📚 Edexcel A-Level Physics: Nuclear Decay and Half-Life (Combined 025) | Edexcel A-Level 物理：核衰变与半衰期（Combined 025）
Radioactivity is a core topic in Edexcel A-Level Physics, requiring you to link nuclear structure, decay equations, exponential behaviour and practical half-life analysis. This Combined 025 revision guide builds up the definitions, equations and exam skills you need.
放射性是 Edexcel A-Level 物理的核心主题，要求你把核结构、衰变方程、指数行为以及半衰期分析联系起来。本 Combined 025 复习指南系统讲解你需要的定义、公式和考试技巧。
1. Nuclear Structure and Notation | 核结构与符号
The nucleus is described by its nucleon number A and proton number Z. A nuclide is written as ᴬᶻX, where A = protons + neutrons, Z = protons, and N = A − Z gives the neutron number.
原子核由核子数 A 和质子数 Z 描述。核素写作 ᴬᶻX，其中 A = 质子 + 中子，Z = 质子数，N = A − Z 是中子数。
For example, uranium-238 is written as ²³⁸₉₂U. It has 92 protons and 238 − 92 = 146 neutrons. Isotopes have the same Z but different A, so they are the same element with different neutron numbers.
例如，铀-238 写作 ²³⁸₉₂U。它有 92 个质子和 238 − 92 = 146 个中子。同位素具有相同的 Z 但不同的 A，因此它们是中子数不同的同一元素。
- Proton number Z determines the element and its chemical identity.
- Nucleon number A determines the total number of protons and neutrons.
- The strong nuclear force holds the nucleus together, but electrostatic repulsion between protons pushes it apart.
- 质子数 Z 决定元素及其化学身份。
- 核子数 A 决定质子和中子的总数。
- 强核力把原子核束缚在一起，但质子之间的静电排斥力会使它分裂。
2. Alpha, Beta and Gamma Decay | α、β 与 γ 衰变
There are three main types of ionising radiation: alpha particles, beta particles and gamma rays. They have different penetrating powers and ionising abilities.
主要有三种电离辐射：α 粒子、β 粒子和 γ 射线。它们具有不同的穿透能力和电离能力。
An alpha particle is a helium nucleus containing 2 protons and 2 neutrons. It has charge +2 and is highly ionising but weakly penetrating. A few centimetres of air or a sheet of paper can stop it.
α 粒子是含有 2 个质子和 2 个中子的氦核。它带 +2 电荷，电离能力强，但穿透能力弱。几厘米空气或一张纸就能阻挡它。
A beta-minus particle is a fast electron emitted when a neutron decays into a proton. It has charge −1. Beta particles are moderately penetrating; a few millimetres of aluminium can stop them.
β⁻ 粒子是中子衰变成质子时释放出的快速电子。它带 −1 电荷。β 粒子穿透能力中等；几毫米铝片就能阻挡它们。
Gamma radiation is an electromagnetic wave emitted by an excited nucleus. It has no mass and no charge. Gamma rays are weakly ionising but highly penetrating, requiring thick lead or concrete to reduce their intensity.
γ 辐射是激发态原子核发射的电磁波。它没有质量和电荷。γ 射线电离能力弱，但穿透能力很强，需要厚铅板或混凝土来降低强度。
3. Beta Decay: β⁻ and β⁺ | β⁻ 与 β⁺ 衰变
In beta-minus decay, a neutron changes into a proton, an electron and an antineutrino. The proton number increases by 1 while the nucleon number stays the same.
在 β⁻ 衰变中，一个中子变成一个质子、一个电子和一个反中微子。质子数增加 1，而核子数保持不变。
¹⁴₆C → ¹⁴₇N + ⁰₋₁e + ν̄
In beta-plus decay, a proton changes into a neutron, a positron and a neutrino. The positron has the same mass as an electron but positive charge. The proton number decreases by 1.
在 β⁺ 衰变中，一个质子变成一个中子、一个正电子和一个中微子。正电子的质量与电子相同，但带正电荷。质子数减少 1。
¹⁸₉F → ¹⁸₈O + ⁰₊₁e + ν
In gamma decay, an excited nucleus releases excess energy as a gamma photon. The nucleon number and proton number do not change.
在 γ 衰变中，激发态原子核以 γ 光子的形式释放多余能量。核子数和质子数均不改变。
²³⁴₉₀Th* → ²³⁴₉₀Th + γ
4. Activity and the Decay Constant | 活度与衰变常数
The activity A of a radioactive source is the number of decays per second. It is measured in becquerels, where 1 Bq = 1 decay per second.
放射源的活度 A 是每秒发生的衰变次数。它用贝克勒尔表示，1 Bq = 每秒 1 次衰变。
Activity is proportional to the number of undecayed nuclei N. The constant of proportionality is the decay constant λ, which has units of s⁻¹.
活度与未衰变原子核的数量 N 成正比。比例常数是衰变常数 λ，单位为 s⁻¹。
A = λN
The decay constant is a property of each isotope. A large λ means the source decays quickly and has a short half-life.
衰变常数是每种同位素自身的性质。λ 大意味着放射源衰变快，半衰期短。
5. The Exponential Decay Law | 指数衰变定律
Radioactive decay is random and spontaneous, but for a large number of nuclei the overall behaviour follows an exponential law. The number of undecayed nuclei after time t is given by the equation below.
放射性衰变是随机且自发的，但对于大量原子核，整体行为遵循指数规律。时间 t 后未衰变的原子核数由下式给出。
N = N₀ e^(−λt)
Here N₀ is the initial number of undecayed nuclei. Since activity is proportional to N, the activity also decays exponentially.
其中 N₀ 是初始未衰变原子核数。由于活度与 N 成正比，活度也按指数规律衰减。
A = A₀ e^(−λt)
Taking natural logarithms gives a straight-line form: ln A = ln A₀ − λt. A graph of ln A against t has slope −λ.
取自然对数可得到直线形式：ln A = ln A₀ − λt。ln A 对 t 作图所得直线的斜率为 −λ。
6. Half-Life | 半衰期
The half-life t½ is the average time taken for half of the radioactive nuclei in a sample to decay. It is also the time taken for the activity to halve.
半衰期 t½ 是样品中一半放射性原子核发生衰变所需的平均时间。它也是活度减半所需的时间。
t½ = ln 2 / λ ≈ 0.693 / λ
This relationship is independent of the initial number of nuclei. After one half-life, N = N₀/2. After two half-lives, N = N₀/4, and after n half-lives, N = N₀/2ⁿ.
该关系与初始原子核数无关。经过一个半衰期后，N = N₀/2。经过两个半衰期后，N = N₀/4；经过 n 个半衰期后，N = N₀/2ⁿ。
| Time elapsed | Fraction of original nuclei remaining |
更多咨询请联系16621398022（同微信）
CommentsMore posts |
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导