5 - Alpha decay, fission and thermonuclear fusion

Summary

Alpha decay, fission and thermonuclear fusion all illustrate in different ways the importance of the phenomenon of quantum-mechanical tunnelling. In α-decay, an α-particle is contained by a barrier, called the Coulomb barrier, formed by the strong attractive potential and the electrostatic repulsive 1/r potential between the a-particle and the rest of the nucleus. The strong dependence of the probability of tunnelling through this barrier on the α-particle's energy is shown to account for the observed systematics of α-decay. In fission there is also a barrier arising from the nuclear and Coulomb forces and the observed spontaneous fission lifetimes are also explained by tunnelling.

The shape of the fission barrier is shown to depend significantly on shell corrections to the liquid drop model. These corrections explain the occurrence of fission isomers. The possibility of a chain reaction following neutron-induced fission and its application in nuclear reactors is then described. Finally the process of thermonuclear fusion of hydrogen within the sun and the possibility of making fusion reactors are discussed. It is shown how the luminosity of the sun can be understood as arising from the weak process: p + p → d + e⁺ + v, which is highly inhibited within the sun by the Coulomb barrier between the protons.

Alpha decay

From the semi-empirical mass formula most nuclei with A > 150 are unstable to decay by α-emission. This is not readily observed until nuclei with A > 200 because of the Coulomb barrier that the α-particles must penetrate by quantum-mechanical tunnelling.