Caesium Caesium (IUPAC spelling; also spelled cesium in American English) is a chemical element; it has Symbol (chemistry), 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 f ...

(₅₅Cs) has 41 known

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

s, ranging in

mass number The mass number (symbol ''A'', from the German word: ''Atomgewicht'', "atomic weight"), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It is appro ...

from 112 to 152. Only one isotope, ¹³³Cs, is stable. The longest-lived

radioisotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable. This excess energy can be used in one of three ...

s are ¹³⁵Cs with a half-life of 1.33 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 the performance of nuclear weapons and the effects of Nuclear explosion, their explosion. Nuclear testing is a sensitive political issue. Governments have often performed tests to si ...

, caesium radioisotopes were released into the

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

, 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, 111.95017(12)# , 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) , 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, 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, 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, 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, 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) , 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 , 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, 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, 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 , , , , (6+) , , -id=Caesium-118m3 , style="text-indent:1em" , ^118m3Cs , colspan="3" style="text-indent:2em" , 65.9 keV , , IT , ¹¹⁸Cs , (3−) , , -id=Caesium-118m4 , style="text-indent:1em" , ^118m4Cs , colspan="3" style="text-indent:2em" , 125.9+X keV , 550(60) ns , IT , ^118m1Cs , (7+) , , -id=Caesium-118m5 , style="text-indent:1em" , ^118m5Cs , colspan="3" style="text-indent:2em" , 195.2+X keV , <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, 43.0(2) s , β⁺ , ¹¹⁹Xe , rowspan=2, 9/2+ , rowspan=2, , - , β⁺, α (<2×10⁻⁶%) , ¹¹⁵Te , -id=Caesium-119m , style="text-indent:1em" , ^119mCs , colspan="3" style="text-indent:2em" , 50(30)# keV , 30.4(1) 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, 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, 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) , 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, 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, 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 , >1 μs , IT , ¹²²Cs , 3+ , , -id=Caesium-122m2 , style="text-indent:1em" , ^122m2Cs , colspan="3" style="text-indent:2em" , 140(30) keV , 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 , 360(20) ms , IT , ¹²²Cs , 5− , , -id=Caesium-123 , ¹²³Cs , style="text-align:right" , 55 , style="text-align:right" , 68 , 122.912996(13) , 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 , 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 , 114(5) ns , IT , ¹²³Cs , (9/2+) , , -id=Caesium-124 , ¹²⁴Cs , style="text-align:right" , 55 , style="text-align:right" , 69 , 123.9122474(98) , 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, 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) , 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 , 900(30) ms , IT , ¹²⁵Cs , (11/2−) , , -id=Caesium-126 , ¹²⁶Cs , style="text-align:right" , 55 , style="text-align:right" , 71 , 125.909446(11) , 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 , ~1 μs , IT , ¹²⁶Cs , (4−) , , -id=Caesium-126m2 , style="text-indent:1em" , ^126m2Cs , colspan="3" style="text-indent:2em" , 596.1(11) keV , 171(14) μs , IT , ¹²⁶Cs , 8−# , , -id=Caesium-127 , ¹²⁷Cs , style="text-align:right" , 55 , style="text-align:right" , 72 , 126.9074175(60) , 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 , 55(3) μs , IT , ¹²⁷Cs , (11/2)− , , -id=Caesium-128 , ¹²⁸Cs , style="text-align:right" , 55 , style="text-align:right" , 73 , 127.9077485(57) , 3.640(14) min , β⁺ , ¹²⁸Xe , 1+ , , -id=Caesium-129 , ¹²⁹Cs , style="text-align:right" , 55 , style="text-align:right" , 74 , 128.9060659(49) , 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 , 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, 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, 3.46(6) min , IT (99.84%) , ¹³⁰Cs , rowspan=2, 5− , rowspan=2, , - , β⁺ (0.16%) , ¹³⁰Xe , - , ¹³¹Cs , style="text-align:right" , 55 , style="text-align:right" , 76 , 130.90546846(19) , 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, 6.480(6) d , β⁺ (98.13%) , ¹³²Xe , rowspan=2, 2+ , rowspan=2, , - , β⁻ (1.87%) , ¹³²Ba , - , ¹³³CsUsed to define the

second The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...

