View source for Isotopes of caesium

{{Short description|none}}
{{Infobox caesium isotopes}}
[[Caesium]] (<sub>55</sub>Cs) has 41 known [[isotope]]s, ranging in [[mass number]] from 112 to 152. Only one isotope, <sup>133</sup>Cs, is stable.<ref name=":0">{{Cite web |title=NNDC {{!}} National Nuclear Data Center |url=https://www.nndc.bnl.gov/ |access-date=2025-02-22 |website=www.nndc.bnl.gov}}</ref> The longest-lived [[radioisotope]]s are <sup>135</sup>Cs with a half-life of 1.33 million years, {{SimpleNuclide|Caesium|137|link=y}} with a half-life of 30.1671 years and <sup>134</sup>Cs with a half-life of 2.0652 years.<ref name="134Cs-JAEA" /> 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 [[Earth's atmosphere|atmosphere]], where caesium is absorbed readily into solution and is returned to the surface of the Earth as a component of [[radioactive fallout]]. Once caesium enters the ground water, it is deposited on soil surfaces and removed from the landscape primarily by particle transport. As a result, the input function{{clarify|date=April 2025}} of these isotopes can be estimated as a function of time.

== List of isotopes ==
{{Anchor|Caesium-111|Caesium-144m1|Caesium-144m2|Caesium-152}}

