Thorium

The decay of ²³²Th (with a half-life of 1.40×10¹⁰ years) to ²⁰⁸Pb is used to date rocks based on the accumulation of the stable daughter product ²⁰⁸Pb. The half-lives of the isotopes between the parent radionuclide ²³²Th and stable endpoint ²⁰⁸Pb all have much shorter half-lives than thorium. Therefore, the amount of ²⁰⁸Pb that accumulates in a sample is determined primarily by the amount of ²³²Th parent radionuclide present when the mineral was formed and the time that has elapsed since the mineral solidified [591]. Another dating method, the ²³⁰Th/ ²³⁴U method, is based on the hypothesis that the sample contains uranium, but no ²³⁰Th at the time of its formation. Then, the age of the specimen is determined mainly by the amount of ²³⁰Th in the specimen. Reliable ages with this method range from several thousand to approximately 350 thousand years [292].

The most precise time and frequency measurements are performed with optical atomic clocks that use as a frequency standard the optical frequency generated as electrons change energy levels. It has been proposed that a nuclear clock, using a nuclear transition could outperform an electron transition. 229mTh, with a half-life of 13.9 h, has been confirmed as a possible candidate for a nuclear clock [592]. The m in 229mTh indicates a metastable state of the isotope. The further development of a nuclear frequency standard will require more precise determinations of the energy and half-life of the isomer.