-4of a moving fragment, but no simple approach seems available to reduce the charge spread of the original fragments. One would therefore like to design a system suchthat exactly the right amount of matter could be interposed be- tween the fission source and the mass-separation device to reduce the most likely charge on each fragment to unity. The appropriate amount of matter is presumably a function of fission fragment mass number, nuclear charge, and original kineiic energy; since these factors all vary widely, the selection of an "appropriate amount" is extremely difficult. The problem can be very simply avoided by using the fissionable material itself as the absorber; if one uses a source of fissionable material that is thicker than one range of fission fragments, there will always be some appropriate depth within the source that will be covered with the proper amount of matter such that any given species will have maximum probability of emerging from the surface with a single charge. Fragments originating at a greater depth will emerge uncharged, or fail to emerge; those originating at a lesser depth will emerge with charges greater than one, The questf6n of the intensity of the singly- charged frag- ment beam from such a thick source remains to be determined; for proper design of an apparatus for mass separation we also require information about the energy spectrum of such a beam. The feasibility of this experimental approach, then, depends 1) on the intensity of the singly-charged fission fragment beam emergent from a thick piece of fissionable material immersed in an available neutron flux, and 2) on the ability to design and build a device for separating such fragments. Deter- mination of the abundance and approximate energy spectrum of the singlycharged fission fragment beam emergent from a thick source constitutes the experimental part of this report. 5001833