No fission products have a half-lifein the range of 100 a–210 ka ... ... nor beyond 15.7 Ma[11] Actinium-225 is a highly radioactive isotope with 136 neutrons.
[15] Another benefit is that the decay chain of 225Ac ends in the nuclide 209Bi,[note 1] which has a considerably shorter biological half-life than lead.
The oxide of 227Ac pressed with beryllium is also an efficient neutron source with the activity exceeding that of the standard americium-beryllium and radium-beryllium pairs.
[27] The medium half-life of 227Ac makes it a very convenient radioactive isotope in modeling the slow vertical mixing of oceanic waters.
The associated processes cannot be studied with the required accuracy by direct measurements of current velocities (of the order 50 meters per year).
Its decay product, 231Pa, gradually precipitates to the bottom, so that its concentration first increases with depth and then stays nearly constant.
231Pa decays to 227Ac; however, the concentration of the latter isotope does not follow the 231Pa depth profile, but instead increases toward the sea bottom.