A radiometric dating method based on the radioactive decay of 232Th, to yield 208Pb + 6He4, with a half-life (see decay constant) of 13 900 million years. The minerals used include sphene, zircon, monazite, apatite, and other rare U/Th minerals. The method is not totally reliable and is usually employed in conjunction with other methods. In most cases the results are discordant as a result of lead loss. The ratio of 208Pb:232Th compared with 207Pb:235U ratio is particularly useful. The Th–Pb system can also be interpreted by means of isochron diagrams similar to those used in the rubidium–strontium method.