Dangers and Uses of Radioactivity

Why ionising radiation is dangerous. It removes electrons from atoms in living cells. That ionisation can damage or alter DNA, which may kill the cell, cause a mutation, or lead to cancer. A very large dose causes radiation sickness and death.

Inside the body against outside the body — and the answer reverses.

• Outside the body, gamma is the most dangerous, because alpha cannot penetrate the skin and beta only partly can. Gamma passes right through and can damage organs.
• Inside the body, alpha is by far the most dangerous, because all its intense ionisation is deposited directly into surrounding tissue over a very short range. Gamma inside the body mostly passes straight out again.

This reversal is a favourite examination point, and the reasoning is the same fact — alpha's short range — seen from two sides.

Safe handling:

• Distance — use tongs, keep sources at arm's length, and store them away from people. Intensity falls rapidly with distance.
• Time — minimise exposure time; dose accumulates.
• Shielding — lead-lined containers for storage, lead aprons and screens in hospitals.
• Never point a source at anyone, and never handle it directly.
• Monitoring — film badges record the cumulative dose received by workers.

Uses, and why the choice of source makes sense:

Smoke alarms — an alpha source ionises the air in a chamber, allowing a small current to flow. Smoke absorbs the alpha particles, the current falls, and the alarm sounds. Alpha is chosen precisely because it will not escape the device.

Thickness monitoring — beta is used for paper, card and aluminium foil. Alpha would be entirely absorbed and gamma would pass through unchanged; only beta is partly absorbed, so the transmitted amount responds to thickness. Rollers adjust automatically to keep the reading constant.

Medical tracers — a gamma emitter with a short half-life is swallowed or injected, and a detector follows it to study organ function or find blockages. Gamma so it escapes the body to be detected, short half-life so it does not remain.

Sterilising — gamma kills bacteria on surgical instruments and food without the heat that would damage them, and works through sealed packaging.

Treating cancer (radiotherapy) — beams of gamma are aimed from several directions so they converge on the tumour, giving it a high dose while surrounding tissue receives much less.

Radiocarbon dating — the proportion of carbon-14 in dead organic material falls at a known rate, so measuring it gives an age.

Example: choosing a source for thickness control of aluminium foil, alpha is stopped entirely by the foil and gamma passes through almost unaffected. Only beta gives a reading that changes with thickness, which is what makes the control loop possible.