BASIC NUCLEAR PHYSICS

Elements

All matter on earth - our bodies and everything around us - is made from more than a hundred basic substances called elements. Some of the most familiar elements include oxygen, nitrogen, hydrogen, and carbon.

Composition of Elements

Elements are composed of atoms, which are in turn made up of sub-atomic particles called protons, neutrons, and electrons. The protons and neutrons form the central nucleus of the atom, and the electrons revolve in a cloud around the nucleus. Elements are classified according to the number of protons in the nuclei of their atoms. For example, atoms with one proton are all hydrogen atoms; atoms with six protons are all carbon atoms.

Structure of the Atom

This figure shows the structure of an atom:

Nuclides

Although the number of protons for each element remains consistent, the number of neutrons in the atom of an element can vary. For instance, in addition to its single proton, the most common form of hydrogen has no neutrons. However, hydrogen can also occur with one neutron, when it is referred to as deuterium, or even with two neutrons, when it is called tritium. These variations of an element are referred to as nuclides.

Hydrogen Nuclides

This figure depicts the nuclides of hydrogen:

Radioactive Decay

Most nuclides are stable; that is, the number of protons and neutrons never changes. In some atoms however, the number of protons and neutrons makes the atom unstable, so it emits excess energy and ejects one or more subatomic particles, or splits into two atoms of smaller elements, in order to become more stable. This process is referred to as radioactive decay.

Ionising and Non-Ionising Radiation

The particles or high-energy rays emitted during the radioactive decay process can pass through matter, and in so doing, can detach electrons from, or ionise, other atoms; it is therefore referred to as ionising radiation. Other types of radiation, such as radiant heat, microwaves, and radio waves, do not change the structure of other atoms, and are referred to as non-ionising radiation.

Radionuclides

Elements that emit ionising radiation are radioactive. An element may have one or more radioactive nuclides; for example, of hydrogen's three nuclides, hydrogen-1 and deuterium are stable, and tritium is radioactive. Carbon has seven nuclides, of which five are radioactive and only two are stable. Radioactive nuclides are referred to as radionuclides.

Half-Life

One of the important characteristics of a radionuclide is its half-life, which refers to the length of time it takes for the amount of radioactivity in a substance to decay to half its original value. That is to say, after one half-life, one half of the radioactivity is left in the substance; the next half-life reduces the remaining amount by half, and so on until there is no radioactivity left. A half-life can be as brief as a fraction of a second for some radionuclides, or as long as billions of years for others.

Fission

As a radionuclide decays, the neutrons that are released may be absorbed by other atoms. When these neutrons are absorbed by a heavier element such as uranium or plutonium, the atomic nuclei may actually split into two or more smaller fragments, which are generally radioactive. This process is called fission, and the smaller fragments are fission products.

The Fission Process

This figure portrays the fission process:

Nuclear Power Reactors

Occasionally fission can occur spontaneously in nature, but it is usually produced in a nuclear power reactor. Nuclear reactors sustain a controlled chain reaction of fissioning atoms, in which neutrons released from atoms that have fissioned will go on to split other atoms. In a nuclear reactor, fission takes place in a moderator, a substance surrounding the nuclear fuel that slows down the neutrons that have been released so that they are more likely to fission other atoms in the fuel. This process produces a continuous source of energy in the form of heat, which is used to create steam to drive turbines that generate electricity.

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