📚 GCSE Edexcel Physics: Radioactive Decay | 爱德思GCSE物理：放射性衰变
Radioactive decay is one of the most predictable topics on the Edexcel GCSE Physics papers: the equations are short, the graphs repeat year after year, and the marking points are almost fixed. This article will guide walks you through the key principles, the three types of nuclear radiation, decay equations, half-life calculations, activity and count rate, real-world applications, hazards and safety, and finally the exam technique that turns easy marks into full marks.
放射性衰变是爱德思GCSE物理试卷中最可预测的考点之一：方程简短、图像年复一年地重复出现、得分点几乎固定。本文将带你系统梳理关键原理、三种核辐射、衰变方程、半衰期计算、活度与计数率、实际应用、危害与安全，最后是能把送分题变成满分的考试技巧。
The topic sits in the "Radioactivity and Particles" section of the specification and is usually assessed in Paper 1 (Physics 1) and in the separate science papers, with a smaller but regular appearance in combined science. Whatever tier you sit, the same core ideas appear again and again.
该主题属于考纲中的"放射性及粒子"部分，通常出现在Paper 1（物理1）以及分科科学试卷中，在综合科学试卷中出现的频率稍低但依然规律。无论你参加基础卷还是高阶卷，同样的核心概念都会反复出现。
1. Understanding Radioactivity | 理解放射性
Radioactivity occurs when an unstable nucleus undergoes a spontaneous change and emits radiation. The process is random: we , meaning we cannot predict exactly when an individual nucleus will decay, nor which nucleus in a sample will decay next. However, with a very large number of nuclei we can describe the overall decay rate statistically, which is what half-life is based on.
放射性出现在不稳定的原子核发生自发变化并发射辐射时。这个过程是随机的：我们既无法准确预测某个具体原子核何时衰变，也无法预测样品中下一个衰变的是哪一个核。但是，当原子核数量极大时，我们可以用统计方法描述整体的衰变速率，半衰期正是建立在这一统计规律之上的。
Unstable nuclei may have too many protons, too many neutrons, or simply too much energy. To reach a more stable arrangement they release particles or electromagnetic waves. The original nucleus is called the parent nucleus, and the resulting nucleus formed after the decay is the daughter nucleus.
不稳定的原子核可能含有过多的质子、过多的中子，或者仅仅是能量过高。为了达到更稳定的状态，它们会释放粒子或电磁波。原来的原子核称为母核，衰变后产生的新原子核称为子核。
Decay is spontaneous, which means it is not triggered by temperature, pressure, chemical bonding or any external factor. This is a favourite exam trap: heating a radioactive sample does not make it decay faster, and cooling it does not slow it down.
衰变是自发的，这意味着它不由温度、压强、化学键或任何外部因素所触发。这是考试中最爱设置的陷阱之一：加热放射性样品不会让它衰变得更快，冷却它也不会让它变慢。
Decay is random per nucleus, but predictable for a large sample.
单个核的衰变是随机的，但大样品整体是可预测的。
2. Types of Nuclear Radiation | 核辐射的类型
There are three main types of nuclear radiation emitted during radioactive decay: alpha (α) particles, beta (β) particles (beta-minus and beta-plus), and gamma (γ) rays. Each type has a distinct nature and is emitted under specific conditions that you must be able to describe.
放射性衰变过程中发射的主要辐射有三种：阿尔法（α）粒子、贝塔（β）粒子（包括β⁻和β⁺）以及伽玛（γ）射线。每种类型具有不同的本质，并在特定条件下产生，你必须能够描述它们。
Alpha (α): a helium nucleus, ⁴₂He, made of 2 protons and 2 neutrons — highly ionising, weakly penetrating.
阿尔法（α）：氦原子核 ⁴₂He，由2个质子和2个中子组成——电离能力极强、穿透能力很弱。
Beta-minus (β⁻): a fast-moving electron, ⁰₋₁e, emitted when a neutron turns into a proton — moderately ionising, moderately penetrating.
