Physics- Radioactivity

An atom of any given element consists of a nucleus containing a number of protons and neutrons. The nucleus is surrounded by electrons.

The half-life of a radioactive isotope is the time taken for half its radioactive atoms to decay.

There are three main types of radiation, called alpha, beta and gamma radiation, which all have different properties. Radiation can damage cells and make them cancerous. Very high doses of radiation can kill cells. It can be detected using photographic film or a Geiger-Muller tube. Radiation badges are used to monitor the level of radiation that people who work with radioactive sources are exposed to.

Radiation has many practical uses. It can be used in medicine to trace where certain chemicals collect in the body, indicating disease, and also in industry, where it can be used to control measuring equipment

The nuclear model

Structure of the atom

Atoms contain three sub-atomic particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus at the centre of the atom, and the electrons are arranged in energy levels or shells around the nucleus.

All the atoms of a given element have the same number of protons and electrons. However, the number of neutrons can vary. Atoms of the same element that have different numbers of neutrons are called isotopes of that element. The diagram shows three hydrogen isotopes.

• The different isotopes of an element have identical chemical properties. Some isotopes, however, are radioactive.
• This means that they give out radiation from their nuclei. This happens all the time, whatever is done to the substance.
• For example, the radiation is still given out if the substance is cooled down in a freezer, or takes part in a chemical reaction.
• There are three main types of radiation emitted from radioactive atoms. These are alpha, beta and gamma radiation

Alpha radiation

Alpha radiation consists of alpha particles. An alpha particle is identical to the nucleus of a helium atom, which comprises two protons and two neutrons.

Beta radiation

Beta radiation consists of high energy electrons emitted from the nucleus. These electrons have not come from the electron shells or energy levels around the nucleus. Instead, they form when a neutron splits into a proton and an electron. The electron then shoots out of the nucleus at high speed.

Gamma radiation

Gamma radiation is very short wavelength - high frequency - electromagnetic radiation. This is similar to other types of electromagnetic radiation such as visible light and X-rays, which can travel long distances.

Alpha radiation

Alpha radiation is the least penetrating. It can be stopped - or absorbed - by just a sheet of paper.

Beta radiation

Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium.

Gamma radiation

Gamma radiation is the most penetrating. Even small levels can penetrate air, paper or thin metal. Higher levels can only be stopped by many centimetres of lead or many metres of concrete.

When radiation collides with molecules in living cells it can damage them. If the DNA in the nucleus of a cell is damaged, the cell may become cancerous. The cell then goes out of control, divides rapidly and causes serious health problems.

The greater the dose of radiation a cell gets, the greater the chance that the cell will become cancerous. However, very high doses of radiation can kill the cell completely. We use this property of radiation to kill cancer cells, and also harmful bacteria and other micro-organisms.

Alpha, beta and gamma radiation

The degree to which each different type of radiation is most dangerous to the body depends on whether the source is outside or inside the body.

If the radioactive source is inside the body, perhaps after being swallowed or breathed in:

• Alpha radiation is the most dangerous because it is easily absorbed by cells.
• Beta and gamma radiation are not as dangerous because they are less likely to be absorbed by a cell and will usually just pass right through it.

If the radioactive source is outside the body:

• Alpha radiation is not as dangerous because it is unlikely to reach living cells inside the body.
• Beta and gamma radiation are the most dangerous sources because they can penetrate the skin and damage the cells inside.

The nuclei of radioactive atoms are unstable. They break down and change into a completely different type of atom. This is called radioactive decay. For example, carbon-14 decays to nitrogen-14 when it emits beta radiation.

It is not possible to predict when an individual atom might decay. But it is possible to measure how long it takes for half the nuclei of a piece of radioactive material to decay. This is called the half-life of the radioactive isotope.

Two definitions

There are two definitions of half-life, but they mean essentially the same thing:

1. the time it takes for the number of nuclei of the isotope in a sample to halve
2. the time it takes for the count rate from a sample containing the isotope to fall to half its starting level

Different radioactive isotopes have different half-lives. For example, the half-life of carbon-14 is 5,715 years, but the half-life of francium-223 is just 20 minutes.