Radiation and Half-Life

Brief notes on... radiation and half-life.

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Detecting Ionising Radiation

Using Photographic Film:

When exposed to ionising radioation, photographic film becomes fogged. This method was first used by Henri Becquerel in 1896 but it is still used today to show when scientists dealing with radioactive materials become exposed to a certain anount of radiation.

The Geiger-Müller Tube:

It is a glass tube containing a special mixture of gases at very low pressure. The inside is lined with an electrically conducting coating. In the centre of the tube, there is an electrode connected to a high voltage supply.

When ionising radiation enters the tube, it causes the gas inside to form ions, allowing a pulse of current to flow from the electrode to the conducting layer. This is detected by an electronic circuit.

The GM tube is linked to a counting circuit or a rate meter which keeps a count of how much ionising radiation has entered the tube or measures the number of ionising events per second.

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Background Radiation

Becquerel - the unit of radioactivity - they measure the rate of unstable nuclei disintegrating per second

Background radiation is low-level ionising radiation that is produced all the time. It has a number of sources; some natural and some artificial.

Natural background radiation from the Earth:

When the Earth was formed, it contained many radioactive isotopes. Some decay quickly and some are still producing radiation. Some of the decay products are also radioactive.

Uranium decays slowly and two of its decay products are Radon and Thoron gases. As they are gases, they can seep out of radioactive rocks. They are dense so they build up in the basements and foundations of buildings.

 

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Background Radiation

Natural background radiation from space:

Supernovae produce very energetic particles and cosmic rays that continuously bombard the Earth. Lower energy cosmic rays are given out by the Sun but our atmosphere gives us fairly good protection from them.

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Background Radiation

Internal Radiation:

The atoms that make up our bodies were formed in the reactions that took place in stars created at the beginning of the Universe. Some of these atoms are radioactive so we carry our own source of radiation. We also breathe in tiny amounts of carbon-14 which is radioactive.

Artificial Radiation:

We use radioactive materials for many purposes, e.g. generating electricity in nuclear power stations. Sometimes the radioactive material leaks into the environment but they are usually small levels. Testing nuclear weapons in the atmosphere has increased the amounts of radioactive isotopes on the Earth.

Radioactive tracers are used in industry and medicine. Radioactive materials are used to treat forms of cancer. The majority of background radiation is natural.

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Radioactive Decay

Radioactive decay is a random process; it is impossible to tell which nuclei will disintergrate at any particular time. Half Life: The half-life of a radioactive sample is the average time taken for half the original mass of the sample to decay. The half life is different for for different radioactive isotopes

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