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Isotopes of caesium

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Caesium (55Cs) has 41 known isotopes, the atomic masses of these isotopes range 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, 137
Cs
 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.

Isotopes of caesium (55Cs)
Main isotopes[1] Decay
abun­dance half-life (t1/2) mode pro­duct
131Cs synth 9.7 d ε 131Xe
133Cs 100% stable
134Cs synth 2.0648 y ε 134Xe
β− 134Ba
135Cs trace 1.33×106 y β− 135Ba
137Cs synth 30.17 y[2] β− 137Ba
Standard atomic weight Ar°(Cs)

Beginning in 1945 with the commencement of nuclear testing, caesium radioisotopes were released into the atmosphere where caesium is absorbed readily into solution and is returned to the surface of the Earth as a component of radioactive fallout. Once caesium enters the ground water, it is deposited on soil surfaces and removed from the landscape primarily by particle transport. As a result, the input function of these isotopes can be estimated as a function of time.

List of isotopes

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Nuclide
[n 1] 		Z N Isotopic mass (Da)[5]
[n 2][n 3] 		Half-life[1]
 Decay
mode[1]
[n 4] 		Daughter
isotope
[n 5][n 6] 		Spin and
parity[1]
[n 7][n 8] 		Isotopic
abundance
Excitation energy[n 8]
112Cs 55 57 111.95017(12)# 490(30) μs p (>99.74%) 111Xe 1+#
α (<0.26%) 108I
113Cs 55 58 112.9444285(92) 16.94(9) μs p 112Xe (3/2+)
114Cs 55 59 113.941292(91) 570(20) ms β+ (91.1%) 114Xe (1+)
β+, p (8.7%) 113I
β+, α (0.19%) 110Te
α (0.018%) 110I
115Cs 55 60 114.93591(11)# 1.4(8) s β+ (99.93%) 115Xe 9/2+#
β+, p (0.07%) 114I
116Cs 55 61 115.93340(11)# 700(40) ms β+ (99.67%) 116Xe (1+)
β+, p (0.28%) 115I
β+, α (0.049%) 112Te
116mCs[n 9] 100(60)# keV 3.85(13) s β+ (99.56%) 116Xe (7+)
β+, p (0.44%) 115I
β+, α (0.0034%) 112Te
117Cs 55 62 116.928617(67) 8.4(6) s β+ 117Xe 9/2+#
117mCs[n 9] 150(80)# keV 6.5(4) s β+ 117Xe 3/2+#
118Cs 55 63 117.926560(14) 14(2) s β+ (99.98%) 118Xe 2+
β+, p (0.021%) 117I
β+, α (0.0012%) 114Te
118mCs[n 9] 100(60)# keV 17(3) s β+ (99.98%) 118Xe (7−)
β+, p (0.021%) 117I
β+, α (0.0012%) 114Te
119Cs 55 64 118.9223 77(15) 43.0(2) s β+ 119Xe 9/2+
β+, α (<2×10−6%) 115Te
119mCs[n 9] 50(30)# keV 30.4(1) s β+ 119Xe 3/2+
120Cs 55 65 119.920677(11) 60.4(6) s β+ 120Xe 2+
β+, α (<2×10−5%) 116Te
β+, p (<7×10−6%) 119I
120mCs[n 9] 100(60)# keV 57(6) s β+ 120Xe (7−)
β+, α (<2×10−5%) 116Te
β+, p (<7×10−6%) 119I
121Cs 55 66 120.917227(15) 155(4) s β+ 121Xe 3/2+
121mCs 68.5(3) keV 122(3) s β+ (83%) 121Xe 9/2+
IT (17%) 121Cs
122Cs 55 67 121.916108(36) 21.18(19) s β+ 122Xe 1+
β+, α (<2×10−7%) 118Te
122m1Cs 45.87(12) keV >1 μs IT 122Cs 3+
122m2Cs 140(30) keV 3.70(11) min β+ 122Xe 8−
122m3Cs 127.07(16) keV 360(20) ms IT 122Cs 5−
123Cs 55 68 122.912996(13) 5.88(3) min β+ 123Xe 1/2+
123m1Cs 156.27(5) keV 1.64(12) s IT 123Cs 11/2−
123m2Cs 252(6) keV 114(5) ns IT 123Cs (9/2+)
124Cs 55 69 123.9122474(98) 30.9(4) s β+ 124Xe 1+
124mCs 462.63(14) keV 6.41(7) s IT (99.89%) 124Cs (7)+
β+ (0.11%) 124Xe
125Cs 55 70 124.9097260(83) 44.35(29) min β+ 125Xe 1/2+
125mCs 266.1(11) keV 900(30) ms IT 125Cs (11/2−)
126Cs 55 71 125.909446(11) 1.64(2) min β+ 126Xe 1+
126m1Cs 273.0(7) keV ~1 μs IT 126Cs (4−)
126m2Cs 596.1(11) keV 171(14) μs IT 126Cs 8−#
