Think of radiation like a bright flashlight. The longer you look at it, the more light you receive. Similarly, the longer you stay near a source, the more radiation you absorb.
Rule of thumb: “Time is a multiplier.” If you cut the time in half, the dose you receive is also halved. This is expressed mathematically as:
\$ D \propto t \$
where \(D\) is dose and \(t\) is exposure time.
Radiation intensity decreases with the square of the distance from the source – the inverse square law. This is like standing back from a campfire: the further you stand, the cooler the heat feels.
\$ I \propto \frac{1}{r^2} \$
Practical tip: If you double your distance from a source, the intensity drops to one‑quarter of what it was.
Shielding blocks or absorbs radiation. Different types of ionising radiation require different materials:
The effectiveness of a shield is described by the half‑value layer (HVL), the thickness that reduces the intensity by 50 %:
\$ I = I_0 \, e^{-\mu x} \$
where \(I_0\) is the original intensity, \(x\) is thickness, and \(\mu\) is the linear attenuation coefficient.
| Radiation Type | Typical Shielding | Half‑Value Layer (HVL) |
|---|---|---|
| Alpha | Paper / 0.1 mm aluminium | ~0.1 mm |
| Beta | Plastic / 1–2 mm aluminium | ~1 mm |
| Gamma / X‑ray | Lead / 5–10 cm concrete | ~1–2 cm (lead) |