Common confusions

So does anything ever hit a nucleus?

Yes, but that's a nuclear reaction, not radioactive decay. Some examples:

• Fission in a reactor: a neutron strikes a uranium-235 nucleus and splits it.
• Fusion in the Sun: hydrogen nuclei crash together hard enough to merge.
• Making radioactive isotopes: stable atoms are hit with neutrons or protons. That's how hospitals get fluorine-18 for PET scans.

None of the five decays work that way. Each one is a nucleus that is already unstable, decaying on its own. Electron capture comes closest, but the electron was already part of the atom.

Can you speed up or slow down radioactive decay?

Basically no. Heat, pressure, chemical reactions and electric fields all deal in energies of a few eV. Nuclear energies are millions of eV, so changes at the atom's scale barely register in the nucleus. That's why half-lives are reliable enough for carbon dating and for dating rocks.

Electron capture is the one tiny exception, because it depends on the atom's own electrons. Beryllium-7's half-life changes by up to about 1% depending on what chemical compound it's in.

What's the difference between gamma rays and X-rays?

Both are photons, which are packets of light. The name only tells you where the photon came from:

• Gamma rays come from the nucleus changing energy level.
• X-rays come from electrons changing energy level, or from electrons being slowed down.

A high-energy X-ray and a low-energy gamma ray can be exactly the same thing.

Where were the electron and neutrino before β decay?

Nowhere. They didn't exist yet. The nucleus doesn't contain electrons waiting to escape. The energy released by the quark changing type is turned into brand-new particles, using E = mc² in reverse. A light bulb is a good comparison: it doesn't hold a stock of photons, it makes them when the electrons in it drop energy.

What happens after the first decay?

The new nucleus is often unstable too, so decays come in chains. Uranium-238 goes through 14 decays (8 alpha and 6 beta-minus, with gamma rays along the way) before it ends up as stable lead-206. That's why uranium ore always contains a little radium and gives off radon gas: they're stops along the chain.

Does being exposed to radiation make you radioactive?

Not from alpha, beta or gamma rays from ordinary sources. They can damage molecules (and DNA) by knocking electrons off atoms, but they don't change the nuclei they pass through. Being X-rayed or standing near a gamma source doesn't make you radioactive.

The real danger is contamination: getting radioactive material itself on or inside you. Neutrons are the exception. They have no charge, so they can slip into nuclei and make them radioactive, which is why materials near a running reactor become radioactive over time.

If a half-life is 10 years, is everything gone after 20?

No. After one half-life ½ is left, after two ¼, after three ⅛, and so on. Each half-life removes half of what's left, not half of the original amount. A common rule of thumb is that after about 10 half-lives (about 1/1000 left) a source has mostly faded.

What this guide simplifies

• The "energy ladders" are a cartoon of the nuclear shell model. Real rungs hold different numbers of nucleons and are unevenly spaced.
• The beta energy spectrum shows the basic shape. It leaves out a correction for the pull of the nucleus's electric charge on the electron.
• The quark diagrams leave out quark colour (the strong force's version of charge) and the way physicists draw antiparticles.
• Nucleons are drawn as little balls packed together. In reality they're fuzzy quantum waves that overlap.