Module 5: Basic Concepts in Radiation Physics - Introduction

HISTORICAL OVERVIEW:

Radiation is intimately connected with atomic structure, a fundamental property of matter, which has challenged scientific and philosophical minds since antiquity.

The major steps which led to our understanding of the structure of the atom are:

• The earliest descriptions visualised all matter as being made up of different proportions of so-called "elements": fire, air, earth and water.
• Leucippus (c 460 BC) assumed that matter was made up of atoms, held to be infinite in number, indivisible, and forever in motion. Democritus (c 460-370 BC) supported and amplified this view.
• John Dalton (1766-1844) formulated an atomic theory: elements are made up of discrete indestructible particles (atoms), which retain their identity through chemical reactions. Molecules consist of atoms of various elements.
• W. K. Roentgen discovers X-rays (1895)
• Henri Becquerel discovers radioactivity (1896)
• Lord Rutherford discovers the atomic nucleus (1974)
• Niels Bohr proposes a dynamic, "planetary model" for the atom
• J Chadwick discovers the neutron (1932)

ATOMIC STRUCTURE:

The atom is made up of a positively charged nucleus, which makes up most of the mass of the atom. Outside the nucleus are found the negatively charged electrons:

• The nucleus consists of
  • protons, with unit positive charge (1.60 x 10^-19 C) and a mass of 1.673 x 10^-27 kg, and,
  • neutrons, which carry no charge, and have a mass of 1.675 x 10^-27 kg, (about the same mass as that of the proton).
• Electrons
  • carry a unit negative charge (-1.60 x 10^-19 C)
  and a mass of 9.11 x 10^-31 kg (roughly 1/2000 times the mass of the proton).

Isotopes:

Isotopes of elements are forms of an element which differ in the number of neutrons in their nuclei. Some isotopes of an element are radioactive, while some are not. Radioactive isotopes all produce radiation to some degree or other.

RADIATION:

Radiation may be defined as the transfer of energy from a source through a vacuum or some medium (commonly, air) to some receiving body. Radiation may be in the form of electromagnetic waves, (such as X-rays) or high-velocity subatomic particles (such as electrons, neutrons or certain nuclei).

Ionizing radiation:

Ionizing radiation is radiation which has sufficient energy to strip electrons away from atoms, thus resulting in chemical changes to molecules. Ionization, if it causes the disruption of a biologically important molecule, can lead to biological damage. It is well to remember that all nuclear radiation (arising from radioactive isotopes) is ionizing, and therefore potentially harmful to living organisms.