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Cambridge IGCSE Physics · 0625

Chapter 5: Nuclear physics — Part 4

Topic 5.2.2 · The three types of nuclear emission

Nature, ionising and penetrating

Emission from a nucleus is spontaneous and random in direction.

β means β− (an electron). β+ is not included.

NatureIonising effectPenetration
αhelium nucleus (42He), charge +2strongeststopped by paper / a few cm of air
βfast electron, charge −1moderatestopped by a few mm of aluminium
γEM wave, unchargedweakestreduced by thick lead / concrete

α ionises strongly because it has a large charge and (typically) high kinetic energy transferred over a short path. γ has no charge and is the most penetrating.

Penetration: alpha stopped by paper or a few centimetres of air, beta stopped by a few millimetres of aluminium, gamma reduced by thick lead or concrete.
Diagram 1: Penetration: alpha stopped by paper or a few centimetres of air, beta stopped by a few millimetres of aluminium, gamma reduced by thick lead or concrete.

Exam Traps

  • Stronger ionising power means weaker penetration — do not say alpha is the most penetrating because it is the most ionising.

Deflection in fields

α and β particles are deflected by electric and magnetic fields (opposite ways because their charges have opposite signs). β is deflected more than α (much smaller mass). γ is not deflected.

Deflection: alpha and beta curving opposite ways in electric or magnetic fields; gamma going straight on, uncharged.
Diagram 2: Deflection: alpha and beta curving opposite ways in electric or magnetic fields; gamma going straight on, uncharged.

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