On this page
In 1899 Ernest Rutherford noticed that the radiation coming from uranium wasn’t all the same. Some of it was stopped by a sheet of paper; some went straight through. He named the two kinds after the first letters of the Greek alphabet, alpha and beta. A year later Paul Villard found a third, even more penetrating kind, which was soon called gamma. More than a century later, those names are still how we sort nuclear radiation.
Why nuclei decay at all
A nucleus is a bundle of protons and neutrons held together by the strong nuclear force, fighting against the electrical repulsion between the positively charged protons. Some combinations of protons and neutrons are stable. Others aren’t — they have too many neutrons, too few, or are simply too big — and they eventually rearrange into something more stable, releasing energy as radiation.
Alpha radiation (α)
An alpha particle is a helium-4 nucleus: 2 protons and 2 neutrons, with a charge of +2.
When a nucleus emits one, it loses 4 from its mass number and 2 from its atomic number — so it becomes a different element, two places earlier in the periodic table.
Example: uranium-238 → thorium-234 + alpha
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
Check: mass numbers 238 = 234 + 4; atomic numbers 92 = 90 + 2.
Alpha decay is typical of very heavy nuclei like uranium, radium, polonium and americium.
- Penetration: very low. Alpha particles are stopped by a sheet of paper, a few centimetres of air, or the dead outer layer of your skin.
- Ionizing power: very high. Being heavy and doubly charged, they rip electrons off everything they pass, dumping their energy in a short distance.
Beta radiation (β)
The common kind, beta-minus, is a fast electron ejected from the nucleus. There are no electrons in a nucleus to begin with; what happens is that a neutron turns into a proton, creating the electron (and an antineutrino) in the process.
So the mass number stays the same, and the atomic number goes up by 1 — the element moves one place later in the table.
Example: carbon-14 → nitrogen-14 + beta
¹⁴₆C → ¹⁴₇N + ⁰₋₁e
This is the decay that makes carbon-14 dating possible. Beta-minus decay is typical of nuclei with too many neutrons, such as many fission products (strontium-90, iodine-131, cesium-137).
There’s also beta-plus decay, where a proton turns into a neutron and emits a positron (the electron’s antimatter twin). The atomic number goes down by 1. Fluorine-18, used in PET scans, decays this way — the “P” in PET stands for positron.
- Penetration: moderate. Stopped by a few millimetres of aluminium or a thick plastic sheet; travels a metre or so in air.
- Ionizing power: moderate.
Gamma radiation (γ)
Gamma rays aren’t particles of matter at all. They are high-energy electromagnetic radiation — like light or X-rays, but much more energetic.
After an alpha or beta decay, the new nucleus is often left in an excited state, and it sheds the extra energy as a gamma ray. Neither the mass number nor the atomic number changes.
Example: cobalt-60 beta-decays to nickel-60 in an excited state, which then emits two gamma rays. That’s why cobalt-60 is used as a gamma source in radiotherapy and for sterilizing medical equipment.
- Penetration: very high. Reduced by several centimetres of lead or a metre or more of concrete, but never completely stopped — only made weaker.
- Ionizing power: low per unit distance — it interacts rarely, which is exactly why it travels so far.
Side by side
| Alpha (α) | Beta-minus (β⁻) | Gamma (γ) | |
|---|---|---|---|
| What it is | Helium-4 nucleus | Electron | Electromagnetic wave |
| Charge | +2 | −1 | 0 |
| Mass number change | −4 | 0 | 0 |
| Atomic number change | −2 | +1 | 0 |
| Stopped by | Paper, skin | Few mm aluminium | Thick lead, concrete (reduced) |
| Ionizing power | High | Medium | Low |
| Deflected by a magnetic field | Yes (slightly) | Yes (strongly, opposite way) | No |
Which is most dangerous?
It depends on where the source is.
- Outside the body, alpha is the least dangerous — your skin stops it. Gamma is the most concerning, because it passes into the body.
- Inside the body (inhaled, swallowed or in a wound), alpha is the most dangerous, because it dumps all its energy into a tiny volume of living tissue. The poisoning of Alexander Litvinenko with polonium-210 in 2006 was a stark example: a pure alpha emitter, harmless in a sealed container, devastating once ingested.
This is also why radon gas matters for health. Radon-222 and its decay products are alpha emitters, and breathing them in exposes lung tissue directly.
Everyday uses
- Alpha: americium-241 in ionization smoke detectors. The alpha particles ionize air between two plates; smoke disrupts that current and sets off the alarm.
- Beta: controlling the thickness of paper and plastic sheets in factories (thicker sheet, fewer betas get through); some cancer treatments.
- Gamma: radiotherapy, sterilizing medical supplies and some foods, and medical imaging tracers such as technetium-99m.
Quick answers
Does radiation make things radioactive? Alpha, beta and gamma radiation generally don’t. Food irradiated with gamma rays doesn’t become radioactive. Neutron radiation, which occurs in reactors, can.
Which is the fastest? Gamma rays travel at the speed of light. Beta particles can approach it; alpha particles are much slower.
Why does beta decay increase the atomic number? Because a neutron inside the nucleus becomes a proton. One more proton means the next element.
Related
How quickly an isotope decays is described by its half-life — see half-life explained, or use the half-life calculator. Each element page also lists every known isotope with its half-life and decay mode.
Advertisement