贝塔负（β⁻）：高速电子 ⁰₋₁e，当中子转变为质子时发射——电离能力中等、穿透能力中等。
Beta-plus (β⁺): a positron, ⁰₊₁e, emitted when a proton turns into a neutron — same mass as an electron but positive charge.
贝塔正（β⁺）：正电子 ⁰₊₁e，当质子转变为中子时发射——质量与电子相同但带正电。
Gamma (γ): a very high-frequency electromagnetic wave, with no mass and no charge — weakly ionising, highly penetrating.
伽玛（γ）：频率极高的电磁波，没有质量也没有电荷——电离能力弱、穿透能力极强。
Notice that only gamma is electromagnetic; alpha and beta are particles. Edexcel frequently asks you to compare them using a table of mass, charge, ionising power and range in air.
请注意，只有伽玛是电磁波；阿尔法和贝塔都是粒子。爱德思经常要求你用质量、电荷、电离能力和空气中射程等项列表比较它们。
3. Alpha Decay | 阿尔法衰变
Alpha radiation is the emission of an alpha particle, which is essentially a helium nucleus containing two protons and two neutrons. It . This occurs mainly in very heavy nuclei such as uranium-238 and radium-226, where the nucleus is too large to be stable.
阿尔法辐射是发射一个α粒子，它实际上是一个含两个质子和两个中子的氦原子核。它主要发生在非常重的原子核中，例如铀-238和镭-226，这些核因为过大而无法稳定存在。
When a nucleus emits an alpha particle, its proton number (atomic number) decreases by 2 and its nucleon number (mass number) decreases by 4. The element therefore changes into a different element, two places to the left in the periodic table.
当一个原子核发射α粒子时，其质子数（原子序数）减少2，核子数（质量数）减少4。因此该元素变成了另一种元素，在周期表中向左移动两格。
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
Alpha particles are strongly ionising because they carry a +2 charge and move relatively slowly, stripping electrons from atoms they pass. They have a very short range in air (about 5 cm) and can be stopped by a sheet of paper or by the dead layer of skin.
α粒子电离能力很强，因为它们带+2电荷且运动相对缓慢，会从经过的原子中剥离电子。它们在空气中的射程很短（约5厘米），一张纸或皮肤的死亡层就能将其挡住。
In alpha decay the daughter nucleus is often itself radioactive, so a decay chain follows until a stable nucleus such as lead is reached. You do not need to memorise the full chain, but you should be able to balance any single equation.
在α衰变中，子核本身常常也是放射性的，因此会形成一条衰变链，直到形成像铅一样的稳定核为止。你不需要记住整条衰变链，但必须能够配平任何一个单一方程。
4. Beta Decay (β⁻ and β⁺) | 贝塔衰变（β⁻与β⁺）
Beta-minus decay happens when a nucleus has too many neutrons relative to protons. A neutron inside the nucleus changes into a proton, and a fast-moving electron (the beta particle) is emitted along with an antineutrino. The proton number increases by 1 while the nucleon number stays the same.
β⁻衰变发生在原子核中子数相对于质子数过多时。核内的一个中子转变为一个质子，同时发射一个高速电子（即β粒子）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
Beta-plus decay happens when a nucleus has too many protons relative to neutrons. A proton changes into a neutron and a positron is emitted. The proton number decreases by 1 and the nucleon number again stays the same.
β⁺衰变发生在原子核质子数相对于中子数过多时。一个质子转变为中子并发射一个正电子。质子数减少1，核子数同样保持不变。
²²₁₁Na → ²²₁₀Ne + ⁰₊₁e
Remember the key difference: alpha changes the element by two places left, beta-minus changes it one place right, and beta-plus changes it one place left. All three must balance both the top numbers (nucleons) and the bottom numbers (protons) on each side of the arrow.
记住关键区别：α使元素向左移两格，β⁻使元素向右移一格，β⁺使元素向左移一格。三者在箭头两侧都必须同时配平上标（核子数）和下标（质子数）。
Beta particles have a moderate ionising power and can travel roughly 1 m in air. They are stopped by a few millimetres of aluminium. Because beta emission produces a new element with the same mass number, isobars (same mass number, different element) are typical products.
