1.2 Interaction of radiation with nuclei

In Section 1.5 of this chapter the interaction of radia­tion with the orbital electrons of atoms was discussed. Radiation may also interact with, and induce changes in, the nuclei of atoms. Such interactions are called nuclear reactions and may be represented by X + x — Y + y. The target nucleus X reacts with the incident particle x to give a product nucleus Y and particle y. The reaction may also be written X (x, y) Y. The particles x and у may include the photon ‘particle’ of 7 rays.

1.3 The compound nucleus

A model to explain what takes place during a nu­clear reaction was put forward by the Danish scientist Niels Bohr in 1936 and is known as the theory of the compound nucleus. Bohr postulated that a nuclear reaction occurs as two distinct events:

• An incident particle is absorbed by the target nu­cleus to form a compound nucleus.

• The compound nucleus disintegrates by ejecting a

particle to leave a final product nucleus.

Bohr’s postulation that the two steps are independ­ent and may be regarded as separate processes is in agreement with the observed facts of nuclear transmutation.

When a nuclear particle enters a nucleus its energy is quickly shared amongst all the nucleons of the nucleus. The compound nucleus is said to be in an excited state. The magnitude of the excitation is the sum of the kinetic energy of the incident particle and its binding energy within the compound nucleus. The excess energy is shared in a random manner. At a given instant the excitation energy may be shared by several nucleons and at a later time it may be shared by some other nucleons; or again it may be concen­trated in one nucleon or group of nucleons. When this happens the one nucleon, or group of nucleons, may have enough energy to break away from the com­pound nucleus. The compound nucleus disintegrates into the product nucleus and an outgoing particle.

As a result of the random manner in which the excitation energy is distributed, the compound nucleus has a lifetime which is relatively long (about 10-l4s) in comparison with the time it takes a particle to travel across the nucleus (about 10-,7s for ‘slow’ neutrons). Thus during its relatively long lifetime the compound nucleus can ‘forget’ how it was formed and so have a disintegration which is independent of its formation.

It should be noted that in a nuclear reaction the product nucleus may be radioactive and subsequently suffer further disintegration through radioactive decay.