The radioactivity distribution patterns produced in the metallurgical processing (particularly casting by consumable arc melting) of thorium metal have been studied. Samples from various locations in the metal, from material deposited on process equipment, and from process area environmental sampling were subjected to gross counting, alfa- and gamma-spectrometry, and growth-decay studies. Separation of thorium and radium and the release of thoron gas are the two important effects observed. A simple gamma-spectrometry photopeak ratio method for determining the enrichment or depletion of radium in thorium samples, along with its calibration, is described. The data indicate extensive removal of the radium daughters from thorium in arc-melting, two successive arc-meltings removing essentially all of the Ra228. The radiological hazard and analysis implications of these radioactivity distribution patterns are discussed.
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