Thorium-231 is a radioactive isotope of thorium, a naturally occurring element. It has a half-life of about 25 hours, meaning it takes that long for half of a sample to decay. This isotope is produced through the decay of uranium-235 and can be found in trace amounts in nature. As a decay product, thorium-231 eventually transforms into protactinium-231, which is another radioactive isotope. Due to its relatively short half-life, thorium-231 is not commonly used in applications like nuclear energy, but it is of interest in research related to nuclear physics and radiochemistry.