Radioactive DecayEdexcel GCSE Physics: Revision notes

Section 1

What types of radiation are emitted from unstable nuclei?

Unstable nuclei emit radiation in a random process — it cannot be predicted when any individual nucleus will decay. The types of radiation are:

Alpha (α) — equivalent to a helium nucleus (2 protons + 2 neutrons)
Beta-minus (β−) — a fast-moving electron emitted from the nucleus
Beta-plus (β+ / positron) — a fast-moving positron emitted from the nucleus
Gamma (γ) — electromagnetic radiation from the nucleus
Neutron radiation — emission of a neutron

Alpha, beta (β− and β+) and gamma are all ionising radiations.

Key termsalpha particlebeta particlegamma ray

Section 2

How do alpha, beta and gamma compare in penetration and ionisation?

Radiation	Penetration	Ionising power
Alpha	Stopped by paper/skin	Most ionising
Beta	Stopped by a few mm of aluminium	Medium ionising
Gamma	Reduced by thick lead/concrete	Least ionising

There is a trade-off: the more easily a radiation type ionises matter (transfers energy to it), the less far it penetrates before losing that energy.

Key termsionisationpenetration

Common mistake

Don't say alpha is 'the most dangerous' in every context — outside the body it's the least penetrating and easily stopped, but inside the body (inhaled/ingested) it is the most ionising and damaging.

Section 3

How do beta decay and nuclear equations work?

β− decay: a neutron in the nucleus becomes a proton, emitting an electron (and an antineutrino)
β+ decay: a proton in the nucleus becomes a neutron, emitting a positron (and a neutrino)

Each type of decay changes the atomic (proton) number and/or mass (nucleon) number:

Decay	Mass number change	Atomic number change
Alpha	−4	−2
β−	0	+1
β+	0	−1
Gamma	0	0

Nuclei that have decayed often rearrange with a loss of extra energy as a gamma ray. Nuclear equations must be balanced so that total mass number and total atomic number are equal on both sides.

Key termsnuclear equation

Example

Alpha decay of Uranium-238 (atomic number 92): ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He — mass number drops by 4, atomic number drops by 2.

Section 4

What is half-life and how is activity measured?

The activity of a radioactive source is the rate at which nuclei decay, measured in becquerels (Bq), where 1 Bq = 1 decay per second. Activity decreases over time as fewer undecayed nuclei remain.

The half-life of a radioactive isotope is the time taken for half of the undecayed nuclei in a sample to decay, or for the activity to fall to half its original value. Although it's impossible to predict when any one nucleus will decay, half-life allows the activity of a very large number of nuclei to be predicted reliably.

Half-life calculations can involve repeatedly halving activity/count over successive half-lives, or reading values from a decay graph.

Key termsactivityhalf-lifebecquerel

Example

A source starts at 800 Bq with a half-life of 3 hours: after 3 hrs it is 400 Bq, after 6 hrs 200 Bq, after 9 hrs 100 Bq.

Must Know

Radioactive decay is random; alpha, β−, β+, gamma and neutron radiation are emitted from unstable nuclei
Alpha = helium nucleus (most ionising, least penetrating); beta = electron/positron; gamma = EM radiation (least ionising, most penetrating)
β− decay: neutron → proton + electron; β+ decay: proton → neutron + positron
Alpha decay reduces mass number by 4 and atomic number by 2; beta decay changes atomic number by ±1, mass number unchanged
Activity is measured in becquerels (Bq); half-life is the time for activity/undecayed nuclei to halve
Individual decay is unpredictable, but half-life predicts the behaviour of large numbers of nuclei reliably

That's the notes covered.

Carry on to the next subtopic.