📚 Particle Physics for GCSE WJEC | GCSE WJEC 物理：粒子物理考点精讲
Particle physics in GCSE WJEC Physics explores the tiny constituents of matter, nuclear instability and the radiation that unstable atoms emit. This topic underpins our understanding of radioactivity, nuclear equations, half-life and the practical uses and dangers of ionising radiation. Mastering these concepts is essential for success in the WJEC Unit 2 examination and provides a foundation for further study in physics.
GCSE WJEC 物理中的粒子物理探讨了物质的微小组成、核不稳定性以及不稳定原子释放的辐射。这部分知识是理解放射性、核反应方程、半衰期以及电离辐射的实际用途与危害的基础。掌握这些概念对于在 WJEC 单元 2 考试中取得成功至关重要，也为后续物理学习打下基础。
1. The Structure of the Atom | 原子结构
Atoms are the smallest units of ordinary matter and consist of a small, dense nucleus surrounded by electrons orbiting in energy levels. The nucleus contains positively charged protons and neutral neutrons, collectively called nucleons. Relative masses and charges are often used at GCSE to simplify calculations.
原子是普通物质的最小单位，由一个微小致密的原子核和按能级轨道运动的电子组成。原子核包含带正电的质子和不带电的中子，统称为核子。在 GCSE 中常使用相对质量和相对电荷来简化计算。
| Particle | Relative mass | Relative charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | 1/1836 (≈0) | –1 | Shells/orbits |
Most of the atom’s mass is concentrated in the nucleus, yet the nucleus occupies only a tiny fraction of the atom’s volume. An atom is electrically neutral because the number of protons equals the number of electrons.
原子的大部分质量集中在原子核内，但原子核只占据原子体积的极小部分。原子是电中性的，因为质子数等于电子数。
2. Isotopes and Nuclide Notation | 同位素与核素符号
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. Their chemical properties are identical, but their physical stability varies; some isotopes are radioactive (radioisotopes).
同位素是质子数相同但中子数不同的同一元素原子。它们的化学性质相同，但物理稳定性不同；一些同位素具有放射性（放射性同位素）。
Nuclide notation expresses an isotope in the form ᴬZX, where A is the mass number (protons + neutrons) and Z is the atomic number (protons). For example, carbon-12 is written as ¹²₆C. Knowing A and Z allows us to calculate the number of neutrons: N = A – Z.
核素符号以 ᴬZX 的形式表示同位素，其中 A 为质量数（质子数+中子数），Z 为原子序数（质子数）。例如碳-12 写作 ¹²₆C。知道 A 和 Z 就能算出中子数：N = A – Z。
3. Radioactive Decay and Stability | 放射性衰变与稳定性
Some nuclei are unstable and undergo radioactive decay to become more stable. This spontaneous process emits ionising radiation (alpha, beta, gamma) and often results in a different element being formed. The rate of decay is random and unaffected by external conditions such as temperature or pressure.
一些原子核不稳定，会发生放射性衰变以变得更稳定。这一自发过程会释放电离辐射（α、β、γ），并常常形成不同的元素。衰变速率是随机的，不受温度、压力等外界条件影响。
The nuclear model explains that stability depends on the neutron-to-proton ratio. Lighter stable nuclei have roughly equal numbers, while heavier stable nuclei require more neutrons than protons to counterbalance electrostatic repulsion between protons.
核模型指出，稳定性取决于中子数与质子数的比值。较轻的稳定核具有大致相等的数量，而较重的稳定核需要比质子更多的中子来抵消质子间的静电排斥。
4. Alpha Radiation | α 辐射
An alpha particle (α) is a helium nucleus consisting of two protons and two neutrons, denoted by ⁴₂He²⁺. Alpha decay reduces the mass number by 4 and the atomic number by 2, transforming the parent nucleus into a new element. Example: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He.
α 粒子是一个由两个质子和两个中子组成的氦原子核，记作 ⁴₂He²⁺。α 衰变使质量数减少 4，原子序数减少 2，母核转变成一个新元素。例如：²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He。
Alpha radiation is strongly ionising because of its large mass and +2 charge, but it has a very short range in air (a few centimetres) and is stopped by a sheet of paper or human skin. It is most dangerous if an alpha-emitting source is ingested or inhaled.
