-16- Fig. 7. Gross activity of cut sections of collector foil, showing predicted positions of light and heavy fission fragments. The general background level is presumed to come from deposition of fission products thermally evaporated from the source, Fig. 8. This background level persists over the entire collector foil. Low charged fragments of mass 140 as measured by Ba 140 have been obscured by decay products from thermally evaporated xel*?, Fig. 9. Low charged fragments of mass 91 as measured by gr7t. Fig. 10. Low charged fragments of mass 97 as measured by Zr?" Fig. 11. Low charged fragments of mass 99 as measured by Mo?”, Fig. 12. Low charged fragments of mass 131 as measured by pst, Fig. 13. Low charged fragments of mass 132 as measured by Tel ??, Fig. 14. Low charged fragments of mass 141 as measured by cel 4} Fig. 15. Low charged fragments of mass 143 as measured by cel 48, Fig. 16. Energy spectrum of singly charged mass 99 fission fragments emerging from a thick uranium source. S081 3845