Caesium (₅₅Cs) has 40 known

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numb ...

s, making it, along with barium and mercury, one of the elements with the most isotopes. The

atomic mass The atomic mass (''m''a or ''m'') is the mass of an atom. Although the SI unit of mass is the kilogram (symbol: kg), atomic mass is often expressed in the non-SI unit dalton (symbol: Da) – equivalently, unified atomic mass unit (u). 1&nb ...

es of these isotopes range from 112 to 151. Only one isotope, ¹³³Cs, is stable. The longest-lived

radioisotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...

s 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, 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 Nuclear fallout is the residual radioactive material propelled into the upper atmosphere following a nuclear blast, so called because it "falls out" of the sky after the explosion and the shock wave has passed. It commonly refers to the radioac ...

. 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 Fission product , style="text-align:right" , 55 , style="text-align:right" , 78 , 132.905451933(24) , colspan=3 align=center, StableTheoretically capable of spontaneous fission , 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 Brachytherapy is a form of radiation therapy where a sealed radiation source is placed inside or next to the area requiring treatment. ''Brachy'' is Greek for short. Brachytherapy is commonly used as an effective treatment for cervical, prost ...

by Isoray, has a

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable at ...

of 9.7 days and 30.4 keV energy.

Caesium-133

Caesium-133 is the only stable

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:

Caesium-134

Caesium-134 has a

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 Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, ...

from nonradioactive ¹³³Cs (neutron capture

cross section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Abs ...

barns A barn is an agricultural building usually on farms and used for various purposes. In North America, a barn refers to structures that house livestock, including cattle and horses, as well as equipment and fodder, and often grain.Allen G. ...

), which is a common fission product. Caesium-134 is not produced via

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...

of other fission product

nuclides A nuclide (or nucleide, from nucleus, also known as nuclear species) is a class of atoms characterized by their number of protons, ''Z'', their number of neutrons, ''N'', and their nuclear energy state. The word ''nuclide'' was coined by Truman ...

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

(β⁻), producing ¹³⁴Ba directly and emitting on average 2.23

gamma ray 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 ...

photons (mean energy 0.698

MeV In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacu ...

Caesium-135

Caesium-135 is a mildly

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...

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 products and the only alkaline one. In most types of nuclear reprocessing, it stays with the medium-lived fission products (including which can only be separated from Cs-135 via

isotope separation Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research (e.g. in chemistry where atoms of "marker" n ...

) rather than with other long-lived fission products. The low

decay energy The decay energy is the energy change of a nucleus having undergone a radioactive decay. Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting ionizing particles and radiation. This decay, or loss of 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 Nuclear fission splits a heavy nucleus such as uranium or plutonium into two lighter nuclei, which are called fission products. Yield refers to the fraction of a fission product produced per fission. Yield can be broken down by: # Individual ...

(e.g. 6.3333% for ²³⁵U and

thermal neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium wi ...

s) but also has the highest known

capture cross section of any nuclide. Because of this, much of the ¹³⁵Xe produced in current

thermal reactor A thermal-neutron reactor is a nuclear reactor that uses slow or thermal neutrons. ("Thermal" does not mean hot in an absolute sense, but means in thermal equilibrium with the medium it is interacting with, the reactor's fuel, moderator and struct ...

s (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 A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. Only two MSRs have ever operated, both research reactors in the United States. The 1950's ...

that continuously removes xenon from its fuel, a

fast neutron reactor A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons (carrying energies above 1 MeV or greater, on average), as opposed ...

, or a nuclear weapon. The xenon pit 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 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 Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, ...

of ¹³⁵Cs are and respectively. 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.
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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 Beta-decay stable isobar (nuclide), isobars are the set of nuclides which cannot undergo beta decay, that is, the transformation of a neutron to a proton or a proton to a neutron within the Atomic nucleus, nucleus. A subset of these nuclides are ...

) 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 Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and ...

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 may refer to: Japan * Fukushima Prefecture, Japanese prefecture ** Fukushima, Fukushima, capital city of Fukushima Prefecture, Japan ***Fukushima University, national university in Japan *** Fukushima Station (Fukushima) in Fukushima, Fukushim ...

nuclear power plant. ¹³⁷Cs beta decays to barium-137m (a short-lived

nuclear isomer A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy higher energy levels than in the ground state of the same nucleus. "Metastable" describes nuclei whose excited states have ...

) then to nonradioactive

barium-137 Naturally occurring barium (56Ba) is a mix of six stable isotopes and one very long-lived radioactive primordial isotope, barium-130, identified as being unstable by geochemical means (from analysis of the presence of its daughter xenon-130 in roc ...

, 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 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 There are 37 known isotopes of iodine (53I) from 108I to 144I; all undergo radioactive decay except 127I, which is stable. Iodine is thus a monoisotopic element. Its longest-lived radioactive isotope, 129I, has a half-life of 15.7 million yea ...

then

isotopes of xenon Naturally occurring xenon (54Xe) consists of seven stable isotopes and two very long-lived isotopes. Double electron capture has been observed in 124Xe (half-life ) and double beta decay in 136Xe (half-life ), which are among the longest measured ...

. 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 Caesium