-9- peaks are clearly defined. The overall fraction of typical singly-charged fragments seemsto be between 0.1% and 0.2% of the total beam for both the light and the heavy groups of fragments. Figures 8 through 15 present the results of several experiments in whieh radiechemical analyses were made for a number of nuclides. The principal difficulty encountered was due to the large amount of fission-produced heat liberated in the source. The resulting elevated temperature frequently induced recrystallization of the enriched uranium fission source foil, which apparently resulted in the release of large quantities of volatile fission products. These products passed down the beam tube as gases and the daughters were deposited on the collector foil. Fission products with rare gas precursors spread over the entire collection foil obscuring the position of those fragments of the same mass separated by the spectrometer. The Balto data (Fig. 8) are presented to illustrate the difficulty. Subsequently, the apparatus was modified to permit the insertion of a thin film, penetrable by energetic fission fragments but not by diffusing gas, at a point in the beam tube prior tothe collimator. The thin film did not sur- vive long (1 week) irradiations and was replaced by a wire grid biased to -90 V about 1 cm from the collector foil to collect daughters of radioactive gaseous fission fragments. The collector foil was grounded. The wire was 0.002 inch in diameter and passed over the collector foil at 1 cm intervals. reduced general background levels to acceptable values, Ba 140 This screen Unfortunately, the was not remeasured with the screen in place; however, the sr?! data were taken with the screen in place and provide a sharp contrast to those of Bal 49 without the screen. There is clear indication in many cases of the resolution of low charged species (+1 to +4). Repetition of the Zr?! 97 and Mo99 data (Figs. 10, 11) with 9001838