Penetration Depth

Key Stage 4

Penetration depth is how far into a material radiation can travel before being absorbed.

Penetration depth depends on the type of material and the type of radiation:

High density materials are difficult to penetrate so penetration depth of a radiation will be low.
High energy radiation can usually penetrate deeper into a material than low energy radiation.

Knowing the penetration depth in different materials is particularly important for ionising radiation to ensure that sources of ionising radiation can be handled safely.

	Alpha particles can travel around 5cm through air (STP) before colliding with and ionising atoms or molecules.

	Beta particles can travel several metres through air (STP) before colliding with and ionising atoms or molecules.

	Gamma-rays can travel an infinite distance through air and the chances of colliding with an atom or molecule are almost non-existent.

	Neutrons have a mean lifetime of around 15 minutes before they decay, so can travel several hundred kilometres depending on their velocity before they decay into a proton and a beta particle while emitting a gamma-ray.

	Alpha particles can be stopped by a thin sheet of paper.

	Beta particles can penetrate paper but are stopped by around 5mm thickness of metal foil.

	Gamma-rays can penetrate paper and sheets of metal foil but cannot penetrate more than a few cm of Lead or a metre of concrete.

	Neutron radiation can penetrate paper and sheets of metal foil but cannot penetrate more than a few cm of Lead or a metre of concrete before they are captured by the nucleus of an atom.