Radioactivity is over a century old, yet it still treats cancer, powers grids and demands strict safety rules. This reference covers AQA GCSE Physics 8463, topic 4.4 Atomic structure.
How the exam treats this topic:
Paper 1 (4.1–4.4) carries this topic. Equation-sheet support depends on the examination series. Practise notation, balanced equations, graphs and explanations as well as calculations.
Background radiation, half-life hazards, uses and fission/fusion are physics only.
Net-decline ratios after several half-lives are Higher Tier.
You must write balanced nuclear equations for single alpha and beta decay (balance atomic numbers and mass numbers; daughter naming not required).
4.1
原子结构；质量数与同位素（4.4.1.1–4.4.1.2）
教学大纲
原子结构；质量数与同位素（AQA 8463 陈述 4.4.1.1-4.4.1.2）。
描述原子结构：由带正电的原子核（含质子和中子）构成，外层电子分布在不同能级上。
记住原子半径的数量级，以及原子核直径小于其 1/10 000，却集中了绝大部分质量。
利用原子序数和质量数确定质子、中子和电子的数量。
定义同位素为具有不同中子数的同一元素原子，并解释正离子是失去外层电子的原子。
来源：Cambridge International 教学大纲
An atom is very small: radius about $1\times10^{-10}$ m. Its structure:
Nucleus: positively charged, with protons and neutrons; most of the atom's mass, but a radius less than 1/10 000 of the atom's.
Electrons: negative, arranged in energy levels. Absorbing electromagnetic radiation moves an electron to a higher level, further from the nucleus; emission moves it to a lower level, closer.
Notation: $\ ^{A}_{Z}X$ where $Z$ = atomic number (protons) and $A$ = mass number (protons + neutrons). In a neutral atom, electrons = protons; atoms have no overall charge.
Isotopes 同位素: atoms of the same element (same $Z$) with different numbers of neutrons (different $A$).
Neutrons in the nucleus = $A - Z$.
Atoms that lose one or more outer electrons become positive ions.
New experimental evidence can change or replace a scientific model:
Before the electron's discovery: atoms were tiny spheres that could not be divided.
Electron discovered → the plum pudding model: a ball of positive charge with negative electrons embedded in it.
Alpha scattering (Rutherford): most alpha particles passed straight through, a few bounced back → the mass and positive charge must be concentrated in a tiny centre → the nuclear model replaced the plum pudding model.
Bohr adapted it: electrons orbit at specific distances; his calculations agreed with observations.
Further work showed the positive charge comes in whole-number units — the proton; Chadwick's experiments (about 20 years later) proved the neutron.
Explain the evidence that changed the model: if the pudding were right, alpha particles should all pass through with small deflections (B1); some bounced almost straight back (B1), which is only possible if the mass and positive charge sit in a tiny, dense, positive nucleus (B1).
4.3
放射性衰变与核辐射（4.4.2.1）
教学大纲
放射性衰变与核辐射（AQA 8463 陈述 4.4.2.1）。
描述放射性衰变为一种随机过程，即不稳定的原子核释放辐射。
定义活度（贝克勒尔）和计数率。
说明α、β、γ和中子辐射的性质，包括穿透能力、在空气中的射程及电离能力。
运用这些性质为特定用途选择最佳放射源。
来源：Cambridge International 教学大纲
Some nuclei are unstable. They give out radiation as they change to become more stable — a random process called radioactive decay 放射性衰变.
Activity 放射性活度: the rate at which a source decays; unit becquerel 贝克勒尔 (Bq).
Count-rate 计数率: detector counts per second, after allowing for background where needed. A detector usually records only some emissions: its count rate is not automatically the source activity in Bq.
Radiation
Identity
Ionising power
Shielding
alpha α
helium nucleus
strong
paper / skin
beta β
fast electron
medium
mm of aluminium
gamma γ
EM radiation
weak
thick lead reduces it
Alpha contains two protons and two neutrons and travels only a few centimetres in air. Beta is emitted when a neutron changes into a proton; its range in air is longer. Gamma has the greatest range of these three and is reduced, not completely stopped, by thick lead or concrete. A nucleus can also emit a neutron; detailed neutron properties are not required here.
Choose a source for a use by matching these properties: alpha for ionisation smoke alarms (smoke reduces the ionisation current); beta for thickness control (partly absorbed by the sheet); gamma for tracers (escapes the body) and sterilising (penetrates packaging and damages microorganisms). A sealed source reduces contamination risk; it does not justify ignoring handling precautions.
Activity from a graph. On a graph of the number of undecayed nuclei against time, draw a tangent at the stated time. Its downward gradient is the rate of decrease in the number of nuclei; activity is the positive magnitude, in Bq. Read two widely separated points on the tangent, not two arbitrary points on the curve.
