View source for Physics:Thorium-232

{{Short description|Isotope of thorium}}
{{Infobox isotope|
 | image =
 | image_caption = 
 | alternate_names =
 | mass_number = 232
 | symbol =Th
 | num_neutrons = 142
 | num_protons = 90
 | abundance = 99.98%<ref name=Nubase2020>{{NUBASE2020}}</ref>
 | halflife = {{val|1.4|e=10|u=years}}<ref name=Nubase2020 />
 | decay_product = Radium-228
 | decay_mass = 228
 | decay_symbol = Ra
 | parent = Uranium-236
 | parent_mass = 236
 | parent_symbol = U
 | parent_decay =a
 | parent2 =Actinium-232
 | parent2_mass = 232
 | parent2_symbol = Ac
 | parent2_decay = b-
 | mass = 232.0380536<ref name=AME2020>{{AME2020 II}}</ref>
 | spin = 0+
 | excess_energy =
 | binding_energy =
 | decay_mode1 =[[Physics:Alpha decay|alpha decay]]
 | decay_energy1 = 4.0816<ref name=NuDat3.0>{{cite web |author=National Nuclear Data Center |title=NuDat 3.0 database |url=https://www.nndc.bnl.gov/nudat3/ |publisher=[[Organization:Brookhaven National Laboratory|Brookhaven National Laboratory]]|access-date=19 Feb 2022}}</ref>
 | decay_mode2 =
 | decay_energy2 =
 | decay_mode3 =
 | decay_energy3 =
 | decay_mode4 =
 | decay_energy4 =
 }}
'''Thorium-232''' ({{SimpleNuclide|Thorium|232}}) is the main naturally occurring [[Physics:Isotopes of thorium|isotope]] of [[Chemistry:Thorium|thorium]], with a relative abundance of 99.98%. It has a half life of 14 billion years, which makes it the longest-lived isotope of thorium. It decays by [[Physics:Alpha decay|alpha decay]] to [[Physics:Isotopes of radium|radium-228]]; its [[Physics:Decay chain|decay chain]] terminates at stable [[Physics:Isotopes of lead|lead-208]].

Thorium-232 is a fertile material; it can capture a neutron to form thorium-233, which subsequently undergoes two successive [[Physics:Beta decay|beta decay]]s to [[Physics:Uranium-233|uranium-233]], which is [[Physics:Fissile material|fissile]]. As such, it has been used in the [[Physics:Thorium fuel cycle|thorium fuel cycle]] in nuclear reactors; various prototype thorium-fueled reactors have been designed; however, as of 2022, thorium has not been used for large-scale commercial nuclear power.

==Natural occurrence==
The [[Physics:Half-life|half-life]] of thorium-232 (14 billion years) is more than three times the [[Earth:Age of the Earth|age of the Earth]]; thorium-232 therefore occurs in nature as a [[Chemistry:Primordial nuclide|primordial nuclide]]. Other thorium isotopes occur in nature in much smaller quantities as intermediate decay products of [[Physics:Uranium-238|uranium-238]], [[Physics:Uranium-235|uranium-235]], and thorium-232.<ref name=WorldNuclearAssociation>{{cite web|url=https://www.nuclear-power.com/nuclear-power/reactor-physics/atomic-nuclear-physics/radioactive-decay/radioactive-decay-chain/radioactive-series-radioactive-cascade/thorium-series/|title=Thorium - World Nuclear Association|publisher=World Nuclear Association|accessdate=19 Feb 2022}}</ref>

Some minerals that contain thorium include [[Chemistry:Apatite|apatite]], sphene, [[Chemistry:Zircon|zircon]], [[Chemistry:Allanite|allanite]], [[Chemistry:Monazite|monazite]], [[Chemistry:Pyrochlore|pyrochlore]], [[Chemistry:Thorite|thorite]], and [[Chemistry:Xenotime|xenotime]].<ref name=USGS>{{cite web|url=https://pubs.usgs.gov/of/2004/1050/thorium.htm|publisher=usgs.gov|title=Thorium|accessdate=19 Feb 2022}}</ref>

==Decay==
[[File:Decay Chain Thorium.svg|thumb|upright=1.25|alt=Ball-and-arrow presentation of the thorium decay series|The 4''n'' decay chain of <sup>232</sup>Th, commonly called the "thorium series"]]
Thorium-232 has a half-life of 14 billion years and mainly decays by [[Physics:Alpha decay|alpha decay]] to [[Physics:Isotopes of radium|radium-228]] with a  [[Physics:Decay energy|decay energy]] of 4.0816 [[Physics:MeV|MeV]].<ref name=NuDat3.0>{{cite web |author=National Nuclear Data Center |title=NuDat 3.0 database |url=https://www.nndc.bnl.gov/nudat3/ |publisher=[[Organization:Brookhaven National Laboratory|Brookhaven National Laboratory]]|access-date=19 Feb 2022}}</ref> The decay chain follows the thorium series, which terminates at stable [[Physics:Isotopes of lead|lead-208]]. The intermediates in the thorium-232 decay chain are all relatively short-lived; the longest-lived intermediate decay products are radium-228 and thorium-228, with half lives of 5.75 years and 1.91 years, respectively. All other intermediate decay products have half lives of less than four days.<ref name=USGS />

