Edexcel A-Level Physics: Radioactive Decay and Half-Life of Radium-226 | Edexcel A-Level 物理：镭-226 的放射性衰变与半衰期

📚 Edexcel A-Level Physics: Radioactive Decay and Half-Life of Radium-226 | Edexcel A-Level 物理：镭-226 的放射性衰变与半衰期

Radium-226 is one of the classic examples used in Edexcel A-Level Physics to illustrate alpha decay, half-life, activity, and the random nature of nuclear processes. This article explains the key definitions, equations, graph skills, and safety considerations you need for exam success, with fully worked calculations based on Ra-226.

镭-226 是 Edexcel A-Level 物理中用于说明 α 衰变、半衰期、活度以及核过程随机性的经典例子。本文讲解考试成功所需的关键定义、方程、图像技能和安全注意事项，并给出基于镭-226 的完整计算示例。

1. The Nature of Alpha Decay | α 衰变的本质

Radium-226 is an unstable isotope with 88 protons and 138 neutrons. To become more stable, its nucleus emits an alpha particle, which is identical to a helium nucleus (2 protons and 2 neutrons). This reduces the proton number by 2 and the mass number by 4, producing radon-222.

镭-226 是一种不稳定同位素，拥有 88 个质子和 138 个中子。为了变得更稳定，它的原子核会释放一个 α 粒子，该粒子等同于一个氦原子核（2 个质子和 2 个中子）。这使得质子数减少 2、质量数减少 4，从而生成氡-222。

The nuclear equation for this alpha decay is shown below.

该 α 衰变的核反应方程式如下所示。

²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He

Alpha particles carry a positive charge and have a high ionising power, but their penetration ability is very low. A thin sheet of paper or a few centimetres of air can stop them.

α 粒子带正电，电离能力强，但穿透能力非常弱。一张薄纸或几厘米的空气就可以阻挡它们。

---

2. Nuclear Notation and Conservation Laws | 核素符号与守恒定律

In nuclear notation, the top number is the mass number (total protons + neutrons) and the bottom number is the proton number. For a nuclear equation, the sum of mass numbers on the left must equal the sum on the right; the same rule applies to proton numbers.

在核素符号中，上标数字是质量数（质子数 + 中子数），下标数字是质子数。对于核反应方程式，左边质量数之和必须等于右边质量数之和；质子数之和也必须守恒。

Check Ra-226: mass number 226 = 222 + 4, and proton number 88 = 86 + 2. This conservation is a common Edexcel mark point.

以镭-226 为例：质量数 226 = 222 + 4，质子数 88 = 86 + 2。这种守恒关系是 Edexcel 考试中常见的得分点。

---

3. Radioactive Decay as a Random and Spontaneous Process | 放射性衰变的随机性与自发性

Radioactive decay is spontaneous, meaning it cannot be influenced by temperature, pressure, or chemical bonding. It is also random, so you cannot predict which nucleus will decay next; you can only state the probability of decay per unit time.

放射性衰变是自发的，意味着它不受温度、压强或化学键的影响。它也是随机的，因此无法预测下一个衰变的是哪个原子核；只能给出单位时间内发生衰变的概率。

This distinction is frequently tested: the decay rate of a sample does not change when the sample is heated, cooled, or placed under pressure.

这个区别经常被考查：样品被加热、冷却或加压时，其衰变速率不会改变。

---

4. Activity and the Decay Constant | 活度与衰变常量

Activity (A) is the number of nuclei that decay per second. It is measured in becquerels (Bq), where 1 Bq = 1 decay per second. The decay constant λ is the probability of an individual nucleus decaying per second.

活度（A）是每秒发生衰变的原子核数量，单位为贝克勒尔（Bq），1 Bq = 每秒衰变 1 次。衰变常量 λ 是单个原子核在每秒内发生衰变的概率。

These quantities are linked by the equation:

这些量通过以下方程联系起来：

A = λN

where N is the number of undecayed nuclei. If you know any two of A, λ, and N, you can calculate the third.

其中 N 是尚未衰变的原子核数量。已知 A、λ、N 中的任意两个，就可以求出第三个。

---

5. The Exponential Decay Law | 指数衰变规律

The number of undecayed nuclei decreases exponentially with time. The same exponential form applies to activity and mass, so for any given sample:

未衰变原子核的数量随时间呈指数减少。活度和质量也遵循相同的指数形式，因此对于任意样品：

N = N₀ e^−λt   and   A = A₀ e^−λt

N₀ is the initial number of nuclei, A₀ is the initial activity, t is the elapsed time, and e is the base of natural logarithms.

N₀ 是初始原子核数，A₀ 是初始活度，t 是经过的时间，e 是自然对数的底。

This law assumes the daughter product is stable or is removed from the sample. For Ra-226, the product Rn-222 is also radioactive, so a full decay-chain treatment would require coupled equations, but the parent decay still follows the same exponential law.

该规律假设子体产物是稳定的或已从样品中移除。镭-226 的产物氡-222 也具有放射性，因此完整衰变链需要耦合方程，但母体衰变仍遵循同样的指数规律。

---

6. Half-Life Definition and Calculations | 半衰期定义与计算

Half-life (t_1/2) is the time taken for the number of und

Published by TutorHao | A-Level Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）