91 - Protactinium
Protactinium oxide. (1)
Crystals of protactinium and thorium metal, prepared
by the van Arkel (chemical vapor transport) process.
The promethium crystal was studied for superconductivity. (2)
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1 - "Matter", Ralph E. Lapp and the editors of LIFE, LIFE
Science Library, 1965.
2 - J. C. Spirlet, "Preparation of the actinide metals", p 361-380, printed in "Actinides in Perspective", N.M. Edelstein ed., Pergamon Press, 1981. I am not sure which piece is protactinium and which is thorium, the text is somewhat ambiguous.
3 - http://www.datadisc.org.uk/expts/radioact.htm - General discussion of an experiment for separating protactinium from uranium with solvent extraction.