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Gamma rays (g) are electromagnetic waves, rather like X rays and radio waves. Thus gamma rays have no mass and no charge. You can find out more about electromagnetic waves from http://www.darvill.clara.net/emag/index.htm. After a nucleus has emitted an a-particle or a b-particle, it may still have too much energy: we say it is in an "excited state". It can get rid of this energy by emitting a pulse of very high frequency electromagnetic radiation, called a gamma ray.

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Gamma rays do not pull electrons off atoms they pass, as a-particles and b-particles do. This means that they do not lose much energy as they travel, as they do not interact as much with the matter they pass.
Therefore, gamma rays have a high penetrating power, and a very long range.

It's worth noting that there is no such thing as a pure g-ray source. Gamma rays are given off by most a-emitters and b-emitters. If we want a source of pure gamma rays, we can get it by using a substance that emits both b and g, and simply keep it in an aluminium container that stops the b-particles.

Useful gamma sources include Technetium-99m, which is used as a "tracer" in medicine. This is a combined b and g source, and is chosen because betas are less harmful to the patient than alphas (less ionisation) and because Technetium has a short half-life (just over 6 hours), so it decays away quickly and reduces the dose to the patient.

Remember, in Gamma decay:- atomic number unchanged atomic mass unchanged.

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Created by Andy Darvill, www.darvill.clara.net,Science teacher at Broadoak Community School, Weston-super-Mare, England