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

{{Isotopes table
| symbol = Cs
| refs = NUBASE2020, AME2020 II<!-- updated 2024–09–29 -->
| notes = m, unc(), mass#, exen#, spin(), spin#, daughter-nst, daughter-st, p, IT, EC, n
}}
|-id=Caesium-112
| rowspan=2|<sup>112</sup>Cs
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 57
| rowspan=2|111.95017(12)#
| rowspan=2|490(30)&nbsp;μs
| [[proton emission|p]] (>99.74%)
| <sup>111</sup>Xe
| rowspan=2|1+#
| rowspan=2|
|-
| [[alpha decay|α]] (<0.26%)
| <sup>108</sup>I
|-id=Caesium-113
| <sup>113</sup>Cs
| style="text-align:right" | 55
| style="text-align:right" | 58
| 112.9444285(92)
| 16.94(9)&nbsp;μs
| p
| <sup>112</sup>Xe
| (3/2+)
| 
|-id=Caesium-114
| rowspan=4|<sup>114</sup>Cs
| rowspan=4 style="text-align:right" | 55
| rowspan=4 style="text-align:right" | 59
| rowspan=4|113.941292(91)
| rowspan=4|570(20)&nbsp;ms
| β<sup>+</sup> (91.1%)
| <sup>114</sup>Xe
| rowspan=4|(1+)
| rowspan=4|
|-
| β<sup>+</sup>, p (8.7%)
| <sup>113</sup>I
|-
| β<sup>+</sup>, α (0.19%)
| <sup>110</sup>Te
|-
| α (0.018%)
| <sup>110</sup>I
|-id=Caesium-115
| rowspan=2|<sup>115</sup>Cs
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 60
| rowspan=2|114.93591(11)#
| rowspan=2|1.4(8)&nbsp;s
| β<sup>+</sup> (99.93%)
| <sup>115</sup>Xe
| rowspan=2|9/2+#
| rowspan=2|
|-
| β<sup>+</sup>, p (0.07%)
| <sup>114</sup>I
|-id=Caesium-116
| rowspan=3|<sup>116</sup>Cs
| rowspan=3 style="text-align:right" | 55
| rowspan=3 style="text-align:right" | 61
| rowspan=3|115.93340(11)#
| rowspan=3|700(40)&nbsp;ms
| β<sup>+</sup> (99.67%)
| <sup>116</sup>Xe
| rowspan=3|(1+)
| rowspan=3|
|-
| β<sup>+</sup>, p (0.28%)
| <sup>115</sup>I
|-
| β<sup>+</sup>, α (0.049%)
| <sup>112</sup>Te
|-id=Caesium-116m
| rowspan=3 style="text-indent:1em" | <sup>116m</sup>Cs<ref group="n" name="order">Order of ground state and isomer is uncertain.</ref>
| rowspan=3 colspan="3" style="text-indent:2em" | 100(60)#&nbsp;keV
| rowspan=3|3.85(13)&nbsp;s
| β<sup>+</sup> (99.56%)
| <sup>116</sup>Xe
| rowspan=3|(7+)
| rowspan=3|
|-
| β<sup>+</sup>, p (0.44%)
| <sup>115</sup>I
|-
| β<sup>+</sup>, α (0.0034%)
| <sup>112</sup>Te
|-id=Caesium-117
| <sup>117</sup>Cs
| style="text-align:right" | 55
| style="text-align:right" | 62
| 116.928617(67)
| 8.4(6)&nbsp;s
| β<sup>+</sup>
| <sup>117</sup>Xe
| 9/2+#
|
|-id=Caesium-117m
| style="text-indent:1em" | <sup>117m</sup>Cs<ref group="n" name="order"/>
| colspan="3" style="text-indent:2em" | 150(80)#&nbsp;keV
| 6.5(4)&nbsp;s
| β<sup>+</sup>
| <sup>117</sup>Xe
| 3/2+#
|
|-id=Caesium-118
| rowspan=3|<sup>118</sup>Cs
| rowspan=3 style="text-align:right" | 55
| rowspan=3 style="text-align:right" | 63
| rowspan=3|117.926560(14)
| rowspan=3|14(2)&nbsp;s
| β<sup>+</sup> (99.98%)
| <sup>118</sup>Xe
| rowspan=3|2(−)<ref name="zheng2021">{{cite journal |last1=Zheng |first1=K. K. |last2=Petrache |first2=C. M. |last3=Zhang |first3=Z. H. |last4=Astier |first4=A. |last5=Lv |first5=B. F. |last6=Greenlees |first6=P. T. |last7=Grahn |first7=T. |last8=Julin |first8=R. |last9=Juutinen |first9=S. |last10=Luoma |first10=M. |last11=Ojala |first11=J. |last12=Pakarinen |first12=J. |last13=Partanen |first13=J. |last14=Rahkila |first14=P. |last15=Ruotsalainen |first15=P. |last16=Sandzelius |first16=M. |last17=Sarén |first17=J. |last18=Tann |first18=H. |last19=Uusitalo |first19=J. |last20=Zimba |first20=G. |last21=Cederwall |first21=B. |last22=Aktas |first22=ö. |last23=Ertoprak |first23=A. |last24=Zhang |first24=W. |last25=Guo |first25=S. |last26=Liu |first26=M. L. |last27=Zhou |first27=X. H. |last28=Kuti |first28=I. |last29=Nyakó |first29=B. M. |last30=Sohler |first30=D. |last31=Timár |first31=J. |last32=Andreoiu |first32=C. |last33=Doncel |first33=M. |last34=Joss |first34=D. T. |last35=Page |first35=R. D. |title=Rich band structure and multiple long-lived isomers in the odd-odd Cs 118 nucleus |journal=Physical Review C |date=21 October 2021 |volume=104 |issue=4 |page=044325 |doi=10.1103/PhysRevC.104.044325 |url=https://www.researchgate.net/publication/355502046 |access-date=29 December 2024}}</ref>
