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

Light charged particles with ranges greater than 6 cm of air produced in the slow neutron fission of U²³³, U²³⁵, and Pu²³⁹ 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 U²³⁵ 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 α-particles. The frequency of emission of these α-particles was found to be 1 in 405±{}30 fissions for U²³³, 1 in 505±{}50 fissions for U²³⁵, 1 in 445±{}35 fissions for Pu²³⁹. 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 α-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 α-emission is about equal to that liberated in binary fission. Possible explanations for α-emission in fission are discussed.