151m

Naturally occurring praseodymium (59Pr) is composed of one stable isotope, 141Pr. Forty radioisotopes have been characterized with the most stable being 143Pr, with a half-life of 13.57 days and 142Pr, with a half-life of 19.12 hours. All of the remaining radioactive isotopes have half-lives that are less than six hours and the majority of these have half-lives that are less than 33 seconds. This element also has 19 meta states with the most stable being 138mPr (2.12 hours), 142mPr (14.6 minutes) and 134mPr (~11 minutes).
The known isotopes of praseodymium range from 121Pr to 161Pr. The primary decay mode of isotopes lighter than 141Pr is positron emission or electron capture to isotopes of cerium, while that of heavier isotopes is beta decay to isotopes of neodymium.

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