-~li- It seems appropriate to consider whether any possible chemical effect may contribute to the discrepancy between mass spectrometric methods of chain-length measurement as exemplified by the work of Armbruster et al. and other methods of deducing chain length as given for example in the paper of Wahl al, i or the papers of Glendenin al. 8 and Bowman, 10 The dis- crepancy appears to be serious enough to indicate that a real phenomenon other than experimental error may be involved, although at very high ion energies chemical effects would seem to be unlikely a priori. The single- charge parabola for mass 99 has been examined in detail by sectioning it into pieces corresponding to 0.25 MeV energy intervals. The energy spectrum of the mass-99 singly-charged ions is shown in Fig. 16. The energy appears to be quite sharply peaked at about 1.6 MeV, and nearly all the beam is included in the energy region between 1.25 and 2.25 MeV. CONCLUSIONS The existence of singly-charged fission fragments emerging from a thick source of fissionable material has been conclusively demonstrated. The abundance of such fragments is small relative to that of the fragments of higher charge; however, with a suitably high flux one could presumably obtain a high enough intensity for investigation, depending upon the nature of the proposed experiment. The practical problem of designing a mass-separator to work on the obtainable particle beam is severe, though not insupereble. The low abundance of singly-charged ions means that one requires either a high neutron flux or a large-area source; either alternative requires the removal of a considerable amount of heat from the source region to avoid the emission of volatile products. A more difficult problem is the design of the separator itself; the high 5001840