Chapter 2 - Post-graduate Course for Those Who Want to Understand Why
I have found that scientists like myself, when talking with the public (too often at the public), start with a lecture on nuclear physics. After all, we are scientists because we find it interesting and we want to share our enthusiasm with others. However, my experience is that the audience's eyes glaze over and we lose them, so I aim to keep the nuclear physics to a minimum necessary for understanding the issues. Thus this chapter is optional and can be skipped.
Each atom can be thought of as a
relatively heavy nucleus, like the sun in our solar
system, surrounded by relatively light orbiting electrons,
like the planets. Each electron has a single negative electrical charge, and
the nucleus has a positive charge equal to the total number of electrons. When
fission occurs, the neutrons that continue the chain reaction are emitted from
the nucleus of the uranium atom. The two fission-product atoms initially have a
great deal of excess energy which they discharge by two processes, one
short-term, one long-term. In the first fraction of a second they move at high
speed through the surrounding material, usually the uranium fuel. In doing so
they disrupt and disorder the material's atomic structure, affecting some of
its properties. This process can be compared to a runaway truck tearing through
a full parking lot.
Even when the fission-product atom comes to rest it still has some excess energy compared with a normal atom of the same element: it is radioactive and sooner or later emits its excess energy as radiation. Radiation is the transfer of energy without the necessity for any intervening material, although radiation can occur through material that is transparent to it, e.g., light through water or glass. Nuclear radiation can be by the emission of particles, including the neutrons and electrons that we have already met, and also alpha-particles. These are the nuclei of helium atoms, particles four times as heavy as hydrogen nuclei, each with two positive charges. However, the radiation of most importance in discussing nuclear issues is gamma-radiation, with properties very similar to those of the better known X-radiation (X-rays).
Gamma- and X-radiation, unlike light and TV-radiation, is termed ionizing radiation because it ionizes the material through which it passess, i.e., it knocks negatively charged electrons out of the atoms leaving them as positively charged ions. The implications when the material is human tissue are discussed later (Chapter 7).
The number of orbiting electrons determines to which chemical element the atom belongs. Chemical processes, such as burning coal - the chemical reaction between carbon atoms in the coal with oxygen atoms in the air - involve only the electrons and not the nuclei. Fission, on the other hand, derives its energy from a nuclear reaction - between a neutron and the nucleus of a uranium atom, and so is properly termed an example of nuclear energy: the earlier terminology "atomic energy" was a misnomer. Other examples are geothermal energy, which results from the radioactive decay of the very low concentration of radioactive atoms throughout the earth; and solar energy. In solar energy, the nuclear reaction in the sun is fusion, the joining together of the nuclei of two light atoms, rather than the splitting of the nucleus of a heavy atom, fission.
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