=V. |last14=Marsh |first14=B. A. |last15=Müller |first15=M. |last16=Mougeot |first16=M. |last17=Naimi |first17=S. |last18=Schweiger |first18=Ch. |last19=Schweikhard |first19=L. |last20=Wienholtz |first20=F. |title=Refining the nuclear mass surface with the mass of Sn 103 |journal=Physical Review C |date=9 January 2025 |volume=111 |issue=1 |article-number=014315 |doi=10.1103/PhysRevC.111.014315|doi-access=free }}</ref> | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/112.pdf 1994] | rowspan=2|490(30)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/113.pdf 1984] | 16.94(9)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/114.pdf 1978] | rowspan=4|570(20)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/115.pdf 1978] | rowspan=2|1.4(8)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/116.pdf 1975] | rowspan=3|700(40)&nbsp;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" | ^116mCs<ref group="n" name="order">Order of ground state and isomer is uncertain.</ref> | rowspan=3 colspan="3" style="text-indent:2em" | 100(60)#&nbsp;keV | rowspan=3 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/116Cs-1.pdf 1977] | rowspan=3|3.85(13)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/117.pdf 1972] | 8.4(6)&nbsp;s | β⁺ | ¹¹⁷Xe | 9/2+# | |-id=Caesium-117m | style="text-indent:1em" | ^117mCs<ref group="n" name="order"/> | colspan="3" style="text-indent:2em" | 150(80)#&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/117Cs-1.pdf 1986] | 6.5(4)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/118.pdf 1969] | rowspan=3|14(2)&nbsp;s | β⁺ (99.98%) | ¹¹⁸Xe | rowspan=3|2(−)<ref name="zheng2021">{{cite journal |last1=Zheng |first1=K. K. |last2=Petrache |first2=C. M. |last3=Zhang |first3=Z. H. |last4=Astier |first4=A. |last5=Lv |first5=B. F. |last6=Greenlees |first6=P. T. |last7=Grahn |first7=T. |last8=Julin |first8=R. |last9=Juutinen |first9=S. |last10=Luoma |first10=M. |last11=Ojala |first11=J. |last12=Pakarinen |first12=J. |last13=Partanen |first13=J. |last14=Rahkila |first14=P. |last15=Ruotsalainen |first15=P. |last16=Sandzelius |first16=M. |last17=Sarén |first17=J. |last18=Tann |first18=H. |last19=Uusitalo |first19=J. |last20=Zimba |first20=G. |last21=Cederwall |first21=B. |last22=Aktas |first22=ö. |last23=Ertoprak |first23=A. |last24=Zhang |first24=W. |last25=Guo |first25=S. |last26=Liu |first26=M. L. |last27=Zhou |first27=X. H. |last28=Kuti |first28=I. |last29=Nyakó |first29=B. M. |last30=Sohler |first30=D. |last31=Timár |first31=J. |last32=Andreoiu |first32=C. |last33=Doncel |first33=M. |last34=Joss |first34=D. T. |last35=Page |first35=R. D. |title=Rich band structure and multiple long-lived isomers in the odd-odd Cs 118 nucleus |journal=Physical Review C |date=21 October 2021 |volume=104 |issue=4 |article-number=044325 |doi=10.1103/PhysRevC.104.044325 |url=https://journals.aps.org/prc/abstract/10.1103/PhysRevC.104.044325 |access-date=29 December 2024}}</ref> | rowspan=3| |- | β⁺, p (0.021%) | ¹¹⁷I |- | β⁺, α (0.0012%) | ¹¹⁴Te |-id=Caesium-118m1 | rowspan=3 style="text-indent:1em" | ^118m1Cs<ref name="zheng2021"/><ref group="n" name="order"/> | rowspan=3 colspan="3" style="text-indent:2em" | X&nbsp;keV | rowspan=3 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/118Cs-1.pdf 1977] | rowspan=3|17(3)&nbsp;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<ref name="zheng2021"/><ref group="n" name="order"/> | colspan="3" style="text-indent:2em" | Y&nbsp;keV | style="text-align:center" | (2021)<ref group="n" | name="notm">Half-life not measured, not included in discovery database</ref> | | | | (6+) | |-id=Caesium-118m3 | style="text-indent:1em" | ^118m3Cs<ref name="zheng2021"/><ref