β粒子电离能力中等，在空气中大约能飞行1米，几毫米厚的铝片就能将其挡住。由于β衰变产生的元素质量数不变，因此产物常是同量异位素（质量数相同、元素不同）。
5. Gamma Emission and Energy Release | 伽玛辐射与能量释放
Gamma radiation is not a particle at all — it is a high-energy electromagnetic wave with no mass and no charge, emitted when a nucleus is left in an excited, high-energy state after alpha or beta decay. Gamma emission usually accompanies the other two types rather than occurring alone.
γ辐射根本不是粒子——它是没有质量、没有电荷的高能电磁波，当原子核在α或β衰变后处于激发的高能态时被发射出来。γ辐射通常伴随另外两种辐射一同出现，而不是单独发生。
⁰₀γ
Because gamma has no charge and no mass, emitting it does not change the proton number or the nucleon number of the nucleus. The nucleus simply loses excess energy and settles into a more stable, lower-energy state.
由于γ没有电荷也没有质量，发射它不会改变原子核的质子数或核子数。原子核只是失去多余的能量，进入更稳定、能量更低的状态。
Gamma rays are very weakly ionising but extremely penetrating because of their short wavelength and high energy per photon. They need thick lead or several centimetres of dense concrete to be significantly reduced, which is why they are used in industrial radiography and cancer radiotherapy.
γ射线电离能力非常弱，但穿透能力极强，这是因为其波长短、每个光子能量高。需要厚铅板或数厘米厚的致密混凝土才能明显削弱它们，这也是它们被用于工业射线照相和癌症放疗的原因。
6. Comparing the Three Radiations | 三种辐射的比较
Edexcel examiners love a comparison table, so make sure you can reproduce the key properties of alpha, beta and gamma from memory in under two minutes.
爱德思考官非常喜欢比较表，因此请确保你能在两分钟内凭记忆再现α、β和γ的关键性质。
| Property | Alpha (α) | Beta (β⁻) | Gamma (γ) |
| Nature | Helium nucleus ⁴₂He | Fast electron ⁰₋₁e | EM wave (photon) |
| Charge | +2 | -1 | 0 |
| Mass | 4 units | ≈1/1840 unit | 0 |
| Ionising power | High | Medium | Low |
| Penetrating power | Low | Medium | High |
| Stopped by | Paper / skin | Few mm aluminium | Thick lead / concrete |
| Range in air | ≈5 cm | ≈1 m | Very large / infinite |
The pattern to remember is inverse: the more strongly a radiation ionises, the less far it penetrates. Alpha loses all its energy over a short distance, while gamma carries its energy deep into material without interacting much.
要记住的规律是反比关系：辐射的电离能力越强，穿透得就越近。α在短距离内就耗尽全部能量，而γ几乎不与物质作用，能将能量带入材料深处。
7. Nuclear Decay Equations and Conservation Laws | 核衰变方程与守恒定律
Every nuclear equation on the Edexcel paper follows two conservation laws: the total nucleon number (mass number) before and after the decay is the same, and the total proton number (atomic number) before and after is the same. Charge and mass-energy are therefore both conserved.
爱德思试卷上的每个核方程都遵循两条守恒定律：衰变前后的总核子数（质量数）相同，衰变前后的总质子数（原子序数）相同。因此电荷与质能都守恒。
A reliable method for balancing equations is to write the numbers as a top row (nucleons) and a bottom row (protons), then treat them like simple arithmetic:
配平方程的一个可靠方法是把数字写成上排（核子数）和下排（质子数），然后把它们当作简单的算术来处理：
Top: 238 = 234 + 4 | Bottom: 92 = 90 + 2
row. Once you see the bottom row as signed numbers, beta-minus and beta-plus become completely routine.
把它应用到β衰变上：上排 14 = 14 + 0，下排 6 = 7 + (-1)。一旦你把下排看作带符号的数，β⁻和β⁺就变得完全机械化了。
If an exam question shows a decay chain, do not panic: balance each step separately. If asked to identify the missing particle, find what is needed to make both rows balance, then match it to α, β⁻, β⁺, γ or a neutron ¹₀n.
