Radioactive Decay — Alpha, Beta and Gamma

This lesson covers the three types of nuclear radiation — alpha ($\alpha$α), beta ($\beta$β) and gamma ($\gamma$γ) — their properties, how they are emitted, and how they affect the nucleus. This is part of the AQA GCSE Combined Science Trilogy specification (8464, section 6.4.2).

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What Is Radioactive Decay?

Radioactive decay is the process by which an unstable nucleus gives out radiation to become more stable. It is:

• Random — you cannot predict when a particular nucleus will decay.
• Spontaneous — it happens on its own without any external trigger.

The rate of decay is not affected by external conditions such as temperature, pressure or chemical reactions.

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Types of Nuclear Radiation

Property	Alpha ($\alpha$α)	Beta ($\beta$β)	Gamma ($\gamma$γ)
What is it?	2 protons + 2 neutrons (a helium nucleus)	A high-speed electron emitted from the nucleus	An electromagnetic wave (very short wavelength)
Symbol	$^{4}_{2}\text{He}$24​He or $^{4}_{2}\alpha$24​α	$^{0}_{-1}\text{e}$−10​e or $^{0}_{-1}\beta$−10​β	$\gamma$γ
Charge	+2	−1	0
Mass	4 (relatively heavy)	Very small (≈ 1/1836 of a proton)	0
Ionising ability	Strongly ionising	Moderately ionising	Weakly ionising
Penetrating power	Low — stopped by a few cm of air or a sheet of paper	Moderate — stopped by a few mm of aluminium	High — only significantly reduced by thick lead or several metres of concrete
Speed	Slow (relative to other radiation)	Fast (up to 90% the speed of light)	Speed of light
Deflected by fields?	Yes (deflected by electric and magnetic fields)	Yes (opposite direction to alpha)	No

graph LR
    subgraph Sources["Radioactive Source"]
        N["Unstable nucleus"]
    end
    N -->|"Alpha (α)"| P["Stopped by paper"]
    N -->|"Beta (β)"| Al["Stopped by aluminium"]
    N -->|"Gamma (γ)"| Pb["Reduced by thick lead<br/>or concrete"]

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Alpha Decay

When a nucleus emits an alpha particle, it loses 2 protons and 2 neutrons.

$^{A}_{Z}X \rightarrow ^{A-4}_{Z-2}Y + ^{4}_{2}\alpha$ZA​X→Z−2A−4​Y+24​α

Effect on the nucleus:

• Mass number decreases by 4 (loses 2 protons + 2 neutrons).
• Atomic number decreases by 2 (loses 2 protons → becomes a different element).

Example

$^{238}_{92}\text{U} \rightarrow ^{234}_{90}\text{Th} + ^{4}_{2}\alpha$92238​U→90234​Th+24​α

Uranium-238 decays to thorium-234 by emitting an alpha particle.

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Beta Decay

In beta decay, a neutron in the nucleus changes into a proton and a high-speed electron (beta particle). The electron is emitted from the nucleus.

$^{A}_{Z}X \rightarrow ^{A}_{Z+1}Y + ^{0}_{-1}\beta$ZA​X→Z+1A​Y+−10​β

Effect on the nucleus:

• Mass number stays the same (a neutron turns into a proton — total nucleons unchanged).
• Atomic number increases by 1 (one extra proton → becomes a different element).

Example

$^{14}_{6}\text{C} \rightarrow ^{14}_{7}\text{N} + ^{0}_{-1}\beta$614​C→714​N+−10​β

Carbon-14 decays to nitrogen-14 by emitting a beta particle.

Exam Tip: In beta decay, the electron comes from the nucleus (a neutron converts to a proton and an electron), NOT from the electron shells. This is a very common error that costs marks.

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Gamma Emission

Gamma radiation is an electromagnetic wave emitted from the nucleus. It often accompanies alpha or beta decay when the nucleus has excess energy after emitting a particle.

• Gamma emission does not change the mass number or atomic number.
• It simply releases excess energy from the nucleus.

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Ionisation

When radiation passes through a material, it can knock electrons off atoms, creating ions. This is called ionisation.

