Nuclear Reactions

A nuclear reaction is a process in which a ‘target’ nucleus is knocked about, and possibly broken up, by bombarding it with a beam of nucleons or other nuclei. A beam of protons or light nuclei (accompanied by some of their atomic electrons in the form of ions) is obtained by subjecting them to a large electric potential difference, which, because of their electric charge, accelerates them to a high energy. Accelerators are either linear, in which a high potential difference is maintained between two points, or circular, in which the particles are constrained by magnetic fields to move in circular orbits and are continually accelerated under the influence of oscillating electric fields. Neutrons, having no electric charge, cannot be accelerated in this way and beams have to be produced using an intermediate nuclear reaction as in the Chadwick experiment. The products of the nuclear reaction are investigated using detectors or counters such as scintillation counters ionization detectors and semiconductor detectors (in which particles excite electrons in a semiconductor into the conduction band, as in solar cells, so giving an electric signal). Nuclear reaction processes take essentially two forms. First, there are what are called direct reactions in which the disturbance of the nucleus by the bombarding particle is not very great. The particle may simply be scattered from the target nucleus without affecting it at all; it may knock a nucleon in the nucleus into a higher energy state, leaving the nucleus excited, or even knock it out; it may ‘pick up’ a nucleon from the nucleus and carry it away or be ‘stripped’ of one of its own component nucleons, leaving a different target nucleus. Second, there are compound nucleus reactions in which the bombarding particle gives up all its energy to the target nucleus and cannot escape thus forming a new (compound) nucleus. Of course, this compound nucleus has a lot of energy and eventually after. a lot of thrashing about and after a relatively long time (perhaps a million or more times longer than the time which would be taken for the bombarding particle simply to pass through the nucleus) this energy is concentrated onto one or more nucleons again which then escape, or the nucleus may get rid of the energy in the form of a photon. Whereas a direct reaction is essentially a continuous process, a compound nucleus reaction has two stages-the formation of the compound nucleus and then, after a while, the emission of one or more nucleons or a photon. The compound nucleus process is also characterized by showing ‘resonant’ behaviour. That is, the probability of it happening is much greater at certain values of the energy of the bombarding particle. This happens when that energy is such that the resultant energy of the compound nucleus coincides with one of its energy levels. Detailed study of these different processes is the preoccupation of many nuclear physicists and gives considerable information about the properties and distribution of nuclear energy levels. There are two other nuclear reaction processes which are of particular interest, namely fusion and fission. A fusion process is one in which two very light nuclei coalesce and energy is released. The best known of these is the fusion process responsible for ‘hydrogen burning’ in stars. In this process a series of nuclear reactions takes place starting with two protons (hydrogen nuclei) joining together to form the heavy hydrogen nucleus (²H) consisting of one proton and one neutron. To achieve this one proton has to convert to a neutron and this happens through the beta decay process which will be discussed in the next section. The heavy hydrogen nucleus then captures another proton, forming the nucleus of an isotope of helium (two protons and one neutron, ³He) and finally two of these nuclei join together and give up two protons so leaving the very stable helium nucleus (⁴He) consisting of two protons and two neutrons. The essential feature of these processes is that the final helium nucleus formed is very stable and a great deal of energy is released in its formation; conversely much energy is needed to knock it to pieces. The energy released in fusing very light nuclei together to form heavier nuclei follows in general terms, in which it can be seen that the binding energy per nucleon moving from very light to heavier nuclei increases, so leading to a release of energy in the fusion process. The magnitude of the energy released in such processes is of the order one million or more times the energy released in a chemical process. For decades physicists have been trying to achieve fusion on a commercial scale as a source of power and are gradually approaching this target. There are two approaches to the problem. One is to confine deuterium (²H) and tritium (³H) nuclei, together with loose electrons, in the form of a plasma (an assembly of ionized atoms and electrons) by powerful magnetic fields at temperatures around l0⁸ K. At this temperature the thermal kinetic energies of the nuclei are such that they can overcome the electric repulsion between themselves and fuse together. The other approach is to use inertial fusion in which a pellet of ²H and ³H is violently compressed using, for example, a laser beam or, as in a hydrogen bomb, a fission explosion. Turning now to nuclear fission, if a very heavy nucleus splits into two lighter nuclei, the binding energy per nucleon is greater in the fission products than in the heavy nucleus so energy is released in the process. Spontaneous fission, is a very rare process, but fission can be induced by bombarding a heavy nucleus such as uranium with neutrons: a neutron is captured by the nucleus and the resultant compound nucleus becomes so unstable that it readily splits apart. What is of extreme importance is that in the fission process a few spare neutrons are also let loose from the fissioning nucleus and these can then provoke fission in neighbouring nuclei and so on and so on. This is the origin of a chain reaction in which, given a sufficiently large collection (critical mass) of closely packed uranium nuclei, one loose nucleon sets in motion an escalating series of fission processes and, hence, a nuclear explosion. Such an explosion is controlled in a nuclear reactor by using control rods (usually of boron or cadmium), which are very good absorbers of neutrons and so limit the extent to which fission takes place. The energy released, again of the order a million or more times chemical energies, is then used to heat a circulating coolant to provide steam to drive a turbine for electric power generation.