1. n. [Formation Evaluation]
An element with an atomic number of 90. The 232Th isotope is radioactive and decays with a half-life of 1.4 * 1010 years through a series of intermediate isotopes to a stable isotope of lead. The intermediate isotopes emit a wide range of gamma rays, the most prominent being that of thallium, 208Tl. It is assumed that formations are in secular equilibrium; that is, the relative proportions of parent and daughter isotopes remain constant, and the measured spectrum is directly related to the amount of 232Th. The concentration in the Earth's crust is about 12 parts per million, ppm, by weight. Thorium-bearing minerals are rare. Thorium is a trace element associated with clays and heavy minerals. It is very immobile so that quantity measured today probably was present at the time of deposition. A log of thorium is presented in parts per million. It is often a good measure of clay content.