Radioactive decay and the three radiations
Introduced| English | Meaning |
|---|---|
| radioactivity/ˌreɪdɪəʊækˈtɪvɪti/ | radioactivity |
| radioactive decay/ˌreɪdɪəʊˈæktɪv dɪˈkeɪ/ | The spontaneous, random change of an unstable nucleus with the emission of radiation. |
| spontaneous/spɒnˈteɪnɪəs/ | Happening randomly and without external trigger, as radioactive decay does. |
| background radiation/ˈbækɡraʊnd ˌreɪdɪˈeɪʃn/ | Ionising radiation present in the surroundings from natural and artificial sources. |
| radon/ˈreɪdɒn/ | A radioactive noble-gas element produced in natural decay chains. |
| cosmic rays/ˈkɒzmɪk reɪz/ | High-energy particles from space that contribute to background radiation. |
| count rate/kaʊnt reɪt/ | The number of events registered by a detector per unit time. |
| alpha/ˈælfə/ | alpha |
| ionising/ˈaɪənaɪzɪŋ/ | Able to remove electrons from atoms or molecules and thereby form ions. |
| beta/ˈbiːtə/ | beta |
| gamma/ˈɡæmə/ | gamma |
Unstable nuclei
- Some nuclei are unstable — sooner or later they break down and shoot out radiation.
- A Geiger counter clicks as it detects each burst.
- This is radioactivity 放射性, and it comes in three types.
Radioactive decay 放射性衰变
- An unstable nucleus breaks down and gives out radiation — radioactive decay.
- A nucleus may be unstable because it has too many neutrons or is too heavy.
- Decay is spontaneous 自发 (you cannot speed it up or slow it down) and random (you cannot say which nucleus is next, or exactly when).
A Geiger counter detects and counts radiation
Radioactive decay
N = N₀·bᵗ
The number of unstable nuclei decays exponentially.
Radioactive decay is spontaneous and random. This means:
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Spontaneous = not triggered and not controllable; random = you cannot say which nucleus decays next or exactly when.
Background radiation 背景辐射 and measuring
- Some radiation is around us all the time — background radiation.
- Main sources: radon 氡 gas (usually the biggest), rocks and buildings, food and drink, cosmic rays 宇宙射线 from space.
- A detector and counter give the count rate 计数率 (counts per second or minute).
- To find a source's true rate, subtract the background:
Deflection of radiation in an electric field
A source reads $250$ counts/min. The background is $30$ counts/min. What is the corrected count rate, in counts/min?
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Corrected = measured − background = $250 - 30 = 220$ counts/min.
The three types of radiation
- Alpha α粒子 (α) — a helium nucleus (charge $+2$): most ionising 电离, least penetrating — stopped by paper.
- Beta β粒子 (β) — a fast electron (charge $-1$): medium — stopped by a few mm of aluminium.
- Gamma γ射线 (γ) — a high-energy EM wave (no charge): least ionising, most penetrating — only reduced by thick lead.
Match each radiation to what stops it.
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Alpha is least penetrating (paper), beta is medium (aluminium), gamma is most penetrating (thick lead only reduces it).
Which radiation is the most ionising but the least penetrating?
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Alpha is the most ionising (big +2 charge, slow and heavy) but is easily absorbed — the least penetrating.
Deflection and decay equations
- Because α and β are charged, fields deflect them — in opposite directions (the lighter β bends more). Gamma (no charge) is not deflected.
- Alpha decay: lose 2 protons + 2 neutrons, so $A$ falls by 4 and $Z$ by 2.
- Beta decay: a neutron becomes a proton + the emitted electron, so $A$ stays the same and $Z$ rises by 1, e.g. $^{24}_{11}\text{Na} \rightarrow {}^{24}_{12}\text{Mg} + {}^{\ \ 0}_{-1}\beta$.
- Gamma: only energy leaves, so $A$ and $Z$ are unchanged.
When a nucleus emits an alpha particle, the nucleon number A and proton number Z change by:
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An alpha particle is 2 protons + 2 neutrons, so A drops by 4 and Z drops by 2.
Gamma radiation is deflected by a magnetic field.
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Gamma rays have no charge, so they are not deflected. Only the charged alpha and beta particles bend in fields.
You've got it
- decay is spontaneous and random; corrected rate = measured − background
- α (He nucleus, +2): most ionising, stopped by paper
- β (electron, −1): medium, stopped by aluminium; γ (EM wave): most penetrating, lead
- α decay: $A{-}4$, $Z{-}2$ · β decay: $A$ same, $Z{+}1$ · γ: no change; only α and β are deflected