Worked example (teacher-written). The approximate tangent passes through $(0\ \text{s},68000)$ and $(200\ \text{s},12000)$.
$$\text{activity} = \frac{\text{decrease in number of nuclei}}{\text{time interval}} = \frac{68000-12000}{200\ \text{s}-0\ \text{s}} = 280\ \text{Bq}$$
This is an estimate from a drawn tangent. AQA June 2024 8463/1H Q09.5 requires the same method on its own graph at 300 s; its official answer is $7.1\times10^{20}$ Bq. Those are different graphs and data.
Check both rows balance ✓ (the daughter's name is not required).
4.4
核反应方程、半衰期及随机衰变（4.4.2.2–4.4.2.3）
教学大纲
核反应方程、半衰期及衰变的随机性（AQA 8463 陈述 4.4.2.2-4.4.2.3）。
书写单α衰变和β衰变的平衡核方程，配平原子序数和质量数。
定义半衰期为原子核数量或计数率减半所需的时间。
根据给定信息或图表确定半衰期。
（仅高中）计算给定半衰期次数后的净下降量，并用比值表示。
来源：Cambridge International 教学大纲
Decay is random: it cannot be predicted for any one nucleus; only the average behaviour of many is predictable.
Half-life 半衰期: the time for (a) the number of nuclei of the isotope in a sample to halve, or (b) the net count rate / activity to fall to half its initial level. Subtract background from detector readings first and keep the detector geometry unchanged.
From a graph: read the time for the count rate to halve — repeat over several halvings and average.
After $n$ half-lives, the fraction remaining is $1/2^n$ (HT: express as a ratio).
Worked example. A sample's activity falls from 800 Bq to 200 Bq in 12 years.
Actual AQA demand, June 2025 8463/1H Q07.4: polonium-210 has a half-life of 138 days. The number of atoms falls from 256 000 to 16 000: four halvings, so the time is $4 \times 138 = 552$ days. Q07.5 compares equal numbers of Po-209 and Po-210 atoms: the longer-lived Po-209 has lower activity. The equal-population condition matters.
Contamination 污染: unwanted radioactive atoms on or inside an object or person. The hazard lasts as long as the atoms are there, decaying on or in the body.
Irradiation 辐照: exposing an object to radiation. The irradiated object does not become radioactive.
Contamination can continue to irradiate tissue while the radioactive atoms remain. Exposure from an external source ends when that source is removed or effectively shielded. Compare the source activity, radiation type, distance, exposure time and whether material is inside the body; contamination is not always the larger dose. External alpha has low penetration and is stopped by skin, but internally its strong ionisation can damage nearby living tissue.
Precautions: hold sources with tongs, keep them at a distance, limit time near them, point them away from people, store in lead-lined boxes. Findings on radiation effects are published and peer-reviewed so they can be checked.
Half-life and hazard: for equal numbers of unstable nuclei, a shorter half-life means greater activity. Amount and exposure conditions also matter. A long-lived source may require secure storage for many years. A medical tracer should remain active long enough for the investigation, then decay quickly to reduce further dose. A smoke-alarm source must remain useful for years.
Medical uses (each = exploration or destruction):
Exploration: a gamma-emitting tracer (e.g. technetium-99m) injected so organs show on a scan; gamma escapes the body; a suitable short half-life limits dose after the scan.
Destruction: focused gamma beams or implanted sources kill cancer cells (radiotherapy); beta for skin conditions.
Evaluating risk: compare the dose and consequence of the procedure against the risk of the illness — with numbers from the question.
4.7
核裂变与核聚变——仅限物理学科（4.4.4）
教学大纲
核裂变与核聚变（AQA 8463 陈述 4.4.4.1-4.4.4.2，仅限物理）。
描述核裂变：一个大而不稳定的原子核吸收中子后分裂，生成产物并释放能量。
解释链式反应，以及受控（反应堆）与不受控（武器）版本的区别。
绘制并解读代表裂变及链式反应的示意图。
描述核聚变：两个轻核结合，部分质量转化为辐射能。
来源：Cambridge International 教学大纲
Fission 核裂变: the splitting of a large, unstable nucleus (uranium-235, plutonium-239).
Spontaneous fission is rare: the nucleus usually absorbs a neutron first.
It splits into two smaller nuclei of roughly equal size, releasing two or three neutrons and gamma rays; energy is released and all products carry kinetic energy.
The released neutrons can cause further fissions — a chain reaction. A reactor controls it (control rods absorb neutrons); a weapon's explosion is an uncontrolled chain.
You must draw or interpret the diagram: neutron in → two fragments + neutrons out → branching chain.
Fusion 核聚变: two light nuclei join to form a heavier nucleus; some mass converts into the energy of radiation. To join, the positive nuclei must approach closely despite their electrical repulsion. Do not describe fusion as chemical bonding or claim that every fusion system is waste-free.