127Cs 55 72 126.9074175(60) 6.25(10) h β+ 127Xe 1/2+
127mCs 452.23(21) keV 55(3) μs IT 127Cs (11/2)−
128Cs 55 73 127.9077485(57) 3.640(14) min β+ 128Xe 1+
129Cs 55 74 128.9060659(49) 32.06(6) h β+ 129Xe 1/2+
129mCs 575.40(14) keV 718(21) ns IT 127Cs (11/2−)
130Cs 55 75 129.9067093(90) 29.21(4) min β+ (98.4%) 130Xe 1+
β− (1.6%) 130Ba
130mCs 163.25(11) keV 3.46(6) min IT (99.84%) 130Cs 5−
β+ (0.16%) 130Xe
131Cs 55 76 130.90546846(19) 9.689(16) d EC 131Xe 5/2+
132Cs 55 77 131.9064378(11) 6.480(6) d β+ (98.13%) 132Xe 2+
β− (1.87%) 132Ba
133Cs[n 10][n 11] 55 78 132.905451958(8) Stable 7/2+ 1.0000
134Cs[n 11] 55 79 133.906718501(17) 2.0650(4) y β− 134Ba 4+
EC (3.0×10−4%) 134Xe
134mCs 138.7441(26) keV 2.912(2) h IT 134Cs 8−
135Cs[n 11] 55 80 134.90597691(39) 1.33(19)×106 y β− 135Ba 7/2+
135mCs 1632.9(15) keV 53(2) min IT 135Cs 19/2−
136Cs 55 81 135.9073114(20) 13.01(5) d β− 136Ba 5+
136mCs 517.9(1) keV 17.5(2) s β−? 136Ba 8−
IT? 136Cs
137Cs[n 11] 55 82 136.90708930(32) 30.04(4) y β− (94.70%)[6] 137mBa 7/2+
β− (5.30%)[6] 137Ba
138Cs 55 83 137.9110171(98) 33.5(2) min β− 138Ba 3−
138mCs 79.9(3) keV 2.91(10) min IT (81%) 138Cs 6−
β− (19%) 138Ba
139Cs 55 84 138.9133638(34) 9.27(5) min β− 139Ba 7/2+
140Cs 55 85 139.9172837(88) 63.7(3) s β− 140Ba 1−
140mCs 13.931(21) keV 471(51) ns IT 140Cs (2)−
141Cs 55 86 140.9200453(99) 24.84(16) s β− (99.97%) 141Ba 7/2+
β−, n (0.0342%) 140Ba
142Cs 55 87 141.9242995(76) 1.687(10) s β− (99.91%) 142Ba 0−
β−, n (0.089%) 141Ba
143Cs 55 88 142.9273473(81) 1.802(8) s β− (98.38%) 143Ba 3/2+
β−, n (1.62%) 142Ba
144Cs 55 89 143.932075(22) 994(6) ms β− (97.02%) 144Ba 1−
β−, n (2.98%) 143Ba
144mCs 92.2(5) keV 1.1(1) μs IT 144Cs (4−)
145Cs 55 90 144.9355289(97) 582(4) ms β− (87.2%) 145Ba 3/2+
β−, n (12.8%) 144Ba
145mCs 762.9(4) keV 0.5(1) μs IT 145Cs 13/2#
146Cs 55 91 145.9406219(31) 321.6(9) ms β− (85.8%) 146Ba 1−
β−, n (14.2%) 145Ba
146mCs 46.7(1) keV 1.25(5) μs IT 146Cs 4−#
147Cs 55 92 146.9442615(90) 230.5(9) ms β− (71.5%) 147Ba (3/2+)
β−, n (28.5%) 146Ba
147mCs 701.4(4) keV 190(20) ns IT 147Cs 13/2#
148Cs 55 93 147.949639(14) 151.8(10) ms β− (71.3%) 148Ba (2−)
β−, n (28.7%) 147Ba
148mCs 45.2(1) keV 4.8(2) μs IT 148Cs 4−#
149Cs 55 94 148.95352(43)# 112.3(25) ms β− (75%) 149Ba 3/2+#
β−, n (25%) 148Ba
150Cs 55 95 149.95902(43)# 81.0(26) ms β− (~56%) 150Ba (2−)
β−, n (~44%) 149Ba
151Cs 55 96 150.96320(54)# 59(19) ms β− 151Ba 3/2+#
This table header & footer:
  1. ^ mCs – Excited nuclear isomer.
  2. ^ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
  3. ^ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
  4. ^ Modes of decay:
    EC: Electron capture
    IT: Isomeric transition
    n: Neutron emission
    p: Proton emission
  5. ^ Bold italics symbol as daughter – Daughter product is nearly stable.
  6. ^ Bold symbol as daughter – Daughter product is stable.
  7. ^ ( ) spin value – Indicates spin with weak assignment arguments.
  8. ^ a b # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  9. ^ a b c d e Order of ground state and isomer is uncertain.
  10. ^ Used to define the second
  11. ^ a b c d Fission product

Caesium-131

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Caesium-131, introduced in 2004 for brachytherapy by Isoray,[7] has a half-life of 9.7 days and 30.4 keV energy.

