Gamma rays are given off by stars, and by some radioactive substances.

They are extremely high frequency waves, and carry a large amount of energy. They pass through most materials, and are quite difficult to stop - you need lead or concrete in order to block them out.

You can find out more about gamma rays at www.darvill.clara.net/nucrad/index.htm

Because Gamma rays can kill living cells, they are used to kill cancer cells without having to resort to difficult surgery.

This is called "Radiotherapy", and works because cancer cells can't repair themselves like healthy cells can when damaged by gamma rays. Getting the dose right is very important!

There's also targeted radiotherapy, where a radioactive substance is used to kill cancer cells - but it's a substance that'll be taken up by a specific part of the body, so the rest of the body only gets a low dose. An example would be using radioactive iodine to treat cancer in the thyroid gland.

Radioactivity is particularly damaging to rapidly dividing cells, such as cancer cells. This also explains why damage is done by radiotherapy to other rapidly dividing cells in the body such as the stomach lining (hence nausea), hair follicles (hair tends to fall out), and a growing foetus (not because of mutations, but simply major damage to the baby's rapidly dividing cells).

Tracers:

Doctors can put slightly radioactive substances into a patient's body, 
then scan the patient to detect the gamma rays and build up a picture 
of what's going on inside the patient. 

This is very useful because they can see the body processes actually working,
rather than just looking at still pictures. 

Example:the picture on the right is a "Scintigram", and shows an asthmatic person's lungs.
The patient was given a slightly radioactive gas to breathe, and the picture was taken using a gamma camera to detect the radiation. The colours show the air flow in the lungs.

In industry, radioactive "tracer" substances can be put into pipes and machinery, then we can detect where the substances go. This is basically the same use as in medicine.

Gamma rays kill microbes, and are used to sterilise food so that it will keep fresh for longer. This is known as "irradiated" food.

Gamma rays are also used to sterilise medical equipment.

Gamma rays cause cell damage and can cause a variety of cancers.

They cause mutations in growing tissues, so unborn babies are especially vulnerable.

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