Physics : P7 - Radioactivity

Radioactive substances have unstable nuclei that become stable by emitting radiation.

Alpha radiation, α, is stopped by paper and travels 2-3 cm in air.

Beta radiation, β, is stopped by aluminium and travels 1-2 m in air.

Gamma radiation, γ, is stopped by many cms of lead and travels many metres in air.

Radioactive decay is random - it is unpredictable.

Rutherford used α particles to probe inside atoms. He found that some of the α particles were scattered through large angles.

The 'plum pudding' model could not explain why some α particles were scattered through large angles.

An atom has a small positively charged nucleus where most of the atom's mass is located.

The nuclear model of the atom correctly explained why some α particles scattered through large angles.

The atomic number is the number of protons in the nucleus. It has the symbol Z.

The mass number is the number of protons plus neutrons in the nucleus. It has the symbol A.

In α decay, the nucleus loses 2 protons and 2 neutrons. They are emitted as an α particle.

In β decay, a neutron in the nucleus changes to a proton and an electron. The fast-moving electron is immediately emitted. 

Gamma rays are high frequency electromagnetic waves. There are no changes in the nucleus.

Radiation can knock electrons out of atoms, This is called ionisation. When an object is exposed to ionising radiation, it is said to be irradiated, but does not become radioactive. Ionising radiation is dangerous for humans as it harms or kills cells.

Radioactive contamination is the unwanted presence of materials containing radioactive atoms on other materials.

Radiation can also be useful. For example, alpha radiation is used in smoke alarms, and beta radiation is used to monitor the thickness of metal foil as it is produced.

The activity of a radioactive source is the number of unstable atoms in the source that decay per second. The count rate (counts per second) decreases as the activity decreases.

The half-life of a radioactive isotope is the average time it takes for the number of nuclei of the isotope in a sample to halve, or the time taken for the count rate to fall to half its initial value.

Carbon-dating is a way to age things that were once living. The half-life of Carbon-14 is 5600 years.