📚 Interaction of Radiation with Matter | 物质与辐射的相互作用
When radiation travels through a medium, it may be absorbed, scattered, or transmitted. The way energy is transferred from radiation to matter forms the basis of many detectors, imaging techniques, and radiation protection strategies.
当辐射穿过介质时，它可能被吸收、散射或透射。辐射向物质传递能量的方式构成了许多探测器、成像技术以及辐射防护策略的基础。
1. Types of Radiation and Their Classification | 辐射类型及其分类
Radiation is broadly divided into ionising and non‑ionising types. Ionising radiation carries enough energy to remove electrons from atoms, while non‑ionising radiation does not.
辐射大致分为电离辐射和非电离辐射。电离辐射携带足够能量将电子从原子中击出，而非电离辐射则不能。
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Ionising radiation: alpha particles, beta particles, gamma rays, X‑rays, and energetic charged particles.
电离辐射：α粒子、β粒子、γ射线、X射线以及高能带电粒子。
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Non‑ionising radiation: ultraviolet, visible light, infrared, microwaves, and radio waves.
非电离辐射：紫外线、可见光、红外线、微波和无线电波。
Alpha particles are highly ionising but have a short range; gamma rays are weakly ionising but highly penetrating. The ionising power and penetrating power are inversely related for directly ionising particles.
α粒子电离能力很强但射程短；γ射线电离能力弱但穿透力极强。对于直接电离粒子，电离能力与穿透能力通常成反比。
2. Attenuation of Radiation in Matter | 辐射在物质中的衰减
When a beam of photons passes through a medium, its intensity decreases exponentially with thickness. The linear attenuation coefficient μ measures how strongly the material absorbs or scatters the beam.
当一束光子穿过介质时，其强度随厚度呈指数衰减。线性衰减系数 μ 用于衡量材料对光束的吸收或散射能力。
I = I₀ e−μx
Here I₀ is the initial intensity, I is the transmitted intensity after passing through a thickness x, and μ has units of m⁻¹ (or cm⁻¹). A larger μ means the material is more effective at reducing the beam intensity.
这里 I₀ 是初始强度，I 是穿过厚度 x 后的透射强度，μ 的单位为 m⁻¹（或 cm⁻¹）。μ 越大，材料对光束强度的削减越有效。
The mass attenuation coefficient μm = μ/ρ, where ρ is the density, is also used because it removes the dependence on physical density and is more convenient for comparing materials.
质量衰减系数 μm = μ/ρ 也常被使用，其中 ρ 为密度，因为它消除了密度的依赖性，便于比较不同材料。
3. Photoelectric Effect | 光电效应
The photoelectric effect dominates at relatively low photon energies (typically a few eV to about 100 keV), especially for materials with high atomic number Z. An incident photon is completely absorbed by an atom, and a bound electron is ejected with kinetic energy.
光电效应在较低光子能量（通常几 eV 到约 100 keV）下占主导，尤其对于高原子序数 Z 的材料。入射光子被原子完全吸收，一个束缚电子以一定动能被发射出来。
Ephoton = φ + Kmax
where φ is the work function (the binding energy of the electron) and Kmax is the maximum kinetic energy of the emitted photoelectron. The interaction requires the photon to have energy at least equal to the binding energy of the electron.
其中 φ 是逸出功（电子的束缚能），Kmax 是发射的光电子的最大动能。这一相互作用要求光子能量至少等于电子的束缚能。
Because the photon transfers all its energy to one electron, the photoelectric effect produces sharp energy peaks in detectors and is exploited in X‑ray imaging and radiation spectroscopy.
由于光子将全部能量转移给单个电子，光电效应会在探测器中产生尖锐的能峰，并可用于 X 射线成像和辐射能谱分析。
4. Compton Scattering | 康普顿散射
Compton scattering occurs when a photon interacts with a loosely bound or free electron. The photon transfers part of its energy to the electron and is deflected with a lower energy (longer wavelength). This is an inelastic scattering process.
康普顿散射发生在光子与弱束缚或自由电子相互作用时。光子将部分能量传递给电子，自身发生偏转并能量降低（波长变长）。这是一个非弹性散射过程。
Applying conservation of energy and momentum gives the change in wavelength:
应用能量守恒和动量守恒可得波长变化：
Δλ = (h / mec) (1 − cosθ)
where θ is the scattering angle of the photon, me is the electron mass, and h/(mec) ≈ 2.43 × 10⁻¹² m is the Compton wavelength.
其中 θ 是光子的散射角，me 是电子质量，h/(mec) ≈ 2.43 × 10⁻¹² m 为康普顿波长。
Compton scattering is the dominant interaction in the intermediate photon energy range (around 100 keV to a few MeV) for many elements, and it degrades image contrast in radiology.
康普顿散射是中间光子能量范围（约 100 keV 到几 MeV）内许多元素的主要相互作用方式，它也会降低放射学影像的对比度。
5. Pair Production | 电子对产生
When a photon with energy greater than 1.02 MeV passes near a heavy nucleus, it may be converted into an electron–positron pair. This process requires the presence of the nucleus to conserve momentum.
当能量大于 1.02 MeV 的光子经过重原子核附近时，可能转化为一个电子-正电子对。该过程需要原子核在场以维持动量守恒。
hf ≥ 2mec² = 1.02 MeV
The threshold energy corresponds to the rest mass energy of the electron and positron. The positron quickly annihilates with an electron, producing two gamma photons of 0.511 MeV moving in opposite directions.
