The Emission of Long-Range Charged Particles in the Slow Neutron Fission of Heavy Nuclei

Abstract

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 $\ensuremath{\alpha}$-particles. The frequency of emission of these $\ensuremath{\alpha}$-particles was found to be 1 in 405\ifmmode\pm\else\textpm\fi{}30 fissions for ${\mathrm{U}}^{233}$, 1 in 505\ifmmode\pm\else\textpm\fi{}50 fissions for ${\mathrm{U}}^{235}$, 1 in 445\ifmmode\pm\else\textpm\fi{}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 $\ensuremath{\alpha}$-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 $\ensuremath{\alpha}$-emission is about equal to that liberated in binary fission. Possible explanations for $\ensuremath{\alpha}$-emission in fission are discussed.