Types of Radioactive Decay
Introduced| English | Meaning |
|---|---|
| alpha/ˈælfə/ | alpha |
| beta/ˈbiːtə/ | beta |
| gamma/ˈɡæmə/ | The most penetrating, needing thick lead or concrete to reduce it. |
| half-life/hɑːf laɪf/ | The time for the number of undecayed nuclei in a radioactive sample to halve. |
Some atoms are unstable — and throw out particles to fix it
- A few kinds of atom have nuclei that are simply too unstable to last.
- To settle down, they decay, hurling out a particle or a burst of energy at random.
- The three flavours are alpha, beta and gamma radiation.
- Knowing which is which tells you how dangerous and how penetrating the radiation is.
Alpha, beta, gamma
- Alpha 阿尔法 ($\alpha$) — a heavy helium nucleus (2 protons, 2 neutrons); slow and highly ionising.
- Beta 贝塔 ($\beta$) — a fast electron flung from the nucleus as a neutron turns into a proton.
- Gamma 伽马 ($\gamma$) — a burst of high-energy photon, no mass or charge, from a nucleus settling down.
- Each is emitted by an unstable nucleus trying to reach a stable state.
Balance a decay
Choose alpha, beta or gamma and see how the nucleus changes and the equation balances.
An alpha particle is:
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An alpha particle is a helium nucleus — 2 protons and 2 neutrons.
A beta particle is a fast ____ emitted from the nucleus.
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Beta decay flings out a fast electron as a neutron becomes a proton.
Penetration and danger
- Alpha is stopped by paper or skin — but dangerous if swallowed (it ionises heavily).
- Beta passes through paper but is stopped by a few millimetres of aluminium.
- Gamma is the most penetrating, needing thick lead or concrete to reduce it.
- The most penetrating is the hardest to shield, but alpha does the most damage once inside.
Which radiation is the most penetrating?
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Gamma is the most penetrating, needing thick lead or concrete to stop.
Select all true statements about radioactive decay.
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Alpha = helium nucleus, stopped by skin; gamma = most penetrating. But alpha (not gamma) is the most damaging inside.
Half-life
- Radioactive decay is random, but on average a sample halves its activity in a fixed half-life 半衰期.
- After one half-life, half the unstable nuclei have decayed; after two, a quarter remain.
- Half-lives range from fractions of a second to billions of years.
- Carbon-dating and nuclear-waste storage both hinge on the half-life.
A sample of $80$ unstable nuclei has a half-life of $2$ years. How many remain after $4$ years?
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$4$ years = two half-lives: $80 \to 40 \to 20$.
Radioactive decay is random, but a sample halves its activity each half-life on average.
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Individual decays are random; the half-life is the average time to halve.
The most penetrating radiation (gamma) is not the same as the most damaging inside the body. Gamma passes right through, so it often does less local damage; alpha, though stopped by skin, is the most ionising and most harmful if it gets inside you.
A radioactive sample has a half-life of $2$ years and starts with $80$ unstable nuclei. How many remain after $4$ years?
- $4$ years is two half-lives.
- After one: $40$; after two: $20$ nuclei remain.
Unstable nuclei decay by emitting alpha (helium nucleus), beta (fast electron) or gamma (photon) radiation. Gamma is the most penetrating; alpha the most ionising (dangerous inside). Decay is random, but a sample halves its activity each half-life — the basis of carbon dating.
Write from 25 nuclei remaining
- A sample starts with 200 unstable nuclei.
- The half-life is 4 hours. Find how many remain after 12 hours.
A sample contains 200 unstable nuclei. The half-life is 4 hours. How many nuclei remain after 12 hours?
Three halvings take 200 to 25.
How many nuclei remain after 12 hours?
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Three half-lives leave one eighth of 200. Other options — 20 nuclei (the lesson remainder); 100 nuclei (the count after only one half-life).