Radioactive decay and the three radiations

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

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

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.

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.

You've got it

Write from a corrected rate of 160 counts per minute

• A detector reads 180 counts per minute. The background is 20 counts per minute.
• Find the corrected rate. Say what stops alpha radiation.