Skoool Kerala

Background Radiation

Some substances give out radiation all the time. They are said to be radioactive.

Radioactive atoms are unstable and will randomly break down.

When this happens the nucleus of some atoms breaks up and gives out radiation.

There are radioactive substances all around us – in the ground, in air, in building materials and in food.

Radiation also reaches us from space.

The radiation from all these sources is called background radiation.

 

Types of Radiation

There are three types of radiation that are emitted by radioactive substances.

Alpha particles – not very penetrating. They are absorbed by a few centimetres of air or thin paper.

Beta particles – moderately penetrating. They pass through air or paper easily, but are absorbed by a few millimetres of metal.

Gamma rays – very penetrating electromagnetic waves. They are absorbed by several centimetres of lead, or several metres of concrete.

As radiation passes through a material it can be absorbed. The thicker the material, the more radiation is absorbed.

 

Dangers of Radiation

When radiation from radioactive substances collides with neutral atoms or molecules these may become ionised. This means they form charged particles called ions.

If molecules inside living cells become ionised they can cause damage, including cancer.

The bigger the dose of radiation, the greater the chance of cancer.
Higher doses of radiation can kill cells.

When sources of radiation are outside the body, alpha radiation is the least dangerous because it cannot penetrate the skin, so is unlikely to reach living cells.

Beta and gamma radiation are the most dangerous because they can penetrate the skin and reach cells of organs, which may absorb them.

Workers who may be exposed to radiation have to wear a radiation badge, which monitors the amount of radiation the person has been exposed to over a period of time.
The badge contains a piece of photographic film. The film darkens if exposed to radiation.
The more radiation a worker has been exposed to, the darker the film goes.

If radioactive sources are inside the body, alpha radiation is the most dangerous because it is strongly absorbed by the cells.
Beta and gamma radiation are less dangerous because they can penetrate the tissue.
 

Half-life

The radiation emitted by a radioactive source gradually gets less as the unstable atoms decay.

The half-life of a radioactive substance is the time it takes for half the radioactive atoms to decay.

So, it is the time for the count rate for the original substance to fall to half its initial value.

A Geiger-Müller tube, or G-M tube, is used to detect and measure radiation. The amount of radiation is measured in counts per second.

The half-life of this substance is 15 minutes.

The more unstable the nuclei the shorter the half-life.

Substances with long half-lives are often the most dangerous because they stay radioactive for many years.

 

A beta source is usually used.

If the sheet stays the correct thickness the detector picks up and displays a steady reading.

If the reading goes up, the sheet is too thin, if it falls the sheet is too thick.

The radioactive sources used are usually gamma emitters, so the radiation passes through the body to a detector and little is absorbed by the cells. They have fairly short half-lives so that they decay quickly to minimise any cell damage.