O. R. Frisch

Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction

ON bombarding uranium with neutrons, Fermi and collaborators1 found that at least four radioactive substances were produced, to two of which atomic numbers larger than 92 were ascribed. Further investigations2 demonstrated the existence of at least nine radioactive periods, six of which were assigned to elements beyond uranium, and nuclear isomerism had to be assumed in order to account for their chemical behaviour together with their genetic relations.

The discovery of fission

THE NEUTRON was discovered in 1932. Why, then, did it take seven years before nuclear fission was found? Fission is obviously a striking phenomenon; it results in a large amount of radioactivity of all kinds and produces fragments that have more than ten times the total ionization of anything previously known. So why did it take so long? The question might be answered best by reviewing the situation in Europe from an experimentalists point of view.

Capture of Slow Neutrons

IT is known that some nuclei have very large capture cross-sections for slow neutrons. This fact may be explained from the point of view of wave mechanics, which also leads to the consequence that the capture cross-section should decrease with 1/v with increasing velocity of the neutrons, so long as the capture probability is constant over the velocity region concerned.

PARITY NONCONSERVATION IN WEAK INTERACTIONS

Publisher Summary This chapter describes parity non-conservation in weak interactions. In quantum theory, the symmetry condition led to the requirement that any non-degenerate wave function should possess either even or odd parity, that is, on being subjected to inversion—the replacement of each co-ordinate by its negative—should become multiplied either by + 1 or by –1, both possibilities leading to the same value. The mixing or interference of the opposite parity states produces asymmetry. It has been shown that parity would be conserved in all transitions, that is, the overall parity of a system would be the same before and after the transition, if all particles formed or destroyed were taken into account. It seemed unlikely that two particles of opposite parity should be similar. Because the interaction on which their decay depended was novel and weak, comparable in weakness with the one that causes β decay, the possibility that parity might not be conserved in those weak interactions was explored.

Die Elementarteilchen der Physik

Von den alten Griechen, mit denen man bei den Elementarteilchen naturgemas anfangen mus, da sie den Atombegriff erfunden haben, mochte ich heute absehen. Im fruhen 19. Jahrhundert glaubten die Wissenschaftler, den eigentlichen Atomen auf der Spur zu sein, namlich den chemischen Atomen. Aber zu Ende dieses Jahrhunderts folgte die Entdeckung des Elektrons und damit die aufregende Erkenntnis, das es zumindest ein Teilchen gibt, das in allen chemischen Atomen enthalten ist. Als dann Rutherford (1911) die Existenz der Atomkerne erkannte, wurde es sehr bald klar, das auch der Wasserstoffkern — der leichteste Atomkern — als ein „wahres Atom“, ein Elementarteilchen, anzusehen ist.