5.2.1Nuclear Physics

Ionising Nuclear and Background Radiation — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions

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Part 1 of 5Ionising radiation

Unstable nuclei

Unstable nuclei Some nuclei are unstable and emit alpha particles, beta particles or gamma rays to become more stable.
- This is called radioactive decay, and it is random.

What ionisation means

What ionisation means Ionisation happens when radiation knocks one or more electrons out of an atom.
The atom loses negative charge, so it is left as a positive ion.

Why it matters

Why it matters Ionisation in living cells can damage molecules such as DNA, which is why these radiations are hazardous.

Background radiation is the low level of ionising radiation that is around us all of the time.

It comes from natural sources that have always existed on Earth and in space, and from man-made sources.

When using a Geiger-Müller tube, first record the count rate with no source present.

Subtract this background count rate from any later measurement to find the count rate from the source alone.

Natural background

Radon gas

Radon gas Uranium in rocks decays to radon, an alpha-emitting gas that seeps into the air and collects in buildings.
It is the largest source for many people, and is dangerous mainly if inhaled in large amounts.

Rocks and buildings

Rocks and buildings Rocks, soil, stone and brick contain traces of radioactive elements such as uranium and thorium.

Food and drink

Food and drink Plants take up radioactive elements from soil, so food and water contain tiny amounts, for example potassium-40 in bananas.

Cosmic rays

Cosmic rays High-speed particles from the Sun and from exploding stars strike the atmosphere and produce gamma radiation,
- and the dose is larger at high altitude.

Living material

Living material All living things contain a very small proportion of radioactive carbon-14.

Medical

Medical X-rays, CT scans, radioactive tracers and radiotherapy all expose patients to extra radiation.

Nuclear industry and weapons

Nuclear industry and weapons Nuclear waste adds little to the general background but is dangerous to those handling it.
Fallout from weapons testing and past nuclear accidents, such as Chernobyl, has spread radioactive material through the environment.

Factor	Effect on background
Type of rock in the area	Granite areas release more radon than many other rocks, so counts are higher
Altitude	Higher places receive more cosmic radiation
Building materials and ventilation	Poorly ventilated buildings on radon-rich ground trap more of the gas
Occupation	Workers in nuclear plants, hospitals or aircraft may receive extra doses

Term	Meaning
Ionisation	The process in which radiation removes electrons from atoms, leaving positive ions.
Background radiation	Ionising radiation that is present in the environment all the time.
Count rate	The number of radiation particles detected each second or minute.
Radon	A radioactive gas formed from the decay of uranium in rocks, which emits alpha particles.
Cosmic rays	High-energy particles from space that strike the Earth's atmosphere.

Thinking background radiation comes only from nuclear power stations. Most of it is natural.

Forgetting to subtract the background count rate when measuring a radioactive source.

Saying that ionisation adds electrons to atoms. It removes them.

Stating that food is a major hazard because it is radioactive. The amounts are tiny.

Believing the background count is the same everywhere. It varies with location, rock type and altitude.

Last updated: 20 October 2026