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

Rubidium-81 (Rb-81) is a radioactive isotope of rubidium with a short half-life of approximately 4.58 hours. It decays by electron capture and positron emission to krypton-81, largely through the metastable state krypton-81m (half-life 13 s, 190 keV gamma line). Rb-81 is not found naturally and is produced in cyclotrons by proton irradiation of krypton. Rubidium-82m (Rb-82m, half-life 6.47 hours) is co-produced in the same irradiation; it decays by electron capture (about 79%) and positron emission (about 21%) to stable krypton-82, emitting gamma lines at 554, 619 and 776 keV. Rb-82m should not be confused with Rb-82 (half-life 75 s), the isotope used for cardiac PET.

Rb-81 is used as the parent nuclide in Rb-81/Kr-81m generators. The short-lived Kr-81m gas (190 keV) is inhaled by the patient for lung ventilation imaging with gamma cameras; because Kr-81m decays within seconds, ventilation studies can be repeated or performed immediately after a perfusion scan. Rb-81 itself is not a PET agent. Rb-82m has been studied as a longer-lived substitute for Rb-82 in cardiac PET but is not in clinical use; in practice it is a radionuclidic by-product that remains on the generator column together with Rb-81.

Because of their artificial production and short half-lives, Rb-81 and Rb-82m are encountered only in controlled environments such as nuclear medicine departments and research laboratories. A gamma spectrum of an Rb-81/Kr-81m generator shows both nuclides: the 190 keV line of Kr-81m, the 446 and 511 keV lines of Rb-81, and the 554, 619 and 776 keV lines of Rb-82m. Their rapid decay makes accidental environmental presence highly unlikely.