group="n" name="order"/> | colspan="3" style="text-indent:2em" | 65.9&nbsp;keV | style="text-align:center" | (2021)<ref group="n" name=notm /> | | IT | ¹¹⁸Cs | (3−) | |-id=Caesium-118m4 | style="text-indent:1em" | ^118m4Cs<ref name="zheng2021"/> | colspan="3" style="text-indent:2em" | 125.9+X&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/118Cs-2.pdf 2021] | 550(60)&nbsp;ns | IT | ^118m1Cs | (7+) | |-id=Caesium-118m5 | style="text-indent:1em" | ^118m5Cs<ref name="zheng2021"/> | colspan="3" style="text-indent:2em" | 195.2+X&nbsp;keV | style="text-align:center" | (2021)<ref group="n" name=notm /> | <500&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/119.pdf 1969] | rowspan=2|43.0(2)&nbsp;s | β⁺ | ¹¹⁹Xe | rowspan=2|9/2+ | rowspan=2| |- | β⁺, α (<2×10⁻⁶%) | ¹¹⁵Te |-id=Caesium-119m | style="text-indent:1em" | ^119mCs<ref group="n" name="order"/> | colspan="3" style="text-indent:2em" | 50(30)#&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/119Cs-1.pdf 1978] | 30.4(1)&nbsp;s | β⁺ | ¹¹⁹Xe | 3/2+ | |-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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/120.pdf 1969] | rowspan=3|60.4(6)&nbsp;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<ref group="n" name="order"/> | rowspan=3 colspan="3" style="text-indent:2em" | 100(60)#&nbsp;keV | rowspan=3 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/120Cs-1.pdf 1977] | rowspan=3|57(6)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/121.pdf 1969] | 155(4)&nbsp;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)&nbsp;keV | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/121Cs-1.pdf 1981] | rowspan=2|122(3)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/122.pdf 1969] | rowspan=2|21.18(19)&nbsp;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)&nbsp;keV | style="text-align:center" | (1987)<ref group="n" | name="conf">Only published in a conference proceeding and not a refereed journal</ref> | >1&nbsp;μs | IT | ¹²²Cs | 3+ | |-id=Caesium-122m2 | style="text-indent:1em" | ^122m2Cs | colspan="3" style="text-indent:2em" | 140(30)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/122Cs-2.pdf 1983] | 3.70(11)&nbsp;min | β⁺ | ¹²²Xe | 8− | |-id=Caesium-122m3 | style="text-indent:1em" | ^122m3Cs | colspan="3" style="text-indent:2em" | 127.07(16)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/122Cs-1.pdf 1969] | 360(20)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/123.pdf 1954] | 5.88(3)&nbsp;min | β⁺ | ¹²³Xe | 1/2+ | |-id=Caesium-123m1 | style="text-indent:1em" | ^123m1Cs | colspan="3" style="text-indent:2em" | 156.27(5)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/123Cs-1.pdf 1972] | 1.64(12)&nbsp;s | IT | ¹²³Cs | 11/2− | |-id=Caesium-123m2 | style="text-indent:1em" | ^123m2Cs | colspan="3" style="text-indent:2em" | 252(6)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/123Cs-2.pdf 2000] | 114(5)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/124.pdf 1969] | 30.9(4)&nbsp;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)&nbsp;keV | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/124Cs-1.pdf 1983] | rowspan=2|6.41(7)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/125.pdf 1954] | 44.35(29)&nbsp;min | β⁺ | ¹²⁵Xe | 1/2+ | |-id=Caesium-125m | style="text-indent:1em" | ^125mCs | colspan="3" style="text-indent:2em" | 266.1(11)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/125Cs-1.pdf 1998] | 900(30)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/126.pdf 1954] | 1.64(2)&nbsp;min | β⁺ | '''¹²⁶Xe''' | 1+ | |-id=Caesium-126m1 | style="text-indent:1em" | ^126m1Cs | colspan="3" style="text-indent:2em" | 273.0(7)&nbsp;keV | style="text-align:center" | (1991)<ref group="n" | name=conf /> | ~1&nbsp;μs | IT | ¹²⁶Cs | (4−) | |-id=Caesium-126m2 | style="text-indent:1em" | ^126m2Cs | colspan="3" style="text-indent:2em" | 596.1(11)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/126Cs-1.pdf 2003] | 171(14)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/127.pdf 1950] | 6.25(10)&nbsp;h | β⁺ | ¹²⁷Xe | 1/2+ | |-id=Caesium-127m | style="text-indent:1em" | ^127mCs | colspan="3" style="text-indent:2em" | 452.23(21)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/127Cs-1.pdf 1971] | 55(3)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/128.pdf 1951] | 3.640(14)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/129.pdf 1950] | 32.06(6)&nbsp;h | β⁺ | '''¹²⁹Xe''' | 1/2+ | |-id=Caesium-129m | style="text-indent:1em" | ^129mCs | colspan="3" style="text-indent:2em" | 575.40(14)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/129Cs-1.pdf 1977] | 718(21)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/130.pdf 1952] | rowspan=2|29.21(4)&nbsp;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)&nbsp;keV | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/130Cs-1.pdf 1977] | rowspan=2|3.46(6)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/131.pdf 1947] | 9.689(16)&nbsp;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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/132.pdf 1953] | rowspan=2|6.480(6)&nbsp;d | β⁺ (98.13%) | '''¹³²Xe''' | rowspan=2|2+ | rowspan=2| |- | β⁻ (1.87%) | '''¹³²Ba''' |-id= | ¹³³Cs<ref group="n">Used to define the second</ref><ref group="n" name="FP">Fission product</ref> | style="text-align:right" | 55 | style="text-align:right" | 78 | 132.905451958(8) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/133.pdf 1921] | colspan=3 align=center|'''Stable''' | 7/2+ | 1.0000 |-id= | rowspan=2|¹³⁴Cs<ref group="n" name="FP" /> | 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" | [https://www.nndc.bnl.gov/discovery/abstracts/55/134.pdf 1940] | rowspan=2|2.0650(4)&nbsp;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)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/134Cs-1.pdf 1940] | 2.912(2)&nbsp;h | IT | ¹³⁴Cs | 8− | |-id= | ¹³⁵Cs<ref group="n">Long-lived fission product</ref> | style="text-align:right" | 55 | style="text-align:right" | 80 | 134.90597691(39) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/135.pdf 1949] | 1.33(19)×10⁶&nbsp;y | β⁻ | '''¹³⁵Ba''' | 7/2+ | |-id=Caesium-135m | style="text-indent:1em" | ^135mCs | colspan="3" style="text-indent:2em" | 1632.9(15)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/135Cs-1.pdf 1962] | 53(2)&nbsp;min | IT | ¹³⁵Cs | 19/2− | |-id= | ¹³⁶Cs | style="text-align:right" | 55 | style="text-align:right" | 81 | 135.9073114(20) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/136.pdf 1951] | 13.01(5)&nbsp;d | β⁻ | '''¹³⁶Ba''' | 5+ | |-id=Caesium-136m | rowspan=2 style="text-indent:1em" | ^136mCs | rowspan=2 colspan="3" style="text-indent:2em" | 517.9(1)&nbsp;keV | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/136Cs-1.pdf 1975] | rowspan=2|17.5(2)&nbsp;s | β⁻? | '''¹³⁶Ba''' | rowspan=2|8− | rowspan=2| |- | IT? | ¹³⁶Cs |-id= | rowspan=2|¹³⁷Cs<ref group="n" name="FP" /> | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 82 | rowspan=2|136.90708930(32) | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/137.pdf 1951] | rowspan=2|30.04(4)&nbsp;y | β⁻ (94.70%)<ref name="137cs-decaymode">{{cite journal |last1=Browne |first1=E. |last2=Tuli |first2=J.K. |title=Nuclear Data Sheets for A = 137 |journal=Nuclear Data Sheets |date=October 2007 |volume=108 |issue=10 |pages=2173–2318 |doi=10.1016/j.nds.2007.09.002|bibcode=2007NDS...108.2173B }}</ref> | ^137mBa | rowspan=2|7/2+ | rowspan=2| |- | β⁻ (5.30%)<ref name="137cs-decaymode"/> | '''¹³⁷Ba''' |-id=Caesium-138 | ¹³⁸Cs | style="text-align:right" | 55 | style="text-align:right" | 83 | 137.9110171(98) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/138.pdf 1943] | 33.5(2)&nbsp;min | β⁻ | '''¹³⁸Ba''' | 3− | |-id=Caesium-138m | rowspan=2 style="text-indent:1em" | ^138mCs | rowspan=2 colspan="3" style="text-indent:2em" | 79.9(3)&nbsp;keV | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/138Cs-1.pdf 1971] | rowspan=2|2.91(10)&nbsp;min | IT (81%) | ¹³⁸Cs | rowspan=2|6− | rowspan=2| |- | β⁻ (19%) | '''¹³⁸Ba''' |-id=Caesium-139 | ¹³⁹Cs | style="text-align:right" | 55 | style="text-align:right" | 84 | 138.9133638(34) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/139.pdf 1939] | 9.27(5)&nbsp;min | β⁻ | ¹³⁹Ba | 7/2+ | |-id=Caesium-140 | ¹⁴⁰Cs | style="text-align:right" | 55 | style="text-align:right" | 85 | 139.9172837(88) | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/140.pdf 1950] | 63.7(3)&nbsp;s | β⁻ | ¹⁴⁰Ba | 1− | |-id=Caesium-140m | style="text-indent:1em" | ^140mCs | colspan="3" style="text-indent:2em" | 13.931(21)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/140Cs-1.pdf 1975] | 471(51)&nbsp;ns | IT | ¹⁴⁰Cs | (2)− | |-id=Caesium-141 | rowspan=2|¹⁴¹Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 86 | rowspan=2|140.9200453(99) | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/141.pdf 1962] | rowspan=2|24.84(16)&nbsp;s | β⁻ (99.97%) | ¹⁴¹Ba | rowspan=2|7/2+ | rowspan=2| |- | β⁻, n (0.0342%) | ¹⁴⁰Ba |-id=Caesium-142 | rowspan=2|¹⁴²Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 87 | rowspan=2|141.9242995(76) | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/142.pdf 1962] | rowspan=2|1.687(10)&nbsp;s | β⁻ (99.91%) | ¹⁴²Ba | rowspan=2|0− | rowspan=2| |- | β⁻, n (0.089%) | ¹⁴¹Ba |-id=Caesium-143 | rowspan=2|¹⁴³Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 88 | rowspan=2|142.9273473(81) | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/143.pdf 1962] | rowspan=2|1.802(8)&nbsp;s | β⁻ (98.38%) | ¹⁴³Ba | rowspan=2|3/2+ | rowspan=2| |- | β⁻, n (1.62%) | ¹⁴²Ba |-id=Caesium-144 | rowspan=2|¹⁴⁴Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 89 | rowspan=2|143.932075(22) | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/144.pdf 1967] | rowspan=2|994(6)&nbsp;ms | β⁻ (97.02%) | ¹⁴⁴Ba | rowspan=2|1− | rowspan=2| |- | β⁻, n (2.98%) | ¹⁴³Ba |-id=Caesium-144m | style="text-indent:1em" | ^144mCs | colspan="3" style="text-indent:2em" | 92.2(5)&nbsp;keV | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/isomers/abstracts/55/144Cs-1.pdf 2009] | 1.1(1)&nbsp;μs | IT | ¹⁴⁴Cs | (4−) | |-id=Caesium-145 | rowspan=2|¹⁴⁵Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 90 | rowspan=2|144.9355289(97) | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/145.pdf 1971] | rowspan=2|582(4)&nbsp;ms | β⁻ (87.2%) | ¹⁴⁵Ba | rowspan=2|3/2+ | rowspan=2| |- | β⁻, n (12.8%) | ¹⁴⁴Ba |-id=Caesium-145m | style="text-indent:1em" | ^145mCs | colspan="3" style="text-indent:2em" | 762.9(4)&nbsp;keV | style="text-align:center" | (2015)<ref group="n" | name=conf /> | 0.5(1)&nbsp;μs | IT | ¹⁴⁵Cs | 13/2# | |-id=Caesium-146 | rowspan=2|¹⁴⁶Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 91 | rowspan=2|145.9406219(31) | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/146.pdf 1971] | rowspan=2|321.6(9)&nbsp;ms | β⁻ (85.8%) | ¹⁴⁶Ba | rowspan=2|1− | rowspan=2| |- | β⁻, n (14.2%) | ¹⁴⁵Ba |-id=Caesium-146m | style="text-indent:1em" | ^146mCs | colspan="3" style="text-indent:2em" | 46.7(1)&nbsp;keV | style="text-align:center" | (2015)<ref group="n" | name=conf />

