from Wiktionary, Creative Commons Attribution/Share-Alike License.

  • noun physics A radioactive isotope of thorium, 22890Th, having a half-life of 1.4 x 1010 years; it occurs naturally, and is a source of nuclear energy

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • noun radioactive isotope of thorium with mass number 228


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