| rowspan=3|
|-
| β<sup>+</sup>, p (0.021%)
| <sup>117</sup>I
|-
| β<sup>+</sup>, α (0.0012%)
| <sup>114</sup>Te
|-id=Caesium-118m1
| rowspan=3 style="text-indent:1em" | <sup>118m1</sup>Cs<ref name="zheng2021"/><ref group="n" name="order"/>
| rowspan=3 colspan="3" style="text-indent:2em" | X&nbsp;keV
| rowspan=3|17(3)&nbsp;s
| β<sup>+</sup> (99.98%)
| <sup>118</sup>Xe
| rowspan=3|(7−)
| rowspan=3|
|-
| β<sup>+</sup>, p (0.021%)
| <sup>117</sup>I
|-
| β<sup>+</sup>, α (0.0012%)
| <sup>114</sup>Te
|-id=Caesium-118m2
| style="text-indent:1em" | <sup>118m2</sup>Cs<ref name="zheng2021"/><ref group="n" name="order"/>
| colspan="3" style="text-indent:2em" | Y&nbsp;keV
| 
| 
| 
| (6+)
|
|-id=Caesium-118m3
| style="text-indent:1em" | <sup>118m3</sup>Cs<ref name="zheng2021"/><ref group="n" name="order"/>
| colspan="3" style="text-indent:2em" | 65.9&nbsp;keV
| 
| IT
| <sup>118</sup>Cs
| (3−)
|
|-id=Caesium-118m4
| style="text-indent:1em" | <sup>118m4</sup>Cs<ref name="zheng2021"/>
| colspan="3" style="text-indent:2em" | 125.9+X&nbsp;keV
| 550(60)&nbsp;ns
| IT
| <sup>118m1</sup>Cs
| (7+)
|
|-id=Caesium-118m5
| style="text-indent:1em" | <sup>118m5</sup>Cs<ref name="zheng2021"/>
| colspan="3" style="text-indent:2em" | 195.2+X&nbsp;keV
| <500&nbsp;ns
| IT
| <sup>118m4</sup>Cs
| (8+)
|
|-id=Caesium-119
| rowspan=2|<sup>119</sup>Cs
| 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)&nbsp;s
| β<sup>+</sup>
| <sup>119</sup>Xe
| rowspan=2|9/2+
| rowspan=2|
|-
| β<sup>+</sup>, α (<2×10<sup>−6</sup>%)
| <sup>115</sup>Te
|-id=Caesium-119m
| style="text-indent:1em" | <sup>119m</sup>Cs<ref group="n" name="order"/>
| colspan="3" style="text-indent:2em" | 50(30)#&nbsp;keV
| 30.4(1)&nbsp;s
| β<sup>+</sup>
| <sup>119</sup>Xe
| 3/2+
|
|-id=Caesium-120
| rowspan=3|<sup>120</sup>Cs
| rowspan=3 style="text-align:right" | 55
| rowspan=3 style="text-align:right" | 65
| rowspan=3|119.920677(11)
| rowspan=3|60.4(6)&nbsp;s
| β<sup>+</sup>
| <sup>120</sup>Xe
| rowspan=3|2+
| rowspan=3|
|-
| β<sup>+</sup>, α (<2×10<sup>−5</sup>%)
| <sup>116</sup>Te
|-
| β<sup>+</sup>, p (<7×10<sup>−6</sup>%)
| <sup>119</sup>I
|-id=Caesium-120m
| rowspan=3 style="text-indent:1em" | <sup>120m</sup>Cs<ref group="n" name="order"/>
| rowspan=3 colspan="3" style="text-indent:2em" | 100(60)#&nbsp;keV
| rowspan=3|57(6)&nbsp;s
| β<sup>+</sup>
| <sup>120</sup>Xe
| rowspan=3|(7−)
| rowspan=3|
|-
| β<sup>+</sup>, α (<2×10<sup>−5</sup>%)
| <sup>116</sup>Te
|-
| β<sup>+</sup>, p (<7×10<sup>−6</sup>%)
| <sup>119</sup>I
|-id=Caesium-121
| <sup>121</sup>Cs
| style="text-align:right" | 55
| style="text-align:right" | 66
| 120.917227(15)
| 155(4)&nbsp;s
| β<sup>+</sup>
| <sup>121</sup>Xe
| 3/2+
|
|-id=Caesium-121m
| rowspan=2 style="text-indent:1em" | <sup>121m</sup>Cs
| rowspan=2 colspan="3" style="text-indent:2em" | 68.5(3)&nbsp;keV
| rowspan=2|122(3)&nbsp;s
| β<sup>+</sup> (83%)
| <sup>121</sup>Xe
| rowspan=2|9/2+
| rowspan=2|
|-
| [[Isomeric transition|IT]] (17%)
| <sup>121</sup>Cs
|-id=Caesium-122
| rowspan=2|<sup>122</sup>Cs
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 67
| rowspan=2|121.916108(36)
| rowspan=2|21.18(19)&nbsp;s
| β<sup>+</sup>
| <sup>122</sup>Xe
| rowspan=2|1+
| rowspan=2|
|-
| β<sup>+</sup>, α (<2×10<sup>−7</sup>%)
| <sup>118</sup>Te
|-id=Caesium-122m1
| style="text-indent:1em" | <sup>122m1</sup>Cs
| colspan="3" style="text-indent:2em" | 45.87(12)&nbsp;keV
| >1&nbsp;μs
| IT
| <sup>122</sup>Cs
| 3+
|
|-id=Caesium-122m2
| style="text-indent:1em" | <sup>122m2</sup>Cs
