alpha therapies. 232Th also very occasionally undergoes spontaneous fission rather than alpha decay, and has left evidence of doing so in its minerals (as trapped xenon gas formed as a fission product), but the partial half-life of this process is very large at over 1021 years and alpha decay predominates. In total, 32 radioisotopes have been characterised, which range in mass number from 207 to 238. After 232Th, the most stable of them (with respective half-lives) are 230Th (75,380 years), 229Th (7,340 years), 228Th (1.92 years), 234Th (24.10 days), and 227Th (18.68 days). All of these isotopes occur in nature as trace radioisotopes due to their presence in the decay chains of 232Th, 235U, 238U, and 237Np: the last of these is long extinct in nature due to its short half-life (2.14 million years), but is continually produced in minute traces from neutron capture in uranium ores. All of the remaining thorium isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes. In deep seawaters the isotope 230Th makes up to 0.04% of natural thorium. This is because its parent 238U is soluble in water, but 230Th is insoluble and precipitates into the sediment. Uranium ores with low thorium concentrations can be purified to produce gram-sized thorium samples of which over a quarter is the 230Th isotope, since 230Th is one of the daughters of 238U. The International Union of Pure and Applied Chemistry (IUPAC) reclassified thorium as a binuclidic element in 2013; it had formerly been considered a mononuclidic element. Thorium has three known nuclear isomers (or metastable states), 216m1Th, 216m2Th, and 229mTh. 229mTh has the lowest known excitation energy of any isomer, measured to be . This is so low that when it undergoes isomeric transition, the emitted gamma radiation is in the ultraviolet range. Different isotopes of thorium are chemically identical, but have slightly differing physical properties: for example, the densities of pure 228Th, 229Th, 230Th, and 232Th are respectively expected to be 11.5, 11.6, 11.6, and 11.7 g/cm3. The isotope 229Th is expected to be fissionable with a bare critical mass of 2839 kg, although with steel reflectors this value could drop to 994 kg. 232Th is not fissionable, but it is fertile as it can be converted to fissile 233U by neutron capture and subsequent beta decay. Radiometric dating Two radiometric dating methods