Light charged particles with ranges greater than 6 cm of air produced in the slow neutron fission of ${\mathrm{U}}^{233}$, ${\mathrm{U}}^{235}$, and ${\mathrm{Pu}}^{239}$ have been studied in detail by coincidence counting methods. In each case particles with a continuous range distribution extending to about 50 cm of air were observed, the distribution showing a broad maximum in the neighborhood of 20 cm. Direct measurement of the energies of the light particles from ${\mathrm{U}}^{235}$ showed a fairly symmetrical distribution about 15 Mev with a maximum energy of about 26 Mev. Comparison of the energy and range distributions shows that all the long-range particles are $\alpha$-particles. The frequency of emission of these $\alpha$-particles was found to be 1 in 405±30 fissions for ${\mathrm{U}}^{233}$, 1 in 505±50 fissions for ${\mathrm{U}}^{235}$, 1 in 445±35 fissions for ${\mathrm{Pu}}^{239}$. No protons were observed, although the apparatus would have detected any with ranges lying between 10 and 100 cm of air. The energy distribution of fission fragments coincident with long-range $\alpha$-particles was also measured. The usual two peaks were observed indicating asymmetric division of mass, but each peak was shifted to a lower energy than is observed in binary fission. Quantitative comparison of the energies involved showed that, on the average, the total kinetic energy carried away in fission accompanied by $\alpha$-emission is about equal to that liberated in binary fission. Possible explanations for $\alpha$-emission in fission are discussed.

• Received 28 June 1950

DOI:https://doi.org/10.1103/PhysRev.80.181