Calcium (20Ca) has 26 known isotopes, ranging from 35Ca to 60Ca. There are five stable isotopes, plus one isotope (48Ca) with such a long half-life that it is for all practical purposes stable. The most abundant isotope, 40Ca, as well as the rare 46Ca, are theoretically unstable on energetic grounds, but their decay has not been observed. Calcium also has a cosmogenic isotope, 41Ca, with half-life 99,400 years. Unlike cosmogenic isotopes produced in the air, 41Ca is produced by neutron activation of solid 40Ca in rock and soil. Most of its production is in the upper metre of the soil column, where the cosmogenic neutron flux is still strong enough. The most stable artificial isotopes are 45Ca with half-life 162.61 days and 47Ca with half-life 4.536 days. All other calcium isotopes have half-lives of minutes or less.