This seems to be conclusive physical evidence for the breaking up of uranium nuclei into parts of comparable size, as indicated by the experiments of Hahn and Strassmann.Įxperiments with thorium instead of uranium gave quite similar results, except that surrounding the neutron source with paraffin did not enhance, but slightly diminished the effect. From this it can be estimated that the ionizing particles must have an atomic weight of at least about seventy, assuming a reasonable connection between atomic weight and effective charge.
Since the longest path of a particle in the chamber was 3 centimeters, and the chamber was filled with hydrogen at atmospheric pressure, the particles must ionize so heavily that they can make 2 million ion pairs on a path equivalent to 0.8 cm of air or less. When the amplifier was connected to an oscillograph, the large pulses could be seen very distinctly on the background of much smaller pulses due to the alpha particles of uranium.īy varying the bias of the thyratron, the maximum size of pulses wasįound to correspond to at least 2 million ion pairs, or an energy loss of 70 Mev of the particle within the chamber. It was checked that the number of pulses depended linearly on the strength of the neutron source this was done in order to exclude the possibility that the pulses are produced by accidental summation of smaller pulses. With the neutron source at a distance of four centimeters from the uranium lining, surrounding the source with paraffin wax enhanced the effect by a factor of two. No pulses at all were recorded during repeated check runs of several hours total duration when either the neutron source or the uranium lining was removed. About 15 particles per minute were recorded when 300 milligram of radium, mixed with beryllium, was placed one centimeter from the uranium lining. The amplifier was connected to a thyratron which was biased so as to count only pulses corresponding to at least 5 x 10 5 ion pairs. Each such particle should produce a total of about 3 million ion pairs.īy means of a uranium-lined ionization chamber, connected to a linear amplifier, I have succeeded in demonstrating the occurrence of such bursts of ionization. In spite of their high energy, these nuclei should have a range in air of a few millimeters only, on account of their high effective charge (estimated to be about 20), which implies very dense ionization. If this picture is correct, one would expect fast-moving nuclei of atomic number 40 to 50 and atomic weight 100 to 150, and up to 100 Mev energy, to emerge from a layer of uranium bombarded with neutrons. The energy liberated in such processes was estimated to be about 200 Mev, both from mass defect considerations and from the repulsion of the two nuclei resulting from the "fission" process. it has been pointed out that this might be explained as a result of a "fission" of the uranium nucleus, similar to the division of a droplet into two. 276 (1939)įrom chemical evidence, Hahn and Strassmann conclude that radioactive barium nuclei (atom number Z = 56) are produced when uranium (Z = 92) is bombarded by neutrons. Frisch on Fission from 1939 Physical Evidence for the Division of Heavy Nuclei under Neutron Bombardment
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