| 1.25(5)&nbsp;μs | IT | ¹⁴⁶Cs | 4−# | |-id=Caesium-147 | rowspan=2|¹⁴⁷Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 92 | rowspan=2|146.9442615(90) | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/147.pdf 1978] | rowspan=2|230.5(9)&nbsp;ms | β⁻ (71.5%) | ¹⁴⁷Ba | rowspan=2|(3/2+) | rowspan=2| |- | β⁻, n (28.5%) | ¹⁴⁶Ba |-id=Caesium-147m | style="text-indent:1em" | ^147mCs | colspan="3" style="text-indent:2em" | 701.4(4)&nbsp;keV | style="text-align:center" | (2015)<ref group="n" | name=conf /> | 190(20)&nbsp;ns | IT | ¹⁴⁷Cs | 13/2# | |-id=Caesium-148 | rowspan=2|¹⁴⁸Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 93 | rowspan=2|147.949639(14) | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/148.pdf 1978] | rowspan=2|151.8(10)&nbsp;ms | β⁻ (71.3%) | ¹⁴⁸Ba | rowspan=2|(2−) | rowspan=2| |- | β⁻, n (28.7%) | ¹⁴⁷Ba |-id=Caesium-148m | style="text-indent:1em" | ^148mCs | colspan="3" style="text-indent:2em" | 45.2(1)&nbsp;keV | style="text-align:center" | (2015)<ref group="n" | name=conf /> | 4.8(2)&nbsp;μs | IT | ¹⁴⁸Cs | 4−# | |-id=Caesium-149 | rowspan=2|¹⁴⁹Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 94 | rowspan=2|148.95352(43)# | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/149.pdf 2017] | rowspan=2|112.3(25)&nbsp;ms | β⁻ (75%) | ¹⁴⁹Ba | rowspan=2|3/2+# | rowspan=2| |- | β⁻, n (25%) | ¹⁴⁸Ba |-id=Caesium-150 | rowspan=2|¹⁵⁰Cs | rowspan=2 style="text-align:right" | 55 | rowspan=2 style="text-align:right" | 95 | rowspan=2|149.95902(43)# | rowspan=2 style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/150.pdf 2017] | rowspan=2|81.0(26)&nbsp;ms | β⁻ (~56%) | ¹⁵⁰Ba | rowspan=2|(2−) | rowspan=2| |- | β⁻, n (~44%) | ¹⁴⁹Ba |-id=Caesium-151 | ¹⁵¹Cs | style="text-align:right" | 55 | style="text-align:right" | 96 | 150.96320(54)# | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/151.pdf 2017] | 59(19)&nbsp;ms | β⁻ | ¹⁵¹Ba | 3/2+# | |-id=Caesium-152 | ¹⁵²Cs<ref name=sumikama2026>{{cite journal |last1=Sumikama |first1=Toshiyuki |last2=Fukuda |first2=Naoki |last3=Kubo |first3=Toshiyuki |last4=Suzuki |first4=Hiroshi |last5=Takeda |first5=Hiroyuki |last6=Inabe |first6=Naohito |last7=Kameda |first7=Daisuke |last8=Ahn |first8=Deuk Soon |last9=Murai |first9=Daichi |last10=Yoshida |first10=Koichi |last11=Kusaka |first11=Kensuke |last12=Yanagisawa |first12=Yoshiyuki |last13=Ohtake |first13=Masao |last14=Shimizu |first14=Yohei |last15=Sato |first15=Yuki |last16=Sato |first16=Hiromi |last17=Otsu |first17=Hideaki |last18=Baba |first18=Hidetada |last19=Lorusso |first19=Giuseppe |last20=Söderström |first20=Pär-Anders |last21=Isobe |first21=Tadaaki |last22=Imai |first22=Nobuaki |last23=Mukai |first23=Momo |last24=Kimura |first24=Sota |last25=Miyatake |first25=Hiroari |last26=Iwasa |first26=Naohito |last27=Yagi |first27=Ayumi |last28=Yokoyama |first28=Rin |last29=Tarasov |first29=Oleg Borisovich |last30=Geissel |first30=Hans |title=Expanding the Isotopic Frontier: Seven New Neutron-Rich Rare-Earth Isotopes Observed at RIKEN RI Beam Factory |journal=Journal of the Physical Society of Japan |date=15 February 2026 |volume=95 |issue=2 |doi=10.7566/JPSJ.95.024202}}</ref> | style="text-align:right" | 55 | style="text-align:right" | 97 | 151.96873(54)# | style="text-align:center" | [https://www.nndc.bnl.gov/discovery/abstracts/55/152.pdf 2026] | 17#&nbsp;ms | | | | {{Isotopes table/footer}}

