Thorium-231

Main isotopes of thorium

iso NA half-life DM DE (MeV) DP 227Th trace 18.68 d α 6.038 5.978 223Ra 228Th trace 1.9116 y α 5.520 224Ra 229Th trace 7917 y α 5.168 225Ra 230Th 0.02% 75400 y α 4.770 226Ra 231Th trace 25.5 h β− 0.39 231Pa 232Th 99.98% 1.405×1010 y α 4.083 228Ra 234Th trace 24.1 d β− 0.27 234Pa
Standard atomic weight (Ar)	7002232037700000000♠232.0377(4)

Although thorium (Th), with atomic number 90, has 6 naturally occurring isotopes, none of these isotopes are stable; however, one isotope, ²³²Th, is relatively stable, with a half-life of 14.05 billion years, considerably longer than the age of the earth, and even slightly longer than the generally accepted age of the universe. This isotope makes up nearly all natural thorium. As such, thorium is considered to be mononuclidic. It has a characteristic terrestrial isotopic composition and thus an atomic mass can be given.

Thirty radioisotopes have been characterized, with the most stable (after ²³²Th) being ²³⁰Th with a half-life of 75,380 years, ²²⁹Th with a half-life of 7,340 years, and ²²⁸Th with a half-life of 1.92 years. All of the remaining radioactive isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes. One isotope, ²²⁹Th, has a nuclear isomer (or metastable state) with a remarkably low excitation energy, recently measured to be 7.6 ± 0.5 eV.

The known isotopes of thorium range in mass number from 209 to 238.

No fission products
have a half-life
in the range of
100–210 k years ...

... nor beyond 15.7 M years

Legend for superscript symbols
₡  has thermal neutron capture cross section in the range of 8–50 barns
ƒ  fissile
m  metastable isomer
№  naturally occurring radioactive material (NORM)
þ  neutron poison (thermal neutron capture cross section greater than 3k barns)
†  range 4–97 y: Medium-lived fission product
‡  over 200,000 y: Long-lived fission product

...
Wikipedia