High Quality Content by WIKIPEDIA articles! Protactinium is a chemical element with the symbol Pa and atomic number 91. Its longest-lived and most abundant naturally-occurring isotope by far, Pa-231, is a decay product of uranium-235 (U-235), and it has a half-life of 32,760 years. Much smaller trace amounts of the short-lived metastable isotope Pa-234m occur as decay products of uranium-238 (U-238). Pa-233 results from the decay of thorium-233 as part of the chain of events used to produce uranium-233 by neutron irradiation of thorium-232.

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