| colspan="3" style="text-indent:2em" | 140(30)&nbsp;keV
| 3.70(11)&nbsp;min
| β<sup>+</sup>
| <sup>122</sup>Xe
| 8−
|
|-id=Caesium-122m3
| style="text-indent:1em" | <sup>122m3</sup>Cs
| colspan="3" style="text-indent:2em" | 127.07(16)&nbsp;keV
| 360(20)&nbsp;ms
| IT
| <sup>122</sup>Cs
| 5−
|
|-id=Caesium-123
| <sup>123</sup>Cs
| style="text-align:right" | 55
| style="text-align:right" | 68
| 122.912996(13)
| 5.88(3)&nbsp;min
| β<sup>+</sup>
| <sup>123</sup>Xe
| 1/2+
|
|-id=Caesium-123m1
| style="text-indent:1em" | <sup>123m1</sup>Cs
| colspan="3" style="text-indent:2em" | 156.27(5)&nbsp;keV
| 1.64(12)&nbsp;s
| IT
| <sup>123</sup>Cs
| 11/2−
|
|-id=Caesium-123m2
| style="text-indent:1em" | <sup>123m2</sup>Cs
| colspan="3" style="text-indent:2em" | 252(6)&nbsp;keV
| 114(5)&nbsp;ns
| IT
| <sup>123</sup>Cs
| (9/2+)
|
|-id=Caesium-124
| <sup>124</sup>Cs
| style="text-align:right" | 55
| style="text-align:right" | 69
| 123.9122474(98)
| 30.9(4)&nbsp;s
| β<sup>+</sup>
| '''''<sup>124</sup>Xe'''''
| 1+
|
|-id=Caesium-124m
| rowspan=2 style="text-indent:1em" | <sup>124m</sup>Cs
| rowspan=2 colspan="3" style="text-indent:2em" | 462.63(14)&nbsp;keV
| rowspan=2|6.41(7)&nbsp;s
| IT (99.89%)
| <sup>124</sup>Cs
| rowspan=2|(7)+
| rowspan=2|
|-
| β<sup>+</sup> (0.11%)
| '''''<sup>124</sup>Xe'''''
|-id=Caesium-125
| <sup>125</sup>Cs
| style="text-align:right" | 55
| style="text-align:right" | 70
| 124.9097260(83)
| 44.35(29)&nbsp;min
| β<sup>+</sup>
| <sup>125</sup>Xe
| 1/2+
|
|-id=Caesium-125m
| style="text-indent:1em" | <sup>125m</sup>Cs
| colspan="3" style="text-indent:2em" | 266.1(11)&nbsp;keV
| 900(30)&nbsp;ms
| IT
| <sup>125</sup>Cs
| (11/2−)
|
|-id=Caesium-126
| <sup>126</sup>Cs
| style="text-align:right" | 55
| style="text-align:right" | 71
| 125.909446(11)
| 1.64(2)&nbsp;min
| β<sup>+</sup>
| '''<sup>126</sup>Xe'''
| 1+
|
|-id=Caesium-126m1
| style="text-indent:1em" | <sup>126m1</sup>Cs
| colspan="3" style="text-indent:2em" | 273.0(7)&nbsp;keV
| ~1&nbsp;μs
| IT
| <sup>126</sup>Cs
| (4−)
|
|-id=Caesium-126m2
| style="text-indent:1em" | <sup>126m2</sup>Cs
| colspan="3" style="text-indent:2em" | 596.1(11)&nbsp;keV
| 171(14)&nbsp;μs
| IT
| <sup>126</sup>Cs
| 8−#
|
|-id=Caesium-127
| <sup>127</sup>Cs
| style="text-align:right" | 55
| style="text-align:right" | 72
| 126.9074175(60)
| 6.25(10)&nbsp;h
| β<sup>+</sup>
| <sup>127</sup>Xe
| 1/2+
|
|-id=Caesium-127m
| style="text-indent:1em" | <sup>127m</sup>Cs
| colspan="3" style="text-indent:2em" | 452.23(21)&nbsp;keV
| 55(3)&nbsp;μs
| IT
| <sup>127</sup>Cs
| (11/2)−
|
|-id=Caesium-128
| <sup>128</sup>Cs
| style="text-align:right" | 55
| style="text-align:right" | 73
| 127.9077485(57)
| 3.640(14)&nbsp;min
| β<sup>+</sup>
| '''<sup>128</sup>Xe'''
| 1+
|
|-id=Caesium-129
| <sup>129</sup>Cs
| style="text-align:right" | 55
| style="text-align:right" | 74
| 128.9060659(49)
| 32.06(6)&nbsp;h
| β<sup>+</sup>
| '''<sup>129</sup>Xe'''
| 1/2+
|
|-id=Caesium-129m
| style="text-indent:1em" | <sup>129m</sup>Cs
| colspan="3" style="text-indent:2em" | 575.40(14)&nbsp;keV
| 718(21)&nbsp;ns
| IT
| <sup>129</sup>Cs
| (11/2−)
|
|-id=Caesium-130
| rowspan=2|<sup>130</sup>Cs
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 75
| rowspan=2|129.9067093(90)
| rowspan=2|29.21(4)&nbsp;min
| β<sup>+</sup> (98.4%)
| '''<sup>130</sup>Xe'''
| rowspan=2|1+
| rowspan=2|
|-
| β<sup>−</sup> (1.6%)
| '''''<sup>130</sup>Ba'''''
|-id=Caesium-130m
| rowspan=2 style="text-indent:1em" | <sup>130m</sup>Cs
| rowspan=2 colspan="3" style="text-indent:2em" | 163.25(11)&nbsp;keV
| rowspan=2|3.46(6)&nbsp;min
| IT (99.84%)
| <sup>130</sup>Cs
| rowspan=2|5−
| rowspan=2|
|-
| β<sup>+</sup> (0.16%)
| '''<sup>130</sup>Xe'''
|-
| <sup>131</sup>Cs
| style="text-align:right" | 55
| style="text-align:right" | 76
