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What is radiation and where does it come from?

Doctor studying an X-ray

Radiation is energy in the form of high-speed particles or waves. Radiation can be thought of as forming a continuous energy spectrum, ranging from low-frequency radio waves to high-frequency cosmic rays. Sunshine, a form of natural radiation, is essential for life on Earth – it is the energy that sustains all plants and animals. The visible light from the sun is medium-frequency radiation.

Exposure to high doses of radiation can cause damage to living tissues, including the human body. Anyone who has suffered from sunburn, for example, has experienced the damaging effects of ultra-violet (UV) radiation on the skin. The high-frequency end of the radiation spectrum is sometimes called ionising radiation, and includes x-rays and gamma rays.

What is radioactivity?

Some atoms are naturally unstable – that is, they continually change or 'decay' until they become entirely new, stable atoms. As unstable atoms decay, they emit ionising radiation. These unstable atoms are said to be radioactive. Radioactive materials may emit different types of ionising radiation, including alpha particles, beta particles and gamma rays.

Nuclear reactors contain a radioactive form of the element uranium. When a neutron – a type of subatomic particle – collides with a uranium atom, the atom can split. This process, known as nuclear fission, releases more neutrons. These neutrons collide with other uranium atoms, triggering a chain reaction, releasing enormous amounts of energy. Nuclear power stations use this energy to create high-pressure steam for generating electricity. The neutrons emitted during nuclear fission are another form of ionising radiation.

Millions of people have benefited from the use of radioactivity – for example in medicine, in the form of x-rays for diagnosis or radiotherapy, in agriculture, and in other sectors, as well as in nuclear energy.

How powerful is radiation?

Different types of radiation vary a great deal in their ability to penetrate materials. X-rays, for example, are powerful enough to pass straight through parts of the human body – a property doctors and dentists regularly exploit.

The main forms of ionising radiation are as follows:

  • gamma rays are high-energy beams similar to x-rays. They travel at the speed of light and only a very dense material, like lead, can slow them down or stop them.
  • alpha particles are high-energy positively-charged particles and are much slower than gamma rays. They also have far less penetrating power. A sheet of paper can stop them.
  • beta particles are negatively charged, much lighter than alpha particles, and much faster moving. They are also more penetrating than alpha particles, but can still be stopped by a layer of aluminium a few millimetres thick.
  • neutron particles are not electrically charged, which allows them to pass straight through a wide variety of materials, so they are highly penetrating. When neutrons come into contact with living tissue and other matter, beta and gamma radiation is emitted.

Several layers of different materials are needed to effectively shield against all forms of ionising radiation.

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