231Pa

Protactinium-231 (231Pa) is a radioactive isotope of the element protactinium (atomic number 91) with a mass number of 231. It is a long-lived alpha emitter and is primarily studied in the fields of nuclear physics, geochemistry, and paleoceanography. The isotope is produced in nature as a daughter product in the decay chain of uranium-235 and is present in uranium-bearing materials only in trace amounts.

The principal decay mode of 231Pa is alpha decay, through which it transforms into actinium-227 (227Ac). The half-life of 231Pa is about 32,700 years, making it one of the long-lived protactinium nuclides. As a heavy actinide, its chemical behavior is similar to other actinides, which influences how it moves and is isolated in environmental and laboratory contexts.

Natural occurrence and production pathways are distinct. In ordinary geological materials, 231Pa is found in extremely small quantities as part of the uranium decay series. It can be produced artificially in reactors or accelerators by neutron irradiation of thorium-230, which yields thorium-231 that beta decays to protactinium-231. In environmental samples, its presence is typically inferred from its decay product and through radiometric dating techniques.

Applications of 231Pa include radiometric dating and tracing processes in marine and geological systems. In paleoceanography, the ratio of 231Pa to 230Th is used to study past ocean circulation patterns because they have different chemical behaviors in seawater. As an alpha emitter, 231Pa also serves as a radiological hazard when ingested, but it contributes minimally to external exposure due to its low activity in natural samples.