Calcium has 6 naturally occuring isotopes (Ca, Ca, Ca, Ca, Ca and Ca), of which Ca is very slightly radioactive with a half-life about 6.4×10 years. Traces of Ca, Ca and Ca are made by cosmic rays striking atoms, and are called cosmogenic nuclides. Ca is made by the neutron activation of Ca 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.