Caesium Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...

(₅₅Cs) has 40 known isotopes, making it, along with

barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...

and

mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...

, one of the elements with the most 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, 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 Nuclear weapons tests are experiments carried out to determine nuclear weapons' effectiveness, yield, and explosive capability. Testing nuclear weapons offers practical information about how the weapons function, how detonations are affected by ...

, caesium radioisotopes were released into the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

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

, - , rowspan=2, ¹¹²Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 57 , rowspan=2, 111.95030(33)# , rowspan=2, 500(100) μs , p , ¹¹¹Xe , rowspan=2, 1+# , rowspan=2, , rowspan=2, , - , α , ¹⁰⁸I , - , rowspan=2, ¹¹³Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 58 , rowspan=2, 112.94449(11) , rowspan=2, 16.7(7) μs , p (99.97%) , ¹¹²Xe , rowspan=2, 5/2+# , rowspan=2, , rowspan=2, , - , β⁺ (.03%) , ¹¹³Xe , - , rowspan=4, ¹¹⁴Cs , rowspan=4 style="text-align:right" , 55 , rowspan=4 style="text-align:right" , 59 , rowspan=4, 113.94145(33)# , rowspan=4, 0.57(2) s , β⁺ (91.09%) , ¹¹⁴Xe , rowspan=4, (1+) , rowspan=4, , rowspan=4, , - , β⁺, p (8.69%) , ¹¹³I , - , β⁺, α (.19%) , ¹¹⁰Te , - , α (.018%) , ¹¹⁰I , - , rowspan=2, ¹¹⁵Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 60 , rowspan=2, 114.93591(32)# , rowspan=2, 1.4(8) s , β⁺ (99.93%) , ¹¹⁵Xe , rowspan=2, 9/2+# , rowspan=2, , rowspan=2, , - , β⁺, p (.07%) , ¹¹⁴I , - , rowspan=3, ¹¹⁶Cs , rowspan=3 style="text-align:right" , 55 , rowspan=3 style="text-align:right" , 61 , rowspan=3, 115.93337(11)# , rowspan=3, 0.70(4) s , β⁺ (99.67%) , ¹¹⁶Xe , rowspan=3, (1+) , rowspan=3, , rowspan=3, , - , β⁺, p (.279%) , ¹¹⁵I , - , β⁺, α (.049%) , ¹¹²Te , - , rowspan=3 style="text-indent:1em" , ^116mCs , rowspan=3 colspan="3" style="text-indent:2em" , 100(60)# keV , rowspan=3, 3.85(13) s , β⁺ (99.48%) , ¹¹⁶Xe , rowspan=3, 4+, 5, 6 , rowspan=3, , rowspan=3, , - , β⁺, p (.51%) , ¹¹⁵I , - , β⁺, α (.008%) , ¹¹²Te , - , ¹¹⁷Cs , style="text-align:right" , 55 , style="text-align:right" , 62 , 116.92867(7) , 8.4(6) s , β⁺ , ¹¹⁷Xe , (9/2+)# , , , - , style="text-indent:1em" , ^117mCs , colspan="3" style="text-indent:2em" , 150(80)# keV , 6.5(4) s , β⁺ , ¹¹⁷Xe , 3/2+# , , , - , rowspan=3, ¹¹⁸Cs , rowspan=3 style="text-align:right" , 55 , rowspan=3 style="text-align:right" , 63 , rowspan=3, 117.926559(14) , rowspan=3, 14(2) s , β⁺ (99.95%) , ¹¹⁸Xe , rowspan=3, 2 , rowspan=3, , rowspan=3, , - , β⁺, p (.042%) , ¹¹⁷I , - , β⁺, α (.0024%) , ¹¹⁴Te , - , rowspan=3 style="text-indent:1em" , ^118mCs , rowspan=3 colspan="3" style="text-indent:2em" , 100(60)# keV , rowspan=3, 17(3) s , β⁺ (99.95%) , ¹¹⁸Xe , rowspan=3, (7−) , rowspan=3, , rowspan=3, , - , β⁺, p (.042%) , ¹¹⁷I , - , β⁺, α (.0024%) , ¹¹⁴Te , - , rowspan=2, ¹¹⁹Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 64 , rowspan=2, 118.922377(15) , rowspan=2, 43.0(2) s , β⁺ , ¹¹⁹Xe , rowspan=2, 9/2+ , rowspan=2, , rowspan=2, , - , β⁺, α (2×10⁻⁶%) , ¹¹⁵Te , - , style="text-indent:1em" , ^119mCs , colspan="3" style="text-indent:2em" , 50(30)# keV , 30.4(1) s , β⁺ , ¹¹⁹Xe , 3/2(+) , , , - , rowspan=3, ¹²⁰Cs , rowspan=3 style="text-align:right" , 55 , rowspan=3 style="text-align:right" , 65 , rowspan=3, 119.920677(11) , rowspan=3, 61.2(18) s , β⁺ , ¹²⁰Xe , rowspan=3, 2(−#) , rowspan=3, , rowspan=3, , - , β⁺, α (2×10⁻⁵%) , ¹¹⁶Te , - , β⁺, p (7×10⁻⁶%) , ¹¹⁹I , - , rowspan=3 style="text-indent:1em" , ^120mCs , rowspan=3 colspan="3" style="text-indent:2em" , 100(60)# keV , rowspan=3, 57(6) s , β⁺ , ¹²⁰Xe , rowspan=3, (7−) , rowspan=3, , rowspan=3, , - , β⁺, α (2×10⁻⁵%) , ¹¹⁶Te , - , β⁺, p (7×10⁻⁶%) , ¹¹⁹I , - , ¹²¹Cs , style="text-align:right" , 55 , style="text-align:right" , 66 , 120.917229(15) , 155(4) s , β⁺ , ¹²¹Xe , 3/2(+) , , , - , rowspan=2 style="text-indent:1em" , ^121mCs , rowspan=2 colspan="3" style="text-indent:2em" , 68.5(3) keV , rowspan=2, 122(3) s , β⁺ (83%) , ¹²¹Xe , rowspan=2, 9/2(+) , rowspan=2, , rowspan=2, , - , IT (17%) , ¹²¹Cs , - , rowspan=2, ¹²²Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 67 , rowspan=2, 121.91611(3) , rowspan=2, 21.18(19) s , β⁺ , ¹²²Xe , rowspan=2, 1+ , rowspan=2, , rowspan=2, , - , β⁺, α (2×10⁻⁷%) , ¹¹⁸Te , - , style="text-indent:1em" , ^122m1Cs , colspan="3" style="text-indent:2em" , 45.8 keV , >1 μs , , , (3)+ , , , - , style="text-indent:1em" , ^122m2Cs , colspan="3" style="text-indent:2em" , 140(30) keV , 3.70(11) min , β⁺ , ¹²²Xe , 8− , , , - , style="text-indent:1em" , ^122m3Cs , colspan="3" style="text-indent:2em" , 127.0(5) keV , 360(20) ms , , , (5)− , , , - , ¹²³Cs , style="text-align:right" , 55 , style="text-align:right" , 68 , 122.912996(13) , 5.88(3) min , β⁺ , ¹²³Xe , 1/2+ , , , - , style="text-indent:1em" , ^123m1Cs , colspan="3" style="text-indent:2em" , 156.27(5) keV , 1.64(12) s , IT , ¹²³Cs , (11/2)− , , , - , style="text-indent:1em" , ^123m2Cs , colspan="3" style="text-indent:2em" , 231.63+X keV , 114(5) ns , , , (9/2+) , , , - , ¹²⁴Cs , style="text-align:right" , 55 , style="text-align:right" , 69 , 123.912258(9) , 30.9(4) s , β⁺ , ''¹²⁴Xe'' , 1+ , , , - , style="text-indent:1em" , ^124mCs , colspan="3" style="text-indent:2em" , 462.55(17) keV , 6.3(2) s , IT , ¹²⁴Cs , (7)+ , , , - , ¹²⁵Cs , style="text-align:right" , 55 , style="text-align:right" , 70 , 124.909728(8) , 46.7(1) min , β⁺ , ¹²⁵Xe , 1/2(+) , , , - , style="text-indent:1em" , ^125mCs , colspan="3" style="text-indent:2em" , 266.6(11) keV , 900(30) ms , , , (11/2−) , , , - , ¹²⁶Cs , style="text-align:right" , 55 , style="text-align:right" , 71 , 125.909452(13) , 1.64(2) min , β⁺ , ¹²⁶Xe , 1+ , , , - , style="text-indent:1em" , ^126m1Cs , colspan="3" style="text-indent:2em" , 273.0(7) keV , >1 μs , , , , , , - , style="text-indent:1em" , ^126m2Cs , colspan="3" style="text-indent:2em" , 596.1(11) keV , 171(14) μs , , , , , , - , ¹²⁷Cs , style="text-align:right" , 55 , style="text-align:right" , 72 , 126.907418(6) , 6.25(10) h , β⁺ , ¹²⁷Xe , 1/2+ , , , - , style="text-indent:1em" , ^127mCs , colspan="3" style="text-indent:2em" , 452.23(21) keV , 55(3) μs , , , (11/2)− , , , - , ¹²⁸Cs , style="text-align:right" , 55 , style="text-align:right" , 73 , 127.907749(6) , 3.640(14) min , β⁺ , ¹²⁸Xe , 1+ , , , - , ¹²⁹Cs , style="text-align:right" , 55 , style="text-align:right" , 74 , 128.906064(5) , 32.06(6) h , β⁺ , ¹²⁹Xe , 1/2+ , , , - , rowspan=2, ¹³⁰Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 75 , rowspan=2, 129.906709(9) , rowspan=2, 29.21(4) min , β⁺ (98.4%) , ¹³⁰Xe , rowspan=2, 1+ , rowspan=2, , rowspan=2, , - , β⁻ (1.6%) , ''¹³⁰Ba'' , - , rowspan=2 style="text-indent:1em" , ^130mCs , rowspan=2 colspan="3" style="text-indent:2em" , 163.25(11) keV , rowspan=2, 3.46(6) min , IT (99.83%) , ¹³⁰Cs , rowspan=2, 5− , rowspan=2, , rowspan=2, , - , β⁺ (.16%) , ¹³⁰Xe , - , ¹³¹Cs , style="text-align:right" , 55 , style="text-align:right" , 76 , 130.905464(5) , 9.689(16) d , EC , ¹³¹Xe , 5/2+ , , , - , rowspan=2, ¹³²Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 77 , rowspan=2, 131.9064343(20) , rowspan=2, 6.480(6) d , β⁺ (98.13%) , ¹³²Xe , rowspan=2, 2+ , rowspan=2, , rowspan=2, , - , β⁻ (1.87%) , ¹³²Ba , - , ¹³³CsUsed to define the

