Radioactivity: Alpha, Beta and Gamma Emission

This lesson explains Radioactivity: Alpha, Beta and Gamma Emission for Cambridge IGCSE Physics 0625. It separates the Core requirements from the additional Supplement work for Extended candidates. Focus on the cause-and-effect explanation and the exact quantities being compared. After the example, use the related practice questions to check what you can do independently.

What are the properties of alpha, beta and gamma radiation?

Radioactive decay is random and spontaneous: you cannot predict which nucleus decays next, and nothing external (temperature, pressure or chemistry) changes the rate. An unstable nucleus emits one of three radiations.

Property	Alpha ($\alpha$)	Beta ($\beta^-$)	Gamma ($\gamma$)
Nature	Helium nucleus $^{4}_{2}\text{He}$ (2p + 2n)	Fast electron $^{0}_{-1}\text{e}$ from the nucleus	Electromagnetic wave
Relative charge	$+2$	$-1$	0
Ionising power	Strongest	Moderate	Weakest
Penetration	Stopped by paper / few cm of air	Stopped by a few mm of aluminium	Reduced by thick lead or concrete
Deflection in fields	Small deflection	Large, opposite direction to alpha	None

The pattern is one chain of logic: alpha is big and doubly charged, so it ionises strongly, loses energy fast and penetrates least. Gamma is uncharged, so it barely ionises and penetrates furthest. Beta sits between. Deflection follows charge and mass: beta bends far more than alpha because it is thousands of times lighter, and it bends the opposite way because its charge is negative.

How do you balance a decay equation?

Both numbers must balance across the arrow: total nucleon numbers equal on both sides, total proton numbers equal on both sides.

In alpha decay, the nucleus emits $^{4}_{2}\text{He}$. So $A$ falls by 4 and $Z$ falls by 2:

$^{226}_{88}\text{Ra} \rightarrow {}^{222}_{86}\text{Rn} + {}^{4}_{2}\text{He}$

In beta-minus decay, a neutron becomes a proton plus an electron, which is emitted. $A$ stays the same; $Z$ rises by 1:

$^{14}_{6}\text{C} \rightarrow {}^{14}_{7}\text{N} + {}^{0}_{-1}\text{e}$

Gamma emission changes neither $A$ nor $Z$; the nucleus simply releases surplus energy after a decay. The element only changes in alpha and beta decay.

Worked example

Polonium-210 ($^{210}_{84}\text{Po}$) decays by alpha emission to an isotope of lead (Pb). (a) Write the balanced nuclide equation. (3 marks) (b) The source is safe to handle inside a sealed plastic container. Explain why. (2 marks)

Solution. (a) Alpha decay: $A$ drops by $4 \rightarrow 206$; $Z$ drops by $2 \rightarrow 82$.

$^{210}_{84}\text{Po} \rightarrow {}^{206}_{82}\text{Pb} + {}^{4}_{2}\text{He}$

Check: $206 + 4 = 210$ ✓ and $82 + 2 = 84$ ✓. (b) Alpha particles have very low penetrating power. The plastic wall (like paper or skin) absorbs them, so no radiation escapes the container.

Original marking points:

• B1: alpha particle written as $^{4}_{2}\text{He}$ (or $^{4}_{2}\alpha$).
• B1: lead nuclide with $A = 206$.
• B1: lead nuclide with $Z = 82$ (equation fully balanced).
• B1: alpha is weakly penetrating / stopped by thin solid material.
• B1: therefore the particles cannot pass through the container wall.

These marking points belong to this original example. They are not an official Cambridge mark scheme.

Common errors and how to correct them

• Beta decay changing the nucleon number. Fix: in $\beta^-$ decay $A$ is unchanged; only $Z$ rises by 1 (neutron → proton + electron).
• Calling the beta particle an orbital electron. Fix: it comes from the nucleus, created when a neutron decays.
• “Gamma is stopped by lead.” Fix: thick lead reduces gamma intensity; mark schemes reject “stopped completely”.
• Ranking penetration and ionisation the same way. Fix: they run in opposite orders. Most ionising (alpha) = least penetrating.
• Saying heating speeds up decay. Fix: decay is spontaneous, unaffected by temperature, pressure or chemical state. State the word “spontaneous”.

How to apply this in an exam

For every decay equation, finish with a visible balance check: add the top numbers on the right and confirm they equal the left, then repeat for the bottom numbers. Write the two little sums next to the equation. Examiners cannot penalise correct working, and the check catches the single most common error in Paper 4 nuclear questions before it costs the accuracy mark.

Where this skill matters

A CS subtopic with a modest tier split. Core candidates (Papers 1 and 3) need the properties table, the random/spontaneous nature of decay and recognition of decay equations. Extended candidates (Papers 2 and 4) must construct balanced equations themselves and explain deflection in electric and magnetic fields using charge and mass. Expect 4-7 marks on theory papers each session. Practical papers do not use live sources; instead Paper 6 may present count-rate data for analysis.