放射性崩壊

Three kinds of radiation 辐射

• An unstable nucleus settles down by throwing out radiation.
• There are three kinds — alpha, beta and gamma — and they behave very differently.
• Telling them apart is the heart of this topic.

What each one is

• α: a helium nucleus ($^{4}_{2}\alpha$), charge $+2e$, mass $\approx 4\ \text{u}$.
• β: a fast electron ($\beta^{-}$) or positron 正电子 ($\beta^{+}$), charge $\mp e$, tiny mass.
• γ: a high-energy photon 光子, no charge, no mass.

Penetration 穿透 and ionising 电离

• α: stopped by paper; strongly ionising.
• β: stopped by a few mm of aluminium.
• γ: only reduced by thick lead; weakly ionising.

In an electric field

• The field pushes charges: α is deflected 偏转 towards the negative plate, β$^{-}$ the opposite way, γ not at all.
• The force is $F = Eq$, so the α feels twice the force of a β — but its mass is thousands of times larger, so it curves far less.
• Same rule for any two charged particles in the same field: compare the forces by charge, the accelerations by $\dfrac{q}{m}$.

Antiparticles 反粒子 and neutrinos 中微子

• Every particle has an antiparticle — same mass, opposite charge (the positron is the electron's).
• β-decay always emits a (anti)neutrino too: nearly massless, no charge, very hard to detect.

Why β is continuous

• α-decay shares energy between two particles → the α has one fixed energy (discrete).
• β-decay shares energy between three (with the neutrino) → β energies range from zero up to a maximum (continuous).

Decay equations

• α: $^{A}_{Z}\text{X} \to {}^{A-4}_{Z-2}\text{Y} + {}^{4}_{2}\alpha$.
• β$^{-}$: $^{A}_{Z}\text{X} \to {}^{A}_{Z+1}\text{Y} + {}^{0}_{-1}\beta + \bar{\nu}_{e}$ (a neutron becomes a proton).
• β$^{+}$: $^{A}_{Z}\text{X} \to {}^{A}_{Z-1}\text{Y} + {}^{0}_{+1}\beta + \nu_{e}$ (a proton becomes a neutron).

Worked example: nitrogen-12 and a field

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