📚 Cesium-137: Radioactive Decay and Applications | 铯-137：放射性衰变及其应用
Cesium-137 is a radioactive isotope that features prominently in Edexcel A-Level Physics and Chemistry. With its well-defined beta decay, gamma emission, and a half-life of about 30 years, it serves as an ideal case study for understanding nuclear equations, activity, half-life calculations, and real-world applications from medical therapy to industrial thickness gauging. This article unpacks every examination-relevant aspect of Cs-137 in a paired English-Chinese format that mirrors the style of an international revision guide.
铯-137 是一种放射性同位素，在 Edexcel A-Level 物理和化学考纲中占有重要地位。它明确的β衰变、γ辐射以及大约 30 年的半衰期，使其成为理解核方程、活度、半衰期计算以及从医疗到工业测厚等实际应用的理想案例。本文以中英对照的复习指南形式，逐一剖析与 Cs-137 相关的所有考试要点。
1. Discovery and Basic Properties | 发现与基本性质
Cesium-137 was first identified in the fission products of uranium during the nuclear era. It is an artificial radioisotope not found in primordial nature. With atomic number 55, it belongs to the alkali metal group, and its mass number of 137 indicates a nucleus containing 55 protons and 82 neutrons. Its physical form in standard school sources is typically a sealed solid salt, such as caesium chloride, encapsulated to prevent leakage.
铯-137 最先在核时代的铀裂变产物中被确认。它是一种人造放射性同位素，不存在于原始自然界中。原子序数为 55，属于碱金属族，质量数 137 表示其原子核含有 55 个质子和 82 个中子。学校放射源中通常将其封装为固态盐（如氯化铯）的密封源，以防止泄漏。
2. Nuclear Notation and Isotopic Data | 核素符号与同位素数据
In examination answers, the correct nuclide notation is 13755 neutron number (N = 137 - 55 = 82) and nucleon number (A = 137). The isotope decays via β- emission; it is neutron-rich relative to the stable isotope Cs-133, which drives the beta-minus process.
考试中正确的核素符号为 13755Cs。考生必须能够从中提取质子数（Z = 55）、中子数（N = 137 - 55 = 82）和核子数（A = 137）。该同位素通过 β⁻ 发射衰变；相对于稳定同位素 Cs-133 而言，Cs-137 中子含量过多，这正是驱动β⁻衰变的原因。
3. Beta-minus Decay of Cesium-137 | 铯-137的 β⁻ 衰变
Beta-minus decay occurs when a neutron in the nucleus transforms into a proton, emitting an electron (β⁻ particle) and an electron antineutrino. In Cs-137, this process increases the atomic number by 1, converting cesium into barium. The emitted beta particle has a continuous energy spectrum up to a maximum of about 0.51 MeV, with an average energy around 0.17 MeV.
β⁻ 衰变发生时，原子核内一个中子转变为质子，同时发射一个电子（β⁻ 粒子）和一个反电子中微子。在 Cs-137 中，这一过程使原子序数增加 1，将铯转变为钡。发射出的β粒子具有连续能谱，最大能量约 0.51 MeV，平均能量约 0.17 MeV。
4. The Decay Equation and Daughter Nucleus | 衰变方程与子核
The direct decay can be written as:
13755Cs → 13756Ba* + 0-1e + ν̅ₑ
s what makes Cs-137 a valuable gamma source. The equation balances both nucleon number (137 = 137 + 0) and proton number (55 = 56 - 1).
直接衰变方程可写作：
13755Cs → 13756Ba* + 0-1e + ν̅ₑ
星号表示子核钡-137 处于激发态。这一点至关重要，因为随后的γ辐射使 Cs-137 成为有价值的γ源。方程在核子数（137 = 137 + 0）和质子数（55 = 56 - 1）上均守恒。
5. Gamma Emission and Metastable Barium-137 | γ辐射与亚稳态钡-137
About 95% of the time, the excited Ba-137 releases its excess energy by emitting a gamma photon of energy 0.662 MeV, transitioning to the ground state. This de-excitation can be expressed as:
13756Ba* → 13756Ba + γ (0.662 MeV)
The metastable state, Ba-137m, has a half-life of only 2.55 minutes. This means that when a Cs-137 source is chemically separated, the short-lived Ba-137m can be extracted as a pure gamma emitter for tracer studies, illustrating the parent-daughter relationship.
大约 95% 的情况下，激发态 Ba-137 会通过发射能量为 0.662 MeV 的γ光子释放多余能量，跃迁至基态。该退激过程可表示为：
13756Ba* → 13756Ba + γ (0.662 MeV)
亚稳态 Ba-137m 的半衰期仅 2.55 分钟。这意味着当 Cs-137 源经过化学分离时，短寿命的 Ba-137m 可被提取为纯γ发射体，用于示踪研究，这恰好体现了母子体关系。
6. Half-life and Decay Calculations | 半衰期与衰变计算
Cesium-137 has a physical half-life (T½) of 30.2 years. The exponential decay law, N = N₀ (½)t / T½ to one-eighth of its original value. Students may be asked to calculate the decay constant λ = ln2 / T½ and use A = λN to interconvert activity and number of nuclei.
