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I apologize if this is an obvious question, but I can't find the answer anywhere. In this page: http://ie.lbl.gov/education/parent/U_iso.htm are listed the isotopes of Uranium. Some of them, for example U238 are listed many times, with "m1" or "m2" attached to the mass number. What does this mean?

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    $\begingroup$ Quick Google search led me to wikipedia's article on metastable isomers. Notation is given there. $\endgroup$ – Kyle Kanos Jan 18 '14 at 17:00
  • $\begingroup$ Thank you! I used google too and found nothing... weird. $\endgroup$ – carllacan Jan 18 '14 at 17:23
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These "metastable states" are excited states of the nucleus that have a non-trivial lifetime (most nuclear excited states decay very quickly).

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    $\begingroup$ I just can't resist adding the truly remarkable example of why not all metastable states decay quickly: Tantalum-180m. It's the only naturally-encountered excited nucleus present in macroscopic quantities on Earth, and it is stabler than its ground state by an incredible factor of at least $10^{18}$. $\endgroup$ – Nicolau Saker Neto Jan 18 '14 at 20:36

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