Long-lived radio-isotopes: Unique signatures of close-by supernovae in the past

The experiments searching in solar system bodies for freshly produced radionuclides from outside the solar system are reviewed, with a special focus on accelerator mass spectrometry. A surplus of ⁶⁰Fe has been found at eight locations in terrestrial oceans as well as in lunar samples. This surplus is attributed to supernova activity within 300 light years from the Sun about 1.5 to 3.0 Ma ago. An intensive search for the r-process nuclide ²⁴⁴Pu in deep-ocean material yielded an upper limit far below the expectation if it were produced exclusively by core-collapse supernovae. Thus, a much less frequent stellar event than core-collapse supernovae must be the site where the heaviest nuclides are produced.