| 130.90546846(19)
| 9.689(16)&nbsp;d
| [[electron capture|EC]]
| '''<sup>131</sup>Xe'''
| 5/2+
|
|-id=Caesium-132
| rowspan=2|<sup>132</sup>Cs
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 77
| rowspan=2|131.9064378(11)
| rowspan=2|6.480(6)&nbsp;d
| β<sup>+</sup> (98.13%)
| '''<sup>132</sup>Xe'''
| rowspan=2|2+
| rowspan=2|
|-
| β<sup>−</sup> (1.87%)
| '''<sup>132</sup>Ba'''
|-
| <sup>133</sup>Cs<ref group="n">Used to define the [[second]]</ref><ref group="n" name="FP">[[Fission product]]</ref>
| style="text-align:right" | 55
| style="text-align:right" | 78
| 132.905451958(8)
| colspan=3 align=center|'''Stable'''
| 7/2+
| 1.0000
|-
| rowspan=2|<sup>134</sup>Cs<ref group="n" name="FP" />
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 79
| rowspan=2|133.906718501(17)
| rowspan=2|2.0650(4)&nbsp;y
| β<sup>−</sup>
| '''<sup>134</sup>Ba'''
| rowspan=2|4+
| rowspan=2|
|-
| EC (3.0×10<sup>−4</sup>%)
| '''<sup>134</sup>Xe'''
|-id=Caesium-134m
| style="text-indent:1em" | <sup>134m</sup>Cs
| colspan="3" style="text-indent:2em" | 138.7441(26)&nbsp;keV
| 2.912(2)&nbsp;h
| IT
| <sup>134</sup>Cs
| 8−
|
|-
| <sup>135</sup>Cs<ref group="n">[[Long-lived fission product]]</ref>
| style="text-align:right" | 55
| style="text-align:right" | 80
| 134.90597691(39)
| 1.33(19)×10<sup>6</sup>&nbsp;y
| β<sup>−</sup>
| '''<sup>135</sup>Ba'''
| 7/2+
|
|-id=Caesium-135m
| style="text-indent:1em" | <sup>135m</sup>Cs
| colspan="3" style="text-indent:2em" | 1632.9(15)&nbsp;keV
| 53(2)&nbsp;min
| IT
| <sup>135</sup>Cs
| 19/2−
|
|-
| <sup>136</sup>Cs
| style="text-align:right" | 55
| style="text-align:right" | 81
| 135.9073114(20)
| 13.01(5)&nbsp;d
| β<sup>−</sup>
| '''<sup>136</sup>Ba'''
| 5+
|
|-id=Caesium-136m
| rowspan=2 style="text-indent:1em" | <sup>136m</sup>Cs
| rowspan=2 colspan="3" style="text-indent:2em" | 517.9(1)&nbsp;keV
| rowspan=2|17.5(2)&nbsp;s
| β<sup>−</sup>?
| '''<sup>136</sup>Ba'''
| rowspan=2|8−
| rowspan=2|
|-
| IT?
| <sup>136</sup>Cs
|-
| rowspan=2|[[Caesium-137|<sup>137</sup>Cs]]<ref group="n" name="FP" />
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 82
| rowspan=2|136.90708930(32)
| rowspan=2|30.04(4)&nbsp;y
| β<sup>−</sup> (94.70%)<ref name="137cs-decaymode">{{cite journal |last1=Browne |first1=E. |last2=Tuli |first2=J.K. |title=Nuclear Data Sheets for A = 137 |journal=Nuclear Data Sheets |date=October 2007 |volume=108 |issue=10 |pages=2173–2318 |doi=10.1016/j.nds.2007.09.002|bibcode=2007NDS...108.2173B }}</ref>
| <sup>137m</sup>Ba
| rowspan=2|7/2+
| rowspan=2|
|-
| β<sup>−</sup> (5.30%)<ref name="137cs-decaymode"/>
| '''<sup>137</sup>Ba'''
|-id=Caesium-138
| <sup>138</sup>Cs
| style="text-align:right" | 55
| style="text-align:right" | 83
| 137.9110171(98)
| 33.5(2)&nbsp;min
| β<sup>−</sup>
| '''<sup>138</sup>Ba'''
| 3−
|
|-id=Caesium-138m
| rowspan=2 style="text-indent:1em" | <sup>138m</sup>Cs
| rowspan=2 colspan="3" style="text-indent:2em" | 79.9(3)&nbsp;keV
| rowspan=2|2.91(10)&nbsp;min
| IT (81%)
| <sup>138</sup>Cs
| rowspan=2|6−
| rowspan=2|
|-
| β<sup>−</sup> (19%)
| '''<sup>138</sup>Ba'''
|-id=Caesium-139
| <sup>139</sup>Cs
| style="text-align:right" | 55
| style="text-align:right" | 84
| 138.9133638(34)
| 9.27(5)&nbsp;min
| β<sup>−</sup>
| <sup>139</sup>Ba
| 7/2+
|
|-id=Caesium-140
| <sup>140</sup>Cs
| style="text-align:right" | 55
| style="text-align:right" | 85
| 139.9172837(88)
| 63.7(3)&nbsp;s
| β<sup>−</sup>
| <sup>140</sup>Ba
| 1−
|
|-id=Caesium-140m
| style="text-indent:1em" | <sup>140m</sup>Cs
| colspan="3" style="text-indent:2em" | 13.931(21)&nbsp;keV
| 471(51)&nbsp;ns
| IT
| <sup>140</sup>Cs
| (2)−
|
|-id=Caesium-141
| rowspan=2|<sup>141</sup>Cs
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 86
| rowspan=2|140.9200453(99)
| rowspan=2|24.84(16)&nbsp;s