铯-137 的物理半衰期（T½）为 30.2 年。指数衰减规律 N = N₀ (½)t / T½ 使考生能够确定经某段时间后的剩余比例。例如，经过 90.6 年（3 个半衰期），活度降为原来的八分之一。学生也可能被要求计算衰变常数 λ = ln2 / T½，并运用 A = λN 在活度与核数目之间进行换算。
7. Activity and the Becquerel | 活度与贝克勒尔
hool Cs-137 source might have an activity of 185 kBq (5 μCi). Exam questions often involve converting half-life into seconds, calculating the number of nuclei originally present, and then finding the initial activity. Remember that the activity of a pure Cs-137 sample decreases very slowly on human timescales, making it a long-lived calibration standard.
放射源活度的单位是贝克勒尔（Bq），1 Bq 表示每秒发生 1 次衰变。典型的学校 Cs-137 放射源活度约为 185 kBq（5 μCi）。试题中常要求将半衰期换算为秒，计算最初存在的核数目，再求出初始活度。需注意，纯 Cs-137 样品的活度在人类时间尺度上衰减极慢，这使它成为一种长寿命的校准标准源。
8. Industrial Applications: Thickness Gauging | 工业应用：厚度测量
One of the most cited applications is non-contact thickness gauging. A Cs-137 gamma source is placed on one side of a material (e.g., paper, metal foil) and a detector on the other. The attenuation of the gamma beam depends on the mass per unit area of the material. Any deviation from the target thickness changes the detected count rate, triggering automatic adjustment in the production line. This method relies on the constant gamma output of the source and does not require physical contact.
最常见的应用之一是非接触式测厚。将 Cs-137 γ源置于材料（如纸张、金属箔）一侧，另一侧放置探测器。γ射线的衰减取决于材料的单位面积质量。任何与目标厚度的偏差都会改变探测器计数率，从而触发生产线自动调节。该方法依赖放射源恒定的γ输出，且无需物理接触。
9. Medical Applications: Radiotherapy and Sterilisation | 医疗应用：放射治疗与灭菌
Although cobalt-60 is more common in teletherapy, Cs-137 has been used in brachytherapy for treating cervical cancer, where small sealed sources are placed near the tumour. More broadly, the gamma rays from large Cs-137 irradiators are employed to sterilise medical equipment, syringes, and sutures. The penetrating power of the 0.662 MeV gamma ray effectively destroys microorganisms without leaving radioactive residue in the sterile product.
尽管钴-60 在外部射束治疗中更常见，但 Cs-137 曾用于近距离治疗宫颈癌，即将小型密封源置于肿瘤附近。更广泛的应用是利用大型 Cs-137 辐照器发出的γ射线对医疗设备、注射器和缝合线进行灭菌。0.662 MeV γ射线具有足够的穿透力，可有效杀灭微生物，且不会在灭菌产品中留下放射性残留。
10. Safety Precautions and Handling | 安全防护与操作
When handling any gamma emitter, the three principles of radiation safety apply: time, distance, and shielding. Exposure time should be minimised; handling is performed with long-handled tongs to maximise distance; and lead or thick concrete shielding attenuates the gamma beam. In schools, Cs-137 sources are stored in lead-lined wooden boxes and must be signed out under strict supervision. The inverse-square law for gamma intensity is routinely demonstrated, and students must know that dose is measured in sieverts (Sv).
操作任何γ发射体时都应遵循辐射安全三原则：时间、距离和屏蔽。应尽量缩短暴露时间；使用长柄钳以最大化距离；铅或厚混凝土屏蔽可衰减γ射线。在学校中，Cs-137 源储存于铅衬木箱，须在严格监管下登记取用。经常演示γ强度的平方反比定律，学生还需知道剂量以希沃特（Sv）为单位。
11. Environmental Impact and Disposal | 环境影响与处置
Owing to its 30-year half-life, Cs-137 is a significant contributor to the long-term radioactivity of nuclear waste and fallout from accidents such as Chernobyl and Fukushima. It can enter the food chain as it mimics potassium in biological systems. Safe disposal requires vitrification or deep geological repositories. Students should be able to discuss why biological half-life can differ from physical half-life due to metabolic excretion.
由于半衰期长达 30 年，Cs-137 是核废料以及切尔诺贝利和福岛等事故放射性沉降物中长期放射性的重要贡献者。它在生物系统中表现得像钾一样，因此可进入食物链。安全处置需要玻璃固化或深地质处置库。学生应能讨论为何生物半衰期会因代谢排泄而与物理半衰期不同。
12. Summary and Exam Tips | 总结与考试技巧
Cesium-137 unifies key nuclear concepts: beta-minus decay, excited nuclear states, gamma photon emission, activity calculations, and the safe use of radioisotopes. In Edexcel examinations, marks are awarded for correctly balanced nuclear equations, accurate unit conversions, and clear explanations of applications. Always link the penetrating range of gamma rays to the specific use—thicker materials for gamma than for beta, and half-life sufficiently long to provide a stable calibration over years. Practising half-life graphs and decay constant arithmetic will strengthen performance in both Physics and Chemistry papers.
铯-137 将β⁻衰变、激发核态、γ光子发射、活度计算以及放射性同位素的安全使用等核心概念串联起来。在 Edexcel 考试中，正确配平的核方程、准确的单位换算以及对应用场景的清晰解释都能获得分数。永远要将γ射线的穿透射程与具体用途联系起来——γ射线比β射线适用于更厚的材料，并且其半衰期足够长，可在多年间提供稳定的校准。练习半衰期图线和衰变常数计算，将提升物理和化学卷面的表现。
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