Isotopes of europium

Naturally occurring europium (₆₃Eu) is composed of two isotopes, ¹⁵¹Eu and ¹⁵³Eu, with ¹⁵³Eu being the more abundant (52.2% natural abundance). While ¹⁵³Eu is observationally stable, ¹⁵¹Eu was found in 2007 to be unstable and undergo alpha decay; its measured half-life of 4.6 × 10¹⁸ years corresponds to 1 alpha decay per two minutes per kilogram of natural europium, so for practical purposes it can be considered stable. Besides the natural radioisotope ¹⁵¹Eu, artificial radioisotopes from ¹³⁰Eu to ¹⁷⁰Eu have been made, with the most stable being ¹⁵⁰Eu with a half-life of 36.9 years, ¹⁵²Eu with a half-life of 13.517 years, ¹⁵⁴Eu with a half-life of 8.592 years, and ¹⁵⁵Eu with a half-life of 4.742 years. All the others have half-lives shorter than 100 days, with the majority shorter than 3 minutes.

This element also has 27 metastable isomers, with the most stable being ^150mEu (12.8 hours), ^152m1Eu (9.3116 hours) and ^152m5Eu (96 minutes). The primary decay mode for isotopes lighter than ¹⁵³Eu is electron capture to samarium isotopes, and the primary mode for heavier isotopes is beta minus decay to gadolinium isotopes. ¹⁵²Eu and ¹⁵⁴Eu can decay either way, as can ^150mEu (meta state only).