| β<sup>−</sup> (99.97%)
| <sup>141</sup>Ba
| rowspan=2|7/2+
| rowspan=2|
|-
| β<sup>−</sup>, [[neutron emission|n]] (0.0342%)
| <sup>140</sup>Ba
|-id=Caesium-142
| rowspan=2|<sup>142</sup>Cs
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 87
| rowspan=2|141.9242995(76)
| rowspan=2|1.687(10)&nbsp;s
| β<sup>−</sup> (99.91%)
| <sup>142</sup>Ba
| rowspan=2|0−
| rowspan=2|
|-
| β<sup>−</sup>, n (0.089%)
| <sup>141</sup>Ba
|-id=Caesium-143
| rowspan=2|<sup>143</sup>Cs
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 88
| rowspan=2|142.9273473(81)
| rowspan=2|1.802(8)&nbsp;s
| β<sup>−</sup> (98.38%)
| <sup>143</sup>Ba
| rowspan=2|3/2+
| rowspan=2|
|-
| β<sup>−</sup>, n (1.62%)
| <sup>142</sup>Ba
|-id=Caesium-144
| rowspan=2|<sup>144</sup>Cs
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 89
| rowspan=2|143.932075(22)
| rowspan=2|994(6)&nbsp;ms
| β<sup>−</sup> (97.02%)
| <sup>144</sup>Ba
| rowspan=2|1−
| rowspan=2|
|-
| β<sup>−</sup>, n (2.98%)
| <sup>143</sup>Ba
|-id=Caesium-144m
| style="text-indent:1em" | <sup>144m</sup>Cs
| colspan="3" style="text-indent:2em" | 92.2(5)&nbsp;keV
| 1.1(1)&nbsp;μs
| IT
| <sup>144</sup>Cs
| (4−)
| 
|-id=Caesium-145
| rowspan=2|<sup>145</sup>Cs
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 90
| rowspan=2|144.9355289(97)
| rowspan=2|582(4)&nbsp;ms
| β<sup>−</sup> (87.2%)
| <sup>145</sup>Ba
| rowspan=2|3/2+
| rowspan=2|
|-
| β<sup>−</sup>, n (12.8%)
| <sup>144</sup>Ba
|-id=Caesium-145m
| style="text-indent:1em" | <sup>145m</sup>Cs
| colspan="3" style="text-indent:2em" | 762.9(4)&nbsp;keV
| 0.5(1)&nbsp;μs
| IT
| <sup>145</sup>Cs
| 13/2#
| 
|-id=Caesium-146
| rowspan=2|<sup>146</sup>Cs
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 91
| rowspan=2|145.9406219(31)
| rowspan=2|321.6(9)&nbsp;ms
| β<sup>−</sup> (85.8%)
| <sup>146</sup>Ba
| rowspan=2|1−
| rowspan=2|
|-
| β<sup>−</sup>, n (14.2%)
| <sup>145</sup>Ba
|-id=Caesium-146m
| style="text-indent:1em" | <sup>146m</sup>Cs
| colspan="3" style="text-indent:2em" | 46.7(1)&nbsp;keV
| 1.25(5)&nbsp;μs
| IT
| <sup>146</sup>Cs
| 4−#
| 
|-id=Caesium-147
| rowspan=2|<sup>147</sup>Cs
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 92
| rowspan=2|146.9442615(90)
| rowspan=2|230.5(9)&nbsp;ms
| β<sup>−</sup> (71.5%)
| <sup>147</sup>Ba
| rowspan=2|(3/2+)
| rowspan=2|
|-
| β<sup>−</sup>, n (28.5%)
| <sup>146</sup>Ba
|-id=Caesium-147m
| style="text-indent:1em" | <sup>147m</sup>Cs
| colspan="3" style="text-indent:2em" | 701.4(4)&nbsp;keV
| 190(20)&nbsp;ns
| IT
| <sup>147</sup>Cs
| 13/2#
| 
|-id=Caesium-148
| rowspan=2|<sup>148</sup>Cs
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 93
| rowspan=2|147.949639(14)
| rowspan=2|151.8(10)&nbsp;ms
| β<sup>−</sup> (71.3%)
| <sup>148</sup>Ba
| rowspan=2|(2−)
| rowspan=2|
|-
| β<sup>−</sup>, n (28.7%)
| <sup>147</sup>Ba
|-id=Caesium-148m
| style="text-indent:1em" | <sup>148m</sup>Cs
| colspan="3" style="text-indent:2em" | 45.2(1)&nbsp;keV
| 4.8(2)&nbsp;μs
| IT
| <sup>148</sup>Cs
| 4−#
| 
|-id=Caesium-149
| rowspan=2|<sup>149</sup>Cs
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 94
| rowspan=2|148.95352(43)#
| rowspan=2|112.3(25)&nbsp;ms
| β<sup>−</sup> (75%)
| <sup>149</sup>Ba
| rowspan=2|3/2+#
| rowspan=2|
|-
| β<sup>−</sup>, n (25%)
| <sup>148</sup>Ba
|-id=Caesium-150
| rowspan=2|<sup>150</sup>Cs
| rowspan=2 style="text-align:right" | 55
| rowspan=2 style="text-align:right" | 95
| rowspan=2|149.95902(43)#
| rowspan=2|81.0(26)&nbsp;ms
| β<sup>−</sup> (~56%)
| <sup>150</sup>Ba
| rowspan=2|(2−)
| rowspan=2|
|-
| β<sup>−</sup>, n (~44%)
| <sup>149</sup>Ba
|-id=Caesium-151
| <sup>151</sup>Cs
| style="text-align:right" | 55
| style="text-align:right" | 96
| 150.96320(54)#
| 59(19)&nbsp;ms
| β<sup>−</sup>
| <sup>151</sup>Ba
| 3/2+#
| 
{{Isotopes table/footer}}