==Caesium-131== Caesium-131 decays purely by electron capture to the ground state of stable xenon-131 with a half-life of 9.69 days; its detectable radiation is the X-rays of xenon, with a maximum energy of 34.5 keV. It was introduced in 2004 for brachytherapy by Isoray.<ref name="131Cs-Isoray">{{cite web |author=Isoray |title=Why Cesium-131 |work=Isoray - Treating Prostate Cancer with Brachytherapy |url=https://isoray.com/why-cesium-131/ |access-date=2017-12-05 |archive-date=2019-06-30 |archive-url=https://web.archive.org/web/20190630105241/https://isoray.com/why-cesium-131/ }}</ref>

==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:

{{Quote|The second, symbol s, is defined by taking the fixed numerical value of the caesium frequency, {{math|Δ''ν''_Cs}}, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom,<ref name="133Cs-SI">Although the phrase 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.&nbsp;130 states: "The effect of this definition is that the second is equal to the duration of {{val|9192631770}} periods of the radiation corresponding to the transition between the two hyperfine levels of the unperturbed ground state of the ¹³³Cs atom."</ref> to be {{val|9192631770}} Hz, which is equal to s⁻¹. }}

==Caesium-134== Caesium-134 has a half-life of 2.0650 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. It is not produced by nuclear weapons because ¹³³Cs is created by beta decay of original fission products 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 stable ¹³⁴Ba after emitting on average 2.23 gamma ray photons (mean energy 0.698 MeV).<ref name="134Cs-JAEA">{{cite web |title=Characteristics of Caesium-134 and Caesium-137 |publisher=Japan Atomic Energy Agency |url=http://c-navi.jaea.go.jp/en/background/remediation-following-major-radiation-accidents/characteristics-of-caesium-134-and-caesium-137.html |access-date=2014-10-23 |archive-date=2016-03-04 |archive-url=https://web.archive.org/web/20160304100452/http://c-navi.jaea.go.jp/en/background/remediation-following-major-radiation-accidents/characteristics-of-caesium-134-and-caesium-137.html }}</ref>