Fission product , style="text-align:right" , 55 , style="text-align:right" , 78 , 132.905451958(8) , colspan=3 align=center, Stable , 7/2+ , 1.0000 , - , rowspan=2, ¹³⁴Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 79 , rowspan=2, 133.906718501(17) , 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 , 2.912(2) h , IT , ¹³⁴Cs , 8− , , - , ¹³⁵Cs Long-lived fission product , style="text-align:right" , 55 , style="text-align:right" , 80 , 134.90597691(39) , 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 , 53(2) min , IT , ¹³⁵Cs , 19/2− , , - , ¹³⁶Cs , style="text-align:right" , 55 , style="text-align:right" , 81 , 135.9073114(20) , 13.01(5) 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) keV , rowspan=2, 17.5(2) s , β⁻? , ¹³⁶Ba , rowspan=2, 8− , rowspan=2, , - , IT? , ¹³⁶Cs , - , rowspan=2, ¹³⁷Cs , rowspan=2 style="text-align:right" , 55 , rowspan=2 style="text-align:right" , 82 , rowspan=2, 136.90708930(32) , rowspan=2, 30.04(4) y , β⁻ (94.70%) , ^137mBa , rowspan=2, 7/2+ , rowspan=2, , - , β⁻ (5.30%) , ¹³⁷Ba , -id=Caesium-138 , ¹³⁸Cs , style="text-align:right" , 55 , style="text-align:right" , 83 , 137.9110171(98) , 33.5(2) 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) keV , rowspan=2, 2.91(10) 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) , 9.27(5) min , β⁻ , ¹³⁹Ba , 7/2+ , , -id=Caesium-140 , ¹⁴⁰Cs , style="text-align:right" , 55 , style="text-align:right" , 85 , 139.9172837(88) , 63.7(3) s , β⁻ , ¹⁴⁰Ba , 1− , , -id=Caesium-140m , style="text-indent:1em" , ^140mCs , colspan="3" style="text-indent:2em" , 13.931(21) keV , 471(51) 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, 24.84(16) 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, 1.687(10) 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, 1.802(8) 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, 994(6) 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) keV , 1.1(1) 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, 582(4) 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) keV , 0.5(1) 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, 321.6(9) 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) keV , 1.25(5) 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, 230.5(9) 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) keV , 190(20) 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, 151.8(10) 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) keV , 4.8(2) 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, 112.3(25) 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, 81.0(26) 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)# , 59(19) ms , β⁻ , ¹⁵¹Ba , 3/2+# ,

Caesium-131

Caesium-131, introduced in 2004 for

brachytherapy Brachytherapy is a form of radiation therapy where a sealed radiation, radiation source is placed inside or next to the area requiring treatment. The word "brachytherapy" comes from the Ancient Greek, Greek word , meaning "short-distance" or "s ...

by Isoray, has a

half-life Half-life is a mathematical and scientific description of exponential or gradual decay. Half-life, half life or halflife may also refer to: Film * Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang * ''Half Life: ...

of 9.7 days and 30.4 keV energy.

Caesium-133

Caesium-133 is the only stable

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 al ...

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

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, wh ...

from nonradioactive ¹³³Cs (neutron capture cross section 29 barns), 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 an atomic nucleus emits a beta particle (fast energetic electron or positron), transforming into an isobar of that nuclide. For example, beta decay of a neutron ...

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 nuclear fission, fission (fission or atomic bomb) or a combination of fission and nuclear fusion, fusion reactions (thermonuclear weap ...

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 ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...

photons (mean energy 0.698 MeV).

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 conside ...

isotope of caesium with a half-life of 1.33 million years. It decays via emission of a low-energy beta particle into the stable isotope barium-135. Caesium-135 is one of the seven long-lived fission products and the only alkaline one. In most types of

nuclear reprocessing 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 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. Except in the Molten salt reactor, where is created as a completely separate stream outside the fuel (after the decay of bubble-separated ). 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 energ ...

, lack of

gamma radiation A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...

, 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 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 wit ...

s) but also has the highest known

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

, 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 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 transmutat ...

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 13.01 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, which is a common fission product. 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 (), producing ¹³⁶Ba directly.

Caesium-137

Caesium-137, with a half-life of 30.17 years, is one of the two principal

medium-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 Ionizing radiation, radiotoxicity, it is necessary to is ...

s, along with ⁹⁰Sr, which are responsible for most of the

radioactivity 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 conside ...

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 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 excited state levels (higher energy levels). "Metastable" describes nuclei whose excited states have Half-life, half-lives of ...

) then to nonradioactive barium-137. Caesium-137 does not emit gamma radiation directly, all observed radiation is due to the daughter isotope barium-137m. 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 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: ** * Half-life, spin, and isomer data selected from the following sources. ** ** ** {{Authority control