There are two kinds of scattering events:

• Elastic scattering In a sense, elastic scattering is not a nuclear reaction because the neutron does not enter the nucleus to form a compound nucleus but is merely deflected by the nuclear fields. Elastic scattering may be regarded as a neutron colliding with a nucleus and rebounding; the total kinetic energy is conserved and the energy exchange be­tween neutron and nucleus may be calculated using the ordinary Newtonian laws of motion. The energy lost by the neutron in the collision depends on the angle through which it is scattered. Elastic scattering is primarily important as a way in which neutrons can lose energy and is discussed further in this context in Section 6 of this chapter.

1.3.1 Absorption reactions

There are two kinds of absorption events, capture and fission reactions:

Capture reactions Here the neutron is retained in the nucleus which then emits a nuclear particle and/or 7 radiation. Examples of neutron capture events follow:

* (n, >) reaction. The emission of the binding energy of the neutron as a 7 ray, the most common neutron capture process, is known as radiative capture.

115[„ + In — H6ln + 7 49 0 49

or In-115 (n, 7) In-116

59Co + In —[1] 60Co + 7

27 0 27

or Co-59 (n, 7) Co-60

In the above, as is often the case in radiative capture, the product nuclei indium 116 and cobalt 60 are both radioactive and emit j3/7 radiations with half life of 54 minutes and 5.3 years respectively.

These are examples of stable materials being trans­formed by neutron capture into radioactive iso­topes. This is known as neutron activation. In a reactor the neutron activation of indium is some­times used to measure the neutron density. Because of the 7 ray that accompanies the 0 radiation, activation of the cobalt present in ferrous materials can cause difficulties with maintenance work and in the eventual decommissioning of the plant.

. • (n, a) reaction:

Юв + In -* ?Li + 4hc

5 0 3 2

or B-10 (n, a) Li-7

This reaction is used in instrumentation to detect neutrons indirectly. Because neutrons are electri­cally neutral they do not ionise atoms to any great extent. However, the alpha particles produced in the neutron reaction with boron cause ample ionisa­tion and are readily detected.

* • (n, p) reaction:

14n + In — 14c + Ip

7 0 6 1

or N-14 (n, p) C-14

Alternatively, as the proton is identical with the nucleus of hydrogen, the reaction may also be written:

]4n + In " 14c + 1h

7 0 6 1

The product nucleus, carbon 14, is radioactive emitting a negative /3 particle and regenerating ni­trogen 14. The reaction is the basis for the carbon dating technique in archaeology: the nitrogen tn the atmosphere is bombarded by neutrons originating in the extra-terrestrial cosmic rays producing, it is presumed, a historically constant concentration of carbon 14 in the atmosphere and hence in plants and other living things. At death, the carbon 14 intake from the air ceases and the concentration in the ‘body’ or plant material reduces through radio­active decay. Knowing the initial concentration and half life (5570 years) of carbon 14, the moment of death can be determined.

Fission reactions The neutron absorption reaction other than capture is fission. The neutron absorbed by the target nucleus induces the resulting compound nucleus to split into usually two parts with the si­multaneous release of some neutrons and considerable energy, primarily in the form of the kinetic energy of the fission products.

Fission is the most important nuclear reaction of all as it is the source of the energy that enables gen­eration of power by nuclear means. The fission process will be discussed in detail in Section 4 of this chapter — firstly it is advantageous to introduce the concept of cross-section.