Four kinds of ionising radiation matter: alpha particles, beta particles, gamma rays and neutrons. Alpha and beta are stopped by paper or thin metal but are dangerous when the source is swallowed, inhaled or lodged in a wound, while gamma rays and neutrons pass through the body and are the external hazard.
Part of Radiation Safety
The four kinds of ionising radiation differ in mass, charge and energy, and those differences decide what stops each one and whether the danger is outside or inside the body.
The Rank Order Of Danger Reverses Between Outside And Inside The Body
The parent article, Nuclear Safety, gives this summary:
| Type | What it is | Stopped by | External danger | Internal danger |
|---|---|---|---|---|
| Alpha | Heavy, slow, positively charged particle | Paper, skin, a few cm of air | Negligible | Extreme |
| Beta | Lighter, faster, negatively charged particle | Heavy clothing, thin metal | Moderate (skin burns) | High |
| Gamma | High-energy electromagnetic wave | Thick concrete, lead, earth | High (whole-body) | High |
| Neutron | Uncharged particle from fission | Water, concrete, soil | High (near reactors) | High |
An alpha source held in the hand does little; the same source in a lung or a wound deposits all its energy in a tiny volume of tissue. The parent article calls this the primary danger from contaminated dust and food. Gamma is the reverse: it is the main hazard from outside, because it passes through a body.
Alpha: A Short-Range Particle That Needs To Be Inside You
Alpha particles stop in a few centimetres of air or in the dead outer layer of skin. Plutonium-239 is the corpus’s example of an alpha emitter that matters: it is a lung-cancer hazard if inhaled and a reason to deny an area, with a half-life of 24,100 years. See Half-Life. The sievert accounts for alpha’s damage by weighting: the parent article says 1 gray of alpha dose is roughly 20 sieverts, against about 1 sievert for 1 gray of gamma or beta. That weighting concerns long-term cancer risk and does not describe what an alpha dose does acutely; see Units of Measurement.
Beta: Skin Burns From Outside, Internal Dose If Swallowed
Beta particles penetrate skin to a depth of a few millimetres. Prolonged skin exposure to beta emitters, such as fallout settled on skin or clothing, can burn. Iodine-131, caesium-137 and strontium-90 are listed in the parent article as sources of concern; each is dangerous mainly because it can be eaten, drunk or inhaled.
Gamma: Penetrating, And Stopped Only By Mass
Gamma rays have no mass and no charge. They are not stopped outright by any thickness; each layer of material removes a fraction. That is why shielding is measured in half-value layers and why mass per square metre, not a material name, is the useful quantity; see Penetration & Shielding and Half-Value Layers.
Neutrons: Rare Outside Reactors And Weapons
The parent article notes neutron radiation is rarely met except near active reactors or nuclear weapon detonations, and is best stopped by hydrogen-rich materials such as water, concrete and polyethylene. For a settlement, the realistic hazards are overwhelmingly gamma and beta from fallout and spent material, and alpha from inhaled or ingested dust. The neutron weighting in sieverts depends on neutron energy, so no single conversion is given here.
Common Mistakes
- “Alpha is harmless.” It is harmless only while outside the body. Dust and food bring it inside.
- “A thick coat stops radiation.” It stops most beta and all alpha, and does almost nothing to gamma.
- Reading a skin burn as gamma. Beta burns on skin contaminated with fallout are a contamination problem, curable by washing; see Decontamination.
Safety
Never handle an unknown source or breathe its dust
A source that cannot be identified should be treated as emitting all four kinds until shown otherwise. Do not pick it up, open it, move it or breathe dust from it. Alpha and beta contamination is invisible and cannot be detected by an improvised electroscope with any confidence; see Improvised Detectors.
What’s Next
- Learn next: Penetration & Shielding — what stops each type
- Combine with: Contamination vs Irradiation — why inside-the-body hazards matter