Type	Ionising power	Why?
Alpha	Strongest	Large, heavy, slow-moving — interacts with many atoms over a short distance
Beta	Moderate	Smaller, faster — fewer interactions per unit distance
Gamma	Weakest	No charge, no mass — rarely interacts with atoms, passes through most material

Exam Tip: There is an inverse relationship between ionising power and penetrating power. Alpha is the most ionising but least penetrating. Gamma is the most penetrating but least ionising.

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Detecting Radiation

Detector	How it works
Geiger-Müller (GM) tube and counter	Radiation enters the tube and ionises gas inside, producing an electrical pulse that is counted
Photographic film	Radiation darkens the film — used in film badges (dosimeters) to monitor exposure
Cloud/bubble chamber	Radiation ionises gas/liquid, producing visible tracks

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Nuclear Equations — Balancing

In any nuclear equation:

• The total mass number (top number) is the same on both sides.
• The total atomic number (bottom number) is the same on both sides.

Worked Example

Complete the equation: $^{226}_{88}\text{Ra} \rightarrow ^{?}_{?}\text{Rn} + ^{4}_{2}\alpha$88226​Ra→??​Rn+24​α

• Mass number of Rn = 226 − 4 = 222
• Atomic number of Rn = 88 − 2 = 86

$^{226}_{88}\text{Ra} \rightarrow ^{222}_{86}\text{Rn} + ^{4}_{2}\alpha$88226​Ra→86222​Rn+24​α

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Radioactive Decay Chain

Some radioactive isotopes decay through a series of steps, emitting alpha or beta particles at each stage, until a stable isotope is reached.

graph TD
    A["Uranium-238<br/>(α decay)"] --> B["Thorium-234<br/>(β decay)"]
    B --> C["Protactinium-234<br/>(β decay)"]
    C --> D["Uranium-234<br/>(α decay)"]
    D --> E["..."]
    E --> F["Lead-206<br/>(stable)"]

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Common Misconceptions

Misconception	Correction
Beta particles come from the electron shells	Beta particles are electrons emitted from the nucleus when a neutron converts to a proton
Gamma radiation is a particle	Gamma is an electromagnetic wave, not a particle
Alpha particles are the most dangerous type of radiation	Alpha is most dangerous only if the source is inside the body; externally, it is the least penetrating and is stopped by skin
Radioactive decay can be speeded up or slowed down	Decay is spontaneous and random — it cannot be affected by temperature, pressure or chemical reactions

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Summary

• Radioactive decay is random and spontaneous.
• Alpha ($\alpha$α): 2 protons + 2 neutrons; strongly ionising; stopped by paper; mass number decreases by 4, atomic number decreases by 2.
• Beta ($\beta$β): a high-speed electron from the nucleus; moderately ionising; stopped by aluminium; mass number unchanged, atomic number increases by 1.
• Gamma ($\gamma$γ): an electromagnetic wave; weakly ionising; reduced by thick lead or concrete; no change to mass number or atomic number.
• Nuclear equations must balance — total mass numbers and atomic numbers are conserved.
• Ionising power and penetrating power are inversely related.

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Worked Example 1 — Completing an Alpha Decay Equation

Complete: $^{210}_{84}\text{Po} \rightarrow ^{?}_{?}\text{Pb} + ^{4}_{2}\alpha$84210​Po→??​Pb+24​α.

Balance mass numbers: $210 = ? + 4 \Rightarrow ? = 206$210=?+4⇒?=206. Balance atomic numbers: $84 = ? + 2 \Rightarrow ? = 82$84=?+2⇒?=82.

$^{210}_{84}\text{Po} \rightarrow ^{206}_{82}\text{Pb} + ^{4}_{2}\alpha$84210​Po→82206​Pb+24​α

Polonium-210 decays to stable lead-206 by alpha emission.

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Worked Example 2 — Completing a Beta Decay Equation

Complete: $^{90}_{38}\text{Sr} \rightarrow ^{?}_{?}\text{Y} + ^{0}_{-1}\beta$3890​Sr→??​Y+−10​β.

Balance mass numbers: $90 = ? + 0 \Rightarrow ? = 90$90=?+0⇒?=90. Balance atomic numbers: $38 = ? + (-1) \Rightarrow ? = 39$38=?+(−1)⇒?=39.

$^{90}_{38}\text{Sr} \rightarrow ^{90}_{39}\text{Y} + ^{0}_{-1}\beta$3890​Sr→3990​Y+−10​β