==Caesium-131==
Caesium-131, introduced in 2004 for [[brachytherapy]] by [[IsoRay|Isoray]],<ref name="131Cs-Isoray">{{cite web |author=Isoray |title=Why Cesium-131 |url=https://isoray.com/why-cesium-131/ |access-date=2017-12-05 |archive-date=2019-06-30 |archive-url=https://web.archive.org/web/20190630105241/https://isoray.com/why-cesium-131/ |url-status=dead }}</ref> has a [[half-life]] of 9.7 days and 30.4&nbsp;keV energy.

==Caesium-133==
Caesium-133 is the only stable [[isotope]] of caesium. The [[SI base unit]] of time, the [[second]], is defined by a [[Caesium_standard|specific caesium-133 transition]]. Since 1967, the official definition of a second is:

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

==Caesium-134==
Caesium-134 has a [[half-life]] of 2.0652 years. It is produced both directly (at a very small yield because <sup>134</sup>Xe is stable) as a [[fission product]] and via [[neutron capture]] from nonradioactive <sup>133</sup>Cs (neutron capture [[Neutron cross-section|cross section]] 29 [[Barn (unit)|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 <sup>134</sup>Xe. It is also not produced by [[nuclear weapons]] because <sup>133</sup>Cs is created by beta decay of original fission products only long after the nuclear explosion is over.

The combined yield of <sup>133</sup>Cs and <sup>134</sup>Cs is given as 6.7896%. The proportion between the two will change with continued neutron irradiation. <sup>134</sup>Cs also captures neutrons with a cross section of 140 barns, becoming long-lived radioactive <sup>135</sup>Cs.

Caesium-134 undergoes [[beta decay]] (β<sup>−</sup>), producing [[Isotopes of barium|<sup>134</sup>Ba]] directly and emitting on average 2.23 [[gamma ray]] photons (mean energy 0.698 [[MeV]]).<ref name="134Cs-JAEA">{{cite web |title=Characteristics of Caesium-134 and Caesium-137 |publisher=Japan Atomic Energy Agency |url=http://c-navi.jaea.go.jp/en/background/remediation-following-major-radiation-accidents/characteristics-of-caesium-134-and-caesium-137.html |access-date=2014-10-23 |archive-date=2016-03-04 |archive-url=https://web.archive.org/web/20160304100452/http://c-navi.jaea.go.jp/en/background/remediation-following-major-radiation-accidents/characteristics-of-caesium-134-and-caesium-137.html |url-status=dead }}</ref>

==Caesium-135==
{{Long-lived fission products}}
Caesium-135 is a mildly [[radioactive]] isotope of caesium with a half-life of 1.33&nbsp;million years. It decays via emission of a low-energy beta particle into the stable isotope barium-135. Caesium-135 is one of the seven [[long-lived fission product]]s and the only alkaline one. In most types of [[nuclear reprocessing]], it stays with the [[medium-lived fission products]] (including {{chem|137|Cs}} which can only be separated from {{chem|135|Cs}} via [[isotope separation]]) rather than with other long-lived fission products. Except in the [[Molten salt reactor]], where {{chem|135|Cs}} is created as a completely separate stream outside the fuel (after the decay of bubble-separated {{chem|135|Cs}}). The low [[decay energy]], lack of [[gamma radiation]], and long half-life of <sup>135</sup>Cs make this isotope much less hazardous than [[Caesium-137|<sup>137</sup>Cs]] or <sup>134</sup>Cs.

Its precursor [[Xenon-135|<sup>135</sup>Xe]] has a high [[fission product yield]] (e.g., 6.3333% for [[Uranium-235|<sup>235</sup>U]] and [[thermal neutron]]s) but also has the highest known [[thermal neutron]] capture cross section of any nuclide. Because of this, much of the <sup>135</sup>Xe produced in current [[thermal reactor]]s (as much as >90% at steady-state full power)<ref name="Groh2004">{{cite web|url=http://canteach.candu.org/library/20041204.pdf|author=John L. Groh|title=Supplement to Chapter 11 of Reactor Physics Fundamentals|year=2004|publisher=CANTEACH project|access-date=14 May 2011|url-status=dead|archive-url=https://web.archive.org/web/20110610131653/http://canteach.candu.org/library/20041204.pdf|archive-date=10 June 2011}}</ref> will be converted to  extremely long-lived (half-life on the order of 10<sup>21</sup> years) {{chem|136|Xe|link=xenon-136}} before it can decay to {{chem|135|Cs}} despite the relatively short half life of {{chem|135|Xe}}. Little or no {{chem|135|Xe}} will be destroyed by neutron capture after a reactor shutdown, or in a [[molten salt reactor]] that continuously removes xenon from its fuel, a [[fast neutron reactor]], or a nuclear weapon. The [[xenon pit]] is a phenomenon of excess neutron absorption through {{chem|135|Xe}} buildup in the reactor after a reduction in power or a shutdown and is often managed by letting the {{chem|135|Xe}} decay away to a level at which neutron flux can be safely controlled via [[control rod]]s again.

