A-Level Physics Revision — Radioactivity

Revise Radioactivity for A-Level Physics with a topic explanation, worked example and common mistakes. Check the board notes for specification differences.

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Topic explanation

What is Radioactivity?

Radioactivity is the spontaneous and random decay of unstable atomic nuclei, which results in the emission of ionising radiation. This topic covers the properties of the three main types of radiation - alpha, beta, and gamma - including their nature, penetrating power, and ionising ability. You will also study the exponential nature of radioactive decay, described by the concepts of half-life, activity, and the decay constant.

Board notes: Radioactivity is a fundamental topic in the nuclear physics section of all A-Level specifications (AQA, Edexcel, OCR). All boards cover the properties of alpha, beta, and gamma radiation, and the mathematics of exponential decay, including half-life and the decay constant. The applications and dangers of radioactivity are also a common focus.

Step-by-step explanation

Worked examples

Worked example

A radioactive sample has a half-life of 10 minutes. If it initially contains 8.0 x 10^12 undecayed nuclei, how many will remain after 30 minutes? 30 minutes is equal to 3 half-lives (30/10 = 3). After 1 half-life, 4.0 x 10^12 remain. After 2 half-lives, 2.0 x 10^12 remain. After 3 half-lives, 1.0 x 10^12 undecayed nuclei will remain.

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Common mistakes

1Confusing the properties of alpha, beta, and gamma radiation. A common mistake is to mix up their relative ionising powers and penetrating abilities. Alpha is highly ionising but has low penetration, while gamma is weakly ionising but has high penetration.
2Thinking that half-life is the time it takes for half of a substance to disappear. Half-life is the time it takes for half of the *unstable nuclei* to decay, or for the *activity* of the sample to halve. The mass of the sample does not halve.
3Using inconsistent units in decay calculations. The decay constant (λ) and time (t) must be in inverse units (e.g., s⁻¹ and s, or year⁻¹ and years) for the exponential decay equations to work correctly.

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Radioactivity exam questions

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Frequently asked questions

What is radioactive decay?
Radioactive decay is a random and spontaneous process where an unstable atomic nucleus loses energy by emitting radiation, such as an alpha particle, beta particle, or gamma ray. The nucleus changes into a different nuclide or a lower energy state.
What is activity of a radioactive source?
Activity is the rate at which nuclei in a radioactive source decay. It is measured in Becquerels (Bq), where 1 Bq is equal to one decay per second.

What is Radioactivity?

Step-by-step explanation

Worked examples

Worked example

Common mistakes

1Confusing the properties of alpha, beta, and gamma radiation. A common mistake is to mix up their relative ionising powers and penetrating abilities. Alpha is highly ionising but has low penetration, while gamma is weakly ionising but has high penetration.

2Thinking that half-life is the time it takes for half of a substance to disappear. Half-life is the time it takes for half of the *unstable nuclei* to decay, or for the *activity* of the sample to halve. The mass of the sample does not halve.

3Using inconsistent units in decay calculations. The decay constant (λ) and time (t) must be in inverse units (e.g., s⁻¹ and s, or year⁻¹ and years) for the exponential decay equations to work correctly.

Frequently asked questions

What is radioactive decay?

Radioactive decay is a random and spontaneous process where an unstable atomic nucleus loses energy by emitting radiation, such as an alpha particle, beta particle, or gamma ray. The nucleus changes into a different nuclide or a lower energy state.

What is activity of a radioactive source?

Activity is the rate at which nuclei in a radioactive source decay. It is measured in Becquerels (Bq), where 1 Bq is equal to one decay per second.