beam shutter opened. Fission fragments were allowed to accumulate on the col- lector foil for periods ranging from a day toa week. The reactor was then shut down, and the collector foil was immediately removed, placed in contact with a large x-ray film, and left for a period ranging between 8 and 48 hours. A sketch representing a typical autoradiograph of the collector foil is shown in Fig. 4; although the contrast was low in the original autoradiograph, it was .possible to see bands of radioactivity. Positions of these bands, in order of ap- pearance from the right of the picture, correspond to the heavy group of fission fragments, charge 1; the light group, charge 1; the heavy group, charge 2; the heavy group, charge 3; and the light group, charge 2, superimposed; the heavy group, charge 4; and the light group, charge 3, superimposed; and others not clearly resolved. The great majority of the beam, as might have been expected, lies in the large region at the left of the picture, containing ions of higher charge. (The original autoradiograph could not be reproduced for publication due to poor contrast.) A certain amount of information is available from inspection of the pat- tern. First, ions of low charge do indeed occur in appreciable abundance. Second, the energy range for low-charged ions is broad but limited, the range being about a factor of 2. Third, the charge on a slowly-moving fission frag- ment is approximately proportional to its energy, since the electrical displace- ment of the bands for ions of low charge is roughly constant. Finally the charge on an energetic ion is approximately proportional to its velocity, since the magnetic deflections of the highly- charged ions do not vary as much as might be expected, Following the autoradiographic examination, the collector foils were cut apart and the pieces analyzed separately for various fission products. The method of cutting depended on a simple geometric property of the parabola. © 9001836