Properties of Ionising Radiation | 电离辐射的性质

📚 Properties of Ionising Radiation | 电离辐射的性质

Ionising radiation is emitted by unstable nuclei as they decay towards stability. Understanding its properties—charge, mass, speed, ionising ability and penetrating power—is central to CIE A-Level Physics, both for explaining detection methods and for comparing alpha, beta and gamma radiation.

电离辐射由不稳定原子核在向稳定状态衰变时发出。理解其电荷、质量、速度、电离能力和穿透能力等性质，是 CIE A-Level 物理的核心内容，既用于解释探测方法，也用于比较 α、β 和 γ 辐射。

1. What is ionising radiation? | 什么是电离辐射？

Ionising radiation has enough energy to remove electrons from atoms or molecules, creating ions. In nuclear physics, the main types are alpha (α) particles, beta (β) particles and gamma (γ) rays.

电离辐射具有足够的能量从原子或分子中剥离电子，从而产生离子。在核物理中，主要类型是 α 粒子、β 粒子和 γ 射线。

Alpha and beta radiations are charged particles, while gamma radiation is an electromagnetic wave. They differ in mass, charge, speed, ionising power and penetrating power.

α 和 β 辐射是带电粒子，而 γ 辐射是电磁波。它们在质量、电荷、速度、电离能力和穿透能力上各不相同。

2. Alpha particles: nature and properties | α 粒子的本质与性质

An alpha particle is a helium nucleus, consisting of two protons and two neutrons. Its symbol is ⁴₂He²⁺, with charge +2e and mass approximately 4u, where u is the atomic mass unit.

α 粒子是氦原子核，由两个质子和两个中子组成。其符号为 ⁴₂He²⁺，电荷为 +2e，质量约为 4u，其中 u 是原子质量单位。

Alpha particles are emitted with typical speeds of about 10⁷ m s⁻¹, much slower than light. They have a large mass and double charge, so they interact strongly with matter.

α 粒子发射时的典型速度约为 10⁷ m s⁻¹，远低于光速。由于质量大且带两个正电荷，它们与物质发生强烈相互作用。

Because of this strong interaction, alpha particles produce dense ionisation along a short track. They lose energy quickly and are therefore easily stopped.

由于这种强相互作用，α 粒子在很短路径上产生密集电离。它们迅速损失能量，因此容易被阻挡。

3. Beta particles: nature and properties | β 粒子的本质与性质

A beta-minus (β⁻) particle is a fast electron emitted when a neutron in the nucleus changes into a proton. It has charge −e and a very small mass, about 1/1836 u. Beta-plus (β⁺) particles are positrons with charge +e.

β⁻ 粒子是原子核内一个中子转变为质子时发射出的高速电子。其电荷为 −e，质量非常小，约为 1/1836 u。β⁺ 粒子是电荷为 +e 的正电子。

Beta particles travel at speeds that can exceed 99% of the speed of light. Their much smaller mass means they cause less intense ionisation than alpha particles but can travel much further in matter.

β 粒子的速度可超过光速的 99%。由于质量小得多，它们的电离能力弱于 α 粒子，但在物质中能传播更远。

A beta particle has a continuous range of kinetic energies up to a maximum value. The missing energy is carried away by an antineutrino (or a neutrino for β⁺ decay).

β 粒子的动能具有连续分布，最大可达某个值。缺失的能量由反中微子（β⁺ 衰变中为中微子）带走。

4. Gamma rays: nature and properties | γ 射线的本质与性质

Gamma radiation is high-energy electromagnetic radiation with no mass and no charge. It is usually emitted by a nucleus after alpha or beta decay when the daughter nucleus is in an excited state.

γ 辐射是一种高能电磁辐射，没有质量和电荷。它通常在 α 或 β 衰变后，子核处于激发态时由原子核发射。

Gamma rays travel at the speed of light, c = 3.00 × 10⁸ m s⁻¹. They do not ionise directly as strongly as charged particles because they interact less frequently, but their penetrating power is much greater.

γ 射线以光速 c = 3.00 × 10⁸ m s⁻¹ 传播。由于相互作用频率较低，它们不像带电粒子那样直接产生强烈电离，但穿透能力要强得多。

Gamma emission does not change the proton number or nucleon number of the nucleus; it only releases excess energy.

γ 发射不会改变原子核的质子数或核子数，只是释放多余的能量。

5. Penetrating power and absorption | 穿透能力与吸收

The three radiations are absorbed very differently. Alpha particles are stopped by a few centimetres of air or a sheet of paper. Beta particles can penetrate paper but are absorbed by about 3–5 mm of aluminium. Gamma rays are only significantly reduced by several centimetres of lead or thick concrete.

三种辐射被吸收的方式差异很大。α 粒子可被几厘米空气或一张纸阻挡。β 粒子能穿透纸，但会被约 3–5 mm 的铝吸收。γ 射线只有通过几厘米厚的铅或厚混凝土才能显著减弱。

Absorption experiments can identify the type of radiation by placing different absorbers between a source and a Geiger–Müller tube. If the count drops with paper only, the source emits alpha; if paper has little effect but aluminium stops it, the source emits beta; if a large count remains after aluminium, gamma is present.

吸收实验可通过在放射源和盖革-米勒管之间放置不同吸收体来鉴别辐射类型。若只有纸使计数下降，则源发射 α；若纸影响很小而铝能阻止，则源发射 β；若铝后仍有大量计数，则存在 γ。

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