如果考题给出了一条衰变链，不要慌：逐步单独配平。如果要求辨认缺失的粒子，先找出让上下两排都配平所需的量，再把它对应到α、β⁻、β⁺、γ或中子 ¹₀n。
8. Half-Life and Its Measurement | 半衰期及其测量
The half-life of a radioactive isotope is the time taken for half of the unstable nuclei in a sample to decay, or equivalently the time taken for the count rate (or activity) to fall to half of its original value. It is a fixed property of each isotope and cannot be changed by heating, cooling or chemical reaction.
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所需的时间，也可以等价地说，是计数率（或活度）下降到原来一半所需的时间。它是每种同位素固定的性质，不能通过加热、冷却或化学反应改变。
ars. This huge range is exactly what makes different isotopes suitable for different jobs.
半衰期的范围极大：钋-214的半衰期约为0.000164秒，而铀-238的半衰期约为4.5 × 10⁹年。正是这种巨大差异，使不同同位素适合不同的用途。
To measure a half-life in the laboratory you record the count rate of a source at regular time intervals using a Geiger-Muller tube and counter, subtract the background count rate first, then plot a graph or halve repeatedly.
在实验室测量半衰期时，你用盖革-米勒管和计数器按固定时间间隔记录放射源的计数率，先扣除本底计数率，然后作图或反复对半递减。
9. Half-Life Calculations and Graphs | 半衰期计算与图像
The simplest method is repeated halving. If a sample starts at 80 g and the half-life is 5 years, then after 5 years there is 40 g, after 10 years 20 g, after 15 years 10 g, and after 20 years 5 g.
最简单的方法是反复对半递减。如果样品初始为80克，半衰期为5年，那么5年后剩40克，10年后剩20克，15年后剩10克，20年后剩5克。
N = N₀ × (½)ⁿ where n = t ÷ t½
factor of 8, which equals 2³, so three half-lives have passed. The half-life is therefore 30 ÷ 3 = 10 minutes.
对于计数率问题，逻辑完全相同。如果计数率在30分钟内从每分钟640次降到每分钟80次，即降到 1/8，而 8 = 2³，说明经过了三个半衰期。因此半衰期是 30 ÷ 3 = 10 分钟。
On a decay graph of count rate against time, the curve is a smooth exponential decay. To find the half-life: read the initial value, halve it, draw a horizontal line to the curve, then a vertical line down to the time axis. Repeat for a second value and average to reduce reading error.
在计数率对时间的衰变图像上，曲线是一条平滑的指数衰减曲线。求半衰期的方法是：读出初始值，取其一半，画水平线交于曲线，再画垂线落到时间轴。再用第二个值重复一次并取平均，以减少读图误差。
Always subtract the background count rate before you start. A common mistake is to halve the raw reading, which gives a half-life that is too long and loses the calculation marks.
开始之前务必先扣除本底计数率。常见错误是直接对原始读数取半，这样算出的半衰期偏长，会丢掉计算分。
10. Activity, Count Rate and the Becquerel | 活度、计数率与贝克勒尔
second. It depends on both the number of unstable nuclei present and the half-life of the isotope: a short half-life means a high activity for the same number of nuclei.
活度是放射源发射辐射的速率，单位是贝克勒尔（Bq），1 Bq = 每秒1次衰变。它同时取决于现存不稳定核的数量和同位素的半衰期：在核数量相同时，半衰期越短，活度越高。
Count rate is what a detector actually measures, in counts per second or counts per minute. It is always lower than the true activity because the detector does not capture every emitted particle and because some radiation is absorbed before it reaches the tube.
计数率是探测器实际测到的数值，单位是每秒计数或每分钟计数。它总是低于真实活度，因为探测器不会捕获每一个发射出的粒子，而且部分辐射在到达计数管之前就被吸收了。
Background → correct count rate = measured − background
Background radiation comes from natural sources such as radon gas, rocks and cosmic rays, and always adds to your reading. Correcting for it is essential in every experiment and calculation involving half-life.