Caesium-133

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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:

The second, symbol s, is defined by taking the fixed numerical value of the caesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom,[8] to be 9192631770 Hz, which is equal to s−1.

Caesium-134

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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 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 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).[9]

Caesium-135

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Long-lived fission products
Nuclide t1⁄2 Yield Q[a 1] βγ
(Ma) (%)[a 2] (keV)
99Tc 0.211 6.1385 294 β
126Sn 0.230 0.1084 4050[a 3] βγ
79Se 0.327 0.0447 151 β
135Cs 1.33  6.9110[a 4] 269 β
93Zr 1.53  5.4575 91 βγ
107Pd 6.5   1.2499 33 β
129I 16.14   0.8410 194 βγ
  1. ^ Decay energy is split among β, neutrino, and γ if any.
  2. ^ Per 65 thermal neutron fissions of 235U and 35 of 239Pu.
  3. ^ Has decay energy 380 keV, but its decay product 126Sb has decay energy 3.67 MeV.
  4. ^ Lower in thermal reactors because 135Xe, its predecessor, readily absorbs neutrons.

Caesium-135 is a mildly radioactive 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, it stays with the medium-lived fission products (including 137
Cs which can only be separated from 135
Cs via isotope separation) rather than with other long-lived fission products. Except in the Molten salt reactor, where 135
Cs is created as a completely separate stream outside the fuel (after the decay of bubble-separated 135
Cs). 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)[10] will be converted to extremely long-lived (half-life on the order of 1021 years) 136
Xe before it can decay to 135
Cs despite the relatively short half life of 135
Xe. Little or no 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 135
Xe buildup in the reactor after a reduction in power or a shutdown and is often managed by letting the 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 8.3 ± 0.3 and 38.1 ± 2.6 barns respectively.[11] 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.[12]

Caesium-136

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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

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Main article: 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 Chernobyl accident and is a major health concern for decontaminating land near the Fukushima nuclear power plant.[13] 137Cs beta decays to barium-137m (a short-lived nuclear isomer) 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

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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

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  1. ^ a b c d Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
  2. ^ "NIST Radionuclide Half-Life Measurements". NIST. Retrieved 2011-03-13.
  3. ^ "Standard Atomic Weights: Caesium". CIAAW. 2013.
  4. ^ Prohaska, Thomas; Irrgeher, Johanna; Benefield, Jacqueline; Böhlke, John K.; Chesson, Lesley A.; Coplen, Tyler B.; Ding, Tiping; Dunn, Philip J. H.; Gröning, Manfred; Holden, Norman E.; Meijer, Harro A. J. (2022-05-04). "Standard atomic weights of the elements 2021 (IUPAC Technical Report)". Pure and Applied Chemistry. doi:10.1515/pac-2019-0603. ISSN 1365-3075.
  5. ^ Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*". Chinese Physics C. 45 (3): 030003. doi:10.1088/1674-1137/abddaf.
  6. ^ a b Browne, E.; Tuli, J.K. (October 2007). "Nuclear Data Sheets for A = 137". Nuclear Data Sheets. 108 (10): 2173–2318. doi:10.1016/j.nds.2007.09.002.
  7. ^ Isoray. "Why Cesium-131". Archived from the original on 2019-06-30. Retrieved 2017-12-05.
  8. ^ Although the phase 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. 130 states: "The effect of this definition is that the second is equal to the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the unperturbed ground state of the 133Cs atom."
  9. ^ "Characteristics of Caesium-134 and Caesium-137". Japan Atomic Energy Agency. Archived from the original on 2016-03-04. Retrieved 2014-10-23.
  10. ^ John L. Groh (2004). "Supplement to Chapter 11 of Reactor Physics Fundamentals" (PDF). CANTEACH project. Archived from the original (PDF) on 10 June 2011. Retrieved 14 May 2011.
  11. ^ Hatsukawa, Y.; Shinohara, N; Hata, K.; et al. (1999). "Thermal neutron cross section and resonance integral of the reaction of135Cs(n,γ)136Cs: Fundamental data for the transmutation of nuclear waste". Journal of Radioanalytical and Nuclear Chemistry. 239 (3): 455–458. doi:10.1007/BF02349050. S2CID 97425651.
  12. ^ Ohki, Shigeo; Takaki, Naoyuki (2002). "Transmutation of Cesium-135 With Fast Reactors" (PDF). Proceedings of the Seventh Information Exchange Meeting on Actinide and Fission Product Partitioning & Transmutation, Cheju, Korea.
  13. ^ Dennis (1 March 2013). "Cooling a Hot Zone". Science. 339 (6123): 1028–1029. doi:10.1126/science.339.6123.1028. PMID 23449572.
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