1.8.5 Selection of Radionuciides. The radionucildes chosen for determination from the particle samples were thoge of greatest concern in worldwide fallout, namely, Sr” and Cs’, plus a sufficient number of others to provide basic data for further investigation of fractiona- tion. In the latter category were Sr*’, y*!, Mo’, Cs'™, ce, Euand u"", The members of this group existed in a variety of forms, ranging from gaseous to relatively nonvolatile species, . during the period of condensation from the flreball. Ca‘’ was determined in conjunction with elemental analyses for calcium and sodium to help in tracing the behavior of the environmental material that forma the major part of the fallout particles. _. Analyses for I'S‘, which were tentatively planned originally, were not carried out because of the limited analytical personnel available, the uncertainties of sample collection for this nuclide, and the relatively lesser interest in its ultimate fate. 21