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| The main types of ionising radiation |
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Alpha radiation: a |
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| Picture 24. Alpha particle. |
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| Alpha radiation is a stream of particles. They are made from two protons and two neutrons - the same as a nucleus of helium.
They have a (relatively) large mass and carry a double positive charge.
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| Alpha particles cause a lot of ionisation.
However they have a very short range in air (a few centimetres) and are easily stopped by anything solid – even a piece of paper will stop alpha radiation.
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| Alpha particles are given out by the nuclei of unstable atoms.
Main source in nature is radon gas which is produced from naturally occurring uranium in the ground.
There are also alpha sources in radioactive and medical wastes.
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Airborne radiation is not a great hazard because it is so short range and will be stopped by clothing. However, contamination of food, water or air supplies is hazardous as the source might then be breathed in or swallowed, giving out the highly ionising radiation inside a person's body.
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Beta radiation: b |
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| Picture 25. Beta radiation |
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| Beta radiation is a stream of fast moving electrons, which have a negative charge. These particles have very little mass (about 7000 times lighter than an alpha particle) and travel close to the speed of light (300 million m/s). |
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| Beta particles tend not to have many collisions. So beta radiation is only weakly ionising.
However, it has a long range in air and It will pass through paper and thin sheets of aluminium or steel. It is stopped by lead or thick pieces of other metals.
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| Beta radiation is given out by unstable atoms.
Potassium-40 in rocks decays by beta decay as does carbon-14. However, the amount of radiation is tiny.
There are also beta sources in radioactive and medical wastes. For example, caesium-137 is used in the treatment of cancer and strontium-90 is found in nuclear waste.
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As with alpha radiation, airborne radiation is not a great hazard: beta is weakly ionising and not very long range. However, contamination of food, water or air supplies is hazardous as the source might then be breathed in or swallowed. Caesium and strontium both have soluble compounds which, if they get into the soil, can be taken up into plants. In this way, radioactive isotopes of these elements could find their way inside our bodies. This is why the waste has to be disposed of safely.
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Gamma radiation: g |
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| Picture 26. Gamma radiation |
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| Gamma radiation is at the high frequency end of the electromagnetic spectrum.
It has a very short wavelength (much less than the radius of an atom) and will pass through atoms with very little chance of being deflected or absorbed.
As part of the electromagnetic spectrum, it travels at the speed of light - 300 million m/s.
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| It will tend to pass through matter without causing much ionisation. It has an extremely long range in air but gets weaker with distance.
It will get through thin samples of most materials. However, its intensity is reduced by lead or very thick pieces of other metals. The thicker the sample, the greater the reduction in intensity.
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| Gamma radiation is given out when unstable radioactive nuclei decay and release energy. It is usually given out in conjunction with alpha and beta radiation and there are many sources in nature. |
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Gamma radiation is only really hazardous if it is very intense. This only happens if sources are deliberately concentrated or after a nuclear explosion.
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X radiation: X-rays |
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| Picture 27. An X-ray machine in hospital. |
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| X-rays are also part of the electromagnetic spectrum. They have a short wavelength but it is longer than the wavelength of gamma radiation. They have less energy and are less penetrating than gamma.
They have no charge and travel at the speed of light - 300 million m/s.
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| They pass through matter and body tissues (hence we can use them to get pictures of bones etc.); however, they are partially absorbed and cause some ionisation. They are not as penetrative as gamma rays. They will get through thin samples of most materials without any noticeable decrease in intensity. However, they are stopped by lead or thick pieces of other metals.
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| Like gamma rays, X-rays are often given out when unstable nuclei decay.
However, they are also given out by atoms of heavy elements (such as tungsten) when they are bombarded with high speed electrons. Hospital X-ray machine use this method as it can be controlled.
X-rays can also be given out, in small doses, by Cathode Ray Tubes (CRT) in TVs and computer monitors (not flat screens).
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X-rays are only really hazardous when they are very intense. This is certainly the case when you have an X-ray in hospital. So, as with many medical procedures, there is an associated risk with having an X-ray. This is why they are only given when necessary.
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| Picture 28. Most radiation damage is caused by UV rays in sunlight. |
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| UV-rays are also part of the electromagnetic spectrum. They have a shorter wavelength than visible light but not as short as X-rays.
They are present in the Sun's radiation. They are partially absorbed by the Earth's atmosphere - especially the layer of ozone in the stratosphere.
They have no charge and travel at the speed of light - 300 million m/s.
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| They are absorbed by matter and body tissue and can cause ionisation. They will get through thin samples of some materials without any noticeable decrease in intensity.
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| The main source of UV rays is the Sun. However, they are also given out by some atoms in emission tubes and UV lamps are sometimes used for security or entertainment. |
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UV rays cause the most harm of all the ionising radiation. This is because they are so intense in sunlight. UV lamps can also damage your eyes and you should never look directly at a source of UV radiation.
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