Calcium
Calcium has 6 naturally occurring isotopes (40Ca, 42Ca, 43Ca, 44Ca, 46Ca and 48Ca), of which 48Ca is very slightly radioactive with a half-life about 6.4×1019 years. Traces of 41Ca, 45Ca and 47Ca are made by cosmic rays striking atoms, and are called cosmogenic nuclides. 41Ca is made by the neutron activation of 40Ca in the upper layers of the Earth's crust and has a half-life of 102,000 years. Since it decays to potassium-41, it is a critical indicator of solar system anomalies.