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Either of two decay products of thorium, mesothorium I, an isotope of radium, or mesothorium II, an isotope of actinium.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
(General Physics) physics obsolete either of the two radioactive elements which are decay products of thorium. Mesothorium I is now called radium-228. Mesothorium II is now called actinium-228
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