mesothorium

noun
meso·​thorium | \¦me|zō, ¦mē|, |sō+\

Definition of mesothorium 

: either of two radioactive products intermediate between thorium and radiothorium in the thorium series or a mixture of the two products obtained usually from thorium minerals (as monazite sand) and used as a substitute for radium especially in luminous paints:

a : an isotope of radium

called also mesothorium 1

symbol MsTh1 or Ra228

b : an isotope of actinium

called also mesothorium 2

symbol MsTh2 or Ac228

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History and Etymology for mesothorium

New Latin, from mes- + thorium

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