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Properties and data of the isotope 232Th.


Thorium-232 is the radioisotope of the element Thorium, whose atomic nucleus has 142 neutrons in addition to the element-specific 90 protons, resulting in a mass number of 232.

See also: list of Thorium isotopes.


General data

Name of the isotope:Thorium-232; Th-232Symbol:232ThMass number A:232Atomic number Z:90 (= number of protons)Neutrons N:142Isotopic mass:232.03806(2) u (atomic weight of Thorium-232)Nuclide mass:231.9886942 u (calculated nuclear mass without electrons)Mass excess:35.45266 MeVMass defect:1.896610952 u (per nucleus)Nuclear binding energy:1766.68172212 MeV (per nucleus)
7.61500742 MeV (average binding energy per nucleon)
Separation energy:SN = 6.4404(11) MeV (first neutron)
SP = 7.605(13) MeV (first proton)
Cross section:σ(n.γ) = 7.37(4) barn (thermal neutron capture cross-section)
σ(n.f) = 3 μb (gap cross-section)
σ(n.α) < 1 μb
Half-life:1.40(1) × 1010 aDecay constant λ:1.569968064977 × 10-18 s-1Spin and parity:0+Charge radius:5.7848(124) femtometer fmYear of discovery:1898


Radioactive Decay

Half-life T½ = 1.40(1) × 1010 a respectively 4.41504 × 1017 seconds s.

Decay modeDaughterProbabilityDecay energyγ energy
α228Ra> 99 %4,0816(14) MeV
SFdiv<< 1 %


Thorium Series


Parent Nuclides

Direct parent isotopes are: 236U, 232Ac, 232Pa.



Thorium-232 is the only primordial isotope of thorium and effectively makes up all natural thorium; other thorium isotopes occur only in traces as relatively short-lived decay products of uranium and thorium. Some minerals that contain the thorium isotope in very small amounts are apatite, sphene, zircon, allanite, monazite, pyrochlore, thorite, and xenotime.

Comparison of the natural Thorium isotopes including isotopic abundance (proportion of the isotope mixture):


Atomic Mass maQuantityHalf-lifeSpin
Isotopic mixture
232,0377 u100 %
Isotope 230Th230,03313(2) u0,02(2) %7.54(3) × 104 a0+
Isotope 232Th232,03806(2) u99,98(2) %1.40(1) × 1010 a0+



Thorium-232, which is more abundant in nature than uranium, is used to artificially produce fissile Uranium-233. Th-232 is able to absorb neutrons and thus transmute them into U-233. The isotope is at the forefront of the uranium-thorium fuel cycle.

Concerns about the limits of the world's available uranium resources initially sparked interest in the thorium fuel cycle. It was envisaged that when uranium reserves were depleted, thorium could be used to produce it. However, uranium was relatively abundant in most countries ... and research into the thorium fuel cycle quickly waned. A notable exception was India's three-stage nuclear energy program. In the 21st century, thorium's potential to improve proliferation resistance and decay characteristics led to renewed interest in the thorium-based nuclear fuel cycle [1].

Th and U isotopes are technical products and parameters that need to be determined analytically in areas such as environmental monitoring or nuclear emergencies. A research group led by Youyi Ni et al. reports on a newer analytical method for the simultaneous determination of these radioactive nuclides in different types of real water samples with good practicability at the same time [2].


Radiation Protection

The German Radiation Protection Ordinance, for example, specifies the following limit values for the isotope Thorium-232 (exemption limits, clearance values and other values as a radioactive or highly radioactive source of radiation (HASS)):

Isotope:Th-232Exemption level:104 BqExemption level per gram:10 Bq/g
Unrestricted handling of solids and liquids.
Surface contamination:0,1 Bq cm-2Half-life:1.4 × 1010 a


Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 142) and isobaric (same nucleon number A = 232) with Thorium-232. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.


OZIsotone N = 142Isobar A = 232


External data and identifiers

CAS:7440-29-1PubChem:ID 23960Adopted Levels, Gammas:NuDat 232Th


Literature and References

[1] - Uguru Edwin Humphrey, Mayeen Uddin Khandaker:
Viability of thorium-based nuclear fuel cycle for the next generation nuclear reactor: Issues and prospects.
In: Renewable and Sustainable Energy Reviews, (2018), DOI 10.1016/j.rser.2018.08.019.

[2] - Youyi Ni, Wenting Bu, Xiaotong Ding et al.:
Automated method for concurrent determination of thorium (230Th, 232Th) and uranium (234U, 235U, 238U) isotopes in water matrices with ICP-MS/MS.
In: Journal of Analytical Atomic Spectrometry, (2022), DOI 10.1039/D1JA00450F.


Category: Isotopes

Last update: 26.11.2022

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