second The second (symbol: s) is the unit of time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds ...

Fission product Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...

, style="text-align:right" , 55 , style="text-align:right" , 78 , 132.905451933(24) , colspan=3 align=center, StableTheoretically capable of

spontaneous fission Spontaneous fission (SF) is a form of radioactive decay that is found only in very heavy chemical elements. The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 56 (e.g., iron-56); spontaneous breakdo ...

, 7/2+ , 1.0000 , , - , rowspan=2, ¹³⁴Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 79 , rowspan=2, 133.906718475(28) , rowspan=2, 2.0652(4) y , β⁻ , ¹³⁴Ba , rowspan=2, 4+ , rowspan=2, , rowspan=2, , - , EC (3×10⁻⁴%) , ¹³⁴Xe , - , style="text-indent:1em" , ^134mCs , colspan="3" style="text-indent:2em" , 138.7441(26) keV , 2.912(2) h , IT , ¹³⁴Cs , 8− , , , - , ¹³⁵Cs , style="text-align:right" , 55 , style="text-align:right" , 80 , 134.9059770(11) , 2.3 x10⁶ y , β⁻ , ¹³⁵Ba , 7/2+ , , , - , style="text-indent:1em" , ^135mCs , colspan="3" style="text-indent:2em" , 1632.9(15) keV , 53(2) min , IT , ¹³⁵Cs , 19/2− , , , - , ¹³⁶Cs , style="text-align:right" , 55 , style="text-align:right" , 81 , 135.9073116(20) , 13.16(3) d , β⁻ , ¹³⁶Ba , 5+ , , , - , rowspan=2 style="text-indent:1em" , ^136mCs , rowspan=2 colspan="3" style="text-indent:2em" , 518(5) keV , rowspan=2, 19(2) s , β⁻ , ¹³⁶Ba , rowspan=2, 8− , rowspan=2, , rowspan=2, , - , IT , ¹³⁶Cs , - , rowspan=2, ¹³⁷Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 82 , rowspan=2, 136.9070895(5) , rowspan=2, 30.1671(13) y , β⁻ (95%) , ^137mBa , rowspan=2, 7/2+ , rowspan=2, , rowspan=2, , - , β⁻ (5%) , ¹³⁷Ba , - , ¹³⁸Cs , style="text-align:right" , 55 , style="text-align:right" , 83 , 137.911017(10) , 33.41(18) min , β⁻ , ¹³⁸Ba , 3− , , , - , rowspan=2 style="text-indent:1em" , ^138mCs , rowspan=2 colspan="3" style="text-indent:2em" , 79.9(3) keV , rowspan=2, 2.91(8) min , IT (81%) , ¹³⁸Cs , rowspan=2, 6− , rowspan=2, , rowspan=2, , - , β⁻ (19%) , ¹³⁸Ba , - , ¹³⁹Cs , style="text-align:right" , 55 , style="text-align:right" , 84 , 138.913364(3) , 9.27(5) min , β⁻ , ¹³⁹Ba , 7/2+ , , , - , ¹⁴⁰Cs , style="text-align:right" , 55 , style="text-align:right" , 85 , 139.917282(9) , 63.7(3) s , β⁻ , ¹⁴⁰Ba , 1− , , , - , rowspan=2, ¹⁴¹Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 86 , rowspan=2, 140.920046(11) , rowspan=2, 24.84(16) s , β⁻ (99.96%) , ¹⁴¹Ba , rowspan=2, 7/2+ , rowspan=2, , rowspan=2, , - , β⁻, n (.0349%) , ¹⁴⁰Ba , - , rowspan=2, ¹⁴²Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 87 , rowspan=2, 141.924299(11) , rowspan=2, 1.689(11) s , β⁻ (99.9%) , ¹⁴²Ba , rowspan=2, 0− , rowspan=2, , rowspan=2, , - , β⁻, n (.091%) , ¹⁴¹Ba , - , rowspan=2, ¹⁴³Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 88 , rowspan=2, 142.927352(25) , rowspan=2, 1.791(7) s , β⁻ (98.38%) , ¹⁴³Ba , rowspan=2, 3/2+ , rowspan=2, , rowspan=2, , - , β⁻, n (1.62%) , ¹⁴²Ba , - , rowspan=2, ¹⁴⁴Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 89 , rowspan=2, 143.932077(28) , rowspan=2, 994(4) ms , β⁻ (96.8%) , ¹⁴⁴Ba , rowspan=2, 1(−#) , rowspan=2, , rowspan=2, , - , β⁻, n (3.2%) , ¹⁴³Ba , - , rowspan=2 style="text-indent:1em" , ^144mCs , rowspan=2 colspan="3" style="text-indent:2em" , 300(200)# keV , rowspan=2, <1 s , β⁻ , ¹⁴⁴Ba , rowspan=2, (>3) , rowspan=2, , rowspan=2, , - , IT , ¹⁴⁴Cs , - , rowspan=2, ¹⁴⁵Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 90 , rowspan=2, 144.935526(12) , rowspan=2, 582(6) ms , β⁻ (85.7%) , ¹⁴⁵Ba , rowspan=2, 3/2+ , rowspan=2, , rowspan=2, , - , β⁻, n (14.3%) , ¹⁴⁴Ba , - , rowspan=2, ¹⁴⁶Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 91 , rowspan=2, 145.94029(8) , rowspan=2, 0.321(2) s , β⁻ (85.8%) , ¹⁴⁶Ba , rowspan=2, 1− , rowspan=2, , rowspan=2, , - , β⁻, n (14.2%) , ¹⁴⁵Ba , - , rowspan=2, ¹⁴⁷Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 92 , rowspan=2, 146.94416(6) , rowspan=2, 0.235(3) s , β⁻ (71.5%) , ¹⁴⁷Ba , rowspan=2, (3/2+) , rowspan=2, , rowspan=2, , - , β⁻, n (28.49%) , ¹⁴⁶Ba , - , rowspan=2, ¹⁴⁸Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 93 , rowspan=2, 147.94922(62) , rowspan=2, 146(6) ms , β⁻ (74.9%) , ¹⁴⁸Ba , rowspan=2, , rowspan=2, , rowspan=2, , - , β⁻, n (25.1%) , ¹⁴⁷Ba , - , rowspan=2, ¹⁴⁹Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 94 , rowspan=2, 148.95293(21)# , rowspan=2, 150# ms 50 ms, β⁻ , ¹⁴⁹Ba , rowspan=2, 3/2+# , rowspan=2, , rowspan=2, , - , β⁻, n , ¹⁴⁸Ba , - , rowspan=2, ¹⁵⁰Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 95 , rowspan=2, 149.95817(32)# , rowspan=2, 100# ms 50 ms, β⁻ , ¹⁵⁰Ba , rowspan=2, , rowspan=2, , rowspan=2, , - , β⁻, n , ¹⁴⁹Ba , - , rowspan=2, ¹⁵¹Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 96 , rowspan=2, 150.96219(54)# , rowspan=2, 60# ms 50 ms, β⁻ , ¹⁵¹Ba , rowspan=2, 3/2+# , rowspan=2, , rowspan=2, , - , β⁻, n , ¹⁵⁰Ba

