Radiacode — Pocket-Sized Next-Generation Geiger Counter and Gamma Spectrometer

Protactinium-231 (Pa-231) is a naturally occurring radioactive isotope of protactinium with a very long half-life of about 32,700 years. It decays primarily by alpha emission to actinium-227, beginning a series of subsequent radioactive decays that eventually leads to stable lead-207. Because of its long half-life, Pa-231 persists in natural uranium-bearing materials for geological timescales.

Pa-231 has no significant commercial or industrial applications because of its rarity and radioactivity. It is mainly used in scientific research. In geochemistry and paleoclimatology, Pa-231 is particularly useful as a natural tracer and, together with thorium-230, is used to study ocean circulation, sedimentation processes, and the history of the Earth's oceans.

Pa-231 occurs naturally as part of the uranium-235 decay series. It is present in extremely small quantities in uranium ores such as uraninite and can also be found in materials and wastes produced during uranium processing. In the environment, trace amounts occur in rocks, soils, sediments, and seawater containing products of natural uranium decay.