α 辐射因质量大且带 +2 电荷而具有很强的电离能力，但在空气中的射程很短（几厘米），可被一张纸或皮肤阻挡。若进入体内吸入或食入 α 放射源，危害最大。
5. Beta Radiation | β 辐射
A beta particle (β⁻) is a high-speed electron ejected from the nucleus when a neutron decays into a proton. This process increases the atomic number by 1 while the mass number remains unchanged. A typical equation: ¹⁴₆C → ¹⁴₇N + ⁰₋₁e.
β⁻ 粒子是从原子核中射出的高速电子，当一个中子衰变成质子时产生。该过程使原子序数增加 1，质量数不变。典型方程：¹⁴₆C → ¹⁴₇N + ⁰₋₁e。
Beta particles are moderately ionising and have a range of up to about a metre in air. They are stopped by a few millimetres of aluminium. Positron emission (β⁺) also exists but is not required for WJEC GCSE.
β 粒子具有中等电离能力，在空气中的射程可达约一米，可被几毫米厚铝板阻挡。正电子发射（β⁺）也存在，但不在 WJEC GCSE 要求范围内。
6. Gamma Radiation | γ 辐射
Gamma radiation is a high-frequency electromagnetic wave emitted after an alpha or beta decay, when the daughter nucleus has excess energy. It is not a particle, has no mass and no charge, and is written as ⁰₀γ in equations.
γ 辐射是在α或β衰变后，子核具有多余能量时发射的高频电磁波。它不是粒子，无质量、无电荷，在方程中写作 ⁰₀γ。
Gamma rays are weakly ionising but extremely penetrating. They can travel many metres in air and require thick lead or several centimetres of concrete to reduce intensity significantly. Gamma emission does not change the mass number or atomic number of the nucleus.
γ 射线电离能力弱但穿透力极强。可在空气中传播数米，需用厚铅板或数厘米厚混凝土才能有效减弱。γ 发射不改变原子核的质量数或原子序数。
7. Nuclear Decay Equations | 核衰变方程
In WJEC exams, you must be able to complete and balance nuclear equations for alpha and beta decay. The total mass number (top number) and total atomic number (bottom number) must be conserved on both sides of the arrow. Always check for conservation when writing equations.
在 WJEC 考试中，你必须能够完成并配平 α 衰变和 β 衰变的核反应方程。箭头两侧的总质量数（上标）和总原子序数（下标）必须守恒。写方程时务必验证守恒。
For alpha decay: parent → daughter + ⁴₂He. For beta-minus decay: parent → daughter + ⁰₋₁e. The daughter element is found by using the periodic table and the new atomic number. Practice with examples like radium-226 undergoing alpha decay to become radon-222.
对于 α 衰变：母核 → 子核 + ⁴₂He。对于 β⁻ 衰变：母核 → 子核 + ⁰₋₁e。通过新原子序数查元素周期表确定子元素。练习题目如镭-226 经 α 衰变后变成氡-222。
8. Half-Life and Decay Curves | 半衰期与衰变曲线
The half-life of a radioactive isotope is the time it takes for half of the unstable nuclei in a sample to decay, or for the activity (count rate) to halve. It is a constant characteristic of each isotope and cannot be altered by physical or chemical means.
放射性同位素的半衰期是指样品中一半不稳定原子核发生衰变，或活度（计数率）减半所需的时间。它是每种同位素的固有属性，不能通过物理或化学手段改变。
On a decay graph (activity versus time), each half-life corresponds to the time interval that reduces the activity to 50% of its previous value. WJEC questions may ask you to determine half-life from a graph or to calculate remaining mass/activity after a given number of half-lives.
在衰变曲线图（活度-时间图）上，每个半衰期对应活度降至前一个值 50% 的时间间隔。WJEC 试题可能会要求你从图中求出半衰期，或计算经过若干个半衰期后剩余的质量/活度。
After n half-lives, the fraction remaining = (½)ⁿ. If a sample starts with 80 g of a radioisotope with half-life 3 days, after 9 days (3 half-lives) the mass left = 80 × (½)³ = 10 g.
经过 n 个半衰期后，剩余份额 = (½)ⁿ。若一样品开始含有 80 克半衰期为 3 天的放射性同位素，9 天（3 个半衰期）后剩余质量 = 80 × (½)³ = 10 克。
9. Background Radiation | 背景辐射
We are constantly exposed to natural and artificial sources of ionising radiation. Background radiation includes cosmic rays, radon gas from the ground, radiation from rocks and buildings, and a small contribution from medical procedures and nuclear power. The typical dose is measured in sieverts (Sv), but at GCSE the count rate (counts per second) is often used.
