Radioactivity - Atoms and Radiation

To understand what happens to radioactive substances when they decay we need to understand the structure of the atoms from which they are made.

The model of an atom that we use to teach at senior high school was developed in the early part of the twentieth century. At the center of the atom is the nucleus; this occupies only a very tiny volume of the whole atom. The nucleus is made up from protons and neutrons that are bound together by nuclear forces. The protons are said to have a mass of 1 atomic mass unit and a small electrostatic charge of +1. Neutrons are electrically neutral and also have a mass of 1 atomic mass unit. The nucleus therefore has an overall positive electrical charge and an atomic mass equal to the number of particles present. Surrounding the nucleus are 'shells' of electrons. These shells are also referred to as 'energy levels'. Electrons have a mass of just 1/1860^th of an atomic mass unit so effectively, they do not contribute much to the mass of an atom. They have a charge of -1 and were the first of the subatomic particles to be discovered. An atom has the same number of electrons as protons so the electrical charges cancel out - atoms are neutral. An atom that is not neutral is not an atom, it is an ion.

In order to harness the power of radioactivity safely, it is important to understand the properties of different types of nuclear radiation. The three types of radiation are named using the first three letters of the Greek alphabet - alpha, beta and gamma. Alpha radiation is in fact a particle containing two protons and two neutrons, you should recognize this as being the same as a helium nucleus. It is the heaviest and slowest-moving type of radiation and can be easily stopped by paper or cloth. Beta radiation is also a particle, it is a fast-moving electron. This is harder to stop; it passes easily through paper and cloth but is stopped by thin sheets of metal such as aluminum. The third type of radiation (gamma radiation) is not a particle, it is a high frequency, short wavelength, high energy electromagnetic wave. It is the most penetrating form of radiation and is very difficult to stop. Alpha and beta radiation can both be deflected by magnetic and electric fields as they are charged particles. Beta particles are deflected more than alpha particles. All three types of radiation are ionising and can therefore damage living tissue. Gamma is the least ionizing whilst alpha is highly ionizing.

The dangers and uses that we make of radioactive sources depends on their penetrating power and half-life. The half-life is a measure of how long it takes for the radiation level to fall by a half, more strongly radioactive materials have shorter half lives.