FISSION OF URANIUM NUCLEI INTO THREE AND FOUR FRAGMENTS

Usually uranium nuclei split into 2 fragments of approximately equal size. In a Wilson chamber or on photographic plates, 2 tracks emerging from one point are observed in this case. The total energy of the fragments is approximately 200 MeV. However, according to the theory of Bohr and Wheeler[^1], the maximum energy that should be released when a nucleus fissions into three charged

FISSION OF URANIUM NUCLEI INTO THREE AND FOUR FRAGMENTS

fragment, would be equal to 220 MeV. Therefore fission into three or more fragments should be considered possible, and experimenters attempted to detect this process. The experiments of Green and Livesey in Cambridge and of Tsien-San Tsiang and others in the Joliot laboratory\({}^{2,3}\) gave positive results.

In the experiments of the French researchers, plates coated with photographic emulsion were used. They were wetted with a 10% solution of uranyl nitrate, dried, and then bombarded with slow neutrons. After development, the plates revealed, first of all, tracks of fragments from ordinary fission into 2 parts. These tracks were arranged along one straight line (the momenta of the fragments are equal in magnitude and opposite in direction). Therefore it was not possible to find the beginnings of the tracks. Branches extend from many tracks owing to collisions with the nuclei of the emulsion. In some cases, however, a branching has a peculiar appearance. It consists of one long and very thin track and must be attributed to a light nucleus. The conservation laws are not fulfilled if one assumes that the thin track is produced by emulsion nuclei that collided with a fragment. Therefore the authors believe that this is a case of fission of a uranium nucleus into three charged fragments. The masses of the heavy fragments are obtained from the conservation laws as 99 and 131. For the third fragment, the authors in one case give 5 or 6, but this is not excluded, since this fragment may be an \(\alpha\)-particle (mass 4). In another case, however, the mass of the third fragment turns out to be equal to 9. The total kinetic energy of triple fission is on average 165 MeV—somewhat greater than in double fission. If one assumes that the excitation energy of the nuclei is of the same order in both cases, then the total energy released in the reaction agrees in magnitude with the theoretical one. The ratio of the probability of triple fission to the probability of fission into 2 fragments is equal to 0.003. It is possible, however, that heavier fragments were not taken into account here because of the impossibility of distinguishing the third fragment from collision nuclei arising in the emulsion.

In addition to triple fission, the indicated researchers succeeded in observing quadruple fission, i.e. fission into four fragments. In this case, either 2 comparatively light and 2 heavy fragments are obtained, or 3 heavy fragments and 1 light one. The probability of fission into 4 fragments is 0.003 of the probability of fission into 2 fragments.

In connection with the experiments of Green and Livesey and of Joliot’s collaborators, Feather\({}^{4}\) in a recently published article considered the question of the emission of \(\alpha\)-particles in the fission of uranium. Feather believes that fission into 2 fragments always occurs, but that after the separation of the fragments from one another they remain for a time of the order of \(10^{-20}\) sec. in a strongly deformed state. At the moment when the deformation is large, the fragments can emit \(\alpha\)-particles. For this it is necessary that the nucleus produced as a result of fission be unstable with respect to \(\alpha\)-decay. The latter can occur for small atomic weights \(A\) at a given number \(Z\). In the known cases, the neutrons in fission are distributed very unevenly, and fragments with a high proton content are observed, although rarely. It is precisely these fragments that should be capable of emitting \(\alpha\)-particles. Feather considers the possible existence of two groups of \(\alpha\)-particles: short-range ones—from the light group of fragments \((A = 90\text{–}100)\), and long-range ones—from the heavy group \((A = 130\text{–}140)\). It should be noted that the mechanism proposed by Feather cannot explain the emission, in addition to two heavy fragments, of particles with weight greater than 4 in triple and quadruple fission.

P. Nemirovsky

References

1. N. Bohr and J. A. Wheeler, Phys. Rev. 56, 426 (1939).
2. Tsien San Tsiang, Ho Zah Weg, Chastel, Vigneron, Phys. Rev. 70, No. 6 (1947).
3. Tsien San Tsiang, Ho Zah Weg, Chastel, Vigneron, C. R. 223, 986 (1946); 223, 1119 (1946); 224, 272 (1947).
4. Feather, Nature 159, 607 (1947).