Radioactive Decay

This lesson covers radioactive decay as required by the AQA GCSE Physics specification (4.4.2). You need to understand what radioactive decay is, why it happens, what types of radiation are emitted, and the concept of random and spontaneous decay.

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What Is Radioactive Decay?

Radioactive decay is the process by which an unstable nucleus releases energy by emitting radiation. The nucleus changes to become more stable.

Key characteristics of radioactive decay:

• It is random — you cannot predict when a particular nucleus will decay. Each unstable nucleus has the same chance of decaying in any given time period, but you cannot say exactly when it will happen.
• It is spontaneous — it happens on its own, without any external trigger. It is not affected by temperature, pressure, chemical reactions, or any other physical conditions.
• It involves the nucleus — radioactive decay is a nuclear process, not a chemical one. It occurs because the nucleus is unstable, not because of the electrons.

Exam Tip: Two key words you must use when describing radioactive decay are random and spontaneous. Random means unpredictable (you cannot say which nucleus will decay next). Spontaneous means it happens on its own (it is not caused by anything external). Use both words in your answer for full marks.

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Why Are Some Nuclei Unstable?

A nucleus becomes unstable when it has:

• Too many neutrons relative to protons.
• Too few neutrons relative to protons.
• Too many protons and neutrons overall (the nucleus is too large).
• An unfavourable ratio of protons to neutrons.

The nucleus is held together by the strong nuclear force, which acts between nucleons (protons and neutrons) at very short distances. However, protons also repel each other due to their positive charges (electrostatic repulsion). If the balance between these forces is not right, the nucleus is unstable and will decay.

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Types of Radiation

When an unstable nucleus decays, it can emit one or more types of radiation:

Type	Symbol	What Is It?	Charge	Mass
Alpha	a (alpha)	2 protons + 2 neutrons (a helium nucleus)	+2	4
Beta	b (beta)	A high-speed electron emitted from the nucleus	-1	Negligible
Gamma	g (gamma)	A high-frequency electromagnetic wave	0	0
Neutron	n	A neutron emitted from the nucleus	0	1

Each type of radiation will be covered in more detail in the next lesson.

graph TD
    A["Unstable Nucleus"] --> B["Alpha Decay<br>Emits 2p + 2n<br>(helium nucleus)"]
    A --> C["Beta Decay<br>Neutron turns into<br>proton + electron"]
    A --> D["Gamma Emission<br>Emits high-energy<br>electromagnetic wave"]
    A --> E["Neutron Emission<br>Emits a neutron"]

    style A fill:#e74c3c,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#2980b9,color:#fff
    style D fill:#9b59b6,color:#fff
    style E fill:#95a5a6,color:#fff

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Radioactive Decay and Changes to the Nucleus

When a nucleus decays, its composition changes:

Alpha Decay

• The nucleus emits an alpha particle (2 protons and 2 neutrons).
• The mass number decreases by 4 and the atomic number decreases by 2.
• The atom becomes a different element (two places lower in the periodic table).
• Example: Uranium-238 decays to Thorium-234 by emitting an alpha particle.

Beta Decay

• A neutron in the nucleus turns into a proton and an electron.
• The electron is emitted at high speed as a beta particle.
• The mass number stays the same and the atomic number increases by 1.
• The atom becomes a different element (one place higher in the periodic table).
• Example: Carbon-14 decays to Nitrogen-14 by emitting a beta particle.

Gamma Emission

• The nucleus emits a burst of electromagnetic radiation (a gamma ray).
• The mass number and atomic number do not change — the nucleus does not transform into a different element.
• Gamma emission often accompanies alpha or beta decay.

Exam Tip: Remember the changes to atomic number and mass number for each type of decay: Alpha: A decreases by 4, Z decreases by 2. Beta: A stays the same, Z increases by 1. Gamma: no change to A or Z. This is essential for writing nuclear equations.

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The Random Nature of Decay

Radioactive decay is a random process. This means:

• You cannot predict which particular nucleus in a sample will decay next.
• You cannot predict when a particular nucleus will decay.
• However, with a large number of nuclei, you can predict the average rate of decay (the activity).

This is similar to rolling dice: you cannot predict which throw will give a six, but with many throws, you can predict that about 1 in 6 throws will be a six.

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Activity

The activity of a radioactive source is the rate at which nuclei decay. It is measured in becquerels (Bq).

1 becquerel = 1 decay per second

For example:

• An activity of 500 Bq means 500 nuclei decay every second.
• An activity of 1 kBq (1 kilobecquerel) means 1000 decays per second.
• An activity of 1 MBq (1 megabecquerel) means 1,000,000 decays per second.

As a radioactive sample decays, the number of unstable nuclei decreases, so the activity also decreases over time.

Exam Tip: Make sure you know the unit of activity: the becquerel (Bq). One becquerel equals one decay per second. This is a simple fact that often appears in multiple-choice questions. Also note that activity decreases over time because there are fewer undecayed nuclei remaining.

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Decay Chains

Sometimes, when a nucleus decays, the daughter nucleus (the new nucleus produced) is also unstable. This means it will decay further, producing yet another nucleus. This sequence of decays is called a decay chain (or decay series).

For example, uranium-238 decays through a long chain of alpha and beta decays, eventually ending up as stable lead-206.

graph TD
    A["Uranium-238<br>(alpha decay)"] --> B["Thorium-234<br>(beta decay)"]
    B --> C["Protactinium-234<br>(beta decay)"]
    C --> D["Uranium-234<br>(alpha decay)"]
    D --> E["Thorium-230<br>(alpha decay)"]
    E --> F["...further decays..."]
    F --> G["Lead-206<br>(stable)"]

    style A fill:#e74c3c,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#f39c12,color:#fff
    style D fill:#e74c3c,color:#fff
    style E fill:#e67e22,color:#fff
    style F fill:#95a5a6,color:#fff
    style G fill:#27ae60,color:#fff

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Background Radiation

We are constantly exposed to low levels of radiation from natural and artificial sources. This is called background radiation. Sources include: