Protactinium231

Protactinium-231, or 231Pa, is a radioactive isotope of the element protactinium (atomic number 91) with a mass of 231. In nature it occurs only in trace amounts in uranium-bearing materials and is produced in the decay chains of uranium-235 as well as by neutron irradiation of thorium-230 in reactors.

The primary decay mode of 231Pa is alpha decay to 227Ac. It has a half-life of about 3.3 × 10^4 years (approximately 33,000 years). The emitted alpha particle carries an energy on the order of 4.6 MeV. The decay product, 227Ac, has a half-life of about 21.8 years and continues through the actinium/thorium decay chain toward stable lead, via a series of alpha and beta decays.

Production and occurrence: in natural settings, 231Pa is generated in uranium decay chains and exists only at trace levels. in nuclear reactors, it can be formed by neutron irradiation of thorium-230 or through other neutron-induced transmutation pathways. Its long half-life and alpha emission make it both a subject of radiochemical study and a useful tracer in certain geochemical applications.

Applications and significance: 231Pa is used in uranium-series dating and as a tracer in paleoceanography. The ratio of 231Pa to 230Th in marine sediments provides information about past ocean circulation and particle export rates. Measurements are typically performed after chemical separation of protactinium followed by alpha spectrometry or mass spectrometric techniques.

Safety: as a long-lived alpha emitter, 231Pa presents radiological hazards if ingested or inhaled and requires appropriate shielding, containment, and radiochemical handling.