A nuclear reactor will also produce much smaller amounts of <sup>135</sup>Cs from the nonradioactive fission product <sup>133</sup>Cs by successive neutron capture to <sup>134</sup>Cs and then <sup>135</sup>Cs.

The thermal neutron capture cross section and [[resonance integral]] of <sup>135</sup>Cs are {{nowrap|8.3 ± 0.3}} and {{nowrap|38.1 ± 2.6 [[barn (unit)|barns]]}} respectively.<ref name="Hatsukawa1999">{{cite journal|title=Thermal neutron cross section and resonance integral of the reaction of135Cs(n,γ)136Cs: Fundamental data for the transmutation of nuclear waste|year=1999|last1=Hatsukawa|first1=Y.|last2=Shinohara |first2=N |last3=Hata |first3=K.|journal=Journal of Radioanalytical and Nuclear Chemistry|volume=239|issue=3|pages=455–458|doi=10.1007/BF02349050|s2cid=97425651|display-authors=etal}}</ref> Disposal of <sup>135</sup>Cs by [[nuclear transmutation]] is difficult, because of the low cross section as well as because neutron irradiation of mixed-isotope fission caesium produces more <sup>135</sup>Cs from stable <sup>133</sup>Cs. In addition, the intense medium-term radioactivity of <sup>137</sup>Cs makes handling of nuclear waste difficult.<ref name="Ohki">{{cite journal|url=http://www.nea.fr/html/pt/docs/iem/jeju02/session6/SessionVI-08.pdf|title=Transmutation of Cesium-135 With Fast Reactors|first1=Shigeo|last1=Ohki|last2=Takaki |first2=Naoyuki|journal=Proceedings of the Seventh Information Exchange Meeting on Actinide and Fission Product Partitioning & Transmutation, Cheju, Korea|year=2002}}</ref>
*[https://web.archive.org/web/20110429095603/http://www.ead.anl.gov/pub/doc/cesium.pdf ANL factsheet]

==Caesium-136==
Caesium-136 has a half-life of 13.01 days.<ref name=":0" /> It is produced both directly (at a very small yield because <sup>136</sup>Xe is [[beta-decay stable isobars|beta-stable]]) as a fission product and via neutron capture from long-lived <sup>135</sup>Cs,<ref>{{Cite web |title=NGAtlas/ZV |url=https://www-nds.iaea.org/ngatlas2/ |access-date=2025-02-22 |website=www-nds.iaea.org}}</ref> which is a common fission product. It is also not produced by nuclear weapons because <sup>135</sup>Cs is created by beta decay of original fission products only long after the nuclear explosion is over. Caesium-136 undergoes beta decay ({{SubatomicParticle|Beta-}}), producing <sup>136</sup>Ba directly.<ref name=":0" />

==Caesium-137==
{{Main|Caesium-137}}
Caesium-137, with a half-life of 30.17 years, is one of the two principal [[medium-lived fission product]]s, along with [[strontium-90|<sup>90</sup>Sr]], 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 disaster|Chernobyl accident]] and is a major health concern for decontaminating land near the [[Fukushima Daiichi nuclear disaster|Fukushima]] nuclear power plant.<ref name="137Cs-Normile">{{cite journal |author=Dennis  |title=Cooling a Hot Zone |journal=Science |volume=339 |issue=6123 |date=1 March 2013 |pages=1028–1029 |doi=10.1126/science.339.6123.1028|pmid=23449572 }}</ref> <sup>137</sup>Cs 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.

<sup>137</sup>Cs 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 [[Nuclear transmutation#Artificial transmutation of nuclear waste|Accelerator Transmutation of Nuclear Waste]]) enables production of neutr<!-- Comment -->ons at high enough intensity to offset and overcome these low capture rates; until then, therefore, <sup>137</sup>Cs must simply be allowed to decay.

<sup>137</sup>Cs has been used as a [[Flow tracer|tracer]] in hydrologic studies, analogous to the use of [[tritium|<sup>3</sup>H]].

==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.

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

== References ==
{{Reflist}}
* Isotope masses from:
**{{NUBASE 2003}}
* Half-life, spin, and isomer data selected from the following sources.
**{{NUBASE 2003}}
**{{NNDC}}
**{{CRC85|chapter=11}}

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

[[Category:Isotopes of caesium| ]]
[[Category:Lists of isotopes by element|Caesium]]