Rubidium-81 (Rb-81) is a radioactive isotope of rubidium with a short half-life of approximately 4.58 hours. It decays primarily through electron capture to produce krypton-81m, a metastable isotope. Due to its rapid decay and emission characteristics, Rb-81 is not found naturally and is typically produced artificially in particle accelerators or nuclear reactors.

Rubidium-81 is used primarily in medical imaging, particularly in positron emission tomography (PET). Its decay product, krypton-81m, is utilized in ventilation studies of the lungs due to its ideal properties as a gaseous tracer. This makes Rb-81 critical in certain diagnostic applications, especially for assessing pulmonary function.

Because of its artificial production and short half-life, Rb-81 is encountered exclusively in controlled environments such as medical facilities and research laboratories where it is generated and used promptly. Its specialized applications and rapid decay make accidental environmental presence highly unlikely.