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The Nature of the Three Types of Radiation
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Ionising Radiation |
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When radiation collides with a neutral atom (or molecule) electron(s) can be knocked off the atom leaving a positively charged species called an ion. This process is called ionisation. |
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The animation below shows radiation hitting an atom with four electrons. The radiation has enough energy to knock an electron off the atom thus leaving only three electrons. The result is a positively charged ion. We say that the atom has been ionised. |
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Alpha particles are 'heavy' and slow moving. When they collide with atoms they can knock off many electrons. We say that alpha particles are strongly ionising. |
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Beta particles are very 'light' and move fairly fast. They are not as strongly ionising as alpha particles as they cannot knock electrons off atoms as easily. |
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Non-ionising Radiation |
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Sometimes the radiation that collides with atoms does not knock any electrons off. In this case the radiation does not ionise the atom so we call the radiation non-ionising. |
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The animation below shows radiation hitting an atom with four electrons. The atom does get 'knocked about' but no electrons are removed. The atom is not ionised. |
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Gamma rays have no mass so they cannot easily knock electrons off atoms. We say that they are weakly ionising. |
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The Penetrating Power of Alpha, Beta & Gamma Radiation |
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When 'heavy' slow moving alpha particles hit a material they knock electrons off many atoms of the material thereby creating a lot of ions. This stops the alpha particles penetrating the material. We say that alpha particles are weakly penetrating. |
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When 'lighter' fast moving beta particles hit a material not many electrons are removed from the material's atoms. Beta particles are not stopped as easily by the material as alpha particles so we refer to beta particles as being moderately penetrating. |
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When gamma rays hit a material they hardly knock any electrons off the atoms in the material. Gamma rays are not stopped easily so we call them strongly penetrating. |
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The diagram below shows the penetrating power of alpha, beta and gamma. |
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1. |
A sheet of paper stops weakly penetrating alpha particles. |
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Living tissue (e.g. a hand) stops moderately penetrating beta particles. |
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Even a metal block cannot stop gamma rays. It takes concrete or lead a few metres thick to stop strongly penetrating gamma radiation. |
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Alpha, Beta and Gamma Radiation in Electric Fields |
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The diagram below shows how alpha particles, beta particles and gamma rays are affected by an electric field. |
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The 'heavy', positively charged, alpha particles are attracted to the negative terminal but still pass through the field. |
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The 'lighter', negatively charged, beta particles are attracted to the positive terminal. They are so light they get deflected more than alpha particles. |
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The gamma rays, with no mass or charge, are unaffected by the field. |
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Summary of the nature of alpha, beta and gamma radiation |
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Radiation |
Ionising Power |
Penetrating Power |
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Alpha |
Strong |
Weak |
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Beta |
Moderate |
Moderate |
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Gamma |
weak |
Strong |
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