It's Elemental: Protactinium - Basic physical and historical information. education.jlab.org/itselemental/ele091.html
LANL: Protactinium - Basic information and history. pearl1.lanl.gov/periodic/elements/91.html
Protactinium - Data tables and historic information. www.scescape.net/~woods/elements/protactinium.html
Radiochemistry of Protactinium - Full text of the monograph by H. W. Kirby (Mound Laboratory, Miamisburg, Ohio). [PDF] lib-www.lanl.gov/cgi-bin/getfile?rc000046.pdf
Visual Elements: Protactinium - General and physical information, source, and key isotopes. www.chemsoc.org/viselements/pages/protactinium.html