阈值能量对应电子与正电子的静质量能。正电子很快与电子湮灭，产生两个方向相反、能量各为 0.511 MeV 的γ光子。
Pair production becomes significant at very high photon energies, especially for absorbers with high atomic number.
电子对产生在极高光子能量下变得显著，尤其对于高原子序数的吸收体。
6. Relative Importance of Photon Interactions | 光子相互作用的相对重要性
Which interaction dominates depends on the photon energy and the atomic number Z of the absorbing material. The following table summarises the general trends.
哪种相互作用占主导取决于光子能量和吸收材料的原子序数 Z。下表概括了一般趋势。
| Interaction | Energy range | Z dependence |
| Photoelectric | Low (eV – ~100 keV) | Strong, ∝ Z⁴ |
| Compton | Intermediate (~100 keV – few MeV) | Weak, ∝ Z |
| Pair production | High (> 1.02 MeV) | Strong, ∝ Z² |
In water or tissue, Compton scattering is often the most important process for gamma rays in radiotherapy energies, while high‑Z materials such as lead enhance photoelectric absorption and pair production, making them useful for shielding.
在水或组织中，对于放疗能量的γ射线，康普顿散射通常最重要；而铅等高 Z 材料会增强光电吸收和电子对产生，因此适合用作屏蔽材料。
7. Exponential Radioactive Decay and Half‑life | 指数衰变与半衰期
Radioactive decay is a statistical process in which the number of undecayed nuclei decreases exponentially with time. The decay law is:
放射性衰变是一个统计过程，未衰变核的数量随时间呈指数减少。衰变定律为：
N = N₀ e−λt
where λ is the decay constant. The half‑life T1/2 is the time taken for half of the original nuclei to decay:
其中 λ 是衰变常数。半衰期 T1/2 是原有核素一半发生衰变所需的时间：
T1/2 = ln 2 / λ
The average lifetime τ is related to λ by τ = 1/λ. The activity A is the rate of decay, given by A = λN, with the becquerel (Bq) as the unit.
平均寿命 τ 与 λ 的关系为 τ = 1/λ。活度 A 是衰变的速率，由 A = λN 给出，单位为贝克勒尔（Bq）。
8. Radiation Dose and Equivalent Dose | 辐射剂量与当量剂量
When radiation is absorbed by a medium, the deposited energy per unit mass is called the absorbed dose D = E/m, measured in grays (Gy), where 1 Gy = 1 J/kg.
当辐射被介质吸收时，单位质量沉积的能量称为吸收剂量 D = E/m，单位为戈瑞（Gy），其中 1 Gy = 1 J/kg。
For biological risk, different types of radiation have different effectiveness per unit dose. The equivalent dose H is defined as:
对于生物风险，不同类型的辐射在单位剂量下有不同的杀伤效果。当量剂量 H 定义为：
H = D × wR
where wR is the radiation weighting factor (1 for photons and electrons, 20 for alpha particles). The unit of equivalent dose is the sievert (Sv).
其中 wR 是辐射权重因子（光子、电子为 1，α 粒子为 20）。当量剂量的单位是希沃特（Sv）。
9. Applications in Medicine and Industry | 在医学与工业中的应用
Photoelectric absorption and Compton scattering are exploited in X‑ray imaging and CT scans, where different tissues attenuate the beam to different extents, producing contrast in the image.
光电吸收和康普顿散射被应用于 X 射线成像和 CT 扫描，不同组织对光束的衰减程度不同，从而在图像中产生对比度。
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Radiotherapy uses gamma rays or electron beams to destroy tumour cells by delivering a high absorbed dose to a localised region.
放疗使用γ射线或电子束，通过向局部区域提供高吸收剂量来摧毁肿瘤细胞。
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PET scans rely on positron emission and subsequent annihilation into two 0.511 MeV photons, which are detected in coincidence to locate active metabolic regions.
PET 扫描依赖正电子发射及其随后的湮灭产生两个 0.511 MeV 光子，通过符合探测来确定活跃代谢区域。
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Industrial radiography uses gamma sources to inspect welds and metal castings for internal flaws.
工业射线照相使用γ源检查焊缝和金属铸件的内部缺陷。
10. Principles of Radiation Protection | 辐射防护原则
Three fundamental principles minimise external radiation exposure: time, distance, and shielding. Reducing exposure time, increasing distance, and placing appropriate shielding between the source and the person all reduce the dose.
减少外照射的三个基本原则：时间、距离和屏蔽。缩短受照时间、增大距离以及在源与人之间放置适当的屏蔽物，都能降低剂量。
Shielding materials are chosen according to the type of radiation: alpha particles are stopped by paper, beta particles by a few millimetres of aluminium, and gamma/X‑rays require dense materials such as lead or concrete.
屏蔽材料根据辐射类型选择：α 粒子用纸即可阻挡，β 粒子用几毫米厚的铝，γ/X 射线则需要铅或混凝土等致密材料。
For internal exposure, containment and preventing ingestion or inhalation are essential, as alpha emitters are especially hazardous inside the body.
对于内照射，密封防护以及防止食入或吸入至关重要，因为α发射体进入体内后尤其危险。
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