Emission of Light Charged Particles in Uranium Fission

In a brief note published in one of the March issues of Nature, L. Green and D. Livesey*) report a phenomenon they have discovered: the disintegration of uranium nuclei under the action of slow neutrons into three parts.

The presence of a third charged particle, in addition to the two ordinarily observed heavy fragments, was discovered in studying the tracks of uranium-fission products in photographic plates. The experiments were carried out as follows: photographic plates, previously immersed in a liquid uranium salt, were irradiated with neutrons from a lithium source bombarded with deuterons and surrounded by paraffin, in which the neutrons were slowed down to thermal velocities. In their experiments the authors used Ilford plates with a special photographic emulsion, in which the concentration of silver bromide had been increased approximately eightfold in comparison with ordinary emulsions. As a result, the density of grains in the tracks of the particles is considerably greater than in the photographic plates used previously. For these experiments an emulsion was chosen that was sensitive to α-particles and heavy ions and insensitive to protons. Altogether 25,000 photographs were examined. According to the authors’ data, for \(80 \pm 4\) fission events there is one fission event with the emission of a third particle.

) Nature 159, No. 4036, 332 (1947). The work was done at the Cavendish Laboratory in Cambridge. Its preliminary results were reported at the conference on elementary particles in Cambridge in July 1946. The authors point out that the same phenomenon was observed by the French physicists Tsien, Chastel, Ho, and Vigneron, who published their work somewhat earlier (in November 1946) in C. R. Acad. Sci., Paris 223*, 986 (1946).

On the basis of the grain density in the tracks of the light particles, two possibilities may be admitted: either these particles are $\alpha$-particles, or they are nuclei of somewhat greater mass. But, as the authors assert, one may completely reject the supposition that the “third” particle consists of recoil nuclei knocked out by uranium fission products as they pass through the emulsion. If such an assumption is adopted, the observed directions of particle emission cannot be reconciled with the law of conservation of momentum.

In photographs with three particles, the range of the light particles and the angle between the tracks of the light and heavy particles were measured. It turned out that the ranges of the light particles lie within a wide interval from $1.7\ \mu$ (equivalent to $2.8\ \mu$ in air) to more than $250\ \mu$ ($45\ \text{cm}$ of air). Most frequently, however, short-range particles were found (with ranges up to $20\ \mu$). A second, considerably lower maximum on the curve of the distribution of particles by range is observed at $150\ \mu$. The relatively small weight of long-range particles is characterized by the following figure: one fission event with emission of a third particle having a range of more than $5\ \mu$ occurs per $340 \pm 30$ fission events (whereas, for light particles of all ranges, one “triple” fission event occurs per $80 \pm 4$ fission events). In studying the angular distribution, no appreciable difference was found between the short-range and long-range light particles. Both are emitted predominantly in a direction perpendicular to the tracks of the heavy fragments.

If it is assumed that the third particle is emitted simultaneously with the two heavy ones, then its track determines the point at which fission occurred. This makes it possible, in the case of “triple” fission, to find with sufficient accuracy the ranges of the heavy fission products. The curve constructed by the authors for the distribution of heavy fragments by range for those cases in which the light particles are long-range has two sharp maxima. The first maximum corresponds approximately to ranges of $10\ \mu$, the second to ranges of $13\ \mu$. The presence of two such peaks on the distribution curve indicates that, in those fission events which are accompanied by emission of a third particle of large energy, the heavy fragments have, for the most part, different masses.

In conclusion, the authors report that in all the 25,000 photographs examined they did not record a single fission event with the emission of more than three particles.

B. T. Geilikman