==Caesium-135== {{Long-lived fission products}} Caesium-135 is a mildly radioactive isotope of caesium with a half-life of 1.33&nbsp;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 {{chem|137|Cs}} which can only be separated from {{chem|135|Cs}} via isotope separation) rather than with other long-lived fission products. As an exception, molten salt reactors create {{chem|135|Cs}} as a completely separate stream outside the fuel (after the decay of bubble-separated {{chem|135|Xe}}). 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)<ref name="Groh2004">{{cite web|url=http://canteach.candu.org/library/20041204.pdf|author=John L. Groh|title=Supplement to Chapter 11 of Reactor Physics Fundamentals|year=2004|publisher=CANTEACH project|access-date=14 May 2011|archive-url=https://web.archive.org/web/20110610131653/http://canteach.candu.org/library/20041204.pdf|archive-date=10 June 2011}}</ref> will be converted to practically stable {{chem|136|Xe|link=xenon-136}} before it can decay to {{chem|135|Cs}} despite the relatively short half-life of {{chem|135|Xe}}. Little or no {{chem|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 {{chem|135|Xe}} buildup in the reactor after a reduction in power or a shutdown and is often managed by letting the {{chem|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 {{nowrap|8.3 ± 0.3}} and {{nowrap|38.1 ± 2.6 barns}} respectively.<ref name="Hatsukawa1999">{{cite journal|title=Thermal neutron cross section and resonance integral of the reaction of135Cs(n,γ)136Cs: Fundamental data for the transmutation of nuclear waste|year=1999|last1=Hatsukawa|first1=Y.|last2=Shinohara |first2=N |last3=Hata |first3=K.|journal=Journal of Radioanalytical and Nuclear Chemistry|volume=239|issue=3|pages=455–458|doi=10.1007/BF02349050|s2cid=97425651|display-authors=etal}}</ref> 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.<ref name="Ohki">{{cite journal|url=http://www.nea.fr/html/pt/docs/iem/jeju02/session6/SessionVI-08.pdf|title=Transmutation of Cesium-135 With Fast Reactors|first1=Shigeo|last1=Ohki|last2=Takaki |first2=Naoyuki|journal=Proceedings of the Seventh Information Exchange Meeting on Actinide and Fission Product Partitioning & Transmutation, Cheju, Korea|year=2002}}</ref> *[https://web.archive.org/web/20110429095603/http://www.ead.anl.gov/pub/doc/cesium.pdf 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, though because of the lower cross-section (see above) and sort half-life, is much less abundant in spent fuel and vanishes quickly. 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 ({{SubatomicParticle|Beta-}}) to ¹³⁶Ba.

==Caesium-137== {{Main|Caesium-137}} Caesium-137, with a half-life of 30.04 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 from several years 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.<ref name="137Cs-Normile">{{cite journal |author=Dennis |title=Cooling a Hot Zone |journal=Science |volume=339 |issue=6123 |date=1 March 2013 |pages=1028–1029 |doi=10.1126/science.339.6123.1028|pmid=23449572 }}</ref> ¹³⁷Cs beta decays to barium-137m (a short-lived nuclear isomer), which in de-excitation to its stable ground state barium-137, usually emits a gamma ray. This process is responsible for all the gamma emission from caesium-137.<ref>{{NNDC}}</ref>

<!-- This whole paragraph is unclear as well as unsourced. --> 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.{{citation needed|date=July 2025}}

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

==Fission-produced isotopes of caesium== The heavier isotopes have half-lives of seconds or minutes. 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 (thus its higher-Z decay products) is often created far from the original site of fission. {{citation needed|date=August 2025}}

== See also == '''Daughter products other than caesium''' * Isotopes of barium * Isotopes of xenon * Isotopes of iodine * Isotopes of tellurium

== References == {{Reflist}}

{{Navbox element isotopes}} {{Authority control}}

Category:Isotopes of caesium Caesium