The following table lists the intermediate decay products in the thorium-232 decay chain:
{| class="wikitable" style="margin:auto;"
!width="80"|nuclide
!width="100"|decay mode
!width="100"|half-life<br/>(''a''=year)
!width="60" |energy released, MeV
!width="100"|product of decay
|-
| align="center"| <sup>232</sup>Th
| align="center"|  α
| align="center"| 1.4{{e|10}} a
| align="center"| 4.081
| align="center"| [[Physics:Radium-228|<sup>228</sup>Ra]]
|-
| align="center"| <sup>228</sup>Ra
| align="center"|  β<sup>−</sup>
| align="center"| 5.75 a
| align="center"| 0.046
| align="center"| [[Physics:Actinium-228|<sup>228</sup>Ac]]
|-
| align="center"| <sup>228</sup>Ac
| align="center"|  β<sup>−</sup>
| align="center"| 6.15 h
| align="center"| 2.134
| align="center"| [[Physics:Thorium-228|<sup>228</sup>Th]]
|-
| align="center"| <sup>228</sup>Th
| align="center"|  α
| align="center"| 1.9116 a
| align="center"| 5.520
| align="center"| [[Physics:Radium-224|<sup>224</sup>Ra]]
|-
| align="center"| <sup>224</sup>Ra
| align="center"|  α
| align="center"| 3.6319 d
| align="center"| 5.789
| align="center"| [[Physics:Radon-220|<sup>220</sup>Rn]]
|-
| align="center"| <sup>220</sup>Rn
| align="center"|  α
| align="center"| 55.6 s
| align="center"| 6.405
| align="center"| [[Physics:Polonium-216|<sup>216</sup>Po]]
|-
| align="center"| <sup>216</sup>Po
| align="center"|  α
| align="center"| 0.145 s
| align="center"| 6.906
| align="center"| [[Physics:Lead-212|<sup>212</sup>Pb]]
|-
| align="center"| <sup>212</sup>Pb
| align="center"|  β<sup>−</sup>
| align="center"| 10.64 h
| align="center"| 0.569
| align="center"| [[Physics:Bismuth-212|<sup>212</sup>Bi]]
|-
| align="center"| <sup>212</sup>Bi
| align="center"| β<sup>−</sup> 64.06% <br> α 35.94%
| align="center"| 60.55 min
| align="center"| 2.252 <br> 6.207
| align="center"| [[Physics:Polonium-212|<sup>212</sup>Po]] <br> [[Physics:Thallium-208|<sup>208</sup>Tl]]
|-
| align="center"| <sup>212</sup>Po
| align="center"|  α
| align="center"| 294.4 ns<ref name=Nubase2020 />
| align="center"| 8.954 <ref name=NuDat3.0 />
| align="center"| [[Physics:Lead-208|<sup>208</sup>Pb]]
|-
| align="center"| <sup>208</sup>Tl
| align="center"|  β<sup>−</sup>
| align="center"| 3.053 min
| align="center"| 4.999 <ref name=NuDat3.0 />
| align="center"| <sup>208</sup>Pb
|-
| align="center"| <sup>208</sup>Pb
| align="center"| stable
| align="center"| .
| align="center"| .
| align="center"| .
|}

===Rare decay modes===
Although thorium-232 mainly decays by alpha decay, it also undergoes [[Physics:Spontaneous fission|spontaneous fission]] 1.1{{e|-9}}% of the time.<ref name=NuDat3.0 /> In addition, it is capable of 
[[Physics:Cluster decay|cluster decay]], splitting into [[Physics:Ytterbium-182|ytterbium-182]], [[Physics:Neon-24|neon-24]], and [[Physics:Neon-26|neon-26]]; the upper limit for the branching ratio of this decay mode is 2.78{{e|-10}}%. [[Physics:Double beta decay|Double beta decay]] to [[Physics:Uranium-232|uranium-232]] is also theoretically possible, but has not been observed.<ref name=Nubase2020 />

==Use in nuclear power==
{{Main|Physics:Thorium-based nuclear power|Physics:Thorium fuel cycle}}

Thorium-232 is not [[Physics:Fissile material|fissile]]; it therefore cannot be used directly as fuel in [[Physics:Nuclear reactor|nuclear reactor]]s. However, {{SimpleNuclide|Thorium|232}} is fertile: it can capture a neutron to form {{SimpleNuclide|Thorium|233}}, which undergoes [[Physics:Beta decay|beta decay]] with a half-life of 21.8 minutes to {{SimpleNuclide|Protactinium|233}}. This nuclide subsequently undergoes beta decay with a half-life of 27 days to form [[Physics:Fissile material|fissile]] [[Physics:Uranium-233|{{SimpleNuclide|Uranium|233}}]].<ref name=WorldNuclearAssociation />

One potential advantage of a thorium-based nuclear fuel cycle is that thorium is more abundant in nature than [[Chemistry:Uranium|uranium]], the current fuel for commercial nuclear reactors. It is also more difficult to produce material suitable for [[Engineering:Nuclear weapon|nuclear weapon]]s from the thorium fuel cycle compared to the uranium fuel cycle. Some proposed designs for thorium-fueled nuclear reactors include the [[Engineering:Molten salt reactor|molten salt reactor]] and a fast neutron reactor, among others. Although thorium-based nuclear reactors have been proposed since the 1960s and several prototype reactors have been built, there has been relatively little research on the thorium fuel cycle compared to the more established uranium fuel cycle; thorium-based nuclear power has not seen large-scale commercial use as of 2022. Nevertheless, some countries such as {{wipe|India}} have actively pursued thorium-based nuclear power.<ref name=WorldNuclearAssociation />

==References==
{{reflist}}

{{Isotope sequence
|element=thorium
|lighter=[[Physics:Thorium-231|thorium-231]]
|heavier=[[Physics:Thorium-233|thorium-233]]
|before=[[Physics:Uranium-236|uranium-236]]<br />[[Physics:Actinium-232|actinium-232]]
|after=[[Physics:Radium-228|radium-228]]
}}

[[Category:Actinides]]
[[Category:Isotopes of thorium]]
[[Category:IARC Group 1 carcinogens]]

{{Sourceattribution|Thorium-232}}