本底辐射来自氡气、岩石和宇宙射线等天然来源，会始终叠加在你的读数上。在任何涉及半衰期实验和计算中，对本底进行修正是必不可少的。
11. Uses and Applications of Radioactivity | 放射性的应用
Medical physics is the biggest application area. Technetium-99m, with a half-life of about 6 hours, is used as a tracer in the body: short enough to decay quickly and limit the dose, yet long enough to complete a scan. Gamma emitters are preferred because they escape the body and can be detected externally.
医学物理是最大的应用领域。半衰期约6小时的锝-99m被用作体内示踪剂：短到可以迅速衰变、限制剂量，又长到足以完成一次扫描。伽玛发射体更受青睐，因为它们能穿出人体并在体外被探测到。
Cancer radiotherapy uses a strong gamma source such as cobalt-60, aimed at a tumour from several directions so that the tumour receives a high dose while healthy tissue around it receives less. Alpha emitters such as americium-241 are used in smoke alarms because alpha ionises the air and the current drops when smoke blocks the path.
癌症放疗使用钴-60等强γ源，从多个方向对准肿瘤，使肿瘤接受高剂量而其周围健康组织接受较低剂量。镅-241等α发射体用于烟雾报警器，因为α使空气电离，当烟雾阻断路径时电流下降。
In industry, gamma sources check welds and pipework for cracks using radiography, beta emitters measure the thickness of paper, plastic and metal foil, and tracers follow leaks in underground pipes or the movement of sediment in rivers.
在工业中，γ源用射线照相法检测焊缝和管道是否存在裂纹，β发射体用于测量纸张、塑料和金属箔的厚度，示踪剂则用于追踪地下管道的泄漏或河流中泥沙的移动。
For dating, carbon-14 (half-life about 5730 years) is used on once-living material such as wood and bone. Because it is constantly replaced while the organism is alive and stops being replaced at death, the remaining activity tells you how long ago it died.
在测年方面，碳-14（半衰期约5730年）用于测定木材、骨骼等曾经有生命的物质。由于生物存活期间碳-14不断被补充、死亡后不再补充，剩余活度就能告诉你它死亡距今多久。
12. Hazards, Safety and Background Radiation | 危害、安全与本底辐射
Ionising radiation damages living cells by breaking chemical bonds and can cause mutations, burns, radiation sickness and cancer. The risk depends on the type of radiation, the activity of the source, the distance from it and the time of exposure. Alpha sources are especially dangerous inside the body, while gamma sources are dangerous externally.
电离辐射通过破坏化学键损伤活细胞，可能导致基因突变、灼伤、辐射病和癌症。风险取决于辐射类型、源的活度、与源的距离以及暴露时间。α源在体内尤其危险，而γ源在体外就很危险。
The three practical precautions you should always quote are: keep the source as far away as possible using tongs, limit the time of exposure, and use appropriate shielding such as lead for gamma or Perspex for beta. Never point a source at yourself or others, and always store sources in a lead-lined container.
你应当始终引用的三条实际防护措施是：用镊子使源尽可能远离、限制暴露时间、使用合适的屏蔽，例如对γ用铅、对β用有机玻璃。绝不要把源指向自己或他人，并且始终把源存放在衬铅容器中。
Background radiation is always present and comes from radon gas in rocks and soil, granite buildings, food and drink (potassium-40 and carbon-14), cosmic rays from space, and a small contribution from nuclear industry and past weapons testing. Radon is usually the largest single contributor.
本底辐射无处不在，来源于岩石和土壤中的氡气、花岗岩建筑、食物和饮水（钾-40与碳-14）、来自太空的宇宙射线，以及核工业和以往核试验的一小部分贡献。氡通常是最大的单一来源。
13. Exam Technique and Common Mistakes | 考试技巧与常见错误
When a question says "explain why the count rate is not exactly half", the expected answer usually involves background radiation, random decay, or statistical variation in small samples. Learn these three phrases and use the one that fits the wording.
当题目说"解释为什么计数
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