我们持续暴露在天然和人为电离辐射源中。背景辐射包括宇宙射线、土壤释放的氡气、岩石与建筑物辐射，以及医疗过程和核电的一小部分贡献。常用剂量单位是希沃特（Sv），但 GCSE 中常使用计数率（每秒计数）。
When conducting experiments, the background count must be measured and subtracted from all readings to obtain the corrected count rate due to the source alone. Radon gas in certain areas contributes significantly to background dose and is a known cause of lung cancer.
进行实验时，必须测量本底计数并从所有读数中扣除，以获得仅由放射源产生的修正计数率。某些地区的氡气对背景辐射剂量贡献显著，是已知的肺癌致因。
10. Uses and Hazards of Radiation | 辐射的应用与危害
Ionising radiation has many important uses. Alpha sources are used in smoke detectors because alpha particles ionise air and are easily stopped. Beta emitters are used for thickness monitoring in paper production. Gamma rays are used to sterilise medical equipment and treat cancer (radiotherapy). Tracers in medicine often involve gamma or beta isotopes with short half-lives to minimise patient exposure.
电离辐射有许多重要用途。α 源用于烟雾探测器，因为 α 粒子电离空气且易被阻挡。β 发射体用于造纸中的厚度监测。γ 射线用于医疗设备消毒和癌症治疗（放射治疗）。医学示踪剂常采用半衰期短的 γ 或 β 同位素，以减少患者暴露。
Hazards arise from ionisation damage to living cells. High doses can cause radiation sickness, mutations and cancer. The risk depends on the type of radiation, dose, exposure time and whether the source is inside or outside the body. Safety measures include shielding, remote handling and limiting exposure time.
危害源于对活细胞的电离损伤。高剂量可导致辐射病、突变和癌症。风险取决于辐射类型、剂量、暴露时间以及放射源位于体内还是体外。安全措施包括屏蔽、遥控操作和限制暴露时间。
11. Nuclear Fission and Fusion | 核裂变与核聚变
Nuclear fission is the splitting of a large, unstable nucleus (e.g. uranium-235 or plutonium-239) after absorbing a neutron, releasing two smaller daughter nuclei, two or three neutrons and a large amount of energy. The released neutrons can cause further fission in a chain reaction, controlled in reactors using control rods and moderators.
核裂变是较大的不稳定原子核（如铀-235 或钚-239）吸收一个中子后分裂成两个较小的子核、两到三个中子并释放大量能量的过程。释放的中子可引发进一步裂变，形成链式反应，在反应堆中通过控制棒和慢化剂来控制。
Nuclear fusion is the joining of two light nuclei (hydrogen isotopes like deuterium and tritium) to form a heavier nucleus (helium) with a mass defect that releases energy. Fusion powers the Sun and requires extremely high temperatures and pressures to overcome electrostatic repulsion. WJEC expects you to compare fission and fusion and recognise fusion as a future potential energy source that does not produce long-lived radioactive waste.
核聚变是两个轻核（氘和氚等氢同位素）结合形成一个较重原子核（氦），因质量亏损而释放能量。聚变是太阳的能量来源，需要极高温度和压力来克服静电排斥。WJEC 要求比较裂变和聚变，并认识到聚变是一种不产生高放长寿命废物的未来潜在能源。
12. Detecting Radiation and Safety | 辐射探测与安全
Radiation is invisible and must be detected using instruments. A Geiger-Müller (GM) tube connected to a counter measures count rate. Photographic film darkens when exposed to radiation and can be used in film badges to monitor cumulative exposure for workers. Cloud chambers show the tracks of ionising particles.
辐射不可见，必须用仪器探测。连接计数器的盖革-米勒（GM）管测量计数率。照相胶片受辐射照射后会变黑，用于胶片徽章来监测工作人员的累积剂量。云室可显示电离粒子的径迹。
Precautions include: using the source for the minimum time necessary, keeping it at arm’s length with tongs/forceps, pointing it away from people, storing it in a lead-lined container, and never eating or drinking near radioactive materials. WJEC practical skills may examine these safety rules.
安全措施包括：尽量缩短使用源的时间，用长柄钳/镊子保持手臂距离，避免指向人，储存在衬铅容器中，不得在放射源附近饮食。WJEC 实验技能题可能考查这些安全规则。
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