K323 / 2027
Radioactivity overview

Topic 2 of 7

Radioactive decay and radiation

During radioactive decay, an unstable nucleus loses energy by emitting radiation. The material contains the radioactive nuclei; the radiation is what leaves them.

A nucleus contains protons and usually neutrons. Nuclear decay changes a nucleus, rather than simply removing an electron from an atom's outer region.

Random events, a predictable average pattern

Random
The exact time when a particular unstable nucleus will decay cannot be predicted. Identical nuclei do not all decay after one fixed waiting time.
Spontaneous
The decay does not require an external trigger. An unstable nucleus can undergo the change on its own.

For a large collection of the same radioactive nuclide, the average rate follows a reproducible pattern. This is compatible with uncertainty about individual nuclei. The half-life describes that population pattern.

The nucleus remaining after an emission is called the daughter nucleus. It may itself be radioactive; one emission does not guarantee a stable final nucleus.

Optional check A large sample of one radioactive nuclide has a reproducible average half-life. What does this tell us about an individual undecayed nucleus?
A large sample of one radioactive nuclide has a reproducible average half-life. What does this tell us about an individual undecayed nucleus?

Ionisation changes atoms along the radiation's path

Ionisation can occur when radiation removes an electron from an atom or molecule, leaving it charged. This can happen in material that the radiation enters, away from the original radioactive nucleus.

Alpha, beta and gamma can all cause ionisation. Compare what each emission is, how densely it ionises along its path and how readily it penetrates a stated material.

Alpha, α

An alpha particle is a helium nucleus: two protons and two neutrons, with charge +2 in proton-charge units. It is not a whole neutral helium atom.

It typically produces strong, dense ionisation over a short path. Its penetration is low: paper or the outer layer of intact skin can stop typical alpha particles.

Beta-minus, β-

A beta-minus particle is a fast electron emitted in a nuclear process, with charge -1. It is produced during that change, rather than being an existing electron ejected from an atomic shell.

Compared with typical alpha radiation, it produces less dense ionisation and penetrates farther. A suitable thin metal absorber can substantially reduce it. The energy and absorber determine how much passes through.

Gamma, γ

Gamma radiation is electromagnetic radiation from a nucleus. It has no electric charge and no rest mass. Its nature connects to the electromagnetic spectrum.

It is highly penetrating. Thick lead or concrete can reduce its intensity, but shielding does not necessarily remove every gamma ray. It can cause ionisation even though its ionisation is less dense in the usual comparison.

Emission type and shielding

Only selected emissions travelling to the right are drawn. Arrows show their direction, not an exact particle count or transmitted fraction. These are typical comparisons: radiation energy, material and thickness matter.

Alpha: low penetration

Alpha: low penetrationThe nature inset shows exactly two positive protons and two uncharged neutrons: an alpha particle is a helium nucleus, not a neutral helium atom. In the shielding schematic, selected emissions travel right from a source towards paper. None is drawn beyond the paper for this typical-alpha example. The indicated particle count and absorber thickness are not a measured transmission result.p+nnp+Helium nucleus2 protons + 2 neutronsCharge +2SourcePaperNo transmittedalpha in thisexampleSchematic: no fixed absorber thicknessTransmission depends on the conditions

Paper can stop typical alpha particles. Low penetration does not make alpha-emitting material harmless, especially if it is inside the body.

Beta-minus: greater penetration than typical alpha

Beta-minus: greater penetration than typical alphaThe nature inset shows a fast electron produced in a nuclear change, not an electron falling from a shell. Selected beta emissions travel right, through paper and towards a suitable aluminium layer. Most shown paths end there and a smaller transmitted set continues. This illustrates reduction, not a universal stopping thickness or a measured fraction.e-Fast electronProduced during decayCharge -1SourcePaperAluminiumSchematic: no fixed absorber thicknessTransmission depends on the conditions

Beta can pass through paper. A suitable thin metal layer substantially reduces it; the amount transmitted depends on the beta energies and the absorber.

Gamma: high penetration, reduced by dense shielding

Gamma: high penetration, reduced by dense shieldingThe nature inset represents electromagnetic radiation emitted from a nucleus. Selected gamma emissions travel right through paper and aluminium towards a thick lead shield. Fewer paths continue beyond the lead: gamma is attenuated, not assumed completely stopped. Arrow counts, spacing and shield thicknesses are schematic, not measured transmission probabilities.Electromagneticradiation from nucleusNo charge; no rest massSourcePaperAluminiumLeadSchematic: no fixed absorber thicknessTransmission depends on the conditions

Thick lead or concrete attenuates gamma radiation. Some can still pass through; neither the drawn thickness nor the transmitted fraction is a universal value.

The absorber comparisons are schematic. Material and radiation energy affect transmission. Gamma radiation is attenuated by shielding rather than necessarily being stopped completely.

Passing through paper does not, by itself, identify an emission as gamma: beta can also pass paper. Use the full set of observations under the stated absorber and source conditions.

Penetration and ionisation describe different properties. A short range does not mean harmlessness, especially if radioactive material is close to living cells. The uses and hazards depend on the source's location and the exposure conditions.