Beryllium-7

　Beryllium is a very light metal with atomic number 4 (proton number 4). The nuclide with a neutron number of 5 is a stable isotope. Although it is not a familiar element, it is known as the main constituent of emeralds. Beryllium is also known as beryllium-7 (7Be), a radioactive nuclide with a neutron number of 3. Beryllium-7 (7Be) is produced when cosmic rays hit nitrogen and oxygen in the atmosphere and destroy these elements (nuclear spallation reaction). Beryllium-7 (7Be) is characterized by its relatively short half-life of 40 days.

　Since beryllium is a metal, when 7Be is produced in the upper atmosphere, it exists attached to other particles. It is then deposited in the oceans with other particles, and is deposited on the seafloor surface while adhering to particles in seawater as well. Since the half-life of 7Be is relatively short (40 days), it is useful to measure the amount of 7Be to investigate the movement (residence time, etc.) of particles in seawater over such a time scale.

　For example, 7Be is rarely detected in layers deeper than the surface mixed layer (~50m). By looking at the rate of decrease of 7Be in the surface mixed layer, we can estimate the residence time of particles in the surface mixed layer.