Caesium-131

Caesium-131, introduced in 2004 for brachytherapy by

Isoray Isoray Inc. (Isoray) is a national isotope-based medical company and the sole producer of Cesium brachytherapy sources, which are expanding brachytherapy treatments for difficult to treat cancers. Isoray is a registered manufacturer with the FDA ...

, 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 The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all ...

of time, the

, is defined by a specific caesium-133 transition. Since 1967, the official definition of a second is:

Caesium-134

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 Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...

and via neutron capture from nonradioactive ¹³³Cs (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 A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions (thermonuclear bomb), producing a nuclear explosion. Both bomb ...

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).

Caesium-135

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 product Long-lived fission products (LLFPs) are radioactive materials with a long half-life (more than 200,000 years) produced by nuclear fission of uranium and plutonium. Because of their persistent radiotoxicity it is necessary to isolate them from man ...

s and the only alkaline one. In most types of

nuclear reprocessing Nuclear reprocessing is the chemical separation of fission products and actinides from spent nuclear fuel. Originally, reprocessing was used solely to extract plutonium for producing nuclear weapons. With commercialization of nuclear power, the ...

, it stays with the medium-lived fission products (including which can only be separated from Cs-135 via isotope separation) rather than with other long-lived fission products. The low decay energy, lack of

gamma radiation A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically s ...

, 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) will be converted to extremely long-lived (half-life on the order of 10²¹ years) before it can decay to despite the relatively short half life of . Little or no 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 The iodine pit, also called the iodine hole or xenon pit, is a temporary disabling of a nuclear reactor due to buildup of short- lived nuclear poisons in the reactor core. The main isotope responsible is 135Xe, mainly produced by natural decay of ...

is a phenomenon of excess neutron absorption through buildup in the reactor after a reduction in power or a shutdown and is often managed by letting the decay away to a level at which neutron flux can be safely controlled via

control rod Control rods are used in nuclear reactors to control the rate of fission of the nuclear fuel – uranium or plutonium. Their compositions include chemical elements such as boron, cadmium, silver, hafnium, or indium, that are capable of absorbing ...

s 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 and respectively. Disposal of ¹³⁵Cs by

nuclear transmutation Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed. A transmutatio ...

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.
ANL factsheet

Caesium-136

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

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 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

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and is a major health concern for decontaminating land near the

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nuclear power plant. 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 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

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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.

References

* Isotope masses from: ** * Isotopic compositions and standard atomic masses from: ** ** * Half-life, spin, and isomer data selected from the following sources. ** ** ** {{Authority control